Early events associated with the excited state proton transfer in 2-(2′pyridyl)benzimidazole
Tarak Nath Burai, Tushar Kanti Mukherjee, Priyanka Lahiri, Debashis Panda, and Anindya Datta
Citation: J. Chem. Phys. 131, 034504 (2009); doi: 10.1063/1.3177457
View online: http://dx.doi.org/10.1063/1.3177457
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THE JOURNAL OF CHEMICAL PHYSICS 131, 034504 共2009兲
Early events associated with the excited state proton transfer
in 2-„2⬘-pyridyl…benzimidazole
Tarak Nath Burai, Tushar Kanti Mukherjee, Priyanka Lahiri, Debashis Panda, and
Anindya Dattaa兲
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
共Received 28 February 2009; accepted 25 June 2009; published online 16 July 2009兲
2-共2⬘-pyridyl兲benzimidazole 共2PBI兲 undergoes excited state proton transfer 共ESPT兲 in acidic
solutions, leading to a tautomer emission at 460 nm. This photoprocess has been studied using
ultrafast fluorescence spectroscopic techniques in acidic neat aqueous solutions, in viscous mixtures
of glycerol with water, as well as in sucrose solutions. The tautomer is found to be stabilized in the
more viscous medium, leading to a greater relative quantum yield as well as lifetime. The long rise
time in tautomer emission is not affected by viscosity though. Rather, it appears to have the same
value as the long component of the decay of the cationic excited state 共Cⴱ兲. In addition to the
subnanosecond lifetime reported earlier, Cⴱ is found to exhibit a decay time of 2 ps. This is assigned
to its protonation to form the nonfluorescent dication in its excited state 共Dⴱ兲 considering the ground
and excited state pKa values reported earlier. An additional rising component of 100 ps is observed
in the region of Cⴱ emission. This is likely to arise from a structural change or charge redistribution
in Cⴱ immediately after its creation and before the phototautomerization. © 2009 American Institute
of Physics. 关DOI: 10.1063/1.3177457兴
I. INTRODUCTION
Proton and hydrogen transfer in the excited state are fundamental processes that have potential applications in lasers,
photostabilization of polymers, and development of fluorescence sensors. So, they have generated a considerable
amount of interest.1–5 Excited state proton transfer 共ESPT兲 in
hydrogen-bonded systems such as benzimidazoles, in particular, has attracted a lot of attention.6,7 The ESPT of the
cationic form of 2-共2⬘-pyridyl兲benzimidazole 共2PBI兲 is
manifested in a distinct dual emission in its aqueous solutions in the pH range of 3.5–0.5 共Scheme 1兲.8 The inclusion
of 2PBI in cyclodextrins hinders the solvent-mediated ESPT
to some extent due to shielding of the fluorophore from
water.9 Systematic studies have been performed on the ESPT
of 2PBI in micelles,10 reverse micelles,11 and nafion film.12
These studies reveal that the ESPT can be brought about in
microheterogeneous media which contain water as a proton
donor along with a nonpolar compartment, separated by a
negatively charged interface. The four important species that
participate in the ESPT process are the normal form of 2PBI
共N兲, the monocationic form 共C兲, the dication 共D兲, and the
phototautomer 共Tⴱ, Scheme 1兲. N and C fluoresce at 380 nm,
while Tⴱ fluoresces at 450 nm. D has been reported to be
nonfluorescent so far. The tautomer emission is associated
with the rise time of approximately 0.8 ns in the microheterogeneous media where ESPT occurs. The slow dynamics
of ESPT of in aqueous micelle and reverse micelles is explained as a result of slow solvation and slowing down of the
proton transfer process as such.13–15 However, a 600–800 ps
growth is observed in neat aqueous solutions at pH 3 as
well.8 The origin of this long rise time is not understood as
proton transfer is known to occur within 150 fs in hydrogenbonded acid-base complexes.16 However, examples of long
time constants in the ESPT process are available as well.
Proton transfer proceeds in 250 ps, for example, in encounter
pairs formed by diffusion of uncomplexed photoacid and
base molecules.17,18 Proton transfer in acetonitrile-water mixture from strong photoacids 6-hydroxyquinolinium and
6-hydroxy-1-methylquinolinium is associated with a rather
long time constant of approximately 1 ns, the process being
diffusion controlled.19 Long time constants of approximately
100 ps have been observed in the perylene quinones, hypocrellin, and calphostin C as a result conformational change
and chain dynamics associated with the proton transfer
a兲
Author to whom correspondence should be addressed. Electronic mail:
[email protected].
0021-9606/2009/131共3兲/034504/8/$25.00
131, 034504-1
SCHEME 1. The species involved in the ESPT of 2PBI.
© 2009 American Institute of Physics
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034504-2
J. Chem. Phys. 131, 034504 共2009兲
Burai et al.
process.20,21 In such cases, the dynamics are expected to be
strongly dependent on viscosity. With this background, one
might contend that the slow dynamics of ESPT, observed in
2PBI in acidic aqueous solutions, arises from a similar molecular motion likely to be the rotation of the two ring systems with respect to each other.8 One way of studying the
coupling between conformational dynamics and ESPT of
2PBI is to attempt to control the mutual rotational motion of
the two rings of 2PBI by varying the viscosity of the
solution.20 Glycerol has been extensively employed as a viscogenic cosolvent to vary solvent viscosity.22 The viscosity
of the aqueous solution has also been increased by the addition of sucrose.23 The effect of thus varying the viscosity and
polarity of the medium on the ESPT of 2PBI has been investigated.
II. EXPERIMENT
A. Photophysical studies
2PBI 共analytical reagent grade兲 from Aldrich and sucrose 共general reagent兲 from Merck have been used as received. Methanol and glycerol 共spectroscopic grade兲 are obtained from Spectrochem, Mumbai, India. D2O and DClO4
from Aldrich have been used as received. For viscositydependence studies, glycerol-water mixtures of varying ratios are prepared. To each, a fixed amount of 2PBI solution is
added. The solutions are acidified by the addition of a fixed
amount of perchloric acid 共HClO4兲 to make the proton concentration in each solution 10−3M. For viscosity dependent
studies with sucrose solution, solid sucrose is added to pH
= 3 solution of 2PBI. The viscosities of the mixtures are
taken from Ref. 24. De-ionized water is distilled twice before
being used as a solvent. The pH of the aqueous solutions is
adjusted using a pH meter through the addition of HClO4
solutions. The 0.5M – 2M solutions of HClO4 are also prepared from its stock solution 共11.66M兲 by proper dilution.
The absorption and fluorescence spectra are recorded on
JASCO V 530 spectrophotometer and Varian Cary Eclipse
fluorimeter, respectively. The excitation wavelength 共␭ex兲 is
290 nm for fluorescence measurements. The absorbance at
this wavelength is kept below 0.1 in order to avoid inner
filter effects. Fluorescence quantum yields 共␾ f 兲 are calculated after proper correction for changes in the absorbance
using literature value of 2PBI.8
1. Time-resolved fluorescence measurements
Time-resolved fluorescence is recorded in a time correlated single photon counting 共TCSPC兲 system from IBH with
␭ex = 295 nm. The fluorescence decays are collected with the
emission polarizer at a magic angle of 54.7°. The data are
fitted to multiexponential functions after deconvolution of
the instrument response function by an iterative reconvolution technique using the JY Horiba IBH DAS 6.2 data analysis
software, where reduced ␹2 and weighted residuals serve as
parameters for goodness of fit.12 The full width at half maximum 共FWHM兲 of the instrument response function is 750 ps
and the resolution is 7 ps per channel.
In the femtosecond upconversion setup 共FOG 100,
CDP兲, the sample is excited at 283 nm, the third harmonic of
a mode-locked Ti:sapphire laser 共Tsunami, Spectra Physics兲
pumped by a 5 W Millennia 共Spectra Physics兲 DPSS laser.
The fundamental beam at 850 nm is frequency tripled in a
nonlinear crystal 关1 mm ␤-barium borate 共BBO兲, ␪ = 25°, and
␾ = 90°兴. The fluorescence emitted from the sample is upconverted in a nonlinear crystal 共0.5 mm BBO, ␪ = 38°, and ␾
= 90°兲 by mixing with the gate pulse, which consists of a
portion of the fundamental beam. The upconverted light is
dispersed in a monochromator and detected using photon
counting electronics. A cross-correlation function obtained
using the Raman scattering from ethanol has a FWHM of
300 fs. The femtosecond fluorescence decays are fitted using
a Gaussian function of the same FWHM for the excitation
pulse.
The time-dependent anisotropy, r共t兲, is constructed from
the decays at parallel and perpendicular directions to that of
the excitation polarization 关I储共t兲 and I⬜共t兲, respectively兴 as25
r共t兲 =
I储 − I⬜
.
I储 + 2I⬜
共1兲
The decays of r共t兲 are fitted to single and multiexponential
functions as required using the formula,
冉 冊
r共t兲 = r0 兺 ai exp −
i
t
,
共 ␶ r兲 i
共2兲
where r0 is the fundamental anisotropy in the absence of
other depolarizing processes such as rotational diffusion, or
energy transfer is related to displacement angle 共␤兲 between
absorption and emission dipoles as follow:
r0 =
冉
冊
2 3 cos2 ␤ − 1
.
5
2
共3兲
B. Computational details
The ground state geometries of the cationic and tautomeric forms of 2PBI at different dihedral angles are optimized in gas phase using the density functional theory with
the Becke3LYP functional in conjugation with 6-31Gⴱ basis
set as implemented in the GAUSSIAN 03 software package.26,27
The first excited singlet state 共S1兲 geometries of cationic and
tautomeric forms of 2PBI at various dihedral angles are optimized in the gas phase using configuration interaction with
singlets 共CIS兲 with 6-31Gⴱ basis set.28 The default options
for the self-consistent field convergence and threshold limits
in the optimization are used.
III. RESULTS AND DISCUSSION
A. The effect of viscosity and polarity on the steady
state spectra
Upon increasing the viscosity of the 2PBI solution by
the addition of glycerol, there is a decrease in the emission
intensity of the cationic species accompanied by an increase
in the emission of the tautomer 共Fig. 1兲. This trend is observed until a composition of 50% glycerol 关Fig. 1共b兲兴. This
behavior can be rationalized on the basis of hindrance of the
rotation of the molecule about the C1 – C1⬘ bond. The decrease in the tautomer emission at even higher concentra-
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034504-3
J. Chem. Phys. 131, 034504 共2009兲
Ultrafast dynamics of ESPT in 2PBI
TABLE I. Ratio of the relative quantum yield of monocation and tautomer
in water-methanol mixture, water-glycerol mixture, and water-sucrose mixture in pH = 3. The dielectric constants and the viscosities of the mixtures are
also provided.
FIG. 1. 共a兲 Absorption spectra of 2PBI in aqueous solution of pH = 3 with 共i兲
no sucrose 共solid line兲 and 共ii兲 35% sucrose solution 共dotted line兲. 共b兲 Fluorescence spectra of 2PBI in glycerol-water 共pH = 3兲 mixtures of various
compositions 关% glycerol 共v/v兲兴. 共c兲 Fluorescence spectra in aqueous solution with pH = 3 with 共i兲 no sucrose and 共ii兲 35% sucrose. All fluorescence
spectra are corrected for fluctuations in the absorbance at ␭ex = 305 nm.
tions of glycerol is likely to be due to the decrease in the
availability of water required for the solvent-mediated ESPT
to take place. Alternatively, this could be due to a change in
the dielectric constant of the medium from 80 to 61. However, at lower glycerol concentrations, the more significant
change is that in viscosity, which increases more than fivefold 共Table I兲.29 The interplay of these factors in affecting the
proton transfer process needs to be understood as pKa values
vary significantly with the polarity of the solvent. In order to
address this point, the experiment has been repeated in aqueous solution at pH = 3 at different concentration of sucrose as
well as methanol-water mixture.23 In the presence of 35%
sucrose, the dielectric constant of the solution changes from
80 to 62, but the viscosity increases four times 共Table I兲.29 In
earlier studies, this increased viscosity has been found to
modify the nonradiative rates 共knr兲 and increase in the excited state lifetime of molecules such as DiA 共4-di-16-ASP;
关4-共4-dihexadecylamino兲styryl兴-N-methylpyridinium
iodide兲.23,30,31
The shape of the absorption spectrum of 2PBI remains
unchanged upon addition of sucrose 关Fig. 1共a兲兴, but the intensity of cation emission at 380 nm decreases and that of
tautomer emission at 460 nm increases concomitantly 关Fig.
1共c兲兴. The invariance of pH of the solution is checked after
the experiment. As has been proposed for the experiment
with glycerol-water mixture, the increase in the relative
quantum yield of tautomer is likely to have arisen from the
freezing of the rotation of the two rings with respect to each
other at high viscosity. Notably, the variation in the relative
quantum yield 共␾T / ␾C兲 共Fig. 2兲 with viscosity follows almost the same trend for the glycerol and sucrose solutions at
low viscosities. However, the increase in the ratio in glycerol
Percentage by weight
Dielectric constant
Viscosity
␾T / ␾C
0
10
20
30
40
Water-methanol solutions
80.37
75.84
71.02
66.01
61.24
1.00
1.33
1.60
1.79
1.84
3.61
4.12
4.59
4.77
4.95
0
10
20
30
40
50
60
Water-glycerol solutions
80.37
77.55
74.72
71.77
68.76
65.63
61.03
1.00
1.29
1.73
3.08
3.65
5.41
10.68
3.61
4.48
5.08
5.88
6.25
6.58
6.71
0
10
20
30
35
Water-sucrose solutions
80.4
78.0
75.4
72.6
71.8
1.00
1.35
1.95
2.95
4.03
3.61
4.13
4.98
5.18
5.21
FIG. 2. 共a兲 Variation in the ratio of relative fluorescence quantum yields of
tautomer and cationic species in glycerol-water 共pH = 3兲 mixtures 共䊊兲, in
sucrose-water solution at different percentage of sucrose 共쎲兲, and in
methanol-water solution 共 ∀ 兲 as a function of logarithm of viscosity. 共b兲
Variation in the ratio of relative fluorescence quantum yields of tautomer
and cationic species in glycerol-water 共pH = 3兲 mixtures 共䊊兲, in sucrosewater solution at different percentage of sucrose 共쎲兲, and in methanol-water
solution 共 ∀ 兲 as a function of dielectric constant ␭ex = 305 nm.
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034504-4
Burai et al.
solutions of higher viscosity is much more than that for sucrose solutions of same viscosity. This may be rationalized in
terms of a greater hydrogen bonding of 2PBI with sucrose,
whereby “blocked structure” might be formed analogous to
the case of 3-hydroxyflavone.32 Such blocked structures are
known to hinder ESPT to a significant extent. Alternatively,
the decrease in the dielectric constant of the solution might
also play an important role here. In order to ascertain the role
of polarity, the experiment has been repeated in methanolwater mixtures at pH = 3. This is a control experiment as the
decrease in the dielectric constant in the presence of 40%
methanol is comparable to that in the presence of 60% glycerol, but the change in viscosity is negligible 共Table I兲. In the
methanol-water mixtures, the ratio of tautomer and cation
fluorescence increases, indicating a role of the polarity in the
process 共Fig. 2兲. The extent of change in the ratio, however,
is significantly lesser in methanol than in glycerol solution
with the same polarity 共Table I兲. Moreover, a plot of the
ratios of quantum yield against the viscosities reveal that at
the low viscosity region, the variation in the ratio with viscosity is almost the same for glycerol-water, methanol-water,
and sucrose-water solutions irrespective of their differences
in polarity 关Fig. 2共a兲兴. However, the plots of the ratio against
dielectric constant for the two viscous systems 共glycerolwater and sucrose-water兲 are distinct from the plot for the
less viscous methanol-water medium even in the water-rich
solutions 关Fig. 2共b兲兴. This behavior indicates the predominance of viscosity dependence in media with polarity that is
not very different from that of water. The polarity dependence is secondary in nature.
B. Quantum chemical calculations in the ground
and first excited singlet states
Quantum chemical calculations have been performed in
order to develop a better rationalization of the experimental
results. As the viscosity dependence is expected to have
arisen due to the involvement of the rotation of the ring
systems relative to each other, conformational analysis has
been performed on the ground state and on the first excited
state of the cation and the first excited state of the tautomer.
The energy of the molecule is computed while changing the
dihedral angle 共N1–C2–C3–N4兲 of the cation, setting the energy associated with zero dihedral angle to zero. The geometries are optimized in the range of dihedral angle from
⫺180° to +180°. The conformation in which the two rings
are coplanar is the most stable one in all the three cases 共Fig.
3兲. In the case of the S0 and S1 states of the cationic form
共C兲, the conformations with dihedral angle of zero and those
with dihedral angles of ⫾180° are degenerate. However, this
is not the case with the tautomer S1 state, where the conformation with dihedral angle of zero is more stable than those
with dihedral angle of ⫾180° by 8 kcal mol−1. This is explained easily in the light of steric interaction between the
N–H bonds in the latter and the absence thereof in the former
configuration. In the case of C and Cⴱ, such steric interaction
does not exist as both the H atoms are on the nitrogen atoms
of the benzimidazole moiety, thereby imparting C2v symmetry to it. From the conformational analysis, the rotational
J. Chem. Phys. 131, 034504 共2009兲
FIG. 3. Energetics for the rotation of cationic form of 2PBI; in the ground
state 共S0兲 共a兲 using B3 LYP/ 6-31Gⴱ and 共b兲 in first excited singlet state 共S1兲
using CIS/ 6-31Gⴱ level of theories and 共c兲 that of tautomer form of 2PBI in
first excited singlet state 共S1兲 using CIS/ 6-31Gⴱ level of theories. The geometry has been optimized at each value of dihedral angle. The energy at
zero dihedral angle is set to zero.
barriers have been determined in each case. These barriers
are 8.5 kcal/mol in the ground state and 17.7 kcal/mol in the
first excited singlet state 共Fig. 3兲, indicating that the rotation
around the pivotal bond of Cⴱ in its excited state is significantly restricted than that in the ground state. This could be
due to the double bond character of pivotal bond in excited
state as the C2–C3 bond distances are found to be 1.46, 1.39
Å in the ground state and excited state geometries of the
molecule, respectively. In highly viscous media, the rotation
of two aromatic rings around the pivotal bond of 2PBI can
get further restricted in the first excited single state as well as
in the ground state as the movement of the rings with respect
to each other is expected to be sluggish. Hence the barrier is
expected to increase in media of higher viscosity. Such viscosity dependent barrier to intramolecular rotational motion
have been reported for the photoisomerization of
3,3⬘-diethyloxadicarbocyanineiodide.33–35 Notably, the barrier is even greater for Tⴱ, indicating that it would be stabilized even further than Cⴱ in a viscous medium. Thus, the
greater yield of the tautomer in viscous media may be rationalized in the light of the higher barrier to rotation. Moreover, the dynamics of the ESPT process may be expected to
be slower in more viscous media if it is coupled with the
rotation of the two rings with respect to each other. An absence of viscosity dependence in the dynamics would indicate that such rotation is not involved in the rate determining
step. In order to understand the effect of viscosity, if any, on
the dynamics of the process, time resolved fluorescence studies have been performed.
C. The slower dynamics of the excited states
The fluorescence decays are recorded over the pH range
of 1–3 in neat water. They are also recorded at pH = 3 at
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034504-5
J. Chem. Phys. 131, 034504 共2009兲
Ultrafast dynamics of ESPT in 2PBI
TABLE II. Temporal characteristics of 2PBI in pH = 3 containing different
percentages of sucrose 共w/w兲 glycerol 共v/v兲 in pH = 3 solution. The data in
the solution with pH = 1 are also shown in this table. These data are obtained
from the TCSPC experiment.
380 nm
460 nm
% glycerol 共v/v兲
␶1
共ns兲
␹2
␶1
共ns兲
␶2
共ns兲
a1
a2
␹2
0
20
30
50
0.83
0.74
0.64
0.58
1.07
1.13
1.17
1.12
0.80
0.56
0.58
0.55
1.50
2.21
2.44
2.96
⫺0.44
⫺0.42
⫺0.38
⫺0.40
1.44
1.42
1.38
1.40
1.05
1.16
1.02
1.08
% sucrose 共w/w兲
0
10
14
28
35
0.86
0.74
0.67
0.68
0.64
1.07
1.01
1.02
1.05
1.10
0.86
0.80
0.79
0.69
0.67
1.38
1.82
2.14
2.40
2.65
⫺0.48
⫺0.51
⫺0.45
⫺0.39
⫺0.42
1.41
1.48
1.45
1.39
1.42
1.09
1.05
1.01
1.10
1.05
pH = 1
0.38
1.05
0.38
1.71
⫺0.41
1.41
1.10
different glycerol and sucrose contents 共Figs. 4 and 5兲. The
decays at pH = 2 – 3 are indistinguishable with a lifetime of
0.70 ns at ␭em = 380 nm, while the decay at pH = 1 is faster
共Table II兲. This is in agreement with the previously reported
trends.8 The lifetime at ␭em = 380 nm decreases at higher
concentration of glycerol and sucrose. At ␭em = 460 nm, the
trace is biexponential with a rise time of 0.80 ns and decay
time of 1.60 ns at pH = 3. The initial part of the decays is
superimpossible 关Fig. 4共a兲兴. This might indicate that the dynamics of the ESPT is not dependent on the viscosity of the
medium unlike in hypocrellin and calphostin C.20,21 Moreover, the rise times of the tautomer emission are more or less
similar to the decays of the cation emission in all the cases.
On a different note, the tautomer lifetime increases by a factor of 2 at 50% glycerol 共v/v兲 and 35% sucrose 共w/w兲 solution 共Table II兲. Thus, the phototautomer is found to be stabilized to a greater extent in viscous solutions as has been
predicted in the quantum chemical calculation. This is manifested further in the variation in log kNR with viscosity 共Fig.
6兲. A decrease is observed at ␭em = 460 nm with an increase
in viscosity, whereas at ␭em = 380 nm, a slight increase in the
saturation is observed. Thus the phototautomer is indeed
found to be stabilized, while the excited state of the cation is
found to be destabilized with increased in viscosity. This
observation bolsters the contention proposed from the steady
state results. The time resolved area normalized emission
spectra 共TRANES兲 in 35% sucrose solution of pH = 3 pass
FIG. 5. The fluorescence decays of 2PBI in aqueous solution pH = 3 containing 共i兲 0% and 共ii兲 35% sucrose solution at 共a兲 ␭em = 380 and 共b兲 460 nm.
共c兲 The fluorescence decays of 2PBI in aqueous solutions at pH = 1 at 380
and 460 nm: ␭ex = 294 nm in all cases.
FIG. 6. Variation in the logarithm of the nonradiative rate 共log kNR兲 of 2PBI
with the viscosity of aqueous sucrose solutions at pH = 3: ␭em = 380 nm
共upper panel兲 and 460 nm 共lower panel兲.
FIG. 4. 共a兲 The fluorescence decays of 2PBI in aqueous solution pH = 3
containing 共i兲 0% and 共ii兲 50% glycerol: ␭em = 380 nm and ␭em = 460 nm.
共b兲 The fluorescence decays of 2PBI in deuterated oxide at pH = 3 at ␭em 380
and 460 nm: ␭ex = 294 nm in all cases.
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034504-6
J. Chem. Phys. 131, 034504 共2009兲
Burai et al.
TABLE III. Temporal characteristics of 2PBI in H2O and D2O at pH = 3.
The acid used is HClO4 in the aqueous solution and DClO4 in the D2O
solution.
380 nm
H 2O
D 2O
460 nm
␶1
共ns兲
␹2
␶1
共ns兲
␶2
共ns兲
a1
a2
␹2
0.83
2.66
1.07
1.13
0.80
2.70
1.50
4.15
⫺0.44
⫺0.38
1.44
1.38
1.05
1.06
through an isoemissive point. This indicates that the ESPT is
a two-state process in the longer time scale.36
The slow conversion to the tautomer indicates a large
energy barrier. To investigate the effects of possible tunneling and to see the kinetic isotope effect, time resolved measurements have been performed in D2O 关Fig. 4共b兲, Table III兴.
At ␭em 460 nm, the rise time is 2.7 ns against the value of 0.7
ns in H2O. In both the cases, the rise times are approximately
equal to the corresponding lifetimes of monocation at ␭em
380 nm, increasing from 0.7 ns in aqueous solution at pH
= 3 − 2.7 ns in deuterated solvent at pH = 3. This indicates
that the decay of the 380 nm emission state should be directly correlated with the rise of the 460 nm state in this time
scale. An evidence of tunneling is not observed 共see Fig. 7兲.
D. Ultrafast dynamics
The ultrafast fluorescence dynamics of 2PBI have been
investigated by the femtosecond fluorescence upconversion
at ␭em = 380 and 460 nm. Aqueous solutions of relatively
mild acidity 共pH = 0.5– 4.5兲 as well as stronger acidity 共molar
concentration of HClO4兲 have been studied in this experiment. The decays in aqueous solution at pH = 7 are superimpossible with the initial parts of the decays obtained by TCSPC and yield the same value of the time constants and
amplitudes 共Table II兲. However, an ultrafast component of 2
ps emerges at lower pH values for ␭em = 380 nm 共Fig. 8,
Table IV兲. The amplitude of this ultrafast component increases with the increase in acidity until it accounts for more
than 50% of the decays at 2M HClO4. This ultrafast component has not been reported in earlier studies. Since its con-
FIG. 7. TRANES of 2PBI in 35% sucrose solution of pH = 3 between time
0 and 3.0 ns at intervals of 0.5 ns. The arrows indicate the direction of an
increase in time.
FIG. 8. Femtosecond fluorescence transients of 2PBI at ␭em = 380 nm in
aqueous solution of 共a兲 pH = 7, 共b兲 pH = 3, and 共c兲 containing 0.5M and 共d兲
containing 2M HClO4: ␭ex = 283 nm. The solid lines denote the lines of best
fit. The fitting parameters are provided in Table II. The cross-correlation
function of the laser pulse is shifted for the sake of clarity.
tribution increases with increase in acidity, it is likely to be a
signature of the dication. However, the dication is known to
be nonfluorescent and the pKa for the protonation of the
monocation is ⫺1.2.7 So, it is unlikely that the 2 ps decay is
due to the decay of the excited state of the dication. However, the pKⴱa for the second protonation of 2PBI is 3.19 as
estimated by the Forster cycle method.7 So, Cⴱ is expected to
get protonated in the pH range studied to form Dⴱ. This
excited state protonation process is expected to be ultrafast
and should contribute to the depletion of the Cⴱ emission. So,
the 2 ps component is ascribed to the decay of Cⴱ to Dⴱ,
which provides an independent depletion pathway for Cⴱ.
Another additional feature is observed in the transients in the
range of the Cⴱ emission in the form of a 100 ps rise time
关Fig. 9共a兲, Table IV兴, which disappears above acid concentration of 0.5M. This rise time persists over the entire range
of Cⴱ emission even though at wavelengths such as 390 nm it
is difficult to resolve due to the presence of contribution
from the longer decay of Cⴱ as well as the long rise and
decay of Tⴱ. This hitherto unreported rise time indicates the
temporal evolution of the cationic excited state. The initially
prepared Cⴱ state seems to undergo either a structural reorganization of a charge redistribution37 or both, before a
modified state, C1ⴱ, is formed and it is this C1ⴱ state that
appears to undergo the actual ESPT process and yield Tⴱ
eventually. The following hypothesis may be proposed regarding the nature of Cⴱ and C1ⴱ: Due to the small activation
energy of rotation of the two rings with respect to each other,
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034504-7
J. Chem. Phys. 131, 034504 共2009兲
Ultrafast dynamics of ESPT in 2PBI
TABLE IV. Early time decay parameters of 2PBI obtained from the fluorescence upconversion experiment. The
lifetimes have been classified in the columns according to their values.
␭em
共nm兲
pH = 7.0
pH = 4.5
pH = 3.0
0.5M HClO4
2.0M HClO4
pH = 3 in D2O. The acid used is DClO4.
380
380
360
380
390
460
490
380
380
360
370
380
390
460
490
␶1
共ps兲
␶2
共ps兲
␶3
共ps兲
2
2
2
2
86
80
100
100
130
100
700
700
700
700
1000
1800
1650
2
2
2
2
2
2
198
48
87
220
162
127
2700
2920
2600
2600
2600
2600
4150
4000
␶4
共ps兲
a1
a2
a3
a4
0.06
0.31
0.17
0.28
0.90
0.45
⫺0.19
⫺0.26
⫺0.36
⫺0.04
0.10
0.49
0.88
1.09
1.08
1.44
1.50
⫺0.40
⫺0.50
800
1000
0.40
0.58
0.53
0.46
0.37
0.42
0.6
0.42
⫺0.15
⫺0.23
⫺0.22
⫺0.25
⫺0.38
⫺0.48
0.62
0.76
0.85
0.83
1.38
1.48
C is likely to exist in a distribution of conformations. Vertical
excitation leads to the formation of Cⴱ in a distribution of
conformations as well. However, in the excited state, the
coplanar conformation is relatively more stable. So, conformational rearrangements occur to lead to a narrower distribution of conformation of Cⴱ and this is what has been referred to as the C1ⴱ state in this discussion. In this argument,
the 100 ps rise in the cation emission would be the time
constant of the conformational rearrangement.
The argument proposed in the previous paragraph finds
some support in the early time fluorescence dynamics in 35%
sucrose solution 共Fig. 9兲. In this medium, the contribution of
the 100 ps rise time at 380 nm is substantially less than that
in the neat aqueous solution. This is what one might expect
in a more viscous medium, where the barrier to rotation
would increase and the process of rotation is hindered. Thus,
from the fluorescence upconversion experiment, it appears
that the 100 ps rise time may be assigned to the conformational relaxation of the Cⴱ state. On a different note, the
transients at 460 nm in the presence and absence of sucrose
are superimpossible 关Fig. 9共b兲兴. This indicates that the viscosity of the media has no role in the slow dynamics of the
ESPT of 2PBI even though the extent of ESPT is greater in
more viscous media due to the greater stability of Tⴱ in such
media.
Time-resolved fluorescence anisotropy decays r共t兲
shown in Fig. 10 of 2PBI in pH = 3 and 35% sucrose solution
are measured at ␭em = 380 nm and at ␭em = 460 nm in order
to better understand the mechanism of the rise time of tautomer. The decays fit well monoexponentially. Fundamental
anisotropy at 380 and 460 nm are 0.22 and 0.10, respectively.
The angle 共␤兲 between absorption and emission transition
FIG. 9. Femtosecond fluorescence transients at pH = 3 in the absence and
presence of 35% of sucrose. 共a兲 ␭em = 380 共b兲 460 nm. ␭ex = 283 nm. The
solid lines denote the lines of best fit.
FIG. 10. Fluorescence anisotropy decay at pH = 3 in the absence 共a兲 and
presence 共b兲 of 35% of sucrose. ␭em = 380 and 460 nm. ␭ex = 283 nm. The
solid lines denote the best fit to the experimental data.
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034504-8
dipole moment is calculated using Eq. 共3兲. These angle values are 33° and 45° at 380 and 460 nm, respectively. The
orientation time constant of approximately 50 ps of 2PBI in
water remains the same in pH = 3 solution as well. However,
in the presence of 35% sucrose solution 共␩ = 4 mPa s兲 in
pH = 3, this increases to 135 ps at ␭em = 380 nm and 280 ps
at ␭em = 460 nm presumably because the tautomer form is
more stable in high viscosity compared to cation of 2PBI due
to the hindrance in the rotation of pivotal bond. Notably, the
r共0兲 value in the region of Cⴱ emission is considerably less
than the expected value of 0.4, indicating a missed ultrafast
component, which is likely to be associated with the early
event of the conformational relaxation.
IV. CONCLUSION
The phototautomer of 2PBI is found to be stabilized and
the excited state of the cation is destabilized at higher viscosity. Consequently, the ratio of the relative quantum yields
of the tautomer and the normal species increases with an
increase in viscosity, as does the ratio of lifetimes. In the
acidic solutions, Cⴱ is found to have an ultrafast decay channel in the formation of the excited state of the nonfluorescent
dication, in addition to ESPT. Moreover, the ESPT is found
to be preceded by a faster process, which is likely to be
conformational relaxation of Cⴱ. The dynamics of ESPT is
found to be independent of viscosity and the formation of Tⴱ
is correlated directly with the decay of conformationally relaxed Cⴱ.
ACKNOWLEDGMENTS
This work was supported by Grant No. SR/S1/PC-19/
2005 of SERC, DST. T.N.B. thanks CSIR for a senior
Research fellowship. The authors are grateful to Professor
G. Naresh Patwari for a useful suggestion and Mr. E. Siva
Subramaniam Iyer for critical reading of the manuscript. The
referees are thanked for insightful comments.
1
J. Chem. Phys. 131, 034504 共2009兲
Burai et al.
M. Carmen Ríos Rodríguez, M. Mosquera, and F. Rodriguez-Prieto, J.
Phys. Chem. A 105, 10249 共2001兲.
2
R. S. Flom and F. P. Barbara, Chem. Phys. Lett. 94, 488 共1983兲.
3
C. Chudoba, E. Riedle, M. Pfeiffer, and T. Elsaesser, Chem. Phys. Lett.
263, 622 共1996兲.
4
M. Carmen Ríos Rodríguez, F. Rodriguez-Prieto, and M. Mosquera,
Phys. Chem. Chem. Phys. 1, 253 共1999兲.
5
S. Takeuchi and T. Tahara, J. Phys. Chem. A 102, 7740 共1998兲.
6
J. Catalan, G. L. J. De Paz, J. C. Del Valle, M. R. Claramount, and T.
Mas, Chem. Phys. 305, 175 共2004兲.
7
J. C. Penedo, J. L. P. Lusters, I. G. Lema, M. C. R. Rodríguez, M.
Mosquera, and F. Rodríguez-Prieto, J. Phys. Chem. A 108, 6117 共2004兲.
8
F. Rodríguez-Prieto, M. Mosquera, and M. Novo, J. Phys. Chem. 94,
8536 共1990兲.
9
M. C. Rath, D. K. Palit, and T. Mukherjee, J. Chem. Soc., Faraday Trans.
94, 1189 共1998兲.
10
T. K. Mukherjee and A. Datta, J. Phys. Chem. B 109, 12567 共2005兲.
11
T. K. Mukherjee, D. Panda, and A. Datta, J. Phys. Chem. B 109, 18895
共2005兲.
12
T. K. Mukherjee and A. Datta, J. Phys. Chem. B 110, 2611 共2006兲.
13
K. Bhattacharyya and B. Bagchi, J. Phys. Chem. A 104, 10603 共2000兲.
14
S. Balasubramanian, S. Pal, and B. Bagchi, Phys. Rev. Lett. 89, 115505
共2002兲.
15
P. Dutta, P. Sen, S. Mukherjee, and K. Bhattacharyya, Chem. Phys. Lett.
382, 426 共2003兲.
16
T. Elsaesser, in Ultrafast Hydrogen Bonding and Proton Transfer Processes in the Condensed Phase, edited by T. Elsaesser and H. J. Bakker
共Kluwer, Dordrecht, 2002兲, pp. 119–153.
17
M. Rini, B. Z. Magnes, E. Pines, and E. T. J. Nibbering, Science 301,
349 共2003兲.
18
O. F. Mohammed, D. Pines, J. Dreyer, E. Pines, and E. T. J. Nibbering,
Science 310, 83 共2005兲.
19
J. L. Pérez-Lustres, F. Rodriguez-Prieto, M. Mosquera, T. A. Senyushkina, N. P. Ernsting, and S. A. Kovalenko, J. Am. Chem. Soc. 129, 5408
共2007兲.
20
K. Das, D. S. English, and J. W. Petrich, J. Am. Chem. Soc. 119, 2763
共1997兲.
21
A. Datta, A. V. Smirnov, J. W. G. Chumanov, and J. W. Petrich, Photochem. Photobiol. 71, 166 共2000兲.
22
B. L. McClain, I. J. Finkelstein, and M. D. Fayer, J. Am. Chem. Soc.
126, 15702 共2004兲.
23
M. M. G. Krishna and N. Periasamy, J. Fluoresc. 8, 81 共1998兲.
24
D. R. Lide, Handbook of Chemistry and Physics, 79th ed. 共CRC, Boca
Raton, 1998兲, Chap. 8, p. 65.
25
J. R. Lakowicz, Principle of Fluorescence Spectroscopy, 3rd ed.
共Springer, New York, 2006兲.
26
C. Lee, W. Yang, and R. G. Parr, Phys. Rev. B 37, 785 共1988兲.
27
M. J. Frisch, G. W. Trucks, H. B. Schlegel et al., GAUSSIAN 03, Revision
C.02, Gaussian, Inc., Wallingford, CT, 2004.
28
J. B. Foresman, M. Head-Gordon, J. A. Pople, and M. J. Frisch, J. Phys.
Chem. 96, 135 共1992兲.
29
G. Akerlof, J. Am. Chem. Soc. 54, 4125 共1932兲.
30
J. B. Birks, Photophysics of Aromatic Molecules 共Wiley, New York,
1970兲, p. 88.
31
P. F. Aramendia, R. M. Negri, and E. S. Roman, J. Phys. Chem. 98, 3165
共1994兲.
32
P. K. Sengupta and M. Kasha, Chem. Phys. Lett. 68, 382 共1979兲.
33
J. M. Hicks, M. Vandersall, E. V. Sitzmann, and K. B. Eisenthal, Chem.
Phys. Lett. 135, 413 共1987兲.
34
S. P. Velsko and G. R. Fleming, Chem. Phys. 65, 59 共1982兲.
35
D. Waldeck and G.R. Fleming, J. Phys. Chem. 85, 2614 共1981兲.
36
A. S. R. Koti, M. M. G. Krishna, and N. Periasamy, J. Phys. Chem. A
105, 1767 共2001兲.
37
D. B. Spry and M. D. Fayer, J. Chem. Phys. 128, 084508 共2008兲.
Downloaded 01 Mar 2012 to 59.162.23.76. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions