Synthesis and photophysical studies of donor-acceptor substituted tetrahydropyrenes

Article abstract of DOI:10.1039/b409411e

The tetrahydropyrene derivatives 2-N,N-dimethylamino-7-nitro-4,5,9,10- tetrahydropyrene (1) and 2-N,N-dimethylamino-7-acetyl-4,5,9,10-tetrahydropyrene (2) were synthesized and characterized. Photophysical properties of these molecules were investigated in several solvents. The absorption spectrum of 1 shows a slight red shift with solvent polarity, whereas that of 2 remains more or less unchanged. Fluorescence spectra of these compounds exhibit large, solvent-polarity-dependent Stokes shifts. The Stokes shifts are correlated to E_T(30) and E_T^N parameters and were quantitatively analyzed by the Mataga-Liptay equation. Both compounds show low fluorescence quantum yields in cyclohexane. Nanosecond flash photolysis studies suggested that the low quantum yield in cyclohexane is due to intersystem crossing to a triplet state. In the case of 2, the fluorescence quantum yields are high in all other solvents. In the case of 1 fluorescence quantum yields are very low in polar solvents and this is explained by invoking a twisted intramolecular charge transfer state. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2005.

Full text of DOI:10.1039/b409411e

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P A P E R
Synthesis and photophysical studies of donor-acceptor substituted
tetrahydropyreneswz
S. Sumalekshmy and K. R. Gopidas*
Photosciences and Photonics Division, Regional Research Laboratory (CSIR), Trivandrum,
6
95 019 India. E-mail: gopidaskr@rediffmail.com; Fax: +91-471-2490186
Received (in Montpellier, France) 21st June 2004, Accepted 30th September 2004
First published as an Advance Article on the web 4th January 2005
The tetrahydropyrene derivatives 2-N,N-dimethylamino-7-nitro-4,5,9,10-tetrahydropyrene (1) and
2
-N,N-dimethylamino-7-acetyl-4,5,9,10-tetrahydropyrene (2) were synthesized and characterized.
Photophysical properties of these molecules were investigated in several solvents. The absorption spectrum of
shows a slight red shift with solvent polarity, whereas that of 2 remains more or less unchanged.
Fluorescence spectra of these compounds exhibit large, solvent-polarity-dependent Stokes shifts. The Stokes
1
N
T T
shifts are correlated to E (30) and E parameters and were quantitatively analyzed by the Mataga–Liptay
equation. Both compounds show low fluorescence quantum yields in cyclohexane. Nanosecond flash
photolysis studies suggested that the low quantum yield in cyclohexane is due to intersystem crossing to a
triplet state. In the case of 2, the fluorescence quantum yields are high in all other solvents. In the case of 1
fluorescence quantum yields are very low in polar solvents and this is explained by invoking a twisted
intramolecular charge transfer state.
In view of the general interest in D-A biphenyls and their
potential applications, we have investigated the photophysical
properties of a few D-A substituted tetrahydropyrene (THP)
derivatives. THPs are bridged biphenyl derivatives where the
1
rotation about the C –C1’ bond is restricted. The phenyl rings
1
. Introduction
Excited state processes taking place in donor (D)-acceptor (A)
substituted biphenyl systems have been studied in great detail
by several groups.
place in D-A biphenyls is an intramolecular charge transfer
20
1
–19
The most prominent photoprocess taking
in the THP system are twisted by 141 (from AM1 calculations):
this value is in-between those reported for D-A fluorenes and
4
(
ICT) from the donor to the acceptor fragment. Other im-
portant issues related to the photophysics of these systems are:
1) the relative energies of the Franck–Condon (FC) and CT
2–8
D-A biphenyls studied earlier by Rettig and others.
Our
(
studies showed that the photophysical properties of D-A THP
systems are very similar to those of D-A biphenyl systems
studied earlier.
Herein we report the synthesis and photophysical properties
of two new THP systems, namely, 2-N,N-dimethylamino-
excited states, (2) the electronic structure and conformation of
the FC and CT excited states, (3) the intramolecular and
environmental factors controlling the radiative and non-radia-
1
tive charge recombination ( CT - S ) process and (4) the
0
nature and role of triplet states, if any. Rettig and co-workers
have recently investigated the excited state ICT processes in a
series of differently twisted 4-N,N-dimethylamino-4 -cyano-
biphenyl systems in great detail. The dipole moments, radiative
rates and torsional relaxations in the excited states were
analyzed by comparison with absorption spectra and CNDO/S
calculations. The electronic nature of these molecules could be
explained in terms of a composite molecular model with the
dimethylaminophenyl moiety acting as the donor unit and the
7
-nitro-4,5,9,10-tetrahydropyrene (1) and 2-N,N-dimethylami-
no-7-acetyl-4,5,9,10-tetrahydropyrene (2), shown in Chart 1.
Compared to the D-A THP and D-A biphenyl systems studied
earlier, these compounds exhibit several differences in their
photophysical behavior. Hence, a detailed study was under-
taken and the results are presented here.
0
2
. Experimental
cyanophenyl moiety acting as the acceptor unit. In the case of
0
2.1. General techniques
4
-N,N-dimethylamino-4 -cyanobiphenyl, in which the two phe-
The melting points are uncorrected and were determined using
1 13
a Mel-Temp melting point apparatus. The H and C NMR
nyl rings are twisted by 391 in the ground state, the first excited
state is shown to be a planar, emissive ICT state, irrespective of
solvent polarity. For 2,6-dimethyl-4-N,N-dimethylamino-
0
4
-cyanobiphenyl, which is twisted by 781 in the ground state,
the first excited state is shown to be a planar, ICT state only in
non-polar solvents. Upon increasing solvent polarity, a highly
twisted, relaxed conformation is populated for this compound,
and this results in a strong decrease in the radiative rates.
w This is publication number PPD-(RRLT)-188 from the Photo-
sciences and Photonics Division, Regional Research Laboratory
(CSIR), Trivandrum.
z Electronic supplementary information (ESI) available: details of the
synthesis and characterization of compounds 1, 2, 4 and 5. See http://
www.rsc.org/suppdata/nj/b4/b409411e/
Chart 1
T h i s j o u r n a l i s & T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 5
N e w J . C h e m . , 2 0 0 5 , 2 9 , 3 2 5 – 3 3 1
325

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3. Results and discussion
3
.1. Absorption spectra
Absorption spectra of 1 and 2 in cyclohexane, benzene and
acetonitrile are shown in Fig. 1. The spectrum in cyclohexane
exhibits vibrational structure. In other solvents the absorption
band is somewhat broadened and the fine structure is lost.
In the case of 1 the absorption maximum exhibits a moderate
red shift with solvent polarity. It can be noted that the onset
of absorption exhibited significant red shifts with increase in
solvent polarity. In the case of 2 the absorption maxima
Scheme 1
spectra were obtained using a 300 MHz Bruker Avance DPX
spectrometer. FT-IR spectra were recorded on a Nicolet Im-
pact 400 D infrared spectrometer. Elemental analyses were
done using a Perkin Elmer Series II 2400 CHN analyzer. The
absorption spectra were recorded on a Shimadzu 3101PC
UV-vis-NIR scanning spectrophotometer. Fluorescence spec-
tra were recorded on a SPEX Fluorolog F112X spectrofluoro-
meter. The fluorescence spectra were corrected for detector
exhibits only slight changes with solvent polarity. In both
R
cases, the integrated absorption intensity edn
a
, which is
proportional to the oscillator strength f, changes only slightly
with solvent polarity (Table 1). This suggests that the electro-
nic and structural nature of the ground and FC excited
states responsible for these absorptions do not vary much
with solvent polarity. The absorption bands of 1 and 2 are ana-
4
response. Fluorescence quantum yields (F
relative to quinine sulfate in 0.5 M sulfuric acid (F
or triphenylpyrylium perchlorate in acetonitrile (F
f
) were determined
logous to the lowest energy bands in D-A biphenyls and the
20
D-A THP systems studied earlier. Hence we assign this band
2
1
f
= 0.546)
= 0.52)
2
2
as arising from an ICT transition, which involves transfer of
charge from the dimethylaminophenyl donor moiety to the
nitrophenyl (or acetylphenyl) acceptor moiety. This assignment
is confirmed by the changes observed in the absorption spectra
upon addition of trifluoroacetic acid (TFA).
Fig. 2 shows the effect of TFA on the absorption spectrum of
1 in cyclohexane. The band due to 1 decreases with simulta-
neous formation of a new band at B325 nm. This new
absorption band is similar to the absorption band of 2-nitro-
4,5,9,10-tetrahydropyrene. Compound 2 also exhibits a similar
behavior. TFA protonates the dimethylamino group in 1 or 2
and prevents the ICT process, leading to the observed changes
in the absorption spectrum.
f
2
3
f
or cresyl violet perchlorate in ethanol (F = 0.56) as reference
materials. Fluorescence lifetimes were determined using a
Edinburgh FL900CD single photon counting system and decay
analysis were carried out using Edinburgh software. Laser flash
photolysis experiments were carried out in an Applied Photo-
physics model LKS–20 laser kinetic spectrometer using a
Quanta Ray GCR–12 series Nd-YAG laser. The excitation
wavelength was 355 nm. Spectroscopic grade solvents were
used throughout.
2
.2. Syntheses
Tetrahydropyrene derivatives 1 and 2 were prepared as shown
in Schemes 1 and 2. For the synthesis of 1, tetrahydropyrene
3
.2. Fluorescence properties
(
3) was nitrated to give the 2,7-dinitro derivative 4, which was
partially reduced followed by methylation using iodomethane
see the ESI for details). The preparation of 2 was achieved by
the reductive methylation of the previously known 2-nitro-
The fluorescence properties of 1 and 2 were investigated in
solvents of varying polarity. The emission spectra in a few
solvents are shown in Fig. 3. The fluorescence spectra in
cyclohexane exhibit vibrational structure for both compounds.
Spectra in other solvents are structure-less and exhibit solva-
tochromic red shifts with increase in solvent polarity. The
emission maxima observed in different solvents and the corre-
sponding Stokes shift values (nabs ꢀ nem ) are presented in
Table 1. It can be seen from Table 1 that the Stokes shifts for
1 and 2 are very large in polar solvents.
(
7
2
-acetyltetrahydropyrene (see ESI for details).
1
Data for 1: mp 219–220 1C; H NMR (CDCl , 300 MHz): d
3
3 2
.85–2.89 (m, 8 H, benzylic), 3.03 [s, 6 H, N(CH ) ], 6.46 (s, 2
1
3
max
max
H, ArH), 7.9 (s, 2 H, ArH); C NMR (CDCl ): d 28.29, 28.56,
3
4
1
1
6
0.24, 109.55, 118.06, 121.17, 134.2, 137.86, 138.03, 144.5,
51.09; IR (KBr): 2934, 2831, 1615, 1506, 1434, 1310, 1191,
ꢀ
1
082, 881, 725 cm ; anal. calcd for C18
.16; N, 9.5; found: C, 73.28; H, 6.22; N, 9.08.
H
18
N
2
O
2
: C, 73.45; H,
Solvent-dependent shifts in the fluorescence spectra can be
attributed to factors such as: (1) dipole-dipole interactions
between solvent and solute, (2) changes in the nature of
emitting state induced by the solvent and (3) specific solvent-
solute interactions such as hydrogen bonding. In order to have
a better understanding of the excited states of 1 and 2 we have
tried to correlate the observed Stokes shifts to the Dimroth
1
Data for 2: mp 174–175 1C; H NMR (CDCl , 300 MHz): d
.5 (s, 3 H, COCH ), 2.89 (s, 8 H, benzylic), 3.0 [s, 6 H,
3
2
3
1
3
3 2
N(CH ) ], 6.46 (s, 2 H, ArH), 7.64 (s, 2 H, ArH); C NMR
(
CDCl ): d 26.3, 28.4, 28.9, 40.4, 109.9, 126.1, 133.6, 134.1,
3
1
36.2, 137.6, 150.7, 197.6; IR (KBr): 2925, 2365, 1672, 1590,
ꢀ1
N
24
1
480, 1430, 1340, 1280, 1190, 1150, 1080, 950, 870 cm ; anal.
21NO: C, 82.44; H, 7.26; N, 4.81; found: C, 82.5;
H, 7.03; N, 5.17.
T T
E (30) and E solvent parameters. The results are plotted in
calcd for C20
H
Fig. 4. In both cases the plots were linear with correlation
coefficients r = 0.92. The linearity of these plots suggests that
dipole-dipole interactions between the solute and solvent are
responsible for the large solvent-dependent shifts observed in
the fluorescence spectra of 1 and 2.
The solvatochromic shifts in the fluorescence maxima were
further analyzed to obtain quantitative information about the
dipole moments of the fluorophores. The Mataga–Liptay
2
5
equation was used:
3
n = ꢀ2m (m ꢀ m )Df/hca + constant
(1)
f
e
e
g
where
Df = [(e ꢀ 1)/(2e + 1)] ꢀ 0.5[(n ꢀ 1)/(2n + 1)]
In the equations m and m
2
2
(2)
e
g
are the excited and ground state
dipole moments, respectively, h is Planck’s constant, c is the
Scheme 2
3
26
N e w J . C h e m . , 2 0 0 5 , 2 9 , 3 2 5 – 3 3 1

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Fig. 1 Absorption spectra of 1 and 2 in cyclohexane (CHX), benzene (Bz) and acetonitrile (ACN).
Table 1 Spectroscopic data for 1 and 2 in different solvents
R
8
edn
M
a
/10
cm
max
ꢀ1
ꢀ2
max
ꢀ1
Compound
Solvent
l
a
/nm
l
f
/nm
nSt/cm
Cyclohexane
Benzene
Toluene
410, 431
429
426
0.92
0.98
0.92
1.0
450, 477
568
563
3425
5700
5700
6800
7000
7800
8000
7700
9100
9800
Diisopropyl ether
Tetrahydrofuran
Chloroform
Dichloromethane
Ethyl acetate
Acetonitrile
Methanol
420
431
588
619
1.0
438
439
0.92
1.0
667
677
428
436
0.97
0.98
0.99
639
722
431
748
Cyclohexane
Benzene
Toluene
363, 379
376
376
1.2
1.2
1.3
1.3
1.2
1.3
1.3
1.3
1.2
399, 411
440
440
1322
3900
3900
4700
5600
6100
6300
7300
9300
Diisopropyl ether
Chloroform
Dichloromethane
Ethyl acetate
Acetonitrile
Methanol
369
382
446
485
380
373
495
488
376
380
518
589
velocity of light, a is the radius of the cavity in which the
fluorophore resides (Onsager radius), Df is known as the
solvent polarity parameter and e and n are the dielectric
constant and refractive index, respectively, of the medium. Df
values for the various solvents were calculated from known
values of e and n. The fluorescence maxima for 1 and 2 were
then plotted against Df and straight line plots were obtained
(
Fig. 5).
From the slopes of theses plots, the dipole moments of the
compounds in the excited state can be calculated if the values
of a and m are known. m for 1 and 2, obtained from AM1
calculations, is 8.27 and 4.63 D, respectively. Using a value of
g
g
ꢀ
10
10
6
.0 ꢁ 10
obtained were 23.7 and 21.6 D, respectively. The very large
values of m indicate significant charge transfer in the excited
e
m for a, we calculated m for 1 and 2; the values
e
state of both the compounds. We thus conclude that fluores-
1
cence of 1 and 2 occurs from a CT state.
3
.3. Fluorescence quantum yield and lifetimes
The fluorescence quantum yields (F ) and lifetimes (t
f
f
) for the
compounds were measured in different solvents. The radiative
Fig. 2 Effect of addition of TFA on the absorption spectrum of 1
in cyclohexane.
N e w J . C h e m . , 2 0 0 5 , 2 9 , 3 2 5 – 3 3 1
327

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Fig. 3 Normalized fluorescence spectra at 298 K of 1 in (a) cyclohexane, (b) benzene, (c) diisopropyl ether, (d) chloroform and (e) acetonitrile and 2
in (a) cyclohexane, (b) benzene, (c) chloroform, (d) acetonitrile and (e) methanol.
(
k = F /t ) and nonradiative [k = (1 ꢀ F )/t ] rate constants
low in CHX. The fluorescence decays in this solvent are
biexponential. We have observed that the values of the two
lifetimes are independent of emission wavelength, but their
relative contributions change with wavelength. For example, in
the case of 1, with lexc = 410 nm and lem = 477 nm, the
contributions due to the short and long components are 92%
and 8%, respectively. When lem was changed to 540 nm, the
contributions change to 75% and 25%, respectively. Similar
r
f
f
nr
f
f
were calculated using the F
f
and t
f
values. The values of F
f
f
, t ,
kr and knr obtained are given in Table 2. As can be seen, the
fluorescence quantum yields exhibit variations with solvent
polarity.
In order to see if these quantum yield variations are due to a
solvent-dependent change in the nature of the excited state, the
following procedure was adopted. The natural fluorescence
rate constant (also known as the Strickler–Berg rate constant,
results were obtained in the case of 2 (lexc = 377 nm, lem
411 nm, t 85% and t 15%; lexc = 377 nm, lem = 475 nm,
t 65% and t 35%). This suggests that the fluorescence band
=
(SB)
r
26,27
k
) for a fluorophore can be calculated using eqn. (3):
1
2
Z
1
2
3
8
pc10 ln 10
ðSBÞ
2
3
f
observed in CHX is a composite of two emission bands,
1
namely the FC and CT states. We tentatively assign the
k
¼
n n
eðnaÞd ln na
ð3Þ
r
NL
component with the short lifetime to the FC state. Since there
are two emitting states with different lifetimes, it is meaningless
In eqn. (3), c is the velocity of light, N is Loschmidt’s constant
L
(
Avogadro’s number), n is the refractive index of the solvent
(SB)
r
R
to calculate k /kr values. Hence these calculations were not
carried out for 1 and 2 in CHX.
used, n is the fluorescence maximum, and e(n )d(lnn ) is the
area under the e(n
is applicable only when the fluorescence occurs from the state
(FC state) populated by the absorption. It can be shown that
the ratio of the radiative rate constant k to the Strickler–Berg
f
a
a
a
) vs. lnn
a
curve. The Strickler–Berg relation
In solvents more polar than CHX, 2 exhibits high fluores-
cence quantum yields. As the polarity increases from benzene
to acetonitrile, F
are single exponential and the lifetime shows a slight enhance-
f
remains constant. The fluorescence decays
r
(SB)
r
20
rate constant k
is given by:
(SB)
r
ment with solvent polarity. The k
r
/k
values for 2 are similar
(
SB)
2
2
in all solvents. These values are slightly greater than 1, indi-
cating that the molecular structure of the FC is only slightly
different from that of the emitting state. The behavior of 2 in
k
r
/k
r
= M
f
/M
a
(4)
If the fluorescence spectrum arises from the same state as that
populated by the absorption process, then M = M and k
= 1 for such systems. In order to see if the fluorescence
takes place from the FC state or from a slightly different state,
we have calculated values of k /k for compounds 1 and 2 in
the various solvents; the values obtained are given in Table 2.
2
–4
a
f
r
/
this respect is similar to that of the D-A biphenyl systems
(
r
SB)
20
k
and D-A THP systems studied earlier. Accordingly, the
dimethylaminophenyl moiety can be considered as the donor
fragment and the acetylphenyl moiety can be labelled as the
acceptor fragment in 2. In the ground state, the donor and
acceptor fragments are twisted by 141. Upon excitation in
polar solvents the FC state undergoes relaxation to a planar
(
r
SB)
r
(
r
SB)
Notice that k /k
r
is not equal to 1 in all solvents, indicating
that the emissions do not arise from the FC states.
An inspection of Table 2 shows that the photophysical data
for the compounds exhibit several differences with those of the
1
CT state, which is responsible for the observed emission. The
CT state is stabilized in polar solvents, leading to an increase
in the lifetime and a decrease of the radiative and non-radiative
1
2
–4
20
D-A biphenyls and D-A THP systems studied earlier. For
1
rates. It is to be noted that the lifetimes of the CT state in
example, the fluorescence quantum yields of 1 and 2 are very
N
T T
Fig. 4 Plots of Stokes shift values of 1 (n) and 2 (m) vs. (A) E (30) and (B) E .
3
28
N e w J . C h e m . , 2 0 0 5 , 2 9 , 3 2 5 – 3 3 1

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3
.4. Laser flash photolysis experiments
f
The very low F values in CHX suggest a facile non-radiative
pathway for 1 and 2 operating only in this solvent. In order to
see if this is due to an intersystem crossing (ISC), we have
carried out laser flash photolysis of 1 and 2. In cyclohexane 1
gives a very strong transient characterized by bleaching in the
3
00–400 nm region and strong absorption in the 500–700 nm
region (Fig. 6). In the case of 2 the transient exhibits strong
bleaching below 400 nm and strong absorption above 450 nm
with a maximum around 550 nm. The transient spectrum was
assigned to the triplet-triplet absorption based on the following
two observations: (1) the lifetime of the transients decrease very
strongly in the presence of oxygen and (2) the transients are
quenched by b-carotene, leading to the formation of a
b-carotene triplet (lmax = 540 nm in benzene). Flash photo-
lysis of these compounds in other solvents listed in Table 2 gave
extremely weak transients on the nanosecond time scale. This
suggests that ISC to the triplets does not contribute signifi-
cantly to excited state deactivation in these solvents.
f
Fig. 5 Plots of n vs. Df for 1 (n) and 2 (m).
t
The quantum yield of triplet formation (F ) for 1 was
determined by the energy transfer method, using b-carotene
as the acceptor. For this experiment, solutions of benzophe-
none in benzene (10 ml) and 1 in cyclohexane (10 ml) were
optically matched at 355 nm and mixed with 0.1 ml of a 1.0
mM solution of b-carotene in benzene. The solutions were flash
photolyzed and the triplet absorptions due to b-carotene at 540
benzene and toluene are comparable to the long-lifetime
component in CHX. This lends support to our assignment of
1
the long-lived state in CHX to a CT state.
For 1, the fluorescence quantum yields are high in benzene
and toluene. In these solvents 1 follows the same photophysical
pathway exhibited by 2. Further increase in solvent polarity
S
nm were monitored. Comparison of plateau absorbances (DA
R
for 1 and DA for benzophenone) following the completion of
sensitized triplet formation, properly corrected for the decay of
f
leads to a decrease in F values. This is associated with a
the donor triplets in competition with energy transfer to
b-carotene, enabled us to determine F for 1. The following
equation was used:
decrease of fluorescence lifetimes and an increase in radiative
and non-radiative rates. Our results suggest that a very strong
deactivation pathway is open to 1 in polar solvents. The
T
2
8
(
r
SB)
kr/k
values differ considerably in the different solvents,
indicating that the excited state structure varies with the
ꢀ
ꢁꢀ
ꢁ
S
DA
kSþb
kRþb ꢀ kR
kRþb
FT ¼ FR
ð5Þ
DAR kSþb ꢀ kS
solvent used for 1.
Table 2 Photophysical data for 1 and 2 in different solvents
a
8 ꢀ1
/10 s
8
ꢀ1
(SB)
8 ꢀ1
/10 s
(SB)
Compound
Solvent
F
f
t
f
/ns
k
r
k
nr/10 s
k
r
r r
k /k
CHX
Bz
Tol
0.03
0.72
0.69
0.34
0.16
0.01
0.28, 1.96
2.38
2.79
0.27
3.0
8.7
1.2
1.1
5.8
2.8
1.6
1.5
1.2
0.9
0.6
—
1.9
2.5
3.0
1.6
2.5
2.7
0.5
DIE
EA
1.13
0.66
2.4
0.31
12.7
30.9
ACN
0.32
CHX
Bz
Tol
0.049
0.74
0.74
0.72
0.74
0.72
0.23, 1.46
1.85
1.77
2.4
0.057
4
1.1
1.4
1.4
1.2
1.0
0.9
3.8
3.2
3.8
2.7
2.2
1.8
—
1.2
1.1
1.1
1.3
1.4
4.2
3.0
2.9
2.5
DIE
EA
2.5
2.9
ACN
a
CHX = cyclohexane, Bz = benzene, Tol = toluene, DIE = diisopropyl ether, EA = ethyl acetate, ACN = acetonitrile.
N e w J . C h e m . , 2 0 0 5 , 2 9 , 3 2 5 – 3 3 1
329

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Fig. 6 Transient absorption spectra of 1 and 2 in cyclohexane at 298 K at different time intervals following the laser excitation. Inset shows the
decay trace at 650 nm for 1 and 550 nm for 2. Spectra a, b, c, d and e were taken at 0.34, 0.84, 4.0, 10.0 and 25.0 ms after excitation.
In eqn. (5), F
R
is the triplet quantum yield of the reference
In the case of 2, in solvents other than CHX, the predomi-
1
nant species present in the excited state is the planar CT state
compound benzophenone, kS + b and kR 1 b refer to the ob-
served rate constants for the growth of b-carotene triplet in the
presence of 1 and benzophenone, respectively, and kR and kS
refer to observed decay rate constants, respectively, of the
triplets of 1 and benzophenone in the absence of quenchers.
(vide supra), as observed in the case of D-A biphenyls. This is
shown in Scheme 3. The molecule in the ground state is twisted
by 141. The FC state formed upon excitation also has the same
(
r
SB)
geometry. k /k
values are not equal to 1, suggesting that the
r
Using this method, we obtained F
T
= 0.97 for 1. Thus F
f
+
emission is not from the FC state. Steady-state fluorescence
spectra and nanosecond lifetime data did not give direct
evidence for the FC state in polar solvents, suggesting that
the FC state has a sub-nanosecond lifetime in polar solvents.
The FC state undergoes planarization and delocalization to
give the CT state as observed in the case of D-A biphenyls
1
(Scheme 3). Since the CT state is stabilized in polar solvents,
the quantum yield and lifetime values are higher in polar
solvents.
F = 1 in cyclohexane, indicating that no other deactivation
t
process takes place for 1 in this solvent. Transient absorptions
due to 2 and b-carotene triplets occurred at the same wave-
length. Because of this the growth kinetics of b-carotene could
1
not be measured properly. Thus, we could not measure F of 2.
T
Since 1 and 2 exhibit similar photophysical properties in
cyclohexane (Table 1), we propose that triplet quantum yields
of 1 and 2 will also be similar in this solvent. Since no tripl-
et was observed in polar solvents, the low fluorescence quan-
tum yields observed for 1 in polar solvents cannot be attributed
to ISC.
For 1, photoprocesses taking place in solvents such as
benzene and toluene are similar to those shown in Scheme 3.
1
The CT state is formed in high yield in these solvents. With
further increase in solvent polarity fluorescence quantum yields
and lifetimes decrease drastically. The behavior of 1 in polar
solvents is similar to that of 4-N,N-(dimethylamino)-2,6-
3
.5. Excited state processes in 1 and 2
0
dimethyl-4 -cyanobiphenyl studied by Rettig and co-workers.
A fast equilibrium between a more planar and a more twisted
As is clear from the results, excited state processes taking place
in 1 and 2 are solvent-dependent. In cyclohexane, excitation
leads to population of an FC state that seems to decay by three
competing pathways: (1) fluorescence to the ground state, (2)
1
rotamer distribution in the CT state is invoked to explain the
photophysics in this case. In the case of 1 also we attribute the
1
value in polar solvents to a twisted CT state. The
low F
twisting can be either at the dimethylamino group as suggested
f
1
cross-over to a closely lying CT state and (3) ISC to a triplet
state (Fig. 7). ISC to the triplet is the major pathway in this
for 4-N,N-dimethylaminobenzonitrile (DMABN) and other
1
solvent. The existence of the CT state is inferred from the
1
biexponential nature of the fluorescence decay. Since the CT
state is not stabilized in non-polar solvents, this state is formed
only in very small amounts in CHX.
Fig. 7 Energy level diagram for 1 and 2 in cyclohexane.
N e w J . C h e m . , 2 0 0 5 , 2 9 , 3 2 5 – 3 3 1
Scheme 3
3
30

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compounds exhibiting dual emission, or at the nitro group. A
third possibility is twisting about the single bond connecting
the two phenyl groups. It must be mentioned here that rotation
about the central C–C bond is restricted in THP systems.
Hence twisting to high angles is not physically possible.
Twisting of the dimethylamino or nitro group is more prob-
able. Since a nitro group can stabilize a negative charge better
can stabilize a negative charge and in this case twisting invol-
ving the nitro group is a predominant excited state deactivation
pathway in polar solvents. With weaker acceptor groups such
as acetyl, this pathway is not at all important.
Acknowledgements
(
compared to the stabilization afforded to a positive charge by
The authors wish to thank the Council of Scientific and
Industrial Research (CSIR), Government of India, and the
Department of Science and Technology (DST), Government of
India, for financial support.
a dimethylamino group), the nitro group may be the twisting
moiety. Twisting reduces delocalization, leading to localization
of the charges. Localized charges are stabilized better in polar
1
solvents and this reduces the energy of the CT states further.
In the case of D-p-A systems such as DMABN, the ground
state is planar. The TICT model invokes a twisting of the
dimethylamino group with respect to the plane of the phenyl
ring in the excited state of DMABN. The D-A biphenyl
systems, on the other hand, are twisted in the ground state
and exhibit very different excited state pathways. D-A biphe-
nyls, which are not sterically crowded, undergo intramolecular
charge transfer and planarization in the excited state. In
sterically crowded D-A biphenyls, ICT in the excited state is
associated with twisting of the phenyl rings to a perpendicular
geometry in polar solvents. Twisting of the D or A substituent
is not an active deactivation pathway in the D-A biphenyl
systems (including the D-A THP systems) studied earlier. The
behavior of 1 suggests that twisting of the acceptor substituent
can also contribute towards the deactivation of the excited
states in these molecules.
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N e w J . C h e m . , 2 0 0 5 , 2 9 , 3 2 5 – 3 3 1
331