Photophysical properties of an asymmetrical 2,5-diarylidene-cyclopentanone dye possessing electron donor and acceptor substituents

Article abstract of DOI:10.1016/j.molstruc.2010.08.016

Spectroscopic and photophysical properties have been measured in a variety of solvents for (2E,5E)-2-(p-cyanobenzylidene)-5-(p-dimethylaminobenzylidene)- cyclopentanone (I), an asymmetrically substituted 2,5-diarylidene-cyclopentanone known to have potent

Full text of DOI:10.1016/j.molstruc.2010.08.016

Journal of Molecular Structure 982 (2010) 121–126
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Photophysical properties of an asymmetrical 2,5-diarylidene-cyclopentanone
dye possessing electron donor and acceptor substituents
⇑
Christopher A. Zoto, Robert E. Connors
Department of Chemistry and Biochemistry, Worcester Polytechnic Institute, Worcester, MA 01609, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Spectroscopic and photophysical properties have been measured in a variety of solvents for (2E,5E)-2-(p-
cyanobenzylidene)-5-(p-dimethylaminobenzylidene)-cyclopentanone (I), an asymmetrically substituted
2,5-diarylidene-cyclopentanone known to have potential for applications utilizing two-photon absorp-
tion (TPA). Compound I was synthesized by a simple procedure utilizing DIMCARB catalyst. Absorption
and fluorescence maxima have been correlated with the ET(30) solvent polarity scale. Theoretical TD-
DFT calculations and Lippert–Mataga analysis demonstrate the internal charge transfer (ICT) nature of
the S0 ? S1 excitation. Radiative and nonradiative decay constants have been determined from fluores-
cence quantum yields and lifetimes. Substantial variation with solvent in the rate of nonradiative decay
is interpreted in terms of a competition between internal conversion and intersystem crossing.
Ó 2010 Elsevier B.V. All rights reserved.
Received 27 July 2010
Accepted 6 August 2010
Available online 14 August 2010
Keywords:
2,5-Diarylidene-cyclopentanones
Fluorescence
Internal charge transfer
Internal conversion
Intersystem crossing
1. Introduction
dimethylaminobenzaldehyde were stirred at room temperature
in the presence of N,N-dimethylammonium-N0,N0-dimethylcarba-
Substituted 2,5-diarylidene-cyclopentanones have been the
subject of numerous studies aimed at elucidating their nonlinear
optical (NLO) properties [1–5]. Recently it has been reported that
the asymmetric push–pull compound, (2E,5E)-2-(p-cyanobenzy-
mate (DIMCARB) catalyst [9,10] to produce (E)-2-(p-dimethylami-
nobenzylidene)-cyclopentanone (1pdbma). This was followed by
the intermolecular base-catalyzed crossed aldol condensation of
1pdbma with p-cyanobenzaldehyde to yield I. Both 1pdbma and
I were purified by silica gel column chromatography employing a
gradient approach with a mixture of hexanes and ethyl acetate.
Purity was confirmed by TLC. 1H NMR spectra (CDCl3, 400 MHz)
of 1pdbma and I agree with previously published data [5].
Absorption spectra were measured with a Shimadzu UV2100U
spectrometer (2-nm band pass). Fluorescence emission spectra
were obtained with a Perkin–Elmer LS 50B luminescence spec-
trometer equipped with a red sensitive R928 phototube. Fluores-
cence quantum yields (Uf) were determined by comparing the
corrected integrated fluorescence spectrum of the sample with
that of a standard (fluorescein in 0.1 N NaOH; Uf = 0.95) according
to [11]
lidene)-5-(p-dimethylaminobenzylidene)-cyclopentanone
(I),
shown in Fig. 1, exhibits efficient two-photon absorption (TPA)
when dissolved in chloroform (CHCl3) [5]. The TPA cross-section
was determined by comparing the two-photon up-converted fluo-
rescence of I to that of a reference compound. These measurements
require knowledge of the one-photon absorption and fluorescence
properties of the target molecule in the solvent used for the
two-photon experiment. This paper reports the photophysical
properties of I in a wide variety of solvent environments. The
results demonstrate that these properties vary significantly with
solvent. Other applications of 2,5-diarylidene-cyclopentanones in-
clude polarity probes [6,7] and fluoroionophores [8]. The work
presented here is expected to be of importance to those interested
in fundamental properties of dyes as well as those considering
practical applications of I and related compounds.
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
Astd
A
n2
n2std
D
Dstd
Uf
¼
Uf;std
ð1Þ
where A is the absorbance at the excitation wavelength, n is the
refractive index of the solvent, and D is the area under the corrected
fluorescence spectrum. Solutions of both the standard and sample
were prepared initially with the absorbance at kmax approximately
equal to 0.5, followed by an accurate tenfold dilution. Fluorescence
spectra used to determine Uf were corrected with correction factors
that were obtained by measuring the spectra of compounds with
2. Experimental and computational methods
Compound I was synthesized by the two step procedure shown
in Fig. 1. First, an equimolar mixture of cyclopentanone and p-
⇑
Corresponding author.
E-mail address: rconnors@wpi.edu (R.E. Connors).
known emission spectra [11]. Fluorescence lifetimes (sf) were
0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.08.016

122
C.A. Zoto, R.E. Connors / Journal of Molecular Structure 982 (2010) 121–126
3. Results and discussion
N
Step 1
-
CO2
3.1. Geometry optimization and spectral calculations
+
H2
O
O
N
Ground state DFT geometry optimization of I was carried out at
the B3LYP/6-311 + G(d,p) level of theory. A minimum energy struc-
ture was confirmed by obtaining all positive frequencies for the
calculated normal modes of vibration. The ground state structure
obtained is nearly planar with ꢀ6° rotation of the substituted phe-
nyl rings. TD-DFT B3LYP/6-311 + G(d,p) spectral calculations were
performed using the optimized ground state structure. The room
temperature absorption spectrum of I in CHCl3 along with the re-
sults of the TD-DFT (PCM = chloroform) calculations are shown in
Fig. 2. Results for the first three computed excitations along with
experimental data are summarized in Table 1. Examination of the
orbitals that make the major contribution to the configuration
interaction (CI) description of the excited state provides insight
to the nature of these excitations. The S0 ? S1 transition is
+
(DIMCARB)
CH2Cl2
H
(H3C)2N
O
(H3C)2N
1pdbma
Step 2
O
Table 1
2.5 % (w/v) NaOH
MeOH
Experimental spectral data and B3LYP/6-311 + G(d,p) TD-DFT calculated results for I.
H
+
1pdbma
Gas phase (calc.)
CHCl3 (calc.)
CHCl3 (expt.)
NC
S1(ICT,
kmax
f
emax
S2(n, pꢁ
p
,
pꢁ
473 nm
0.73
)
515 nm
0.94
468 nm
0.61
3.20 ꢂ 104 (Mꢃ1 cmꢃ1
)
O
)
kmax
f
426 nm
0.00
403 nm
0.00
Not observed
S3(
kmax
f
p
,
pꢁ
)
C
N
(H3C)2N
367 nm
0.84
376 nm
0.86
322 nm
I
0.8 0.2
emax
2.27 ꢂ 104 (Mꢃ1 cmꢃ1
)
Fig. 1. Reaction scheme for the preparation of I.
obtained with a Photon Technology International TM-3 time re-
solved spectrofluorometer with pulsed nitrogen/dye laser excita-
tion. All solutions were degassed with N2 to prevent excited state
quenching by molecular oxygen. DFT and TD-DFT quantum chemi-
cal calculations were carried out using the Gaussian 03 program
[12]. Solvent effects were computed using the Self-Consistent Reac-
tion Field (SCRF) Polarizable Continuum Model (PCM) options. The
DFT gas phase geometry was used in the solvent calculations with-
out further optimization.
40000
30000
20000
10000
0
1.2
0.8
0.4
0
200
300
400
500
600
Wavelength (nm)
Fig. 2. Absorption spectrum and TD-DFT results for I in CHCl3.
Fig. 3. Molecular orbitals of I.

C.A. Zoto, R.E. Connors / Journal of Molecular Structure 982 (2010) 121–126
123
Table 2
Spectroscopic properties of I in various solvents.
Solvent
kabs (nm)
kf (nm)
mabs (cmꢃ1
)
mf (cmꢃ1
)
ET(30)
D
f
Dm )
(cmꢃ1
Carbon tetrachloride
Carbon disulfide
Toluene
446
464
455
457
438
458
465
446
459
442
469
460
467
464
454
464
471
456
471
473
473
472
473
513
535
558
569
580
651
625
643
641
631
603
678
673
643
718
740
731
721
647
648
644
659
693
22,422
21,552
21,978
21,882
22,831
21,834
21,505
22,422
21,786
22,624
21,322
21,739
21,413
21,552
22,026
21,552
21,231
21,930
21,231
21,142
21,142
21,186
21,142
19,487
18,688
17,923
17,583
17,234
15,360
16,000
15,550
15,600
15,850
16,597
14,760
14,863
15,560
13,930
13,520
13,673
13,868
15,450
15,432
15,530
15,177
14,420
32.4
32.8
33.9
34.3
34.5
37.4
38.0
38.1
38.1
38.5
39.1
39.4
40.5
40.7
42.2
43.2
45.1
45.6
48.4
50.2
50.7
51.9
55.4
0.0119
ꢃ0.0007
0.0131
0.0031
0.1669
0.2104
0.1867
0.1996
0.1581
0.1709
0.1491
0.2391
0.2124
0.2171
0.2843
0.2752
0.2637
0.3054
0.2769
0.2642
0.2746
0.2887
0.3093
2935
2864
4055
4299
5597
6474
5505
6872
6186
6774
4725
6979
6550
5992
8096
8032
7558
8062
5781
5710
5612
6009
6722
Benzene
Diethyl ether
Tetrahydrofuran
o-Dichlorobenzene
Ethyl acetate
Ethyl benzoate
n-Butyl acetate
Chloroform
Cyclopentanone
Pyridine
Dichloromethane
Acetone
Dimethylformamide
Dimethyl sulfoxide
Acetonitrile
Isopropanol
n-Butanol
n-Propanol
Ethanol
Methanol
observed at 468 nm in CHCl3 and is predicted by TD-DFT to be a
strong transition appearing at 473 nm in the gas phase and
515 nm in CHCl3, arising from the HOMO ? LUMO orbital configu-
ration. Examination of the HOMO and LUMO of I depicted in Fig. 3
the gas phase and 403 nm in CHCl3. Absorption to this state has
not been observed owing to its forbidden nature. Excitation to S3
is predicted to occur at 367 nm in the gas phase and 376 nm in
CHCl3 with major CI configurations HOMO-1 ? LUMO, HOMO ? -
reveals the internal charge transfer (ICT)
p
,
pꢁ nature of this
LUMO + 1, corresponding to a p,
pꢁ transition delocalized over the
absorption, in that electron density is transferred from the elec-
p
entire molecule. The band observed at kmax = 322 nm is assigned to
tron donor side of the molecule in the HOMO to the electron accep-
tor side of the molecule in the LUMO. The S0 ? S2 transition is
computed to be a forbidden n, pꢁ excitation arising from HOMO-
2 ? LUMO where HOMO-2 is a nonbonding orbital localized on
the carbonyl oxygen. The calculated wavelengths are 426 nm in
this computed excitation.
Absorption and fluorescence properties of I were examined in
23 solvents and the results are presented in Table 2. A survey of
absorption and fluorescence spectra of I in six of these solvents
which vary in polarity and hydrogen bonding strength (nonpolar,
polar aprotic, and protic) is shown in Fig. 4. The absorption and
fluorescence maxima of I in wavenumbers are plotted against the
ET(30) empirical solvent polarity scale in Fig. 5. It is seen that with
increasing polarity there is a small shift towards the red (batho-
chromic shift) in absorption and a much larger shift in fluores-
cence. Further, protic solvents do not appear to be anomalous in
the absorption plot, but they clearly fall on a separate line in the
fluorescence plot. Previous studies of molecules similar to I have
MeOH
2-PrOH
CHCl3
24000
(a)
22000
20000
Ether
Toluene
CCl4
(b)
18000
16000
14000
12000
30
35
40
45
50
55
60
ET (30)
400
600
800
Wavelength (nm)
Fig. 5. Plot of (a) absorption and (b) fluorescence maxima of I in various solvents
against ET(30). Solid symbols represent aprotic solvents; open symbols represent
protic solvents.
Fig. 4. Normalized absorption and fluorescence spectra of I in various solvents.

124
C.A. Zoto, R.E. Connors / Journal of Molecular Structure 982 (2010) 121–126
noted that there is a transfer of electron charge to the carbonyl
oxygen following excitation from S0 to S1 causing the molecule to
become a stronger base in the excited state. The increased basicity
in S1 results in a stronger hydrogen bond with protic solvents
[13,14] which is manifested in Fig. 5 by a shift of the fluorescence
trend line for alcohols relative to other solvents.
Fig. 7 displays a plot of Uf against the maximum frequency of
fluorescence for I in the solvents listed. The appearance of the plot
is reminiscent of Fig. 11 of our previous work [17] where a similar
plot is shown for an unsubstituted 2,5-diarylidene-cylopentanone
(2,5-bis-(5-phenyl-penta-2,4-dienylidene)-cyclopentanone). It is
seen that Uf reaches a maximum (CHCl3, Uf = 0.30), approximately
in the middle (16,597 cmꢃ1) of the frequency range plotted
(13,000–20,000 cmꢃ1), then falls off towards higher frequencies
(nonpolar solvents: CCl4, Uf = 0.019) and towards lower frequen-
cies (polar protic: MeOH, Uf = 0.003 and aprotic solvents: ACN,
Uf = 0.004). Related to this result is the plot shown in Fig. 8 of knr
against the frequency of fluorescence. Here a minimum is observed
for knr in the mid frequency range with an increase in knr towards
higher and lower frequencies. The data for the aprotic solvents de-
scribes a trend in knr that is nearly parabolic over the frequency
range of fluorescence emission. The knr rates for I in alcohols are
higher than the rates for I in aprotic solvents that have similar
Fig. 6 shows the Stokes shift, the difference in the absorption
~
m
~
(
A) and fluorescence (
m
F ) maxima, plotted against the solvent’s
orientation polarization function (
function is given by
D
f). The orientation polarization
ð2Þ
ð
e
ꢃ 1Þ
ðn2 ꢃ 1Þ
D
f ¼
ꢃ
ð2
e
þ 1Þ ð2n2 þ 1Þ
where
e
is the dielectric constant and n is the index of refraction for
the solvent. Again, separate lines are drawn for alcohols and non-
alcohols. The positive slope is evidence that the dipole moment of
I is larger in the excited electronic state than in the ground state.
The change in dipole moment in going from S0 to S1 can be deter-
mined by application of the Lippert–Mataga equation
2D
l2
Table 3
~
~
mA
ꢃ
mF
¼
D
f þ constant
ð3Þ
Photophysical properties of I in various solvents.
hca3
Solvent
Uf
sf
(ns)
kf (sꢃ1
)
knr (sꢃ1
)
where
D
l
=
le
ꢃ
lg is the difference between the excited state and
the ground state dipole moments, h is Planck’s constant, c is the
speed of light in vacuum, and a is the Onsager cavity radius for
the spherical interaction of the dipole in a solvent [15,16]. The
Onsager radius was computed by B3LYP/6-311 + G(d,p) to be
5.50 Å. Application of the Lippert–Mataga equation to the data for
1
2
3
4
5
6
7
8
Carbon tetrachloride
Carbon disulfide
Toluene
0.019
0.034
0.05
0.066
0.074
0.027
0.29
0.09
0.18
0.14
0.3
0.034
0.051
0.17
0.0095 0.23
0.0066 0.18
0.17
0.28
0.56
0.76
0.89
0.18
2.18
0.90
1.24
1.01
2.08
0.31
0.41
1.29
1.12 ꢂ 108 5.77 ꢂ 109
1.21 ꢂ 108 3.45 ꢂ 109
8.93 ꢂ 107 1.70 ꢂ 109
8.68 ꢂ 107 1.23 ꢂ 109
8.31 ꢂ 107 1.04 ꢂ 109
1.50 ꢂ 108 5.41 ꢂ 109
1.33 ꢂ 108 3.26 ꢂ 108
1.00 ꢂ 108 1.01 ꢂ 109
1.45 ꢂ 108 6.61 ꢂ 108
1.39 ꢂ 108 8.51 ꢂ 108
1.44 ꢂ 108 3.37 ꢂ 108
1.10 ꢂ 108 3.12 ꢂ 109
1.24 ꢂ 108 2.31 ꢂ 109
1.32 ꢂ 108 6.43 ꢂ 108
4.13 ꢂ 107 4.31 ꢂ 109
3.67 ꢂ 107 5.52 ꢂ 109
Benzene
Diethyl ether
Tetrahydrofuran
o-Dichlorobenzene
Ethyl acetate
Ethyl benzoate
n-Butyl acetate
Chloroform
Cyclopentanone
Pyridine
Dichloromethane
Acetone
non-alcohols in Fig. 6 yields a
dipole moment calculated by B3LYP/6-311 + G(d,p),
excited state dipole moment e = 27.0 D is obtained. The large in-
crease in dipole moment supports the ICT assignment for the
S0 ? S1 transition.
D
l
= 16.0 D. Using the ground state
l
g = 11.0 D, an
9
l
10
11
12
13
14
15
16
3.2. Fluorescence quantum yields, lifetimes, and decay constants
N,N-
Fluorescence quantum yields (Uf), lifetimes (sf), and radiative
(kf) and nonradiative (knr) decay constants are presented in Table
3. The decay constants were determined from
Dimethylformamide
Dimethyl sulfoxide
Acetonitrile
Isopropanol
n-Butanol
n-Propanol
Ethanol
Methanol
17
18
19
20
21
22
23
0.0074 0.19
0.0036 0.17
0.026
0.017
0.013
3.89 ꢂ 107 5.22 ꢂ 109
2.12 ꢂ 107 5.86 ꢂ 109
1.24 ꢂ 108 4.64 ꢂ 109
7.73 ꢂ 107 4.47 ꢂ 109
7.22 ꢂ 107 5.48 ꢂ 109
4.36 ꢂ 107 7.10 ꢂ 109
2.08 ꢂ 107 7.67 ꢂ 109
0.21
0.22
0.18
kf ¼ Uf
=
sf
ð4Þ
ð5Þ
ꢀ
ꢁ
0.0061 0.14
0.0027 0.13
1
Uf
knr
¼
ꢃ 1 kf
11
7
0.3
0.2
0.1
10000
8000
6000
4000
2000
0
9
14
10
8
5
4
13
12
3
2
1
15
17
19
20
21
6
16
22
18
23
0
12000
14000
16000
νf (cm-1)
18000
20000
-0.1
0
0.1
0.2
0.3
0.4
Δf
Fig. 6. Lippert–Mataga plot for
I
in various solvents. Solid symbols represent
Fig. 7. Quantum yield of I in various solvents. Circles represent aprotic solvents;
aprotic solvents; open symbols represent protic solvents.
diamonds represent protic solvents.

C.A. Zoto, R.E. Connors / Journal of Molecular Structure 982 (2010) 121–126
125
Table 4
8.00x109
6.00x109
4.00x109
2.00x109
0.00x100
TD-DFT computed energy gaps between the lowest (n, pꢁ) and (
p,
pꢁ) states and
23
6
experimental knr
.
22
CCl4
Toluene
CHCl3
T3(n, pꢁ)–T1(
p
,
pꢁ
pꢁ
)
)
7138 cmꢃ1
4465 cmꢃ1
57.7
7234 cmꢃ1
4592 cmꢃ1
17.0
8112 cmꢃ1
5384 cmꢃ1
3.37
18
1
21
16
17
S2(n, pꢁ)–S1(
p,
knr ꢂ 10ꢃ8/sꢃ1
19
20
15
2
pꢁ)–T (n, pꢁ) energy gap relative to kBT. In this
pꢁ and n, pꢁ states in
12
13
tude of the S1(p,
mechanism, spin–orbit coupling between p,
different spin manifolds and vibronic coupling within the same
spin manifold are operative in promoting intersystem crossing.
3
4
5
8
9
10
14
pꢁÞ SOC ! Tmðn; pꢁÞ VC ! T1ð
p
;
pꢁ
pꢁ
Þ
Þ
11
7
S1ð
p
p
;
;
S1ð
pꢁÞ VC ! Smðn; pꢁÞ SOC ! T1ð
p;
12000
14000
16000
18000
20000
νf (cm-1)
A solvent induced increase in the spacing between S1/T1 and the
appropriate intermediate n, pꢁ state(s) attenuates the degree of
state mixing which in turn reduces the rate of S ? T intersystem
crossing [22]. Both the thermally activated mechanism and the
vibronic spin–orbit coupling mechanism are favored over direct
Fig. 8. Nonradiative decay constant plotted against fluorescence maxima of I in
various solvents. Circles represent aprotic solvents; diamonds represent protic
solvents.
spin–orbit coupling between S1(p, p,
pꢁ) and T1( pꢁ) because of
positions for their fluorescence maxima, suggesting a hydrogen
bonding influence on the nonradiative decay of I. In discussing
the major routes for nonradiative decay in aprotic solvents, we
separate the data in Fig. 8 into two regions, the region from the
minimum to the low frequency side (region 1) and the region from
the minimum to the high frequency side (region 2). In region 1, knr
the well established understanding that the matrix element for
spin–orbit coupling between states of different orbital configura-
tion is greater than the matrix element for spin–orbit coupling be-
tween states of the same orbital configuration [23]. The smooth
decrease in knr shown in region 2 is consistent with a gradual, sol-
vent induced increase in the spacing between S1/T1 and higher en-
ergy (n, pꢁ) states. Table 4 provides theoretical support for this
interpretation with the results of TD-DFT calculations modeled in
CCl4, toluene, and CHCl3 solvent environments. It is seen that there
is an inverse relation between the magnitude of the computed en-
increases from 3.3 ꢂ 108 sꢃ1 (o-dichlorobenzene,
m )
f = 16,000 cmꢃ1
to 5.9 ꢂ 109 sꢃ1 (acetonitrile,
m
f = 13,868 cmꢃ1). In region 2, knr de-
creases from 5.8 ꢂ 109 sꢃ1 (CCl4,
m
f = 19,487 cmꢃ1) to 3.4 ꢂ 108 sꢃ1
(CHCl3, m
f = 16,597 cmꢃ1). Noting that knr = kic + kisc, where kic is the
rate of internal conversion from S1 to S0 and kisc is the rate of inter-
system crossing from the singlet to the triplet manifold of states,
we believe that the variation in knr shown in Fig. 8 can be attrib-
uted to opposing behavior for these two rates with respect to sol-
ergy gaps between the lowest (n, pꢁ) and ( pꢁ) states and the
p,
experimental values for knr. At this level of theory, the qualitative
trend calculated is expected to be more meaningful than the nu-
meric values calculated for the (n, pꢁ)–( pꢁ) energy gaps. It is pos-
p,
vent polarity. In shifting from nonpolar (high
(low f), kic increases while kisc decreases. In region 1, where knr in-
creases with a decrease in f, the increase in kic dominates the de-
crease in kisc; whereas in region 2, where knr decreases with a
decrease in f, the decrease in kisc dominates the increase in kic.
mf) to polar solvents
sible that a combination of thermally activated and vibronic spin–
orbit coupling mechanisms contribute to solvent dependent S ? T
intersystem crossing observed for I at room temperature.
The radiative rates (kf) vary less with solvent and, although
somewhat scattered, show a general trend toward lower values
m
m
m
The order of magnitude increase in knr found in region 1 is
attributed to the energy gap law for internal conversion which pre-
dicts an exponential dependence of kic on the S0–S1 energy gap,
with lower mf, as predicted by Einstein’s treatment of electronic
transitions [24].
(D
E).
4. Conclusion
kic
¼
a
expðꢃb
D
EÞ
ð6Þ
The spectroscopic and photophysical properties of I have been
found to vary considerably with solvent. The natures of the low ly-
ing excited states have been assigned with the assistance of exper-
imental and theoretical data. The behavior of knr with respect to
fluorescence wavelength and solvent polarity can be divided into
two regions. In region 1, the increase in the rate of internal conver-
sion dominates the decrease in the rate of intersystem crossing;
whereas in region 2, intersystem crossing is the dominant channel
of decay from S1.
According to the energy gap law, kic is expected to increase as
the S0–S1 energy gap decreases due to greater vibrational overlap
(Frank–Condon factor) between the S0 and S1 states [18].
In region 2, where intersystem crossing is the major nonradia-
tive decay channel, we believe that the solvent modulated location
of (n, pꢁ) states relative to the S1( pꢁ) and T1( pꢁ) states influ-
p, p,
ences the rate of S ? T intersystem crossing. The positions of (n,
pꢁ) states and ( pꢁ) states behave differently under the influence
of a change in solvent polarity. Whereas (n, pꢁ) states undergo a
hypsochromic shift with increased solvent polarity; (
pꢁ) states
undergo a bathochromic shift [19]. Data presented in Fig. 5 and Ta-
ble 2 confirm the bathochromic shift expected for S1(
pꢁ). A
mechanism involving thermally activated intersystem crossing
from S1(
pꢁ) to a higher lying 3(n, pꢁ) state has been offered to ex-
p,
The variation in the rates of internal conversion and intersystem
crossing are interpreted to be related to solvent induced changes in
energy gaps on the singlet and triplet spin manifolds.
p,
p,
p,
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