Photochemistry of porphyrins and their metal
complexes in solution and organized media
David G. Whitten
Department of Chemistry, Univeraityof North Carolina, Chapel Hill, North Carolina 27514
Con~n~
I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
H. Porphyrins as Photosensitizers in Solution: Electronic Energy Transfer . . . .
A. Metalloporphyrin excited states . . . . . . . . . . . . . . . . . . . .
B. Triplet energy transfer with porphyrins and metalloporphyrins . . . . . .
C. Intramolecular triplet energy transfer in metalloporphyrin complexes . . .
D. Luminescent triplet metalloporphyrins as scnsitizers in quantum chain
processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IlL Photoinduced redox reactions of porphyrins . . . . . . . . . . . . . . . .
A. General redox patterns . . . . . . . . . . . . . . . . . . . . . . . .
B. Electron transfer quenching and exciplex formation . . . . . . . . . . .
C. Photoreduction and oxidation of the porphyrin ring . . . . . . . . . . .
IV. Photoreactions of porphyrins in organized media . . . . . . . . . . . . . .
A. General behavior ofporphyrins in monolayers and micelles . . . . . . . .
B. Ligand photoejection and exchange in ruthenium and iron complexes in monolayer assemblies and micelles . . . . . . . . . . . . . . . . . . . . .
C. Photooxidation of surfactant protoporphyrins in monolayer films, organized
monolayer assemblies and micelles . . . . . . . . . . . . . . . . . . .
V. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
107
108
108
109
1I0
I 16
121
121
122
123
127
127
131
133
136
L Introduction
The photochemical reactions of porphyrins and their metal complexes and
related compounds (chlorophylls, chlorins, etc.) have received much attention
from workers in a wide variety o f areas approaching the subject with various
interests and points of view. It is beyond the scope of this article to attempt to
review the broad topic ofporphyrin photochemistry. In fact, a resurgence of interest in porphyrin chemistry has led to two excellent review series 1.2 both of which
include reviews on the general area of porphyrin photochemistry 3 as well as
some specific topics such as the photochemistry of chlorophyll and its analogs
in membranes as related to photosynthesis. 4 In the present review, we will
restrict our discussion to a limited number of photoproccsses that have been
studied extensively in our laboratories. Much of this work focuses on the special
relationship between the metal and. porphyrin macrocycle in metaIloporphyrin
complexes. In particular, this relationship is manifested in processes involving
light induced energy transfer and redox phenomena. The material covered in this
article will therefore be limited compared to other recent reviews and will deal
largely with work done by our group and investigations related to these studies.
© 1978, by Verlag Chemie International, Inc.
107
108
D. G. Whitten
This review is divided into three main areas. The first two deal with lightinduced reactions of porphyrins and their metal complexes in solution, in
particular energy transfer and electron transfer reactions of the porphyrin. The
third section reviews the reactions of porphyrins in organized media including
monolayer films, monolayer assemblies and micelles and will deal with nonphotochemical processes as well as photoreactions.
II. Porphyrins as photosensitizers in solution: electronic energy transfer
The commonly studied porphyrins fall into two general groups distinguished
by the substitution pattern on the porphyrin ring. The naturally occurring and
biologically important porphyrins such as protoporphyrin IX have substituents
on the eight t-positions of the four pyrrole rings and hydrogen at the four
"bridge" positions and are referred to as octaalkylporphyrins. The synthetically
easy-to-prepare but non-naturally occurring tetraphenylporphyrins and tetraarylporphyrin have hydrogens on the fl-pyrrole positions and aromatic groups at
the bridges. Although there are characteristic red shifts in absorption and
luminescence spectra on going from the octaalkyl to the tetraphenylporphyrins,
the shifts are small even for the free-base porphyrins since the phenyl groups in
most complexes are largely twisted out of the porphyrin plane thus minimizing
both steric and conjugative interactions, a Free-base porphyrins exhibit characteristic fluorescence and phosphorescence from the lowest singlet and triplet states
respectively. The fluorescence has a lifetime in the nanosecond region and can be
observed in both solution and rigid glasses. 3 The triplets of most free-base
porphyrins do not phosphoresce in room temperature solutions but they can be
easily observed by conventional (micro-second) flash spectroscopic techniques;
typical lifetimes are in the range 100-500 tzsec. Interestingly, a significant
deuterium isotope effect has been observed for triplet lifetime for only the N - - H
protons of free-base porphyrins. 6'~ It has been suggested that relaxation of the
triplet to ground state is coupled with interconversion of different tautomeric
forms in the free base. 7 Typical singlet energies for octaalkylporphyrins are in
the range 47-48 kcal tool- x while that for tetraphenylporphyrin is 45 kcal mol- z.
Triplet energies are generally 6--7 keal mol-X lower than the singlet energies. "~
A. Metalloporphyrin excited states
A great number of porphyrin metal complexes have been prepared, particularly during the last 15 years. In many eases, there has been at least a cursory
investigation of excited state properties for these complexes such as the determination of luminescence spectra and lifetimes. In general, for most metal
complexes, it is clear that the excited states are best described as discrete singlet
and triplet states associated basically with the porphyrin ligand but whose
lifetimes and reactivities are strongly affected by the contained metal. While it is
beyond the scope of this review to cover in detail the luminescence behavior of
the numerous complexes prepared to date, a few generalizations are worth men-
Photochemistry of porphyrins and their metal complexes
109
tioning. The complexes of closed-shell diamagnetic metal ions such as Cd(II),
Mg(II), Zn(II), and Sn(IV) are strongly fluorescent but do not phosphoresce in
solution; triplet states are easily detected by flash photolysis and have lifetimes
in the 50-500/~sec range, a'e Complexes of most open-shell metal ions of the first
transition period are either non-fluorescent or very weakly fluorescent and their
triplet lifetimes are very short in solution. In contrast, complexes of open-shell
but diamagnetic metal ions of higher transition periods (e.g. Pd(II), Pt(II),
Ru(II)) show both intense fluorescence and phosphorescence in solution at room
temperature) ,8-z: Triplet lifetimes for some of these complexes range upwards
into the msec range. In several cases, the singlet-triplet splittings are small enough
to allow appreciable repopulation of the singlet such that delayed fluorescence is
a significant channel for excited state deactivation. 9,~z A major part of this
review will deal with photoreactions in which the strongly luminescent triplet of
Pd(II), Pt(II), or Ru(II) is an intermediate.
B. Triplet energy transfer with porphyrins and metalloporphyrins
Since several free-base and metaUoporphyrins have prominent uv and visible
absorption spectra, small singlet-triplet splittings, high intersystem crossing efficiencies and long excited triplet lifetimes, they should be excellent sensitizers for a
variety of photoprocesses. In fact, their use has been relatively limited until
recently. It has been recognized that porphydns play an important role in a number of light induced biological disorders, many of which also involve molecular
oxygen. In several cases, it has been suggested that the key step involves energy
transfer from porphyrin or metaUoporphyrin triplets to ground state (triplet)
oxygen with production of excited singlet oxygen, z2-1~ Hematoporphyrin IX,
protoporphyrin IX, and some metalloporphyrins have been widely used as sensitizers of singlet oxygen in more conventional photo-oxygenation reactions.
That metaUoporphyrins can function well as classical photosensitizers has been
demonstrated in studies of sensitization of azobenzene and thioindigo dyes. zs.z6
Detailed studies of triplet energy transfer by several groups over the last 20
years have had a major impact in elucidating mechanistic details of photochemical reactions and determining properties of excited states. Although several
different types of molecules have been studied as donors and acceptors, including
ketones, lanthanide complexes and other spectroscopically or chemically reactive
systems, one of the most widely used probes has been the cis-trans isomerization
of olefins such as the stilbenes and related compounds. 17-2° These compounds
have been used both as donors and, more widely, as acceptors of triplet excitation
and indeed their use has led to the observation in several cases of unexpected
new phenomena. Probably, the most interesting phenomena are those occurring
where the stilbenes are used as triplet energy acceptors from potential donors
having lower triplet energies than the indicated or "spectroscopic" triplets of the
stilbenes, zT-za The "' spectroscopic" or vertical triplets of the stilbenes have been
estimated from singlet-triplet absorption spectra to be 48 and 57 kcal tool-z for
trans and cis isomers respectivelyZV'~°; those of the 1,2-diphenylpropenes
(~-methylstilbenes) are indicated as 50 and 56 kcal tool -z for trans and cis
110
D. G. Whitten
isomers. 17 Since, as indicated earlier, the porphyrins and their metal complexes
have excited triplets in the 40-45 kcal tool- x range,a their convenient excitation
in the visible region where the stilbenes do not absorb and their relatively long
triplet lifetimes make them particularly suitable low energy sensitizers for
investigation of triplet energy transfer phenomena occurring with stilbenes and
related systems. In investigations conducted in these laboratories, we have
investigated energy transfer phenomena in the metalloporphyrin-stilbene system
in both intra- and intermolecular situations.
C. Intramolecular triplet energy transfer in metalloporphyrin complexes
The zinc, magnesium and cobalt (III) porphyrins are well suited for use in a
study of intramolecular energy transfer since the metal in the simple porphyrin
complex is not fully coordinated. Thus, zinc porphyrins readily accept one
additional nitrogen, phosphorous or oxygen ligand while the corresponding
magnesium and cobalt complexes can accept up to two apiece. 21-2a Although
ligand exchange is indicated to be very rapid for zinc and magnesium complexes
in both the ground and excited state, 2a the cobalt (III) porphyrins do not readily
exchange ligands and it is easy to isolate the diligated complex, MPL2. The
stilbene-like ligands that were chosen for the study of intramolecular energy
transfer were 1-(~-naphthyl)-2-(4-pyridyl) ethene (NPE) and 4-stilbazole. In a
preliminary study, it was found that both of these olefins undergo cis-trans
isomerization under direct and sensitized photolysis. 24 Results with different
sensitizers and the observation of weak singlet-triplet spectra indicate that triplet
levels for the uncomplexed 4-stilbazole isomers are similar to those of the
stilbenes while slightly lower energies are indicated for the isomers of NPE. 25
Both NPE and 4-stilbazole isomers form complexes with zinc and magnesium
porphyrin complexes in which the long-wavelength transitions (porphyrin
~r--,~r* bands) are essentially identical to those of pyddine or piperidine
complexes. Since excess 4-stilbazole or NPE must be added to solutions containing the zinc and magnesium porphyrins, it is not possible to determine the
effect complex formation produces on the ligand transitions; however, for the
cobalt (III) complexes, which do not undergo exchange, a subtraction spectrum
indicates that the ligand ~r~ ~r* transitions are shifted to longer wavelengths
(for NPE the ~,~x shifts from 331 to 353 nm with a corresponding but smaller
shift in the band onset) and that excitation energies may be lowered by several
kcal tool- 1. Similar behavior has been observed for ligand olefins in other metal
complexes.
Studies ofintermolecular energy transfer with the stilbenes and 1,2-diphenylpropenes indicate that these olefins quench sensitizers having energies in the
range 40-45 kcal tool -1 with rate constants in the range k = 104-107 1 tool -1
s-1. la Therefore, at the very high effective acceptor concentrations assumed
for the intramolecular stilbazole-porphyrin or NPE-porphyrin complexes, one
would anticipate energy transfer to dominate radiationless decay of the metalloporphyrin triplet with efficient cis-trans isomerization of the bound ligand as a
consequence. Since photostationary states reported for the stilbenes with
Photochemistry o f porphyrins and their metal complexes
trans-NPE
111
trans-4-stilbazole
sensitizer in the 40-45 kcal m o l - 1 energy range are 50--70% cis, one might expect
that intramolecular sensitization should involve efficient conversion in both
directions. In fact, irradiation o f N P E and 4-stilbazole complexes o f zinc and
magnesium porphyrins (etioporphyrin I and mesoporphyrin I X dimethylester
complexes) does lead to efficient isomerization of the olefinic ligand but the
results are quite different from those obtained in intermolecular studies. 2s First
o f all, although the trans to cis efficiencies are moderate in several cases (Table 1)
Table 1. I$omerization of ligand-olefats in metalloporphyrin complexe~ .~
Porphyrit~
Zn etio I
Zn etio I
Zn eao I
Zn meso IX
Mg etio I
Mg etio I
Ligand.Olef~
O,-.t
O~-,c
Stilbene
4-Stilbazole
NPE
NPE
4-Stilbazole
NPE
0.01
0.4
6.6 + 1
6.6 _+ 1
0.17
3+ 1
0.001
0.001
0.2
.PhotoStationary
State
99% trans
96°7, trans
96% trans
99% trans
95% trans
• Reprinted with permission from D. G. Whitten, P. D. Wildes, and C. A.
DeRosier, I. Amer. Chem. Soc., 94, 7811 (1972). Copyright by the American
Chemical Society.
b Dcgassed benzene solutions irradiated at 25-28 ° with 405 and 436 nm
light.
"5 x 10-s M.
a5 x 1 0 - 3 M.
for the complexed ligand, the cis to trans efficiencies are much greater and in
severalcases substantiallyexceed unity,the anticipated limitfor an isomerization
occurring via radiationlessdecay of the olefintriplet.The photostationary states
wcrc trans-richin all cases and far from the values obtained in intermolecular
sensitization. Perhaps the most striking feature of the intramolecular photoreaction is the finding by flash photolysis that the tripletlifetime of the metalloporphyrin is unaffected by complex formation or the occurrence of the
isomerization process. In factthe triplet-tripletabsorption spectrum obtained for
the complex is unchanged from that obtained for complexes with ligands
112
D. G. Whitten
possessing no low excited states. Several experiments indicate that the metalloporphyrin is the precursor for the isomerization and that the reaction does
require complexation between the metaUoporphyrin and the olefin. For example,
it is found that addition of substancas quenching the porphyrin triplet strongly
quench the isomerization. ~s Addition of bases such as pyridine which compete
for the metal coordination site reduces the extent but does not completely
eliminate the isomerization. The observation that trans-rich photostationary
states are obtained might tend to suggest the intermediacy of radicals or radical
ions which might initiate a radical chain-induced isomerization in the energetically
favored c/a to trans direction. However, such a mechanism, though difficult to
absolutely rule out, appears to be unlikely for several reasons. The quantum
yield for such a process would likely increase rapidly with c/s olefin concentration
yet the limiting quantum yields are reached at concentrations in the 1 × 10 -3 M
range for NPE and are essentially invariant at higher concentration. If a radical
ion were involved in a chain process, an increase in the efficiency with solvent
polarity might be anticipated; however, a change to solvents more polar than
benzene decreases the efficiency in the cases studied to date. Further considerations of the various possibilities for metalloporphyrin ligand electron transfer
indicate that such processes should be highly energetically unfavorable from the
lowest singlet and triplet.
The fact that isomerization occurs readily with the metalloporphyrin localized
triplet as a necessary precursor but without causing observable quenching of the
triplet suggest that the metaUoporphyrin is a photocatalyst and that a mechanism
involving use of the triplet in promoting the isomerization should also include its
regeneration. Although, as indicated earlier, porphyrin to olefin energy transfer
should be likely with the complexes investigated, a simple energy transfer
followed by isomerization concurrent with non-radiative decay can be ruled out
as the probable mechanism. However, a mechanism involving reversible energy
transfer is possible and in fact appears to be the most likely explanation for the
observed phenomena. The basis for the reversible energy transfer mechanism lies
in the particular triplet energy surface for the stilbenes and other 1,2-diarylethylenes and the relatively long triplet lifetimes for these olefins. A potential energy
surface such as indicated in Figure 1 is believed to occur for several olefins
similar to stilbene; the key feature is the broad minimum or double minimum
with a small-barrier occurring between transoid and twisted geometries. For
stilbene triplets, several paths for deactivation are evidently possible. Unassisted
decay evidently occurs only from the twisted state resulting in formation of c/s
and trans ground states in an approximately 3:2 ratio. However, several
molecules can quench the excited state in competition with non-radiative decay
in processes which yield different c/s: trans ratios. Quenching by azulene 2o and by
/~-carotene,26 which is believed to occur by triplet energy transfer, results in
selective production of only trans-stilbene. This has been taken as evidence that
quenching by energy transfer involves only the transoid form of the triplet,
which has a large potentialenergy gap to the ground state,as a potential donor. 2°
In contrast, quenching by oxygen, which probably does not involve energy
Photochemistry of porphyrins and their metal complexes
113
E
0
i
90"
RANS
C15
Figure 1. Proposed triplet (upper) and ground state (lower) potential energy curves
for stilbene as a function of angle of twist about the carbon-carbon central
bond. Reproduced with permission from J. A. Mercer-Smith and D. G.
Whitten, J. Amer. Chem. Soc., in press. Copyright by the American
Chemical Society.
transfer, shortens the triplet lifetime without affecting the decay ratio; here
presumably only the twisted form is q u e n c h e d ) ° Quenching by nitroxide
radicals, also a process not involving energy transfer, leads to a slightly c/s-rich
stationary stateY The key feature in the intramolecular ligand-olefin-porphyrin
complexes is that the metalloporphyrin triplet should lie below the transoid
triplets o f NPE and 4-stilbazole; thus once energy is transferred to produce the
olefin-localized triplet state, the bound metalloporphyrin should be able to
quench the olefin triplet by energy transfer with selective production o f trans
ground state and metalloporphyrin triplet in a process analogous to the intermolecular eases occurring with azulene and B-carotene as quenchers. The overall
mechanism for the process is described by eqs. (1)-(I0) where L * = c/s-ligandolefin and L t = trans-ligand-olefin and MP = metalloporphyrin.
MP-L
Mp1,_
c
Lc
Mp1,_LC
h. , M p 1 , _
k,,
Lt
MP - U a*
MP a * - U + L
~
MP a * - L ~ ~
Mpa,_Lt
MP-
,
(2)
Lt
(3)
MP - L '3.
(4)
*~, M P k,
(1)
Lc
k, ~ M p a . _
MP a* - L ~ ~
MP a * -
MP-
Lc
L to*
(5)
M p a , - Lt
(6)
k, ~ M p a , _
L ~ + Lt
(7)
MP-
L~
(8)
ko, M P -
Lt
(9)
L ta* ..k~,~ ~ ( M P -
L t) + (1 - ~ ) ( M P -
L ~)
(lO)
114
D. G. Whitten
The key steps are the "uphill" energy transfer, eqs. (4) and (5), the reverse
energy transfer eq. (6), and ligand exchange eq. (7). An expression for the
cis --> trans quantum yield using this mechanism may be easily derived (eq. (11)).
k4[k, + kT(L~)][ke + ak~o]4,~o
~c-.tffi [ks + kT(LC)][ks + k~o][k, + ks] - kT(LC)kek4
(11)
Since ligand exchange is rapid compared to metalloporphyrin triplet decay
(k~(L~) >> ks) and the observation of no quenching of the porphyrin triplet by the
olefin-ligand indicates ks >> k~o, the relationship reduces to the much simpler
eq. (12):
(12)
~..,, = k,~,,Jk8
Although a value for k, cannot be measured for NPE or 4-stilbazole,
estimated values (vide supra) based on intermolvcular examples suggest that
k~ > k8; thus with the large indicated value of ~,-o for both magnesium and zinc
porphyrins a predicted value of ~c-.t > 1 can be estimated in accord with the
observed results. 25
A more quantitative picture of the process as well as confirmation of eqs.
(I)-(10) as the mechanism for the photocatalysis can be gained by examination of
intramolecular photosensitization with some modified derivatives of NPE and
4-stilbazole. These ligand olefins, 1-(a-naphthyl)-2-(4-pyridyl) propene (PPP) are
analogs of the methyl-substituted stilbene, 1,2-diphenylpropene. This olefin
undergoes photochemical cis-trans isomerization under direct and sensitized
irradiation in processes similar to those occurring with the stilbenes. 28 Some
major differences occur, however, and these can be attributed to slightly different
excited state potential energy surfaces for the diphenylpropvnes. Figure 2
indicates an approximate potential surface for the triplet as a function of the
angle of rotation about the olefinic bond. The major difference is the presence of
only a single energy minimum corresponding to the near 90 ° twisted geometry.
u5
0
TRANS
90"
C IS
Figure 2. Proposed triplet (upper) and ground state (lower) potential energy curves
for 1,2-diphenylpropene as a function of angle of twist about the central
carbon-carbon bond.
Photochemistry o f porphyrins and their metal complexes
115
The consequence of the single minimum near the ground state maximum is a
very short lifetime for the triplet; unlike the stilbenes, quenching of planar
excited states is not possible for 1,2-diphenylpropene and no "azulene" effect
occurs.2'.8° Thus, for metalloporphyrin complexes of NPP and PPP, the mechanism described by eqs. (1)-(10) would have to be modified to delete the reverse
energy transfer step (eq. (6)). As a consequence, if the isomerization described
above for NPE and 4-stilbazole occurs via energy transfer, the use of NPP and
PPP as ligands should result in some pronounced changes including ff,*t values
lower than unity and the observation of quenching of the porphyrin triplet for
these complexes.
~H
CH3
H3
trans-PPP
trans-NPP
The predicted changes are in fact readily observable with zinc porphyrin
complexes of NPP and PPP (Table 2)) 5,28 The triplet lifetime of the zinc
Table 2. Photoisomerization of 1,2-diphenylpropene analogs
as ligands in zinc etioporphyrin complexesa'~
Ligand-Olefin ~
~Pc-.t
~t~c
PhotoStationary
State
NPP
PPP
0.4
0.1
0,2
0.05
31 70 cis
30% c/s
Reprinted with permission from D. G. Whitten, P. D.
Wildes, and C. A. DeRosier, J. Amer. Chem. Soc., 94, 7811
(1972). Copyright by the American Chemical Society.
Degassed benzene solutions irradiated at 25-28 ° with 504
and 436nm light; [zinc etio], 5 × 10 -5 M.
c Concentration, 5 x 10 -3 M.
etioporphyrin I-cis-NPP complex is reduced to ca. { of the value obtained with
other zinc etioporphyrin I-nitrogen base complexes indicating the introduction
of a new channel for non-radiative decay. Quantum yields for the isomerization
of both NPP and PPP are substantial but below unity in all cases suggesting that
no chain process is involved for these olefins. The $o*t for the zinc etio-cis NPP
complex is 0.4; assuming a 1:1 decay ratio from the NPP excited state this
116
D.G. Whiten
indicates that ca. 807o of the excitation energy delivered to the complex is
degraded by nom'adiative decay of the ligand-olefin triplet. This agrees well with
the calculated value of 84% estimated from a comparison of relative triplet
lifetimes of the c/s-NPP and pyridine complexes. A value for the uphill energy
transfer step (eq. (4)) can be estimated from the same data to be 1.1 × 10' s -1.
Since the potential surface from the c/s sides of NPP and NPE should be very
similar, the values for k, for both complexes should be quite similar. Substitution
of the value for NPP in eq. (12) leads to a "predicted" value for ~c-.t for NPE of
5.S in good agreement with the measured value of 6.6. An interesting aspect of
the study of intramolecular triplet energy transfer in the zinc porphyrin-NPP
complexes was the finding through a study of temperature effects on the quenching process, eq. (4), that the activation energy for the process is only 2.5 kcal
tool- 1.25 Since the difference in "spectroscopic" triplets of zinc porphyrin and
olefin is estimated to be 10-12 kcal tool -~, it is clear that the triplet populated
must not be the cisoid triplet but more likely one of twisted geometry. Not
surprisingly the value of AS* for the process is - 34 eu.
D. Luminescent triplet metalloporphyrins as sensitizers
in quantum chain processes
Having observed a quantum chain process mediated by reversible intramolecular transfer of triplet excitation in the zinc and magnesium porphyrinligand olefin complexes, we thought it would be of interest to determine whether
an intermolecular counterpart of such a process could be observed. Intermolecular quantum chain reactions have been reported for conjugated diene
isomerization processes, ax-~s These reactions occur at high diene concentrations
and are indicated to involve energy transfer between diene triplets and ground
states. Although such a process could conceivably occur at very high stilbene
concentrations, we have seen no evidence for it with conventional sensitizers. The
mechanism proposed for the intramolecular quantum chain process is insensitive
to stilbene concentration above the point where energy transfer to the stilbene is
dominant and the limiting value for the quantum chain is reached at relatively
low stilbene concentrations. Although a number of compounds having energies
in the 40-45 kcal tool-x range could have potentially mediated such a process,
the palladium (II) and platinum (II) porphyrins appeared to be particularly
attractive candidates for such a study for a number of reasons. These complexes,
in which the porphyrin is bound to a d 8 metal ion, are very stable as square
planar complexes and there is little tendency to bind extra ligands, x°.xl Thus the
use of simple stilbenes and 1,2-diphenylpropenes with these porphyrins should
involve no ground state interactions and any sensitization observed should be
purely due to dynamic interaction in the excited state. The strong phosphorescence of these complexes coupled with long excited state lifetimes in solution
indicate that even relatively slow bimolecular energy transfer processes should
occur with moderate efficiencyand be readily detectable,x°'xx
Preliminary experiments indicated that palladium (II) octaethylporphyrin
(PdOEP), palladium (If)mesotetraphcnylporphyrin (PdTPP), and platinum (II)
Photochemistry of porphyrins and their metal complexes
117
m e s o t e t r a p h e n y l p o r p h y r i n ( P t T P P ) do n o t f o r m complexes with the stilbenes or
1,2-diphenylpropenes i n the g r o u n d state i n benzene or p e n t a n e solution, a4
I r r a d i a t i o n o f b e n z e n e or p e n t a n e solutions o f t h e p a l l a d i u m or p l a t i n u m p o r p h y tins with the stilbenes or d i p h e n y l p r o p e n e s leads to cis-trans isomerization o f
the ol¢fin as the only detectable reaction. F o r sensitization o f the 1,2-diphenylpropenes the observed results are completely i n accord with those to be expected
for a conventional sensitization process as indicated in eqs. (13)-(17). 3~ Dynamic
quenching of the metalloporphyrin triplets can be followed by quenching of
MP
~'
MP a*
M P 1.
, MP" (+he)
' M P a*
(13)
(14)
M P a* + c/s-DPP
) D P P a* + M P
(15)
M P a* + trar~-DPP
) D P P a* + M P
(16)
) ~(trans-DPP) + (l-a)(cis-DPP)
(17)
D P P a*
phosphorescence and the quenching constants obtained (Table 3) are in good
agreement with those obtained for the diphenylpropenes with aromatic hydrocarbons and ketones having comparable triplet energies. The photostafionary
Table 3. Quenching of metalloporphyrin triplets by stilbenes and 1,2.diphenylpropene: .~
Sensitizer
])dOEP
(pentane)
PdTPP
(benzene)
PdOEP (benzene)
~,°~sec
555
Method
~-
T
3.28 x 10e
1.37 x 108
1.75 x l0 s
1.00
0.993
0.995
3.34 x 108
1.52 x 10e
5.32 x l0 s
1.03 x lOs
5.02 x IOs
0.982
0.946
0.986
0.998
1.98 x 10s
9.69 x 108
1.00
0.809
eis-stilbcne
k~°
correlation
coefficient
trans-stilbcne
k~
correlation
coefficient
io c
0.999
cis-l,2-diphenylpropen¢
ke
correlation
coefficient
trans-l,2-diphenylpropene
kq
correlation
coefficient
io ~
500
PtTPP
(benzene)
vo a
358
54
io c
io c
I
I
6.68 x 10s
6.81 x l0 s
0.995
Reprinted with permission from J. A. Mercer-Smith and D. G. Whitten, J. Amer. Chem.
Sot., in press. Copyright by the American Chemical Society.
b Degasscd solutions, rate constants determined by linear regression analysis.
c Measured by phosphorescence intensity quenching.
d Measured by reduction of triplet lifetime as determined by photolysis.
• Units of k~ are I tool- 1 s- 1.
118
D. G. Whitten
states (Table 4) are cis-rich and the measured values for P d O E P sensitization
correlate well with those calculated using the mechanism outlined in eqs.
(13)--(17). The measured quantum yield, ~o_., = 0.37, is reasonable for a normal
process and it can be used to estimate a value of 0.84 for the PdOEP intersystem
crossing etficiency.
Table 4. Photostationary
atates for metalloporphyrin sensitized isomerization o f
stilbenea and 1,2-diphenylpropenes ~
Photostationary State, ~ cis
Sensitizer
Stilbene
1,2-Diphenylpropene
PdOEP
PdTPP
PtTPP
17
68
16
37
Degassed benzene solutions, averages of
several concentrations.
Sensitization o f the stilbenes by PdOEP, PdTPP, and P t T P P is more complicated than that for the diphenylpropenes and cannot be adequately explained by
the simple mechanism outlined above. Although the results suggest a quantum
chain process is occurring (Table 5), the behavior in the intermolccular case is
not so simple as that observed intramolecularly with the zinc and magnesium
porphyrins. 34 First o f all relatively rapid quenching o f metalloporphyrin triplets
is observed by both isomers (Table 3). This contrasts with the intramolecular
situation where no quenching o f metalloporphyrin triplets is observed (vide
supra). The quenching is viscosity-dependent as indicated by an approximately
Table 5. Quantum yields for metalloporphyrin sensitization of stilbene
isomerization ~
Sensitizer
Processb
PdO EP
PdTPP
PtTPP
~c~t
1.58 _+ 0.03c
0.30 + 0.01c
1.36 + 0.3"
0.24 :i: 0.01~
1.34 _+ 0.01f
~J~c
Reprinted with permission from J. A. Mercer-Smith and D. G.
Whitten, J. Amer. Chem. Soe., in press. Copyright by the American
Chemical Society.
b Degassed benzene solutions, porphyrin concentrations 5 I00 x 10-5 M; irradiation wavelengths 435 nm for PtTPP and 546 nm
for PdOEP and PdTPP.
c Concentration 0.186 M.
a Concentration 1.44 M.
* Concentration 0.646 M.
I Concentration 2.84 M.
Photochemistry of porphyrins and their metal complexes
119
threefold increase when the solvent is changed from benzene to the less viscous
pentane. Also, in contrast with the intramolecular eases described above, the
observed photostationary states (Table 4) are less rich in trans isomer than the
95-9970 obtained for the NPE and 4-stilbazole complexes. However, the photostationary states for the stilbenes are trans-rich compared to those obtained for
the 1,2-diphenylpropenes and they are dearly much more trans-rich than
predicted using known decay ratio and the mechanism described by eqs. (13)--(17).
The fact that quantum yields for the stilbene isomerization sensitized by
triplets of Pd(II) and Pt(II) porphyrin triplets exceed unity in all three cases
examined (Table 5) indicates a chain process is occurring. However, the
observation of net quenching of the porphyrin triplet suggests that some nonradiative decay by stilbene triplets is occurring. The fact that both quenching
constants and photostationary state composition are influenced by viscosity
suggests that either a discrete excited state complex or at least a solvent-caged
encounter complex is playing a role as indicated in eqs. (18)-(20) where st =
stilbene. If simple porphyrin to stilbene energy transfer were to occur with
MP a* + st ~
(MPa*---st)
(MP---st) 7 - - - * (MP---st a*)
(MP---st a*)
~ st a* + MP
(18)
(19)
(20)
production of a kinetically "free" stilbene triplet, there would be no possibility
of reverse energy transfer or a quantum chain mechanism since the porphyrin
concentrations used (max 5 x 10 -s M) are too Iow to permit "'dynamic'"
quenching to compete with non-radiative decay (k ~ 107 s-l). 85 If an excited
porphyrin-stilbene complex or cage encounter assembly exists in which equilibration between essentially non-perturbed stilbene and porphyrin triplets occurs, an
equilibrium constant can be calculated for the PdOEP-stilbene system:
[stilbene]8*
= 8 x 10 -~
K~q = [porphyrin]a,
Considering relative decay rates for metalloporphyrin and stilbene triplets of
2 x 10a and 107 s -1 a5 respectively, it can be predicted that decay from the
stilbene triplet should be an important process and in fact dominate porphyrin
decay by ca. 10 to 1. Therefore, it is easy to explain qualitatively why quenching
should occur; however, the fact that no quantitative agreement is observed
suggests that the interaction between porphyrin and stilbene in the excited state
may not be a simple one. If the differences between results obtained in benzene
and pentane can be ascribed purely to viscosity effects,a6 the increase in quenching constant and the fraction cis in the photostationary state can be attributed to
enhanced formation o f " f r e e " stilbene triplets in the latter solvent. This suggests
that the stilbene-porphyrin excited state complex or cage encounter complex
must have either a strong preference for decay to trans-stilbene, an equilibrium
120
D. G. Whitten
constant other than that calculated above or a much longer lifetime for the
stilbene triplet in the complex.
Two mechanisms which can account for the observed results are outlined
below:
Mechanism L
MP
h" , MP 1.
' MP a*
MP a* + trans
k't , [MP + st a*]
MP a* + c i s
kg , [ M P + s t a*]
I M P + s t a*]
t'*t, M P + s t 3.
[MP + st a*]
k, , Mpa, + trans
st a*
,~t
MP a*
a(trans) + [l-e] (cis)
*[ , MP
Mechanism IL
MP ~
MP 1.
, MP a*
MP a* + trans
kl , [MP, st] a*
MP a* + cis
kg , [MP, st] a*
IMP, st] a*
k~t, MP + st a*
[MP, st] a*
k,
[MP, st] a*
~ (l-~) cis + a(trans)
Mpa, + trans
*°~ ~ MP + (1-fl) cis + [3 trans
For mechanism I in which [MP + sP*] represents a loose or cage-encounter
complex, the relationships given by eqs. (21)-(23) can be readily derived.
4~.t = 4~,~ kp + ¢k~t
kst
(21)
~,.c = ¢ , ~ (1 - a)
(22)
[trans]
=
k~ a + k f l k , t
(23)
The mechanism allows for the quantum chain process, the observed viscosity
effect, and the trans-rich photostationary state. Calculated and measured
photostationary states are in reasonable agreement. However, values of ,~t.o
calculated using the well-established value of 0.41 f o r , and measured ~ o are
substantially higher than the measured values with PdOEP and PdTPP. Thus, it
appears that an additional decay channel beyond that outlined in mechanism I
must be occurring. Mechanism II, in which IMP, st] °* is used to represent a
"tighter" excited state complex than the simple encounter complex of mechanism
I, adds an additional decay channel, kox, which likely involves selective production
Photochemistry of porphyrins and their metal complexes
121
of trans-stilbene ground states. This allows for a lower ~t,~ than mechanism I
predicts and also can account partially for the enhanced quenching in the
intermolecular case as compared to the intramolecular complexes described in
Section II C. Otherwise, mechanism II predicts qualitatively similar results to
those of eqs. (21)-(23) obtained for mechanism I but with expressions that are
somewhat more complex.
In conclusion, the intermolecular sensitization ofstilbene isomerization by the
platinum and palladium porphyrin triplets appears to involve several complications which are described most simply by mechanism II in which a discrete
complex intervenes. The complex evidently can dissociate to give free stilbene and
free metalloporphyrin triplets but it also has competitive non-radiative process of
its own. Apparently, the complex formation induces a decrease in the nonradiative rate of the stilbene-localized triplet. Consideration of redox potentials
for stilbcne and the metalloporphyrins indicate that the complex is almost
certainly not strongly charge-transfer in nature. Although there is clearly not
enough evidence available to permit a description of the complex, a reasonable
possibility appears that complex formation involves a configuration in which the
~.-electron systems of trans-stilbene and the metalloporphyrin overlap in something close to a "sandwich" arrangement. This would be in agreement with the
known tendency of metalloporphyrins to associate in ground and excited states 87
and also in accord with lack of evidence for such a complex in the intramolecular
complexes (where direct bonding of the olefin-ligand to the metal through
nitrogen precludes such an interaction). 25 A complex of this sort would be
anticipated to have a longer lifetime, particularly if complexation stabilizes the
trans form of the stilbene relative to the twisted configuration. Decay of the
complex might be expected to give almost exclusively trans-stilbene.
The results of the inter- and intramolecular sensitization studies with
metalloporphyrin triplets as potential energy donors suggest that triplet energy
transfer can be considerably more complicated than previous experiments have
suggested. It appears reasonable that complex formation of other than a charge
transfer type can play a general role, particularly where relatively long-lived
excited states are involved in low energy sensitization processes. If formation of
such a complex involves an alteration of excited state potential surfaces in the
donor or acceptor as is observed here for stilbene, appreciable modification of
photoreactivity may occur.
Ill. Photoinduced redox reaction of porphyrins
A. General redox patterns
It is now fairly well established through a number of different investigations
that porphyrins and their metal complexes undergo a series of (generally)
reversible one-electron oxidations and reductions, as For metal complexes these
reactions can involve either the metal or the ring as the site of the redox reactions
and it has been found in several cases that interesting phenomena involving
"redox isomerism" and cooperativity can result via these reactions. *~.45 In
122
D. G. Whitten
recent years, it has been found that excited state redox reactions can occur as a
general process for both organic molecules and metal complexes. In several
cases, these reactions have been found to involve very low activation energies and
in examples where the excited state energy exceeds the energy required to drive
the ground state redox process, excited state quenching via redox can proceed at
rates very close to diffusion-controUed. .7.48 Given the rich array of redox levels
available for porphyrins and metailoporphyrins and their long-lived excited
states, it is not surprising that quenching of porphyrin and metalloporphyrin
excited states by electron donors and acceptors is a prominent process.
B. Electron transfer quenching and exciplex formation
Although quenching of porphyrin excited states by both electron donors and
acceptors has been observed, the more thoroughly studied process has been
quenching by electron acceptors. It has been found that both excited singlets and
triplets can be quenched readily at rates which correlate with ease of quencher
reduction. 4a'5° In polar solvents such as N-methylformamide, acetonitrile, and
ethanol, it is clear that both singlet and triplet quenching by electron acceptors
such as nitroaromatics, pyridinium ions, and quinones produces free ions which
generally recombine with back electron transfer to yield ground states of the
starting material as outlined in eqs. (24)-(27). Rates of quenching are generally
p
h , , p1.
, pa,
px. + A"
' P f + A .~-1
p a . + A"
, P f + A .n-x
P* + A . ~-x
, P+A"
(24)
(25)
(26)
(27)
near diffusion controlled provided the energy of the excited state equals or
exceeds the energy of the redox state as determined by eq. (28) developed by
En+,A,,-I = Elt2(P/P +) - EltS(An-1/A) + 0.10 + 0.10eV
(28)
Weller and co-workers for aromatic systems. For example, quenching of the
luminescent triplet of PdOEPby the electron acceptor paraquat 2 + (N,N'-dimethyl4,4'-bipyridine) occurs to give the products PdOEP + and paraquat + with a
rate constant k = 1.5 x 109 1 mol -x s-1. 5x The bimolecular back reaction,
which can be observed by flash photolysis, occurs with a rate constant k = 1.4 x
10~ 1 mo1-1 s -I. Excited state electron transfer reactions with free base and
metalloporphyrins can be followed by fluorescence or phosphorescence quenching in the case of luminescent complexes. For non-luminescent, but long lived
triplets such as free base porphyrins or the zinc (II), tin (IV), or magnesium (II)
complexes, direct quenching can be easily observed by flash photolysis, s2
Interestingly, however, we have recently found that even short-lived nonluminescent excited states not detectable by flash spectroscopic techniques can be
quenched by electron transfer to yield unstable ionic products. 63 For example,
we have found that Ru(TPP) (pyridine)2, which does not emit measurably at
room temperature in N,N-dimethylformamide, can be quenched by 0.05 M
Photochemistry of porphyrins and their metal complexes
123
Ru(NHa)] + to yield transient spectral changes consistent with formation of
Ru(TPP) (pyddine)~" and Ru(NHa)~ +. The transients formed decay with secondorder kinetics yielding a value k = 1.2 x 10a 1 tool -1 s -1 for the back electron
transfer process. 53 The possibility of quenching very short lived non-luminescent
excited states demonstrated by this reaction offers the prospect of examining
numerous electron transfer processes not previously observable.
In non-polar solvents, and especially for neutral acceptors, quenching of
porphyrin excited states readily occurs, but in many cases no production of ions
can be observed. 4',52 In several cases, electron transfer quenching doubtless
occurs but the initial product is an ion-pair whose separation to free ions is
inhibited in low dielectric constant media. 52 With singlets of zinc, magnesium,
and free-base porphyrins quenching by electron acceptors in non-polar solvents
produces no transient detectable by conventional (microsecond) flash photolysis.
Evidently, the ion-pair has a lifetime too short to permit its detection on this
time scale. For quenching of triplet states of zinc and magnesium porphyrins, the
situation appears more complicated both for the quenching process and with the
behavior and type of transient formed.
As mentioned above, quenching of free base, zinc, and magnesium porphytins by electron acceptors occurs at near diffusion-controlled rates provided the
"redox state" lies at or lower than the energy of the porphyrin triplet. From the
behavior observed with several other types of systems, it would be anticipated
that rates of quenching should fall off when the energy of the redox state exceeds
the triplet energy of the substrate porphyrin and that the fall off should give a
linear relationship between log kq and AE with a slope = - 1/2.3RT. Although a
monotonic decrease in k~ with increasing AE is observed, the fall-off is not as
rapid as expected; and in several cases a near diffusion controlled quenching rate
is observed where AE is several kcalmol-L 49'5° In several cases, triplet
quenching produces relatively long lived transients having spectra similar to but
slightly different from the porphyrin triplet. 49'5° These transients appear to be
themselves excited states since they can be quenched by substances such as
azulene or tetracene having very low-lying triplets (in contrast the ions produced
in polar solvents cannot be quenched). The most reasonable explanation for the
above results is that quenching of the porphyrin triplets in non-polar solvents can
occur by either complex formation or electron transfer. The product of the
quenching can be properly called an "exciplex" in both cases but it is clear that
formation of the exciplex in the cases where the "redox state" lies well above the
triplet in energy must involve other than exclusively charge transfer interactions.49
Although we have observed such behavior with metalloporphyrins and a variety
of acceptors, the results do not seem a property of porphyrins alone since
quenching of the anthracene triplet by the same acceptors leads to similar
results.~,, so
C. Photoreduction and oxidation of the porphyrin ring
Although, in many cases, the photoreduction and oxidation reactions of
porphyrins and their metal complexes consist simply of reversible electron
124
D. G. Whitten
transfer reaction, there are several examples of reactions where the ring can be
photooxidized or reduced. This section will deal with two specific examples: the
photoreduction of tin (IV) porphyrins which appears to be a fairly general
reaction for a number of different tin porphyrins and certain other metal
complexes and the photooxidation of protoporphyrin free base which appears to
be a fairly specific process but one of moderate biological significance.
The tin (IV) porphyrin photoreduction can be mediated by irradiating
solutions of the metalloporphyrin in the presence of a variety of reducing agents
including SnCIa.2H20, 54.55 N,N-dimethylaniline, 56 triethylamine 56 and other
aliphatic amines) 7 The reaction produces in consecutive steps first chlorins
(ring-reduced dihydroporphyrins) and subsequently the corresponding victetrahydroporphyrin or/sobacteriochlorin, eq. (29). We have studied the reaction
H
H
H
(29)
most thoroughly with SnCI2 as the reducing agent. B5Under these conditions, we
find that the porphyrin triplet is the reaction precursor since it is efficiently
quenched by SnCla while the fluorescent singlet is unaffected under the reaction
conditions. When octaalkyl porphyrins such as tin (IV) complexes ofetioporphyrin I, mesoporphyrin IX or octaethylporphyrin are reacted, an intermediate in
which the bridge positions have been reduced can be detected. With these
compounds, reduction apparently proceeds via electron transfer quenching
(whether one or two electrons are transferred in the initial step is uncIear)
followed by proton transfer to yield a bridge-reduced species which subsequently
rearranges to give the chlorin. The second reduction (evidently) proceeds by an
analogous mechanism. The proton source in the SnC12.2H20/pyridine reactions
is the water of hydration; deuterium incorporation, predominantly into the
bridge positions, results when SnCI2.2D20 is used. 55
The reaction proceeds with very low quantum efficiency when the octaalkyl-
Photochemistry of porphyrins and their metal complexes
125
porphyrin complexes are used.ss W h e n MS-tctraphenylporphyrin tin (IV) is the
substrate the quantum yield is approximately I00 times greater,s8 Although
tetraphenylporphyrin complexes are generallyeasier to reduce than the corresponding octaalkylporphyrins,g8 the greater case of reduction does not appear
itselfto simply account for the greater efficiencysince under the reaction
conditions used even the octaethylporphyrin tin (IV) tripletis more than 9 0 3
quenched by SnCl2. Preferential reactivity of the intermediates to chlorin
products and/or greaterstabilityof the intermediatesvery likelydoes play a role.
Interestinglyitis found that simple mctaIlationof MS-tctraphcnylporphydn by
SnCI2.2H20 in dcgassed pyridine solutionleads to quantitativeformation of the
chlorin in the dark.se Here, presumably as with octaalkylporphyrins,the first
product produced by treatment of the free-base with SnCI2 is the porphyrin tin
(II) complex which is a "redox isomer" of the porphyrin tin (IV) dianion.
Although the porphyrin tin (II) complex can be isolated from octaethylporphydn
in the absence of water, metallation of OEP with SnCI2.2H20 leads directly to
the tin (IV) porphyrin and not to the chlorin suggesting a different partitioning of
intermediates for the different porphyrins.
As mentioned above, we have observed similar reactivity for tetraphenylporphyrin with a variety of reducing agents including SnCI2, tricthylamine and
N,N-dimethylaniline. Although we have not performed extensive experiments to
determine intermediates involved in the reaction with amines, a similar mechanism involving the amine as a quencher of the mctalloporphyrin triplet by electron
transfer appears reasonable. In related experiments, Harel and Manassen s7 have
observed intermediates detectable by esr produced by photolysis of Sn (IV)
tetraphenylporphyrin in benzene-N-methylpyrrolidine. They suggest the role of
porphyrin free radicals in the reduction which are formed by decay ofa porphyrinamine exciplex. Interestingly, they have found that intermediates occurring in the
tin tetraphenylporphyrin photoreduction can be trapped chemically by added
substrates such as nitrobenzene. 57
The photooxidation of protoporphyrin in the presence of molecular oxygen
has been known for some time although details of the reaction and structure of
the products have only recently been determined.Sa'sa Inhoffen and co-workers 60
were able to show that the major products formed in solution were the isomeric
hydroxy aldehydes as shown in eq. (30). The mechanism for the reaction is likely
as shown in eqs. (31)-(34) where PP = protoporphyrin IV dimethyl ester and
PPO2 is the photooxidation product "photoprotoporphyrin" consisting of the
two isomers shown in eq. (30). The initial adduct is believed to be a cyclic product
of oxygen 1,4-addition to the diene unit of rings 1 or 2. 60 The reaction proceeds
with relatively low efficiency(~ ranges from 0.006--0.03 depending upon solvent) ~a
and evidently is retarded under conditions where more reactive acceptors of
singlet oxygen are present, la,1~
Several features of the photooxidation of protoporphyrin are unique and
interesting. Whereas most porphyrin photooxidations result in a net bleaching in
which ultimately colorless products are formed, the protoporphyrin reaction
proceeds in good yield to form the indicated products which are relatively stable
126
D. G. Whitten
~
CH=CH2 CHa
OH/cHO
CH=CH2 CHa
H
3
C
~
CH=CH2
+
H s C ~ - - C H a
CH2
I
CH2
I
COOCHa
HaC ~ ~ ~ ' - C H a
CH2
1
CH2
I
CH2
CH2
CH2
CH2
I
I
I
I
COOCH.~ COOCH3
COOCHa
CHa
CHa
I
/~,~OH/CHO
(Z.)
2 CH3
COOCHa
i
pp
'~ , pp1,
ppa, + O2
PP + O12"
"Adduct" ~
,
,
,
'
ppa,
PP + O~*
"Adduct"
PPO2
(3o)
CH
U
CH2
(31)
(32)
(33)
(34)
to further irradiation. Interestingly the oxidation essentially stops with incorporation of one molecule of 02 even though a similar reaction to yield a doubly
oxidized product would seem reasonable. The failure of the photoproduct to
react with oxygen photochemically could be due to an inertness of the photoproduct to reaction with singlet oxygen or to a failure of the photoproduct to
sensitize singlet oxygen. While there is presently no evidence regarding the
former possibility, it appears that the latter may play a role. Although the
photoproduct is strongly fluorescent, we have been unable to detect a triplet for
it by flash photolysis. 61 Since singlet oxygen is produced only via triplet semitizers, 62'6a the presence of a very short-lived triplet for photoprotoporphyrin
could account at least in part for the product stability. The ir spectrum of the
photoproduct indicates strong intramolecular hydrogen bonding in the hydroxyaldehyde 6° which could play a role in rapid deactivation of the triplet.
In our reinvestigation of the protoporphyfin photooxidation, we have
Photochemistry of porphyrins and their metal complexes
127
observed formation of a second product having a "porphyrin-like" electronic
sp~trum but one whose absorption maxima are shifted ca. 22 nm to longer
wavelengths. Although this product, which appears to be a formyl derivative
formed via 1,2 photoaddition of O2, is a very minor product in solution photooxidation, it is produced in relatively higher proportions in the reaction of
surfactant protoporphyrins in films, micelles, and monolayers which will be
discussed in the next section. This product is not formed from the normal
photoproduct in the reaction and we have not been able to effect its interconversion with the normal product (in either direct) by treatment with acid, alumina,
silica gel or other potential catalysts.
IV. Photoreactions of lm~hyrins in organized media
,4. General behavior of porphyrins in monolayers and micelles
One of the chief goals of much of the research involving porphyrins, their
metal complexes, and related molecules has been the construction of relatively
simple model systems which simulate the behavior of the more complex biological
systems in which porphyrins or related molecules such as the chlorophylls serve
as prosthetic groups. The development of simple models has been hampered to a
great extent by the strong dependency of the behavior of the porphyrins upon
environment. For example, the reversible binding of molecular oxygen to ferrous
porphyrins in the hemoproteins such as hemoglobin and myoglobin is not readily
simulated by simple iron (II) porphyrins in solution. The recent preparation of
successful models for hemoprotein behavior has only been accomplished by the
construction of porphyrins containing special substituents such that (a) a hydrophobic "pocket" not containing a tightly-bound ligand is present and (b) bimolecular contact of oxygenated iron (II) porphyrins culminating in the formation of
oxygen bridged (/~-oxo) dimers is inhibited, e4-66 For some time there has been
evidence that in concentrated solutions as well as in solid or colloidal systems
chlorophylls and related pigments can form aggregates having properties quite
different from isolated monomers.67 Recently, some of the reaction centers from
photosynthetic organisms have been investigated and it has been suggested that
the active " t r a p " probably involves a pair (or perhaps even a trimer) s6-7° of
chlorophyll molecules which donate an electron to an acceptor molecule following excitation of the dimcric site by energy transfer from the light absorbing or
"bulk" pigments. The luminescence of chlorophyll and related pigments is
particularly solvent dependent; for example, in non-polar solvents almost no
fluorescence is observed while a change to polar solvents or addition of coordinating molecules is accompanied by a dramatic increase in the fluorescence. 71
Although some information concerning the state of aggregation and the immediate environment of the complex has been obtained in certain cases, it is often
difficult to obtain definitive evidence as to the origin of the luminescence changes
and the structure of the complexes in solution. Studies of chlorophyll and some
metalloporphyrins in membranes and colloidal systems clearly suggest that
aggregation plays a major role and that the structure and size of these aggregates
128
D. G. Whitten
are effected strongly by polar ligands coordinating with the metal. 4 Chlorophyll
itself is well known to exhibit surfactant properties such that monolayer films are
formed by spreading on a water surface. Langrnuir TM found that these films can
be transferred to various rigid supports and that the usual solution fluorescence
disappeared in films and assemblies. Subsequent work with chlorophyll 73-~s has
shown that rapid degradation of the chlorophyll occurs in the films unless
precautions are taken. 78 In the films formed from pure chlorophyll the molecule
is folded such that both the bulk of the porphyrin ring and the phytol chain are
in the "hydrophobic" region and "liquid-like" properties are reported. 7°-76
When the phytol chains of chlorophyll are replaced by methyl groups as in
methyl chlorophyllide, monolayers are also formed but these are less stable and
absorption spectra indicate more interaction between the porphyrin rings. 74'76
It has also been found that mixed monolayers and membranes containing
chlorophyll can be formed. For example bacteriochlorophyU can diffuse into
membranes with a dramatic shift in the fluorescence from the solution value
(785-790 nm) to the in vivo maximum at 885 nm. 77 In contrast to the earlier
reports by Langmuir,TM Krasnovsky TM reports that solid films of chlorophyll
fluoresce with a A,.~xnear the in vivo value.
Techniques for the preparation, manipulation and study of monolayer films
and assemblies have been greatly developed and refined during recent years. 7~'al
We have recently begun an investigation of porphyrin chemical and photochemical reactivity in films, micelles and monolayer assemblies using the
techniques to isolate porphyrins and their metal complexes in a controlled
environment. We have found these techniques particularly useful in the preparation of reactive intermediates by ligand photoejection such that these intermediates are "trapped" and can be diverted from their usual solution reactions. We
have also examined interfacial reactions occurring between porphyrins immobilized in a micelle, film or assembly and reagents and substrates present in a
solution or vapor phase.
In our investigations, we have prepared and utilized the surfactant and
porphyrins shown below, 1-5. Some investigations have already been carried out
with monolayer assemblies generated from these compounds. Results of these
investigations give some idea of what sort of information can be obtained
regarding environmental effects on luminescence and reactivity. For example, it
has been found that both films and assemblies can be readily formed from the
free base of 1. However, both the absorption and fluorescence spectra obtained
for the assemblies of I are quite different from those obtained in dilute solutions
of 1 or other free base porphyrins at room temperature.82 The spectra are
however very similar to those obtained for concentrated solutions at low
temperatures ( ~ -80°C) where thermodynamic data suggest a dimerization is
occurring. A study of the surface area/molecule gives a value of ,-, 56 A 2 for
closely packed (23 dyne/era) spread films on water which is about ½ the anticipated area of 120 A 2 obtained from crystallographic data. Thus the results
suggest that the films contain dimeric units where 2 porphyrin molecules are
stacked face to face. 82 This evidently results from a packing phenomenon and
Photochemistry of porphyrins and their metal complexes
129
should probably occur with most metal complexes as well. Thus with complexes
of 1, formation of monolayer assemblies allows construction of dimedc units
having known and controlled geometry. Dimedc structures are also formed when
1 is incorporated into micelles with octyl tdmethylammonium bromide (CTAB)
as a host. The absorption spectrum in the CTAB micelles is nearly identical to
that obtained in the assemblies. A dimeric structure, based on surface pressurearea measurements and absorption spectra, is also indicated for the free bases of
2and 3.
Surface-pressure area isotherms have also been measured for some of the
metal complexes. Although by no means all of the possible luminescent complexes of I have been prepared or studied, it appears that packing into dimeric
units is a general phenomenon more or less independent of the central metal ion.
CH2CHa CHa
CH2CHa
CHa
CH2
I
ll2
CH2
[
CH2
I
COODHC
CH2
I
CH2
I
C02OHC
CH2
[
CH2
I
COO--C18Ha7
COOC~Ha~
1
i
2
COOC18Ha7
Ha7C~802C
COOC~aHa7
COOClaHa7
3
130
D. G. Whitten
(~xsHsl
Cx~Hax [ ( - ~ [
I
CO
I
CO
I
t,,~'~--NH
" Y
~
a , a )t
Cx,Hal
I
CO
I
NH--CO--ClsHal
a # , am=Isomer
4
CO2~DHC
C02--DHC
C02DHC
Photochemistry of porphyrins and their metal complexes
131
Preliminary experiments suggest that 2 and 3 and their metal complexes also
exist in dimeric units although more study will be necessary to determine the
precise spatial relationship between adjacent chromophores. Apparently in 1-3
the area of the hydrophilic porphyrin chromophore is too large to be effectively
balanced by the hydrophobic hydrocarbon side chains. The stacking of two
porphyrin rings face-to-face, perhaps in a staggered arrangement, results in a
better balanced and more stable assembly.
That such geometry can control reactivity is indicated by a recent study of the
iron (III) complexes of I and 3. 88 Both iron complexes are clearly monomeric in
solution and show "normal" solution spectra. However, when monolayer
assemblies are constructed, the assemblies show spectra nearly identical to the
"p-oxo-dimer," PFe-O-FeP, ofthe parent porphyrins. Extraction of the porphytin from the assemblies confirms that reaction of the monomer to give the
p-oxo-dimer has occurred. A careful study of the process indicates that the dimer
is formed rapidly in the spread films on water, sa Evidently both the rate of
reaction is accelerated and the equilibrium is dramatically shifted to favor the
dimer in the film.
In an effort to avoid dimerization, the free base and iron complexes of 4 have
recently been prepared, s' With the free base of 4 excellent films and monolayer
assemblies can be obtained for mixtures with arachidic acid. Here the absorption
spectrum is nearly identical to that obtained for dilute solutions and most
evidence indicates assemblies in which the porphyrin is monomerically dispersed
are obtained. 8' The iron (III) complex of 4 also exhibits spectra in assemblies
that are nearly identical to those obtained in dilute solution. More importantly,
the iron (III) complex of 4 does not form p-oxo-dimer either in thin films or in
the assemblies. The fact that 4 is monomerically dispersed in the assemblies has
enabled us to prepare the corresponding iron (I]) complexes which, in the
assemblies, exhibit properties which may make them reasonable models for
hemoprotein systems,s4
B. Ligand photoejection and exchange in ruthenium
and iron complexes in monolayer assemblies
The photoejection of ligands such as carbon monoxide has been investigated
for a large number of metal complexes, including iron porphyrins. In many cases,
ligand photoejection generates a very reactive species which may react further
with a variety of reagents, especially potential new ligands or electron donors)
In solution studies, we found that the highly thermally stable CO complexes of
ruthenium (II) porphyrins undergo photolysis in the presence of reagents such as
pyridine, aliphatic amines, ethers or water to yield the corresponding complex in
which the oxygen or nitrogen ligand has replaced CO. 9 For several of the oxygen
ligands such as tetrahydrofuran, the complexes are unstable and in the dark in
the presence of even small amounts of CO they revert to the starting CO complex.
Although the reaction proceeds with very low overall quantum efficiency, primary
ligand ejection appears to be a moderately efficient process since an intermediate
subsequent to the phosphorescent state can be readily detected by flash photolysis.
132
D. G. Whitten
Attempts to isolate the CO-free ruthenium porphyrin in the absence of potential
ligands have been only marginally successful since even under reduced pressure
recapture of CO or scavenging of traces of potential ligands occurs, g,85
The possibility of isolating porphyrins and their metal complexes in monolayer films or assemblies suggested that ligand photoejection processes might
proceed under these conditions to afford different results and products not
obtainable in solution. Thus in the absence of a solvent, cage recombination
processes should be minimized and the retardation of diffusion might together be
expected to facilitate isolation of the reactive intermediates not isolable in
solution. Toward this goal, we prepared surfactant derivatives of Ru (II) CO
mesoporphyrin IX as the bis(dihydrocholesteryl) and bis(octadecyl) esters.
These complexes afforded good monolayer assemblies when spread as mixtures
with dodecanoic (arachidic) acid. Assemblies of the ruthenium porphyrin CO
complex gave spectra nearly identical to those obtained in solution,aa On
irradiation of these assemblies in vacuum with visible light, there is a spectral
change similar to that occurring transiently in solution under flash photolysis;
the product formed is stable indefinitely under vacuum but reacts instantaneously with CO to regenerate the initial spectrum. Admission of nitrogen to the
species generated from photolysis of the CO complex in the assemblies produces
still a further change in the visible spectrum. The product from nitrogen addition
can itself be photolyzed in vacuum to regenerate the species formed by photolysis
of the CO complex. A product having the same spectrum as that produced by
photolysis of the CO complex in vacuum and subsequent addition of nitrogen
can be produced by photolysis of the CO complex in a nitrogen stream, as The
results are most consistent with photolysis of the CO complex to yield an isolable
ruthenium porphyrin in the assemblies in which a reactive site at the metal is
generated. The overall chemistry is summarized in the Scheme below. Similar
hv
N2 Stream
PRuCO
h, ,
co
PRu
vgguum
,
1
~' -~ PRuN2
~,
[
( VagUUlll
hv
CO atmosphere
]
reactivity has been observed when oxygen replaces nitrogen as the substituting
agent, ae The PRu also reacts rapidly with ligands such as pyridine in the
assemblies to generate a product having a spectrum nearly identical to that of the
pyridinate complex generated by photolysis of the CO complex in solution. 9 The
PRuN2 complex prepared in the assemblies also reacts rapidly with pyridine
vapour or pyridine from an aqueous pyridine solution contacted with the assemblies to yield what is evidently the pyridine complex.
The most interesting aspect of these experiments is the isolation and selective
reaction of the reactive PRu species which is afforded by the monolayer technique
in this case. Thus what is only an intermediate in solution can be isolated in the
Photochemistry of porphyrins and their metal complexes
133
assemblies and made to react selectively with nitrogen in oxygen in reactions we
have not yet observed in solution. An obvious choice for extending the study
is an investigation of the corresponding iron (II) porphyrins. As mentioned
above, the strong tendency to dimerize has been a perplexing problem in the case
of the iron porphydns since the iron (III) porphyrins rapidly and irreversibly
form reoxodlmers in the assemblies. 8a Fortunately, we find that assemblies can
be formed from the iron (III)"picket fence" porphyrin tetrahexadecanoic acid
amide 4 in which the porphyrin is monomerically dispersed and ~oxodimers are
not formed. Preliminary investigations show that in the assemblies the iron (III)
bromide is readily reduced by pipiridine vapor to yield a relatively stable iron (II)
complex. Our experiments in this area are still incomplete but preliminary
evidence suggests that labile CO and O2 adducts of the iron (II) complex are
readily and reversibly generated by exposure of the complex to moderate
pressures of these gases, a~
C Photooxidation ofsurfactant protoporphyrins in monolayer
films, organized monolayer assemblies and micelles
As outlined in Section III-C, the oxidation of protoporphyrin IX free base is
a prominent and relatively unusual photooxidation reaction whose mechanism
involves several steps and at least two electronically excited species, the porphyrin
triplet and ringlet excited oxygen. We felt that this reaction might be especially
attractive to study in membrane-like organized media for a number of reasons.
For example, it might be anticipated that restriction of diffusion or molecular
motion might either inhibit the reaction entirely or lead to different products.
Secondly, if the reaction were to occur, the rate and efficiency might be quite
medium-sensitive and it should be especially interesting to determine the role and
effectiveness of potential quenchers and scavengers in the different environments.
The surfactant photoporphyrin IX used in this study was the bis(dihydrocholesteryl) ester, Proto IX(DHC)=. Both the Proto IX(DHC)= and the corresponding dimethylester, Proto IX DME, were found to undergo the "normal"
photooxidation in air-saturated methylene chloride solution to yield predominantly the green hydroxyaldehyde6°; a small amount of the " r e d " product in
which a formyl group has replaced the vinyl group was also obtained but it was
estimated to be no more than a few percent ( ~ 1.5) of the total obtained. The
Proto IX(DHC)= was found to form stable films and monolayer assemblies in
mixtures with dodecanoic acid; however, the spectra obtained indicated that the
porphyrin was dimeric in the assemblies.a2 Irradiation of both the assemblies and
films led to the formation of green photooxidation products. The product
formed in the assemblies was separated by medium-pressure liquid chromotography; although the product consisted chiefly of the two products formed in
solution photolysis there was substantially more (ca. 40°7o of the total) of the
" r e d " product. The major product was the mixture of hydroxyaldehyde
isomers. 61 Photolysis of Proto IX(DHC)2 in monolayer assemblies was carried
out under a variety of conditions. Rapid formation of the photooxidation
product occurred for a single outer layer ofporphyrin or for assemblies containing
134
D. (7. Whitten
the porphyrin "submerged" under several layers of dodecanoic acid. Photooxidation of assemblies immersed in water proceeds at comparable rates to those
obtained for the same assemblies surrounded by air. That the reaction does
require oxygen was verified by the finding that assemblies irradiated under
vacuum are photostable, sl The finding that what are evidently formyl products
are formed in enhanced proportions in the monolayer films and assemblies is
especially striking and illustrative of the influence the monolayer environment
can produce. The formyl product likely arises from 1,2-addition of singlet oxygen
as shown in eqs. (35) and (36). 1,4-Addition to the diene unit to form the
0~0
Os)
O~O
+ CH2=O
(36)
hydroxyaldehyde is reasonably the preferred path in unhindered non-viscous
solutions. However, in the monolaycr films and assemblies,it is likelythat the
tight packing into dimeric units inhibits the 1,4-addition while stillallowing
the less favorable 1,2-addition to occur and thus giving enhancement of the
formyl/hydroxyaldehyde product ratio.
Both protoporphyrin esters were investigated in micellar solution. It was
found that both Proto IX D M E and Proto IX(DHC)2 can bc incorporated into
the micelles formed from cctyltrimethylammonium bromide (CTAB). Both
porphyrins exhibited spectra very much like those obtained for the monolaycr
assemblies suggesting that the porphyrins are also dimeric in the micellcs.
Irradiation of CTAB-Proto 1X(DME) micellcs in water leads to slow oxidation
of the porphyrin to form what appears spectrally to be predominantly the
hydroxyaldehydemixture. In contrast, irradiation of the CTAB-Proto IX(DHC)2
solutions under similar conditions led to even slower product formation. The
pronounced retardation of the reaction in micelles is probably due to unfavorable
partitioning of oxygen between the micelles and bulk solvent. The micelles are
small and probably contain much less than one oxygen molccule/micelle. Oxygen
can move freelyacross the micclle-solutioninterfaceas evidenced by the lack of
observable triplet-tripletabsorption from the porphyrins in undegasscd micelles
following flash excitation. However, excited oxygen likely escapes from the
miccllcsrapidly and has littleprobabilityof re-enteringa micellebefore deactivation. The differences in reactivity between the Proto IX D M E and Proto
Photochemistry of porphyrins and their metal complexes
135
IX(DHC)= could be due to different positioning of the two porphyrins in the
micelles. It would appear reasonable that the surfactant Proto IX(DHC)= might
have the porphyrin more anchored near the edge of the micelle while the
nonsurfactant Proto IX DME might be more "dissolved" in the hydrophobic
region.
The effect of quenchers on the photolysis in monolayer assemblies has been
investigated using two different types of quenchers. 1-(p-Methoxyphenyl)-6phenyl-l,3,5-hexatriene has been used as a potential quencher of the porphyrin
triplet while cholesterol has been used as a potential singlet oxygen scavenger.
The triene quenches protoporphyrin triplets in solution with a rate constant,
k~ = 5 × 10 81 tool -z s -1 as measured by flash photolysis for both esters.
Cholesterol is a reasonable singlet oxygen scavenger as indicated by its role as the
primary acceptor in the disorder porphyria. 18':* Interestingly, we find that the
triene is ineffective as a quencher even in very high concentrations. For example,
an assembly prepared with Proto IX (DHC)=, triene and dodecanoic acid in a
1 : 2: 3 ratio undergoes photooxidation at the same rate (in air) as those of assemblies with no tdene, el In contrast, assemblies prepared from Proto IX(DHC)2
cholesterol and dodecanoic acid in a 1: 2:3 ratio react at about half the rate of
assemblies without cholesterol. Cholesterol is also affective at retarding the rate
of photooxidation of Proto IX D ME in solution and Proto IX (DHC)= in spread
films on water surface. A reasonable explanation of the "nonquenching" effect
of the triene could be that even though energy transfer quenching of the porphyfin triplet occurs in the assemblies, the triene triplet may have a sufficient
lifetime and energy to sensitize singlet oxygen. In contrast, cholesterol scavenging
of singlet oxygen should definitely retard photooxidation of the porphyrin. In an
effort to investigate the effective "range" of singlet oxygen in the assemblies, we
have varied the distance between porphyrin and cholesterol by constructing
assemblies of different configuration. As mentioned above, with the assemblies
containing porphyrin and cholesterol in the same layer, both in high "concentra=
tion" there is ca. 50% quenching of the photooxidation. When assemblies are
constructed with comparable "concentrations" of cholesterol and Proto
IX(DHC)= in adjacent layers in hydrophilic contact there is a reduction of the
rate of photooxidation of ca. 35%. However, when two or more layers of
dodecanoic acid separate the layers containing porphyrin from those containing
cholesterol, there is no measurable retardation of the photooxidation. We are
currently investigating the use of other quenchers including species with triplets
lower than singlet oxygen, quenchers such as nitrogen, which do not quench by
energy transfer, and more powerful scavengers of zO=.
Acknowledgment
Much of the research described in this review is the work of students and
postdoctoral associates who have worked in these labs during the last eleven
years. Especially important contributions are due to the following: J. C. Yau,
136
D. G. Whitten
I. G. L o p p , P. D. Wildes,
Schmehl and B. E. Horsey.
( G r a n t G M 15,238) and the
0079) for financial support
F. R. Hopf, F. A. Carroll, J. Mercer-Smith, R. H.
We are grateful to the N a t i o n a l Institutes o f H e a l t h
U.S. A r m y Research Office ( G r a n t D A A G 2 9 - 7 6 - G o f various portions o f the w o r k described.
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