Structural Requirements for Intramolecular Charge-Transfer
in Excited State of 4-(9-Anthryl)-N,N-dimethylaniline
Aleksander Siemiarczuk
Institute of Physical Chemistry, Polish A c a d e m v of Science, Kasprzaka 44, 01-224 Warsaw,
Poland
Jacek Koput
Photochemistry Group, A . Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland
A n d r z e j Pohorille
Department of Biophysics, Institute of Experimental Physics, University of Warsaw,
Zwirki i Wigury 93, 02-089 Warsaw, Poland
Z. Naturforsch. 37a, 598—606 (1982); received January 11, 1982
Excited state kinetics and other photophysical features of 4-(9-anthryl)-N,N-dimethylaniline
and two model compounds have been thoroughly examined in order to establish the structural
conditions for highly polar excited state formation.
Also quantum chemical calculations b y means of I N D O / S CI and P C I L O methods have been
performed to obtain potential energy curves, dipole moments and atomic charge densities for the
ground and lowest excited singlet states as a function of the angle of twist between anthryl and
dimethylaniline subunits.
Both the experimental and theoretical results confirm the previously proposed TICT (Twisted
Intramolecular Charge-Transfer) model predicting the perpendicularity between electron donor
and acceptor moieties as a condition for nearly full charge-separation in excited states for some
classes of aromatic compounds.
Introduction
Formation
conformer of D M A B N is responsible for the lower
of
intramolecular
charge-transfer
(ICT) in e x c i t e d 4 - ( 9 - a n t h r y l ) - N , N - d i m e t h y l a n i l i n e
[1, 2, 3] has been p r e v i o u s l y e x p l a i n e d in t e r m s of
t h e T I C T model (Twisted I n t r a m o l e c u l a r ChargeT r a n s f e r E x c i t e d State) [4, 5, 6]. T h e model seems
t o be applicable t o v a r i o u s classes of i n t r a m o l e c u l a r
donor-acceptor molecules ( D - A ) e x h i b i t i n g a n e a r l y
full electron transfer f r o m D t o A in s u f f i c i e n t l y
polar solvents [6]. T h e T I C T model p r e d i c t s t h a t
in some cases, w hen D a n d A are linked t o g e t h e r
b y a f o r m a l l y single bond, t h e m o s t
favourable
c o n f o r m a t i o n for the e x c i t e d state I C T is perpen-
e n e r g y polar emission whereas t h e higher energy
fluorescence
arises f r o m t h e u n t w i s t e d precursor.
S u c h a n assignment w'as m a d e a f t e r investigation
of several m o d e l c o m p o u n d s in w h i c h the N(CÜ3)2
g r o u p is r o t a t e d t o the planar or perpendicular
c o n f o r m a t i o n . I n t h e case of a n t h r y l derivates of
dimethylaniline,
I, I I and I I I
(Fig. 1), none of
steric restrictions imposed did p r e v e n t the relaxat i o n in t h e e x c i t e d state, b u t t h e y allowed at least
t o e x c l u d e t h e r o t a t i o n of the N(CH3)2 group as the
process leading t o t h e polar e m i t t i n g state [4]. S o m e
indirect conclusions derived f r o m kinetic d a t a led
d i c u l a r i t y of the ^-electronic s y s t e m s of D a n d A ,
minimizing t h u s their overlap. I t has b e e n s h o w n
CH3\N/CH3
H3CXN/CH3
I
II
H
3
C ^ C H
in t h e case of d i m e t h y l a m i n o b e n z o n i t r i l e ( D M A B N )
t h a t its anomalous double luminescence could be
satisfactorily e x p l a i n e d in t e r m s of r o t a t i o n of t h e
N(CH3)2 g r o u p in the e x c i t e d s t a t e t o t h e perpendicular c o n f o r m a t i o n w i t h respect t o t h e benzene
ring [7, 8]. I t has been established t h a t t h e t w i s t e d
Reprint requests to Dr. A. Siemiarczuk, Institute of
Physical Chemistrv, Polish Academv of Science, Kasprzaka
44, 01-224 Warsaw/Polen.
III
Fig. 1. 4-(9-anthryI)-N,N-dimethylaniline (I), 4-(9-anthryl)3,5-dimethvl-N,N-dimethylaniline (II) and l-methyl-5(9-anthryl)-indoline (III).
0340-4811 / 82 / 0600-0555 $ 01.00/0. - Please order a reprint rather than making your own copy.
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LIS t o t h e h y p o t h e s i s of p e r p e n d i c u l a r i t y b e t w e e n
m t h r a c e n e a n d dimethylaniline ( D M A ) moieties in
the e q u i l i b r a t e d I C T s t a t e [4, 6]. I n t h e m e a n t i m e ,
B a u m a n n proposed an a l t e r n a t i v e model based on
Iiis electrooptical m e a s u r e m e n t s in absorption a n d
^mission, w h i c h h a v e been interpreted in t e r m s of
•nhanced p o l a r i z a b i l i t y u p o n e x c i t a t i o n [9j. A s he
concluded, none of s t r u c t u r a l changes claimed for
I, I I a n d I I I in t h e T I C T model are necessary t o
explain t h e n a t u r e of t h e equilibrated I C T s t a t e
and t h e kinetics of its f o r m a t i o n . T h e latter, accordn g t o B a u m a n n , m a y be e x c l u s i v e l y d u e t o s o l v e n t
reorientation a r o u n d a polarizable rigid dipole represented b y t h e e x c i t e d molecule.
I n order t o e x p l a i n t h e observed p h e n o m e n a w e
present some n e w spectroscopic a n d kinetic d a t a
and results of I N D O / S a n d P C I L O calculations
concerning transition energies a n d the g r o u n d s t a t e
energy of I as a f u n c t i o n of t h e angle of t w i s t bet w e e n a n t h r y l a n d D M A subunits.
Materials
Syntheses
of
4-(9-anthryl)-N,N-dimethylaniline
(I), 4 - ( 9 - a n t h r y l ) - 3 , 5 - d i m e t h y l - N , N - d i m e t h y l a n i l i n e
(II) a n d l-methyl-5-(9-anthryl)-indoline (III) were
p e r f o r m e d b y D r . A . K r o w c z y r i s k i a n d are described
elsewhere [4].
T h e solvents were of fluorescence or spectroscopic
grade. 2 - m e t h y l t e t r a h y d r o f u r a n ( M T H F ) w a s distilled o v e r L i A l Ü 4 before use t o eliminate a n y contents of w a t e r .
T h e solutions were d e a e r a t e d b y f r e e z e - p u m p thaw technique.
Experimental
Steady-state
fluorescence
measurements w e r e
p e r f o r m e d w i t h a l a b o r a t o r y - b u i l t J a s n y - t y p e spect r o f l u o r i m e t e r w i t h a r e g u l a t e d t e m p e r a t u r e equipm e n t [10]. T h e a p p a r a t u s m a y be used f o r c h e c k i n g
of t h e s a m p l e t r a n s m i t t a n c e a t t h e w a v e l e n g t h o f
e x c i t a t i o n s i m u l t a n e o u s l y w i t h emission measurem e n t s in t h e w h o l e t e m p e r a t u r e range.
F l u o r e s c e n c e rise a n d d e c a y curves were recorded
w i t h a s a m p l i n g t e c h n i q u e w i t h nanosecond pulsed
N2 laser e x c i t a t i o n . Fluorescence lifetimes were
d e t e r m i n e d b y m e a n s of a least squares m e t h o d
c o m b i n e d w i t h a c o n v o l u t i o n of a laser pulse w i t h
a n a s s u m e d fluorescence response f u n c t i o n .
599
A l s o a dye laser pumped b y N2 laser pulses was
used in order to extend the Avavelength of excitation towards the red.
Results and Discussion
1. Kinetics
I n t h e p r e v i o u s p a p e r [4] a strong influence of
t e m p e r a t u r e o n t h e s h a p e a n d position of t h e
fluorescence
spectra in n - b u t a n o l w a s reported.
A r a p i d increase of t h e fluorescence b a n d w i d t h in
some t e m p e r a t u r e region w a s a t t r i b u t e d t o t h e
a p p e a r a n c e of a higher e n e r g y anthracene-like
emission superposing t h e polar one. T h e fluorescence
b a n d b r o a d e n i n g could be also caused b y t h e intera c t i o n s of polar s o l v e n t molecules w i t h e x c i t e d
solute molecules. T h i s is t h e case w h e n t h e solvent
r e l a x a t i o n t i m e is of t h e order of t h e
fluorescence
lifetime, so t h e molecule does n o t emit f r o m t h e
e q u i l i b r a t e d e x c i t e d s t a t e . T h e fluorescence spect r u m is c o m p o s e d of m a n y o v e r l a p p i n g b a n d s w i t h
slightly different t r a n s i t i o n energies reflecting different stages of solvent reorientation a r o u n d e x c i t e d
molecules. T h e kinetics of t h e fluorescence d e c a y
is u s u a l l y n o n - e x p o n e n t i a l a n d depends on t h e
emission w a v e l e n g t h [12, 13].
I n order t o o b t a i n b e t t e r spectral resolution w e
m e a s u r e d t i m e - r e s o l v e d fluorescence spectra a t t h e
temperatures at which steady-state
fluorescence
b r o a d e n i n g h a d been o b s e r v e d . T h e s e spectra are
c o m p o s e d of t w o distinct b a n d s [4, 1 1 ] indicating
t h e e x i s t e n c e of t w o e m i t t i n g f o r m s of intramolecu l a r origin, r a t h e r t h a n t h e superposition of m a n y
s p e c t r a d u e t o stepwise reorientation of solvent
molecules. T h e s o l v e n t r e l a x a t i o n p r o b a b l y also
c o n t r i b u t e s t o t h e b a n d broadening, h o w e v e r , it is
v e r y u n l i k e l y t h a t this process is responsible for t h e
a p p e a r a n c e o f t w o distinct bands.
T h e i n t e r p r e t a t i o n h a s b e e n confirmed b y a n
e x a m i n a t i o n of t h e rise a n d d e c a y
fluorescence
c u r v e s d e t e c t e d a t t h e short- a n d l o n g w a v e l e n g t h
edge of t h e fluorescence s p e c t r u m [4]. W e h a v e
o b s e r v e d a clear correlation b e t w e e n t h e l o n g - w a v e l e n g t h risetime a n d t h e s h o r t - w a v e l e n g t h d e c a y t i m e a n d h a v e p r o p o s e d a model of consecutive
steps l e a d i n g t o t h e I C T e x c i t e d state f o r m a t i o n :
x 5 x * J b Y *
^ kx
X
lCy ,
Y
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(1)
600
A. Siemiarczuk, J. Koput, and A. Pohorille • Structural Requirements for Excited State ICT 600
where X * denotes a locally excited molecule
( A * - D ) , Y * the polar structure of the relaxed excited molecule ( A - - D + ) * , kx and ky the total deactivation rate constants and kxy the rate of intramolecular relaxation.
T h e lifetime of the short-wavelength local emission decreases rapidly at higher temperatures
(lower viscosity). Thus in
1/t* = kx + kxy,
(2)
kxy = k ™ e x p ( — E X y l R T ) is the dominating term.
I n [4] we reported the a c t i v a t i o n energy Exy
for I I in n-butanol to be considerably lower t h a n
for the other compounds; now after additional
measurements and recalculation we h a v e obtained
practically the same values for all three derivatives
(Figure 2). This identity indicates t h a t the intramolecular conversion rate is most probably controlled by the viscosity of the solvent.
Basing on the dependence of the spectral halfwidth of the fluorescence band in n-butanol on
temperature (see Fig. 2 in [4]) and applying the
kinetic scheme (1), one can t r y t o estimate the rate
of conversion kxy. T h e m a x i m a l broadening of the
fluorescence
band is observed for compound I I
at 183 K . For I and I I I it occurs at 209 K and
203 K , respectively. Assuming t h a t the h a l f w i d t h
reaches its m a x i m u m value in the temperature
interval in which both X and Y species give a
comparable contribution to the total emission intensity, one gets the approximate relation
^
+ k^j. -(- kxy
and
x y
^^{k\+kl
T
0X
fit to l/Tx = kx +
T
).
0Y
°K
Fig. 2, the following estimations for kxy
tained :
k\y (209 K ) ^
Ty
ns
are ob-
7 x 10 7 s - 1 ,
k ™ (203 K ) ^ 14 X 10 7 s - i .
(4)
TX
k~exv(-Exy/RT).
kl]{ 183 K ) ^ 22 x 10 7 s - i ,
T a k i n g appropriate values from T a b l e 1 and from
Compound
Fig. 2. Temperature dependence of the short-wave fluorescence decay-times ( • ) and the long-wave fluorescence
rise-times ( o ) in n-butanol. Solid lines represent the best
kxy1c%
(kf -f- Afir + kxy) (kf -f- k^T)
finally
k
r 1 / io- 3 K _1
k?
Assuming t h a t the activation energies for all three
derivatives are within the range of 3 -I- 4 kcal/mole,
one gets the limiting values for the preexponential
f a c t o r k™ presented in Table 2. These constants
kt
k*T
kvnT
-
1.2
107 s- 1
I
294
213
123
0.76
0.80
-
11.1
19.3
24.6
-
6.8
4.1
-
2.2
II
294
223
113
0.66
0.18
-
29.0
35.0
~ 21
-
2.3
0.5
-
-
1.1
III
290
203
113
0.53
0.22
-
17.0
13.6
13.9
-
3.1
1.6
-
-
2.8
Table 1. Some photophysical
parameters of I, II and I I I in
n-butanol.
-
2.4
-
6.0
-
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Siemiarczuk, J. Koput, and A. Pohorille • Structural Requirements for Excited State ICT
Table 2. Estimated limiting values o f absolute rates o f
conversion.
kfja-i
"ompound
EXy = 3 kcal/mole
ion
I
II
III
8.4 X i o n
2.4 X lOn
Exy
= 4 kcal/mole
1012
13.0 x 1012
2.8 x 1012
fall into the range of 1 0 1 1 -f- 1 0 1 3 s _ 1 , corresponding
thus t o t y p i c a l rates of molecular vibrations. Since
the preexponential f a c t o r for c o m p o u n d I I is considerably greater t h a n for I and I I I , it seems v e r y
probable t h a t the I C T excited state corresponds
to a twisted g e o m e t r y of our molecules.
I n order to determine t h e p h o t o p h y s i c a l parameters of the polar e x c i t e d state in t e r m s of t h e
kinetic model (1), t h e t e m p e r a t u r e effect u p o n
fluorescence a n d lifetime has been t h o r o u g h l y investigated. M T H F has been used as a s o l v e n t t o
observe pure I C T fluorescence in a wide range of
temperatures. A t high t e m p e r a t u r e s kxy^>kx,
and
the q u a n t u m yield of I C T fluorescence is s i m p l y
given b y
0 y
=
k%Ty.
T /10 K"
T
2.
tA
O
(5)
r-
1.
From the experimental 0 y a n d r y v a l u e s t h e r a t e
constant of I C T fluorescence could be determined.
As can be seen from F i g . 3, k\ is t h e r m a l l y a c t i v a t e d
for all three derivatives, a n d generally w e can express this dependence as
k\ ^tf
+ kfexvi-E^RT).
(6)
L o w values of $ « 2 x 10 7 s " 1 , < 3 X 10 6 s~i a n d
< 10 7 s _ 1 for I, I I a n d I I I , respectively) indicate
t h a t the radiative process is rather s t r o n g l y forbidden for the conformation a t equilibrium. T h i s
is in good agreement w i t h the T I C T model, in w h i c h
the perpendicularity b e t w e e n donor a n d a c c e p t o r
moieties should diminish the transition m o m e n t
considerably. T h e r m a l a c t i v a t i o n of a l o w e n e r g y
torsional vibration a r o u n d the donor-acceptor b o n d
would produce an additional transition i n t e n s i t y
which is strongly dependent on t h e a m p l i t u d e of t h e
vibration. T h e value of t h e a c t i v a t i o n e n e r g y of
-7
v.5
3
vi"
N>* .3
T'VlO'V1
t/i 3.
o
\
\
>>-
K
1.
vi 1 .7
Fig. 3. 0v/Ty ( o ) and (l/ry) - (0y/ry) ( A ) versus l/T7 for
I (a), I I (b) and I I I (c) in M T H F . Dashed lines represent
the low and high temperature limit of (1 /ry) — (0y/ry).
rViO'V
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601
602
A. Siemiarczuk, J. Koput, and A. Pohorille • Structural Requirements for Excited State ICT 602
(-^130 c m - 1 )
corresponds t o small v i b r a t i o n a l
2. Structural Diversity in the Ground State
q u a n t a a n d is consistent w i t h the model.
The
nonradiative
r a t e constant
should
be
expressed in t h e h i g h t e m p e r a t u r e limit as
[9]. C o n t r a r y t o our i n t e r p r e t a t i o n he claimed that
Kr={llTy)-<t>ylTy.
(7)
A s it c a n be seen f r o m F i g . 3, the plot of t h e r . h . s .
of (7) versus T - 1 e x h i b i t s a m i n i m u m , indicating
t h a t t h e f o r m u l a is n o t v a l i d in t h e whole temperat u r e range. B y t h e introduction of an e x a c t expression for <&y i n t o (7), one gets
(1 fry) - (Vyfry) =
(k{
- f - k^r)
( k
+ K
-(-
y
'"nr
-(-
^
-
+
k^j.
noncooperative
tion
at
the
edge
of
the
long-wave
absorption
A c c o r d i n g t o B a u m a n n , such a n e x c i t a t i o n selects
t h e molecules s t r o n g l y s o l v a t e d a l r e a d y in theii
g r o u n d s t a t e , a n d a f t e r e x c i t a t i o n such an oriented
s o l v e n t s h e a t h induces t h e redistribution of elect r o n i c c h a r g e w i t h o u t a n y a c c o m p a n y i n g changes
in
( 0 y f r y )
molecular
geometry.
Therefore,
the
question
arises w h e t h e r t h e molecules e x c i t e d in such a w a y
(k?+k*T)(k«{ + kl
kyf~>
w i t h i n t h e model of
s o l v e n t r e l a x a t i o n [12, 13]. H e f o u n d t h a t excita-
kXykvm
(8)
+ ^nr
kxn
F'or
e x c i t e d state, a n d t h a t all kinetic p h e n o m e n a are
explainable
(FC) s t a t e , w i t h a dipole m o m e n t n e a r l y equal tc
in t h i s a p p r o x i m a t i o n
fry)
responsible f o r t h e h i g h l y polar structure in the
t h a t of t h e e m i t t i n g e q u i l i b r a t e d s t a t e [6, 9].
Since kxy > k% -+- k*T in a broad t e m p e r a t u r e range,
(1
a n increase of p o l a r i z a b i l i t y u p o n e x c i t a t i o n m a y be
shoulder p r o d u c e s a s t r o n g l y polar F r a n c k - C o n d o n
kXykf
-
T h e role of s o l v e n t reorientation in the kinetic
b e h a v i o u r has been s t r o n g l y stressed b y B a u m a n n
(9)
are r e a l l y identical s t r u c t u r a l l y w i t h those emitting
in t h e e q u i l i b r a t e d I C T state.
(I a n d I I I ) , the expression (9) simplifies
to
T h e t e m p e r a t u r e d e p e n d e n c e of t h e absorption
s p e c t r a r e p o r t e d b y O k a d a et al. [2] shows t h a t a
(1 fry) ~ (&yfry) = (*? + Kr)
If
k
decrease of t e m p e r a t u r e causes s t r o n g broadening
v
(9a)
KXy
crease of t h e l o n g - w a v e l e n g t h tail. T h e y interpret
< A 4 (II), w e h a v e
(1 fry) ~ i®yfry)
of t h e v i b r a t i o n a l s t r u c t u r e a n d a n intensity in-
(*? •+ Kr)
t h i s e f f e c t in t e r m s of double p o t e n t i a l
kv
(9 b)
KXy
minima
in t h e g r o u n d state, suggesting t h a t w i t h lowering
t e m p e r a t u r e a s t r u c t u r e f a v o u r i n g partial electron
A t s u f f i c i e n t l y high t e m p e r a t u r e s t h e first t e r m in
t r a n s f e r is stabilized. S u c h stabilization, however,
(9a) a n d (9b) can be neglected a n d t h e relation (7)
m i g h t be c o n n e c t e d either w i t h a n intramolecular
is o b e y e d (left side o f respective plots in F i g u r e 3).
c o o r d i n a t e and/or w i t h t h e distribution of surround-
For I and III,
ing s o l v e n t molecules.
is m a r k e d l y dependent on tem-
kynx
p e r a t u r e w i t h a n a c t i v a t i o n e n e r g y Evm of ca. 3.8
and
1.0 kcal/mole,
respectively,
while
for
II
it
T h e e x c i t a t i o n spectra of I presented in F i g . 4
e x h i b i t a strong t e m p e r a t u r e e f f e c t in accordance
with
a m o u n t s t o o n l y ca. 0.3 kcal/mole.
A t lower t e m p e r a t u r e s t h e first t e r m in (9 a) a n d
Okada's
findings.
similar b e h a v i o u r .
Compound
The excitation
II,
(9 b) becomes more significant, so t h a t Exy — E\
however,
f o r I a n d I I I a n d Exy
e x c e p t f o r a v e r y slight change at t h e red edge of
The
estimated
— EynT for I I can be e s t i m a t e d .
values
of
Exy
are
within
1 . 1 4 - 1 . 3 kcal/mole f o r all d e r i v a t i v e s a n d are sig-
are a l m o s t insensitive t o
I I I reveals a
spectra of
temperature,
t h e s p e c t r u m (Figure 5).
Therefore,
it
seems
that
red-edge
excitation
n i f i c a n t l y lower t h a n those o b t a i n e d in n-butanol.
selects molecules w h i c h are more planar in their
V i s c o s i t y d a t a f o r M T H F in our t e m p e r a t u r e range
ground
w e r e not a v a i l a b l e , b u t b y a n a l o g y w i t h t e t r a h y d r o -
m a x i m u m of t h e a b s o r p t i o n b a n d . T h e enhanced
s t a t e as c o m p a r e d
to excitation
at
the
f u r a n a n d other ethers one m i g h t e x p e c t t h e Arrhe-
p l a n a r i t y causes a n a d d i t i o n a l charge-transfer both
nius f a c t o r for t h e M T H F
con-
in t h e g r o u n d a n d F C locally e x c i t e d state, b u t
(for
s t r o n g charge-separation b e t w e e n D a n d A is still
siderably
THF E
v
lower
than
^ 2 kcal/mole).
that
v i s c o s i t y t o be
for
n-butanol
u n f a v o u r a b l e in v i e w of 7r-electronic d e r e a l i z a t i o n
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A. Siemiarczuk, J. Koput, and A. Pohorille • Structural Requirements for Excited State ICT
603
of excitation: 6.5 ^ 0.5 ns and 1 4 . 5 ± 0 . 5 ns corresponding to the high and low energy emission,
respectively. Therefore, the "planar" conformers
are not structurally identical with those responsible
for I C T fluorescence.
W e are dealing thus in the ground state with a
distribution of conformers differing b y the twist
angle between anthryl and D M A . None of them is
structurally identical with the molecule in the
equilibrated I C T state. In such a case, the efficiency of populating of the I C T emitting state
should depend on the structure of the species we
are exciting. According to (1) we can describe the
quantum yield of Y fluorescence as
@y(Vexc)
V «lO'Vcm"1
Fig. 4. Excitation spectra of I in n-butanol at 198 K
(
) and at 103 K (
).
=
(fxy{vexc)
(10)
where
kxy(^exc)
<fxy ( l ' e x e ) —
in such a flat system. The highly polar structure [9]
observed in the F C excited state for this " p l a n a r "
population must be due also to strong solvation
occurring already in the ground state.
T o establish whether the red-edge excitation is
leading directly to the I C T emitting state, we
recorded decay curves of the fluorescence of I,
excited at 430 nm b y means of nanosecond dyelaser pulses. The estimated lifetimes in butanolic
glass are ca. 8 ns when excited at 430 nm as well as
at 337 nm. A t a higher temperature (200 K ) we
observed two decays independent of the wavelength
Oy,
kx(Vexc) 4~ kXy{vexc)
and d y is the intrinsic yield of Y fluorescence.
Excitation at the red-edge of the absorption band
(vi, " f l a t " conformers) and around maximum (v 2 ,
more twisted forms) yields the ratio
<Py{vl)
_
(fyih)
kXy{vi)
kXy{v
2)
A t higher temperatures
so that
lim
00
®y(h)
0y(~V
kx(v2)
+
kxyjh)
kx{V\)
+
kXy{Vl)
kx(vexc),
kxy(vexc)
1.
2
(11)
(11a)
)
W h e n relaxation is slowed down at lower temperatures, kxy{vexc)
<1 kx(vexc),
then
lim
—
0y{v2)
kxy{v2)
kx(h)
Since t x i n ) = t x (v2), therefore
@y(v
r™
23
25
27
29
0 «lO'Vcm"1
Fig. 5. Excitation spectra of II in n-butanol at 198 K
(
) and at 113 K (
).
<Py{v2)
1)
kxy
in)
kxy{v2)
(lib)
I n Fig. 6 the ratio of the fluorescence intensity
excited at n = 23000 c m - i and i>2 = 27000 c m - 1
is plotted versus temperature. The ratio decreases
monotonically with lowering temperature to the
point of glass formation and then starts rising
because of the overlap with emission originated
from the primary excited state X * . If relation ( l i b )
is valid, the results from Fig. 6 indicate that the
rate constant of intramolecular conversion, kXy, is
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A. Siemiarczuk, J. Koput, and A. Pohorille • Structural Requirements for Excited State ICT 604
604
1.5
U5 *
r-1 E
u.
S
1.0-
o*
31
LL
0.5
150
200
250
300
T/K
Fig. 6. Ratio of the fluorescence intensities of I excited at
430 nm and 366 nm as a function of the temperature in
M T H F . Fluorescence signals have been corrected for
sample absorbance and lamp intensity.
considerably lower for excitation of a more planar
t h a n a more twisted conformer. Since the intramolecular relaxation in the excited state seems to
be governed b y the viscosity of the solvent, the
preexponential factors in the Arrhenius formula
are most probably responsible for the differences
in k x y observed for the different conformers.
Quantum Chemical Calculations
Energies of electronic transitions, dipole moments
and atomic charges as a function of the angle of
t w i s t a between the anthryl and D M A moieties
were calculated b y means of the I N D O / S C I method
described elsewhere [14, 15]. The CI procedure was
performed over about 200 monoexcited configurations.
T h e P C I L O method [16, 17] was used in addition
to the I N D O / S method to construct a potential
energy curve as a function of a in the ground state.
T h e I N D O / S method has a spectroscopic parametrization which optimizes the electronic energy
and is not appropriate for the conformational
analysis where the nuclear part of the energy is
essential.
I n both methods, the bond lengths were t a k e n
as 1.41 for all C — C and C — N bonds and 1.1 for
C — H . T h e bond angles were 120° in the ring as well
as for C - N - C .
T h e distribution of the localized double bonds
in the P C I L O method was chosen in such a w a y
as to minimize the calculated energy.
Fig. 7. Potential energy curves for the ground (So) and two
lowest excited singlet states (Si and S2) as a function of a.
D o t t e d and dashed lines represent the potential curves
after including the term (12) into S2 and the term (13) into
So for ethyl ether (
) and acetonitrile (
) as
solvents.
E x c i t e d singlet state potential curves wrere obtained b y addition of the excitation energy to the
ground state energy derived from P C I L O calculations at a given value of a.
T h e ground state and t w o lowest excited state
potential curves are demonstrated in Figure 7.
T h e 1 L b anthracene-like state has been omitted
here as spectroscopically unobservable. The S i
state, originating from the anthracene-like x L a state,
exhibits a moderate I C T character dependent on a
(Fig. 8 and 9). Most interesting are the properties
of the S2 state, the transition to which is carrying
a v e r y small oscillator strength with a maximum
v a l u e of 0.027 for a = 30° and vanishing at 0° and
90°. This state exhibits a v e r y strong I C T character
growing rapidly at a = 90°. In an isolated molecule
S2 is a l w a y s higher t h a n the anthracene-like S i state,
e x c e p t for the region nearby oc = 0°, where, how-
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A. Siemiarczuk, J. Koput, and A. Pohorille • Structural Requirements for Excited State ICT
605
a classical f o r m u l a based on Onsager's r e a c t i o n
field a p p r o x i m a t i o n [18, 19] in a simplified f o r m as
used b y M a t a g a [20] neglecting t h e p o l a r i z a b i l i t y
of t h e solute m o l e c u l e :
Jl/D
20
(12)
^solv^e2/«3)/*
w h e r e /ue d e n o t e s t h e e x c i t e d state dipole m o m e n t ,
e t h e dielectric c o n s t a n t of t h e solvent, a the radius
of O n s a g e r ' s c a v i t y t a k e n a f t e r M a t a g a [2] as 5 Ä
and U = ( e - l ) / ( 2 e + l ) .
A s it c a n be seen f r o m F i g . 7, t a k i n g 2£Soiv
( E q . (12)) i n t o consideration leads t o a strong
decrease of t h e I C T s t a t e e n e r g y , locating this s t a t e
b e l o w t h e S i p o t e n t i a l c u r v e for t h e h i g h l y t w i s t e d
c o n f o r m a t i o n . T h i s inversion occurs in t h e full r a n g e
of s o l v e n t polarities where I C T emission has b e e n
o b s e r v e d e x p e r i m e n t a l l y (e ^ 5 -f- 37).
60*
30*
90*
Fig. 8. Calculated dipole moments of the Si ( • ) and S2 ( o )
state as a function of a.
T o c o m p a r e t h e I C T transition f r e q u e n c y w i t h
e x p e r i m e n t a l d a t a , the destabilization e n e r g y of
t h e F C g r o u n d s t a t e has been also estimated w i t h i n
Onsager's a p p r o x i m a t i o n [19, 20]:
^dest^e2/«3)
ever, a v e r y
m o m e n t of
close t o t h a t
as well as
(15.2 -f-19.3
(fe-fn),
(13)
strong steric barrier exists. T h e dipole
S2 calculated a t 90° (20.3 D ) is v e r y
e s t i m a t e d b y O k a d a et al. [2] (ca. 18 D )
t o t h a t reported b y B a u m a n n [9]
D depending on t h e d e r i v a t i v e ) .
w h e r e / n = (n 2 — l)/(2% 2 -f- 1) and n is t h e r e f r a c t i v e
Since a n I C T emission has been o b s e r v e d o n l y in
sufficiently polar solvents, t h e energetic stabilizat i o n d u e t o electrostatic interactions w i t h s o l v e n t
molecules m i g h t be reponsible f o r a n inversion of S i
a n d S2 • T o check for this effect, w e h a v e a p p l i e d
t h e emission f r o m S i , t a k e n s i m p l y as t h e transition
n u m b e r of the solvent.
T r a n s i t i o n frequencies h a v e been calculated for
a = 90° a c c o r d i n g
to the T I C T
model
and
are
c o m p a r e d w i t h e x p e r i m e n t a l ones in T a b l e 3. A l s o
e n e r g y a t a = 45° f r o m I N D O / S calculations,
v e r y well t h e
fluorescence
fits
transition in nonpolar
solvents. D e s p i t e of an excellent agreement, it m u s t
b e e m p h a s i z e d , h o w e v e r , t h a t t h e Onsager radius
s t r o n g l y influences t h e calculated
i£Soiv
and
-finest-
I f one w o u l d a p p l y a q u a n t u m - c h e m i c a l a p p r o a c h
[21] t o c a l c u l a t e t h e s o l v a t i o n e n e r g y w i t h
the
s a m e radius of t h e c a v i t y as w e h a v e used here,
Table 3. Calculated and experimental fluorescence transition energies of I.
Solvent
.012
.003
-,0H
-.055
-.002
-.015
-.006
-.027
cC = 6 0 *
d - 90*
Fig. 9. Changes in net atomic charges upon excitation to
Si at a = 60° and to S 2 at a = 90°.
Acetonitrile
Ethyl ether
n-hexane
a
J'calc
cm-1
cm-1
17,800
a
16,700
21,800
a
21,500
23,500
b
23,600
S 2 - > S 0 transition energy from INDO/S calculations with
£soiv and £"dest included (a = 90°).
Pure S i - > S o transition energy calculated b y INDO/S
method (a = 45°).
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606
A. Siemiarczuk, J. Koput, and A. Pohorille • Structural Requirements for Excited State ICT 606
the obtained value of ESoiv would be three times
larger t h a n t h a t evaluated from the classical
formula (12).
T h e value of a — 5 A used in this paper seems
t o be justified since it gave a reasonable estimation
of the excited state dipole moment, when the same
classical Onsager's reaction field approach h a d been
applied to the analysis of the fluorescence spectral
shifts in solvents of different polarity [2].
I n Fig. 9 changes in net atomic charges upon
excitation t o Si and S2 are presented for the
equilibrium conformation of the excited states
(a = 60° for S i and 90° for S 2 ). While in S i there
is only partial electron transfer from the D M A
moiety to the anthryl one, t h a t does not exceed
0.12 e, in the twisted S 2 state we have almost a full
electronic charge flow (Zlg, = 0.9e), as it has been
originally predicted b y the T I C T model (see, for
example, Fig. 5 in [6]).
T h e radiative transition to S 2 is forbidden because
of A 2 s y m m e t r y of this state in the C 2 v point group
in planar and perpendicular conformation. I n the
T I C T model the observed non-zero transition probability is attributed to torsional vibrations around
the single bond connecting the donor and acceptor,
w h a t leads t o break-up of s y m m e t r y and so gives
the TT-electronic coupling between both subunits.
A s it has been pointed out in the previous section,
[1] T . Okada,
T. Fujita, M. K u b o t a ,
S. Masaki,
N.
Mataga, R . Ide, Y . Sakata, and S. Misumi, Chem.
P h y s . Letters 14. 563 (1972).
[2] T . Okada, T. Fujita, and N . Mataga, Z. Physik. Chem
N F 101, 57 (1976).
[3] E . A . Chandross, in The Exciplex, ed. M. G o r d o n and
W . R . W a r e , Academic Press, New Y o r k 1975.
[4] A . Siemiarczuk, Z. R . Grabowski, A . K r ö w c z y n s k i ,
M. Asher, and M. Ottolenghi, Chem. Phys. Letters 51,
315 (1977).
[5] Z . R . Grabowski, K . Rotkiewicz, and A . Siemiarczuk,
J . Luminescence 18/19, 420 (1979).
[6] Z . R . Grabowski, K . Rotkiewicz, A . Siemiarczuk, D . J.
Cowley, and W . Baumann, N o u v . J . Chim. 3, 443
(1979).
[7] K . Rotkiewicz,
K . H . Grellmann,
and
Z . R . Grabowski, Chem. Phys. Letters 1 9 , 3 1 5 (1973).
[8] K . Rotkiewicz, Z. R . Grabowski, and A . Krowczyriski,
J . Luminescence 12/13, 877 (1976).
[9] W . Baumann, F . Petzke, and K . - D . Loosen, Z . Naturforsch. 34a, 1070 (1979).
kfV exhibits an activation energy of ca. 130 c m - 1
which is a typical value for small vibrational quanta.
A p p r o x i m a t i n g the potential energy curve S 2
modified b y E s o i v term nearby the minimum b y a
parabolic function, the harmonic frequency m a y be
estimated as 150 c m - 1 , being surprisingly close to
the experimental value.
The results of quantum chemical calculations
demonstrate the possible existence of a strongly
polar I C T excited state which exhibits the features
predicted b y the T I C T model [6]. I t seems to be
v e r y unlikely, t h a t the enhanced polarizability of
the locally excited state is exclusively responsible
for the I C T state formation [9]. The enhanced
polarizability in the excited state m a y promote
solvent-induced charge-separation instantaneously
after light absorption, but then, the system will
certainly proceed along intra- and intermolecular
coordinates to reach the minimum on the potential
energy hypersurface.
A cknowledge ment
W e are greatly indebted to Professor Zbigniew
R . Grabowski for his interest and stimulating discussions.
T h e work was done under project 03.10.7 of the
Polish A c a d e m y of Science.
[10] J . Jasny, J . Luminescence 17, 149 (1978).
[11] A . Siemiarczuk, Ph. D . Thesis, Warsaw 1978.
[12] W . R a p p , H . - H . Klingenberg, and H . E . Lessing, Ber.
Bunsen. Phys. Chem. 75, 883 (1971).
[13] H . E. Lessing and M. Reichert, Chem. Phys. Letters
46, 111 (1977).
[14] J . R i d l e y a n d M. Zerner, Theor. Chim. A c t a 32, 111
(1973).
[15] J . K o p u t , t o be published.
[16] S. Diner, J . P . Malrieu, and P. Claverie, Theor. Chim.
A c t a 13, 1 (1969).
[17] S. Diner, J . P . Malrieu, F. Jordan, and M. Gilbert,
Theor. Chim. A c t a 15, 100 (1969).
[18] L. Onsager, J . Amer. Chem. Soc. 58, 1483 (1936).
[19] W . L i p t a y , in Modern Quantum Chemistry, ed. 0 .
Sinanoglu, A c a d e m i c Press, N e w Y o r k 1966.
[20] N. Mataga, in The E x c i p l e x , ed. M. Gordon, and W .
R . W a r e , A c a d e m i c Press, N e w Y o r k 1975.
[21] A . T. A m o s and B. L. Burrows, A d v . Quantum Chem.
7, 289 (1973).
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