Fluorescence and photoisomerization studies of p-nitrophenyl-substituted ethenylindoles

Article abstract of DOI:10.1002/poc.987

The synthesis, electronic absorption, fluorescence (λ_f, λ_eX, Φ_f, τ_f) and photoisomerization (Φ_t→c, photostationary state composition) properties of 3-(4-nitrophenylethenyl-E)-NH-indole (1), 3-(4- nitr

Full text of DOI:10.1002/poc.987

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2006; 19: 43–52
Published online 30 August 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.987
Fluorescence and photoisomerization studies
of p-nitrophenyl-substituted ethenylindoles
Anil K. Singh* and Prasanta K. Hota
Department of Chemistry, Indian Institute of Technology, Bombay, Powai, Mumbai 400 076, India
Received 24 January 2005; revised 20 June 2005; accepted 5 July 2005
ABSTRACT: The synthesis, electronic absorption, fluorescence (ꢀf, ꢀex, Èf, ꢁf) and photoisomerization (Èt ! c
,
photostationary state composition) properties of 3-(4-nitrophenylethenyl-E)-NH-indole (1), 3-(4-nitrophenylethenyl-
E)-N-ethylindole (2) and 3-(4-nitrophenyl ethenyl-E)-N-benzenesulfonylindole (3) in organic solvents of varying
polarity are reported. The absorption maximum of these compounds undergoes a moderate red shift with increasing
solvent polarity. However, the fluorescence maximum becomes highly red shifted with increasing solvent polarity.
Whereas 1 and 2 show broad fluorescence bands, 3 exhibits dual fluorescence. Further, 1 and 2 fluoresce much more
efficiently than 3. Correlation of the Stokes shift with solvent polarity parameters such as Áf and ET(30) and excited-
state dipole moment indicate a highly polar excited state for 1–3. Time-resolved fluorescence studies show that the
fluorescence decays are single- and multi-exponential type, depending on the solvent polarity. Further, 1 and 2 do not
show photoisomerization on irradiation. However, 3 is photoactive and shows efficient photoisomerization in non-
polar heptane. The sensitivity (ꢂ) of the photoreaction is determined in various solvent in terms of the Hammett plot,
which showed that the excited states involved are electron deficient in nature and consequently stabilized more by an
electron sufficient polar solvent and electron donating substituent. These results led us to suggest the existence of
three types of excited states, namely the locally excited state, the intramolecular charge-transfer excited state and the
conformationally relaxed intramolecular charge-transfer excited state in the photoprocesses of these compounds.
Copyright # 2005 John Wiley & Sons, Ltd.
KEYWORDS: indole; ethene; fluorescence; photoisomerization; charge transfer
INTRODUCTION
twisted perpendicular species (P*) is weakly polar and
follows the non-radiative pathway to the ground state. On
the other hand, when the excited state has more polar
character, its rapid stabilization causes considerable en-
ergy gap between the P* and charge-transfer (CT) excited
species, which leads to efficient fluorescence. Substitu-
ents can raise or lower the torsional energy barrier and
thus the fluorescence quantum yields for substituted
stilbene compounds are altered. In nitro-substituted stil-
benes, butadienes and hexatrienes and in their stiffened
derivatives, a highly polar excited-state species is be-
lieved to be formed upon excitation, which becomes
stabilized more than the P* state even in relatively non-
polar solvents17–22 and such compounds follow the radi-
ative pathway rather than the phantom excited state (P*)
pathway. Recently, we reported large solvatochromic/
dual fluorescence in nitro-substituted diarylbutadienes.23–26
These systems in general, however, fluoresce very
inefficiently.
The photochemistry and photophysics of 1,2-dipheny-
lethene has been extensively examined and reviewed.1–6
This is partly because 1,2-diphenylethene (stilbene) and
its longer homologues (collectively known as ꢃ,!-diphe-
nylpolyenes) serve as a model for photobiologically
significant linear polyenes such as retinal, carotenoids
and their lower homologues.7–12 Additionally, these
compounds exhibit interesting electro-optical properties
that can find applications in sensors, optical brighteners,
laser dyes, optical data storage systems, photoconductors,
photochemical cross-linking of polymers, non-linear
optics, etc.13–16 In this context, photoinduced intramole-
cular charge transfer in donor–acceptor conjugated poly-
enes such as stilbene and its derivatives have attracted a
great deal of attention in recent years. It is found that the
singlet excited state (S1) of trans-stilbene is highly
substituent and solvent polarity sensitive and is relatively
stabilized in polar solvents. Upon excitation, the S1 state
of trans-stilbene competes with its activated twisting to
a perpendicular (P*) species, which subsequently leads
to its cis–trans photoisomerization. The double bond
It is widely accepted that in such push–pull molecules
the solvent polarity-driven dual fluorescence can occur
owing to the emission from an initially prepared planar
locally excited state (LE) and from a CT state formed
through subsequent conformational relaxation. Thus,
various mechanisms have been proposed in order to
explain the photo-induced charge separation in push–pull
*Correspondence to: A. K. Singh, Department of Chemistry, Indian
Institute of Technology, Bombay, Powai, Mumbai 400 076, India.
E-mail: retinal@chem.iitb.ac.in
Copyright # 2005 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 43–52

44
A. K. SINGH AND P. K. HOTA
systems, which include a twisted intramolecular charge-
transfer (TICT) mechanism,27,28 planarized intramolecu-
lar charge-transfer (PICT) state or pseudo-Jahn–Teller
state29,30 and rehybridized intramolecular charge-transfer
state (RICT).31,32
solvents were of AR or UV grade and further dried and
distilled. The light petroleum used was of the b.p. 60–
80 ꢁC fraction. Melting-points were determined on a
Veego melting-point apparatus. UV–visible measure-
ments were made on Shimadzu UV-160A spectrophot-
ometer. FTIR spectra in KBr discs were recorded on a
Nicolet Impact 400 spectrophotometer. 1H NMR spectra
in CDCl3 using TMS as internal standard were recorded
on a Varian VXR 300 MHz FT-NMR instrument. CHN
analyses were performed on a Theoquest CE 1112 Series
CHNS autoanalyzer. HPLC analyses were performed on
a Beckman instrument consisting of a Beckman Model
110A pump and a Model 340 organizer fitted with a
wavelength-selective absorbance detector. The fluores-
cence spectra of 1–3 in all the solvents at 298 K were
recorded on a DM1B microprocessor-controlled Spex-
112 Fluorolog spectrofluorimeter equipped with Spex-
1932 F accessories. The samples were excited at their
absorption wavelength maximum (ꢀabs). The fluores-
cence quantum yield (Èf) at 298 K was determined by
taking Rhodamine B in ethanol as standard (Èf, 0.69),33
and using the following equation: È ¼ Èref(ꢅ2ref/ꢅ2) ꢂ
(ODref/OD) ꢂ (A/Aref), where ꢅ and ꢅref are the refractive
indices of the solvents, OD and ODref are the optical
densities, È and Èref are the fluorescence quantum yields
and A and Aref are the areas of the fluorescence bands of
the compound and the reference standard, respectively.
Time-resolved fluorescence decay measurements were
carried out using a high repetition rate picosecond laser
coupled to a time-correlated single photon counting
(TCSPC) spectrometer (Hamamatsu 2809). Samples
were excited in the absorption band of the compounds
at 315 nm by vertically polarized picosecond laser pulses
(frequency-doubled Ti:sapphire laser). The emission
was collected at the peak in the 350–600 nm region.
The typical peak count was 1000–5000. The quality of
the exponential fits was evaluated by the reduced ꢆ2 value
(ꢃ1.2). The excited-state dipole moments for all the
While the excited states of nitrostilbenes and their N,N-
dimethyl derivatives have been extensively studied, little
attention has been paid to the N-substituted acceptor
systems. To the best of our knowledge, the photophysical
studies of ethenes bearing indole as one of the donor
substituents are limited. Therefore, it was thought desir-
able to investigate such compounds to obtain more infor-
mation regarding the nature of the CT excited states in
donor–acceptor ethenes. In this context, we prepared p-
nitrophenyl-substituted ethenylindoles, namely, 3-(4-
nitrophenylethenyl-E)-NH-indole (1), 3-(4-nitropheny-
lethenyl-E)-N-ethylindole (2) and 3-(4-nitrophenylethe-
nyl-E)-N-benzenesulfonyl indole (3) (Scheme 1) and
examined the effect of N-substituted electron-acceptor
and electron-donor substituents on the absorption, fluor-
escence and photoisomerization properties of these push–
pull compounds. The nitroaryl group (ꢄp: þ0.81) can act
as a strong acceptor and the indole moiety can act as an
electron donor. Further, the donor ability can be altered
by putting an electron-releasing ethyl group (ꢄp: ꢀ0.14
for CH3) or an electron-withdrawing group such as
benzenesulfonyl (ꢄp: þ0.73 for SO2CH3) by replacing
hydrogen at the nitrogen atom. Hence, these compounds
represent novel push–pull ethenes, which have been
examined for their excited-state properties.
EXPERIMENTAL
General methods
Indole-3-aldehyde and p-nitrophenyl acetic acid from
Aldrich Chemical Co. USA, were used as received. All
NO2
NO2
CHO
3
iii
i
N
1
N
H
N
H
SO2C6H5
ii
NO2
2
N
C2H5
Scheme 1. Synthetic scheme for 1–3. Reagents and reaction conditions: (i) p-nitrophenylacetic acid, pyridine–piperidine,
reflux, 100 ꢁC, 8 h; (ii) potassium-tert-butoxide, tert-butyl alcohol, ethyl bromide, reflux, 12 h; (iii) benzenesulfonyl chloride,
acetone, K2CO3, r.t., 3 h
Copyright # 2005 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 43–52

STUDIES OF p-NITROPHENYL-SUBSTITUTED ETHENYLINDOLES
45
compounds were calculated using the ground-state
dipole moments (determined by the semiempirical AM1
method,34 geometry optimization, convergence lim-
it ¼ 0.000 000 1, iteration limit ¼ 32 357) and the slopes
obtained from the Lippert–Mataga equation:35 ꢇa ꢀ ꢇf ¼
{[2(ꢈe ꢀ ꢈg)2/hca3]F(",ꢅ)}, where ꢇa ꢀ ꢇf is the Stokes’
shift, ꢈe and ꢈg are the excited-state and ground-state
dipole moments, respectively, ꢈe ꢀ ꢈg ¼ Áꢈ is the
change in dipole moment, h is Planck’s constant, c is
the velocity of light, a is the Onsager cavity radius and
F(",ꢅ) ¼ Áf is the solvent polarity parameter. Further,
Áf ¼ (" ꢀ 1)/(2" þ 1) ꢀ (ꢅ2 ꢀ 1)/(2ꢅ2 þ 1), where " is the
relative permittivity and ꢅ is the refractive index36 of the
solvent. For Onsager radius parameter a, we used a value
(m, 1H, C-7), 7.47 (s, 1H, H—C2), 7.51 (d, J ¼ 16.4 Hz,
—
1H, —C CH—ArNO ), 7.61 (d, J ¼ 8.4 Hz, 2H, —Ar),
—
2
7.99–8.02 (m, 1H, —H—C4), 8.22 (d, J ¼ 8.7 Hz, 2H, —
ArNO2), 8.34 (s, br, 1H, —NH). Elemental analysis.
calcd for C16H12N2O2 (264.3): C, 72.71; H, 4.57;
N,10.60. Found: C, 72.57; H, 4.54; N, 10.78%.
3-[4-Nitrophenylethenyl-E]-N-ethylindole (2). Com-
pound 1 (0.5 g, 0.002 mol) was taken in freshly distilled
tert-butyl alcohol (20 ml) and potassium tert-butoxide
(0.2 g, 0.002 mol) in a two-necked round-bottomed flask
fitted with a reflux condenser. The reaction mixture was
stirred for 30 min at room temperature. Ethyl bromide
(2 ml, 0.01 mol) was added to the reaction mixture and
stirring was continued. Awhite suspension was formed. It
was further refluxed at 100 ꢁC for 12 h. The progress of
the reaction was monitored by TLC (15% ethyl acetate in
light petroleum, Rf ¼ 0.45). The reaction mixture was
cooled to room temperature, poured into ice-cold water
and kept at 4 ꢁC for 1 day. A yellow crystalline product
was filtered. It was further purified by column chromato-
graphy (silica gel, 5% ethyl acetate in light petroleum).
Yield 80%; Rf ¼ 0.45 [ethyl acetate–light petroleum
(1.5:8.5)]; HPLC, tR ¼ 6.66 min [LiChrosorb Si-60,
5 mm, 250 ꢂ 4 mm i.d., ethyl acetate–hexane (1.5:8.5),
flow rate 1.1 ml minꢀ1, detector wavelength 405 nm];
m.p. 135–136 ꢁC; UV–visible (MeOH) ꢀmax (nm) (",
l molꢀ1 cmꢀ1): 418 (18 823); FTIR ꢇmax (cmꢀ1): 1596,
˚
of 7.3 A, which is a reported value for a similar ethene
(N,N-dimethylamino-p-nitrostilbene).22 For all electronic
spectroscopic studies, 2.0 ꢂ 10ꢀ5 M solutions were used.
For photoisomerization studies, 1.0 ꢂ 10ꢀ3 M solutions
of 1–3 were irradiated at 365 nm using a 400 W medium-
pressure mercury lamp (Applied Photophysics, London,
UK) equipped with a monochromator. Irradiated solu-
1
tions were analyzed using HPLC and H NMR spectro-
scopy. The photoisomerization quantum yield (Èt ! c
)
was determined using the potassium ferrioxalate actino-
metry method.37 The amount of trans photoisomer that
disappeared during irradiation was determined by HPLC
analysis. The number of quanta absorbed was determined
by irradiating 2 ml of the ferrioxalate solution (0.006 M)
and assuming the quantum yield of formation of Fe2þ ion
to be 1.21. Using the percentage disappearance of trans
isomers, action plots were drawn.
1331 (NO2), 1627 (C C); 1H NMR: ꢉ1.51(t,
—
—
J ¼ 7.32 Hz, 3H, CH3), 4.21(q, J ¼ 7.32 Hz, 2H, CH2),
—
7.12 (d, J ¼ 16.1 Hz, 1H, —CH C—ArNO ), 7.23–7.34
—
(m, 2H, C-5, C-6), 7.38 (s, 1H, C-7), 7.40 (s, 1H, C-2),
—
2
7.49 (d, J ¼ 16.4 Hz, 1H, —C CH—ArNO ), 7.59 (d,
—
2
Syntheses
J ¼ 8.7 Hz, 2H, Ar), 7.98–8.00 (m, 1H, C-4), 8.20 (d,
J ¼ 8.7 Hz, 2H, —ArNO2). Elemental analysis. Calcd for
C18H16N2O2 (292.3): C, 73.99; H, 5.51; N, 9.58. Found:
C, 73.99; H, 5.09; N, 9.51%.
3-(4-Nitrophenylethenyl-E)-NH-indole (1). 3-For-
mylindole (1.45 g, 0.01 mol) was taken in freshly distilled
pyridine (10 ml) along with piperidine (0.6 ml) and p-
nitrophenylacetic acid (1.81 g, 0.01 mol) in a round-
bottomed flask fitted with a reflux condenser. The reac-
tion mixture was heated at around 100 ꢁC for 6 h. The
progress of the reaction was monitored by TLC (15%
ethyl acetate in light petroleum, Rf ¼ 0.2). The reaction
mixture was cooled to room temperature, poured into ice-
cold water and treated with 100 ml of dilute hydrochloric
acid to remove excess of pyridine from the reaction
mixture. The brick red product was filtered and purified
by column chromatography (silica gel, 10% ethyl acetate
in light petroleum). Yield 19%; Rf ¼ 0.2 [ethyl acetate–
light petroleum (1.5:8.5)]; HPLC, retention time
tR ¼ 8.3 min [LiChrosorb Si-60, 5 mm, 250 ꢂ 4 mm i.d.,
ethyl acetate–hexane (2:8), flow rate 2 ml minꢀ1, detector
wavelength 405 nm); m.p. 142–144 ꢁC; UV–visible
(MeOH) ꢀmax (nm) (", l molꢀ1 cmꢀ1: 413 nm (20 000);
3-[4-Nitrophenylethenyl-E]-N-benzenesulfonylin-
dole (3). Compound 1 (0.1 g, 0.4 mmol) in acetone
(10 ml) and anhydrous potassium carbonate (0.5 g,
4 mmol) were taken in a round-bottomed flask fitted
with a reflux condenser. The reaction mixture was stirred
at room temperature for 30 min, then cooled to 0 ꢁC in a
crushed-ice bath and benzenesulfonyl chloride (0.1 ml,
0.8 mmol) was added dropwise to the reaction mixture
and stirring was continued. The progress of the reaction
was monitored by TLC (silica gel, 15% ethyl acetate in
light petroleum). The product was filtered and the organic
solvent was evaporated under reduced pressure. The
light-yellow compound was purified by column chroma-
tography (silica gel, 5% ethyl acetate in light petroleum).
Yield 85%; Rf ¼ 0.4 [ethyl acetate–light petroleum
(1.5:8.5)]; HPLC: tR ¼ 13 min [LiChrosorb Si-60, 5 mm,
250 ꢂ 4 mm i.d., ethyl acetate–hexane (1.0:9.0), flow
rate, 1.1 ml minꢀ1 at detector wavelength 365 nm];
m.p.: 177–178 ꢁC; UV–visible (MeOH) ꢀmax (nm) (",
FTIR "(cmꢀ1): 3363 (NH), 1591,1342 (NO ), 1637 (C
—
—
2
1
—
C); H NMR: ꢉ7.17 (d, J ¼ 16.4 Hz, 1H, —CH C—
—
ArNO2), 7.25–7.33 (m, 2H at C-5 and C-6), 7.41–7.45
Copyright # 2005 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 43–52

46
A. K. SINGH AND P. K. HOTA
l molꢀ1 cmꢀ1): 367 (16 561); FTIR ꢇmax (cmꢀ1): 1597,
near sulfonyl, benzene proton), 7.81 (2H, d, J ¼ 9 Hz,
Ar), 7.99 (2H, d, J ¼ 9 Hz, ArNO2), 8.17 (1H, d, H at —
In—C-5), 8.23 (1H, d, H at —In—C-6).
1334 (NO ) 1640 (—C C—), 1183(—S ¼ O); 1H
—
—
2
—
NMR: ꢉ7.22 (d, J ¼ 18 Hz, 1H, —CH C—ArNO ),
—
2
—
7.29 (d, J ¼ 18 Hz,1 H, —C CH—ArNO ), 7.34–7.43
—
2
(m, 2 H, H at C-5 and C-6), 7.44–7.56 (m, 3H, benzene-
sulfonyl proton), 7.62 (d, J ¼ 9 Hz, 2 H, Ar), 7.81–
7.87 (m, 2H, near sulfonyl, benzene proton), 7.92 (m,
2H, H—C-7 and H—C-2), 8.05 (m, 1H, H—C-4), 8.22
(d, J ¼ 9 Hz, 2 H, ArNO2). Elemental analysis. Calcd for
C22H16N2O4S (404): C, 65.33; H, 3.98; N, 6.92, S, 7.92.
Found: C, 65.20; H, 3.88; N, 6.82; S 7.95%.
RESULTS AND DISCUSSION
Absorption and fluorescence studies
Absorption and fluorescence spectral data for 1–3
in organic solvents of different polarity are given in
Table 1. Typical absorption spectra of 1 and 3 are shown
in Fig. 1. A moderate red shift in the absorption max-
imum (ꢀabs max) with increasing solvent polarity is
observed for all the compounds. In protic polar solvents
such as methanol, the ꢀabs max is comparable to that in the
aprotic polar solvents. This indicates the absence of
ground-state hydrogen bond interactions in these com-
pounds. The moderate red shift in ꢀabs max can, therefore,
be due to mesomeric effects and intramolecular charge
transfer from the donor to the acceptor moiety. The
ground-state dipole moment (1, 8.50 D; 2, 8.91 D; 3,
7.86 D) as determined by the Hyperchem semiempirical
AM1 method indicates that in the ground state com-
pounds 1–3 are polar in nature.
3-[4-Nitrophenylethenyl-Z]-N-benzenesulfonylin-
dole (cis-3). A solution of trans-3 [0.01 g in 10 ml of
heptane–1,4-dioxane mixture (1:1, v/v)] was irradiated
for 2 h with a medium-pressure mercury lamp using a
glass filter with 350/400 nm cut-off (transmittance at
350 nm 50% and at 400 nm 100%). The reaction mixture
was concentrated in vacuum and the cis product was
isolated by conventional preparative TLC, which pro-
vided 3 mg of cis-3. Yield 50% (with respect to trans);
TLC Rf ¼ 0.42 [ethyl acetate–light petroleum (1.5:8.5)];
HPLC tR ¼ 12 min (13 min for trans) (LiChrosorb Si-60,
5 mm, 250 ꢂ 4 mm i.d., 10% ethyl acetate–hexane, flow
rate, 1.1 ml minꢀ1 at detector wavelength 365 nm); UV–
visible (MeOH) ꢀmax (nm): 363; 1H NMR: ꢉ6.78 (1H, s,
In contrast to a rather moderate solvent polarity effect
on their ꢀabs max, 1–3 show a marked influence of solvent
polarity on their fluorescence maximum (ꢀf max). Typical
fluorescence emission and excitation spectra of 1 and 3
are shown in Figs 2 and 3. Compounds 1 and 2 show a
—
H at —In—C ), 7.17 (1H, d, J ¼ 8 Hz, —CH C—
—
2
—
ArNO ), 7.19 (1H, d, J ¼ 8 Hz, —C CH—ArNO ),
—
7.28–7.40 (2H, m, H at —In—C-4 and —C-7), 7.42–
7.50 (3H, m, benzenesulfonyl proton), 7.52–7.68 (2H, m,
2
2
Table 1. UV–visible absorption and fluorescence emission data for 1–3
Compound
Solvent
ꢀabs max (nm)
(ꢀf max) (nm)
ꢀex max (nm)
Stokes shift (cmꢀ1
)
Èf
1
n-C7H16
CCl4
Dioxane
THF
EtOAc
CH3OH
DMF
CH3CN
n-C7H16
CCl4
Dioxane
THF
EtOAc
CH3OH
DMF
CH3CN
n-C7H16
CCl4
Dioxane
THF
393
403
404
414
409
413
428
410
403
413
416
423
414
418
432
419
357
366
369
368
367
367
376
369
511
543
554
581
583
567
638
642
509
531
565
592
592
558
642
650
402
402
403
407
403
416
425
405
398
406
404
421
413
418
426
410
356
364
371
372
371
372
374
369
5876
6397
6702
6943
7297
6577
7691
8814
5167
5381
6339
6749
7263
6002
7562
8482
3198
8165
8197
8723
8966
8932
9501
10237
0.051
0.311
0.516
0.457
0.089
0.003
0.040
0.009
0.076
0.521
0.869
0.324
0.090
0.003
0.093
0.001
0.004
0.007
0.047
0.121
0.095
0.002
0.149
0.066
2
3
403, 424, 520
407, 428, 522
418, 529
542
EtOAc
CH3OH
DMF
547
417, 546
585
593
CH3CN
Copyright # 2005 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 43–52

STUDIES OF p-NITROPHENYL-SUBSTITUTED ETHENYLINDOLES
47
Figure 1. UV–visible absorption spectra of (top)
1
and (bottom) 3 in (a) heptane, (b) 1,4-dioxane and (c)
methanol
Figure 2. Fluorescence spectra of (top) 1 and (bottom) 3 in
(a) heptane, (b) 1,4-dioxane, (c) tetrahydrofuran, (d) metha-
nol, (e) dimethylformamide and (f) acetonitrile
single fluorescence band in all the solvents examined.
However, this band is red-shifted with increasing solvent
polarity. On the other hand, 3 shows three fluorescence
bands located at 403, 424 and 520 nm in the non-polar
solvent heptane. In medium-polarity and polar solvents,
however, the fluorescence emission bands of 3 at the
shorter wavelengths are minimized and the longer wave-
length fluorescence emission band at 520 nm is red-
shifted. On going from the non-polar solvent heptane to
the aprotic polar solvent acetonitrile, the longer wave-
length fluorescence emission bands of 1–3 are red-shifted
by 131, 141 and 73 nm, respectively. Such large solvato-
chromic shifts of the fluorescence emission are charac-
teristic of nitro-substituted arylindolic ethenes. It may be
noted that the ethenyl compounds bearing a p-aminophe-
nyl group, e.g. 3-(4-aminophenylethenyl-E)-NH-indole,
show only a very moderate solvent polarity effect on
their absorption and fluorescence spectra.26 Hence, the
remarkable solvatochromic effect in 1–3 is due to the
electron-withdrawing nature of the nitro group. It is
therefore suggested that in their excited state the nitro-
substituted compounds 1–3 interact strongly with the
polar environment.
lifetimes of these compounds are shown in Table 2. The
fluorescence decay curves for 1 and 3 in 1,4-dioxane are
shown in Fig. 4. The fluorescence decays are multi-
exponential in non-polar heptane. However, 1 and 2
show single-exponential decay in medium-polarity and
polar solvents. This indicates that more than one type of
singlet excited state is involved in the photoprocesses of
these compounds. Two of the excited species have shorter
life times (0.1–1 ns), whereas the third one has a longer
lifetime (1–4 ns). The amplitude data suggest that the
shorter lifetime species dominates over the longer life-
time species.
A plot of Stokes shift vs solvent polarity parameters
such as Áf35 and ET(30)38 are shown in Figs 5 and 6,
respectively. The change in excited-state dipole moment
(Áꢈ) for 1–3 is calculated to be 15.87, 16.45 and 16.33 D,
respectively. Hence, the excited-state dipole moment (ꢈe)
for 1–3 is 24.37, 25.36 and 24.19 D, respectively. From
the plot of Stokes shift vs ET(30), it is observed that all
three ethenes show a large ÁG value with maximum
191.3 kcal molꢀ1 for 1 and minimum 183.8 kcal molꢀ1
for 3 (Table 3) (1 kcal ¼ 4.184 kJ). This means that the
singlet excited states of 1–3 are highly polar in nature
and, hence, a strong interaction of the excited state of
The excitation spectra of 1 and 3 are shown in Fig. 3.
The excitation spectrum is similar to the absorption
spectrum of these compounds, which indicates that the
fluorescence emissions in these compounds originate
from a single ground-state species. The excited-state
Copyright # 2005 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 43–52

48
A. K. SINGH AND P. K. HOTA
excited state (LE), intramolecular charge-transfer excited
state (ICT) and conformationally relaxed intramolecular
charge-transfer excited state (CRICT) as found in other
donor–acceptor compounds23–29. In 1 and 2, all three
species are found in non-polar solvents, whereas in
medium-polarity and polar solvents, the CRICT species
is predominantly formed. Hence, the structure of the
singlet excited state is highly sensitive to specific solute–
solvent interactions and the fluorescing species may not
be the same in every polar solvent. The extent of
conformational relaxation of the locally excited singlet
state and the resultant charge transfer depended greatly
on the nature of the surrounding media.
Photoisomerization
Direct irradiation of a solution of 1 and 2 in heptane, 1,4-
dioxane and methanol does not yield any photoisome-
rized product, as revealed by the UV–visible absorption
and the HPLC analyses. There was no change in the UV–
visible spectrum of 1 and 2 even after 8 h of irradiation.
The HPLC analysis showed only one peak with
tR ¼ 8.3 min (20% ethyl acetate–hexane, 2.0 ml minꢀ1
)
and tR ¼ 6.66 min (15% ethyl acetate in hexane,
1.1 ml minꢀ1) corresponding to 1 and 2, respectively.
On the other hand, 3, having an electron-withdrawing
phenylsulfonyl substituent at the indolic nitrogen, on
irradiation undergoes trans–cis photoisomerization as
evidenced by the UV–visible absorption and the HPLC
analyses of the photomixture. The HPLC analysis of the
photomixture of 3 showed the presence of two compo-
nents, one with tR ¼ 13.3 min corresponding to trans-3
and the other with tR ¼ 12.3 min due to cis-3 (10% ethyl
acetate in hexane, flow rate 1.1 ml minꢀ1) (Fig. 8). A high
photoisomerization quantum yield (Èt ! c, 0.50 in hep-
tane, 0.31 in 1,4-dioxane and 0.21 in methanol) in all the
three solvents is observed for 3. The photostationary state
composition is given in Table 4.
Figure 3. Fluorescence excitation spectra of (top) 1 and
(bottom) 3 in (a) heptane, (b) 1,4-dioxane and (c) methanol
these compounds with the solvent molecules is expected.
A plot of the Èf against solvent polarity parameter ET(30)
is shown in Fig. 7. The Èf for 1–3 increases with
increasing solvent polarity, with a maximum in 1,4-
dioxane, (1, 0.516; 2, 0.869; 3, 0.047). However, Èf falls
off drastically in polar solvents with a minimum in
methanol for 1–3 (1, 0.003; 2, 0.003; 3, 0.002). The Èf
for 3 is lower than those for 1 and 2. The existence of
large solvatochromic fluorescence and multiple fluores-
cence decay behaviors indicates the presence of more
than one excited state in the photoprocesses of these
compounds (Scheme 2). These states can be locally
The efficiency of trans–cis isomerization decreases on
increasing the solvent polarity. On absorption of light,
these ethenes yield a highly polar excited state. In polar
Table 2. Fluorescence lifetime and amplitude data for 1–3
Lifetime [ꢁ (ns)]/amplitude (ꢃ)
Compound
Solvent
ꢁ1
ꢃ1
ꢁ2
0.224
—
—
0.223
—
—
0.728
0.901
0.594
ꢃ2
0.255
—
—
0.254
—
—
0.359
0.445
0.170
ꢁ3
ꢃ3
ꢁav (ns)
ꢆ2
1
n-C7H16
Dioxane
CH3OH
n-C7H16
Dioxane
CH3OH
n-C7H16
Dioxane
CH3OH
0.036
—
—
0.016
—
—
0.045
0.138
0.026
0.725
—
—
0.714
—
—
0.472
0.519
0.791
1.814
2.68
0.10
1.023
2.707
0.15
3.189
2.067
4.359
0.020
1.0
1.0
0.034
1.0
1.0
0.169
0.036
0.039
0.120
2.68
0.10
0.104
2.707
0.15
0.847
0.547
0.293
1.32
1.02
1.10
1.35
1.08
1.20
1.18
1.23
1.09
2
3
Copyright # 2005 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 43–52

STUDIES OF p-NITROPHENYL-SUBSTITUTED ETHENYLINDOLES
49
Figure 6. Plot of Stokes shift vs solvent polarity parameter
ET(30) for (diamonds) 1, (circles) 2 and (triangles) 3 in organic
solvents of varying polarity
indicates an efficient deactivation process other than
trans–cis isomerization for trans-3 from its excited sing-
let state.
The singlet excited-state energy for these compounds
is calculated by taking the intersection wavelength of
fluorescence excitation and emission spectra in heptane
and methanol and using the approximate equation
S1 ꢀ S0 ¼ hc/ꢀ ¼ 1.24 keV/ꢀ, where, h (Planck’s con-
stant) ¼ 6.62 ꢂ 10ꢀ34 J s, c (velocity of light) ¼
3 ꢂ 108 m sꢀ1 and ꢀ ¼ wavelength in nanometers. From
heptane to methanol, it is found that the singlet excited-
state energy varies from 2.66 to 2.74 eV for 1 and 2,
whereas it is 3.04–3.26 eV for 3. This indicates that the
singlet-state energy of 3 is 0.52 eV higher than those of 1
and 2 (Scheme 3).
The activation energy barrier to the phantom excited
state (P*) in trans-stilbene is known to be increased in the
presence of a donor substituent or donor–acceptor con-
jugation effect.39–41 Hence, such compounds follow the
radiative pathway rather than the P* pathway. This is also
corroborated by their relatively higher Èf. Here, we
expect that compared with 1 and 2, 3 has a lower energy
barrier to the P* state because of the electron-withdraw-
ing nature of —SO2C6H5. Hence, 3 follows both the
radiative and the P* pathways, whereas 1 and 2 follow the
radiative pathway predominantly.
Figure 4. Fluorescence decay profiles for (top) 1 and (bot-
tom) 3 in 1,4-dioxane
Again, the trends observed for Èf for 1 and 2 and Èf
and Èt ! c for 3 indicate competitiveness of the observed
photoprocesses, i.e. fluorescence emission and photoi-
somerization. It can be suggested that photoisomerization
of 3 occurs from its singlet excited state. Hence 3 is
photochemically active, whereas 1 and 2 are photoche-
mically stable (Scheme 4).
Linear free energy relationship
Figure 5. Lippert–Mataga plot: Stokes shift vs solvent po-
larity parameter (Áf) for (diamonds) 1, (circles) 2 and
(triangles) 3 in organic solvents of varying polarity
The sensitivity (ꢂ) towards the formation of CRICT state
and photoisomerization process is determined by the
Hammett concept of linear free energy relationship
(LFER)42,43 and using the following equation:
solvent, a net stabilization of the excited state can cause a
large energy barrier for the photoisomerization process.
Hence, the efficiency of trans–cis isomerization is lower
in polar solvents. The small Èt ! c for 3 in polar solvents
logðk=k0Þ ¼ ꢄꢂ
Copyright # 2005 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 43–52

50
A. K. SINGH AND P. K. HOTA
Table 3. Dependence of fluorescence spectral data on solvent polarity parameters
ET(30)
Áf
ꢈ (D)
Compound
R2
Slope
R2
Slope
Áꢈ (D)
ꢈg (D)
ꢈe (D)
1
2
3
0.94
0.95
0.80
191.3
189.9
183.8
0.94
0.89
0.90
5914
6351
6265
15.87
16.45
16.33
8.50
8.91
7.86
24.37
25.36
24.19
2
˚
R , linear correlation; Áꢈ, dipole moment change as obtained by Lippert–Mataga plot by taking Onsager radius 7.3 A and using dielectric constant (") and
refractive index (ꢅ) of the solvent; parameters are calculated without considering solvent heptane, owing to deviation from the Lippert–Mataga and ET(30) plots
for 3 (Figs 5 and 6); ꢈg is the ground-state dipole moment as calculated by the Hyperchem semiempirical AM1 method.
Figure 8. HPLC traces: (a) before and (b) after the irradia-
tion of 3 in 1,4-dioxane
Figure 7. Plot of fluorescence quantum yield (Èf) vs ET(30)
value for (circles) 1, (squares) 2 and (asterisks) 3 in organic
solvents of varying polarity
Table 4. Photostationary state (PSS) composition and
photoisomerization quantum yield (Èt ! c) of 1–3
LE
PSS composition (%)
ICT
Compound
Solvent
E
Z
(Èt ! c
)
CRICT
P*
1
n-C7H16
Dioxane
CH3OH
n-C7H16
Dioxane
CH3OH
n-C7H16
Dioxane
CH3OH
—
—
—
2
3
—
—
—
48.28
93.42
94.83
51.72
6.58
5.17
0.50
0.31
0.21
Scheme 2. Three-state kinetic scheme applicable to p-
nitrophenylindolic ethenes. LE, locally planar excited state;
ICT, intramolecular charge-transfer excited state; CRICT,
conformationally relaxed intramolecular charge-transfer ex-
cited state; P*, perpendicular double bond twisted excited-
state species (phantom excited state)
more stable CT excited state, from which fluorescence
occurs. These relaxation processes result in a Stokes shift
(ÁE).
where k is the rate constant when a substituent is present
in the molecule, k0 is the rate constant when the sub-
stituent is hydrogen atom, ꢄ is the Hammett substituent
constant and ꢂ is the sensitivity of the reaction.
Hence the above Hammett equation can be used as ÁE/
2.3kT ¼ ꢄꢂ, where, k is the Boltzmann constant
(1.38 ꢂ 10ꢀ23); T is the absolute temperature, ꢄ is the
relative value between the nitro and the substituent
On excitation to the singlet excited state, the trans
configuration undergoes a conformational relaxation ow-
ing to the solvent–solute interaction, which leads to a
present on the indolic nitrogen (ꢄNO ꢀ ꢄR). Sensitivity
2
(ꢂ) of photoprocesses in various solvent with increasing
solvent polarity is calculated from the slope of the plot,
Copyright # 2005 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 43–52

STUDIES OF p-NITROPHENYL-SUBSTITUTED ETHENYLINDOLES
51
Table 5. Sensitivity (ꢂ) of the CRICT state for 1–3 in solvents
of increasing solvent polarity
Solvent
ꢂ
R2
No. of points
S1
n-C7H16
CCl4
Dioxane
THF
EtOAc
DMF
CH3CN
4.29
3.08
2.24
2.41
2.19
2.55
2.18
0.99
0.95
0.99
0.99
0.98
0.97
0.98
3
3
4
4
4
4
4
P*
hv
S0
Scheme 3. Plausible energy profiles which show the de-
crease in the energy barrier between S1 and P* states in 3.
Dashed line, energy profile in the absence of substituent;
solid line, energy profile in the presence of an electron-
withdrawing substituent, SO2C6H5
NO2
hv
Figure 9. Plot of Stokes shift/2.3kT vs relative Hammett ꢄ
constant (ꢄNO ꢀ ꢄR): (diamonds) heptane, (squares) CCl4,
2
N
R
(closed triangles) 1,4-dioxane, (open circles) THF, (open
triangles) ethyl acetate, (closed circles) DMF and (plus)
acetonitrile
N
R
i or ii or iii
NO2
NO2
1 : R =H
2 : R = C2H5
of an electron-withdrawing substituent, such as benzene-
sulfonyl. Similarly, the cationic excited state is stabilized
more in the presence of polar solvents rather than the
non-polar solvents n-heptane and CCl4. Therefore, a high
ꢂ value is observed in non-polar solvents, whereas in
medium-polarity and polar solvents a comparable ꢂ value
is observed. In other words, the cationic excited state is
less stabilized in the presence of an electron-withdrawing
substituent and in an non-polar solvent. Therefore, the
excited state is close to the P* state, which leads to
efficient photoisomerization in the case of 3.
hv
N
i or ii or iii
N
NO2
SO2C6H5
SO2C6H5
3
Scheme 4. Photochemical changes of 1–3 in (i) heptane, (ii)
1,4-dioxane and (iii) methanol
drawn between (ÁE/2.3kT) vs relative ꢄ-values of
the substituent (ꢄNO ꢀꢄR) (Table 5 and Fig. 9) with
2
the assumption that all the molecules lying on the same
Hammett plot belong to the same type of charge-transfer
species. It is found that on increasing the solvent polarity,
the value of ÁE/2.3kT increases, which indicates strong
interaction of the excited state of these molecules with
polar solvents. All points on the Hammett plot lie on the
straight line, except for 3 in the non-polar solvents n-
heptane and CCl4, when we consider the shorter wave-
length emission maximum for calculation of the Sto-
kes’shift/2.3kT parameter. This deviation for 3 is because
of the presence of a species different from the CRICT
species in heptane and CCl4. The ꢂ value is negative,
which indicates the formation of an electron-deficient
cationic excited-state species. Hence the excited state is
stabilized more in the presence of an electron donor
substituent, such as ethyl, but destabilized in the presence
CONCLUSION
Absorption and fluorescence studies together with corre-
lation of these spectroscopic properties with various
solvent polarity parameters reveal the highly polar nature
of the singlet excited state of push–pull ethenylindoles.
Time-resolved fluorescence studies together with
Lippert–Mataga, ET(30) and LFER plots show that these
compounds contain three type of excited species, namely
LE, ICT and CRICT, the formation of which depended
greatly on the solvent polarity and the substituent present
on the indolic nitrogen atom. The LFER plots indicate a
cationic excited state, which is stabilized more in the
presence of an electron-releasing substituent such as
Copyright # 2005 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 43–52

52
A. K. SINGH AND P. K. HOTA
16. Muthuraman M, Masse R, Nicoud JF, Desiraju GR. Chem. Mater.
2001; 13: 1473–1479.
17. Johnson ID, Thomas EW, Cundall RB. J. Chem. Soc., Faraday
Trans 2 1985; 81: 1303–1315.
18. Lin CT, Guan HW, McCoy RK, Spangler CW. J. Phys. Chem.
1989; 93: 39–43.
19. Gruen H, Gorner H. J. Phys. Chem. 1989; 93: 7144–7152.
20. Sonoda Y, Kwok WM, Petrasek Z, Ostler R, Matousek P, Parker
AW, Phillips D. J. Chem. Soc., Perkin Trans. 2 2001; 308–314.
21. Singh AK, Mahalaxmi GR. Proc. Natl. Acad. Sci., Sect. A: Phys.
Sci. (India) 2000; 70: 1–26.
22. Lapouyade R, Kuhn A, Letard JF, Rettig W. Chem. Phys. Lett.
1993; 208: 48–58.
23. Singh AK, Mahalaxmi GR. Photochem. Photobiol. 2000; 71:
387–396.
ethyl and in medium-polarity and polar organic solvents.
Compounds 1 and 2 are photochemically stable, whereas
3 is photoactive and undergoes efficient photoisomeriza-
tion in the non-polar solvent heptane. Further, the trend of
Èf and Èt ! c in 3 is inversely related. This indicates that
the photoisomerization in 3 occurs from the singlet
excited state. The energy barrier to photoisomerization
is controlled by the substituents and solvents. Thus,
ethenylindoles exhibit fluorescence due to CT states
and it is possible to alter the absorption, fluorescence
and photoisomerization properties of these compounds
by changing the donor ability of the indole moiety. Such
push–pull systems can be useful for developing materials
having newer opto-electronic properties.
24. Singh AK, Darshi M, Kanvah S. J. Phys. Chem. A 2000; 104: 464–
471.
25. Singh AK, Kanvah S. J. Chem. Soc., Perkin Trans. 2 2001; 395–
401.
26. Singh AK, Hota PK. Indian J. Chem. 2003; 42B: 2048–2053.
27. Pines D, Pines E, Rettig W. J. Phys. Chem. A 2003; 107: 236–242.
28. Yang JS, Liau KL, Wang CM, Hwang CY. J. Am. Chem. Soc.
2004; 126: 12325–12335.
Acknowledgments
29. Iyichev YV, Kuhnle W, Zachariasse KA. J. Phys. Chem. A 1998;
102: 5670–5680.
30. Zachariasse KA. Chem. Phys. Lett. 2000; 320: 8–13.
31. Sobolewski AL, Domcke W. Chem. Phys. Lett. 1996; 250: 428–
436.
A research fellowship to P.K.H. from the University
Grants Commission, New Delhi, is gratefully acknowl-
edged. Thanks are also due to Professor N. Periasamy for
his help in time-resolved fluorescence studies.
32. Sobolewski AL, Sudholt A, Domcke W. J. Phys. Chem. A 1998;
102: 2716–2722.
33. Rabek JF. Experimental Methods in Photochemistry and Photo-
physics, Part 2. Wiley: Chichester, 1982; 746–790.
34. Dewar MJS, Zoebisch EG, Healy EF, Stewart JJP. J. Am. Chem.
Soc.1985; 107: 3902–3909.
REFERENCES
35. Lippert E, Luder W, Moll F, Nagele H, Boos H, Prigge H,
Blankenstein IS. Angew. Chem. 1961; 73: 695–706.
36. Lide DR (ed). CRC Handbook of Chemistry and Physics
(79th edn). CRC Press: Boca Raton, FL, 1998–1999; Section 8–
120.
37. Rabek JF. Experimental Methods in Photochemistry and Photo-
physics, Part 2. Wiley: Chichester, 1982; 944–946.
38. Reichardt C. Solvents and Solvent Effects in Organic Chemistry.
Verlag Chemie: Weinheim, 1990; 285–338.
1. Zeichmeister L. Cis–trans Isomeric Carotenoids—Vitamin A and
Aryl Polyenes. Academic Press: New York, 1962; 158–188.
2. Saltiel J, Zafiriou OC, Megarity ED. J. Am. Chem. Soc. 1968; 90:
4759–4760.
3. Allen MT, Whitten D. Chem. Rev. 1989; 89: 1691–1702.
4. Waldeck DH. Chem. Rev. 1991; 91: 415–436.
5. Gorner H, Kuhn HJ. Adv. Photochem. 1995; 19: 58–115.
6. Arai T, Tokumaru K. Adv. Photochem. 1995; 20: 1–57.
7. Salem L. Acc. Chem. Res. 1979; 12: 87–92.
39. Saltiel J, Waller AS, Sears DF Jr, Hoburg EA, Zeglinski DM,
Waldeck DH. J. Phys. Chem. 1994; 98: 10689.
40. Papper V, Pines D, Likhtenshtein G, Pines E. J. Photochem.
Photobiol. A: Chem. 1997; 111: 87–96.
41. Yang JS, Chiou SY, Liau KL. J. Am. Chem. Soc. 2002; 124: 2518–
2527.
42. March J. Advanced Organic Chemistry, Reactions, Mechanism
and Structure (4th edn). Wiley: New York, 1999; 273–286.
43. Papper V, Likhtenshtein GI. J. Photochem. Photobiol. A: Chem.
2001; 140: 39–52.
8. Findlay JBC, Pappin DJC. Biochem. J. 1986; 238: 625–642.
9. Birge RR. Biochim. Biophys. Acta 1990; 1016: 293–327.
10. Gartner W, Towner P. Photochem. Photobiol. 1995; 62: 1–16.
11. Pepe IM. J. Photochem. Photobiol. A: Biol. 1999; 48: 1–10.
12. Dugave C, Demange L. Chem. Rev. 2003; 103: 2475–2532.
13. Meir H. Angew. Chem. Int. Ed. Engl. 1992; 31: 1399–1420.
14. Verbiest T, Burland DM, Jurich MC, Lee VY, Miller RD, Volksen
W. Science 1995; 268: 1604–1606.
15. Lambert C, Stadler S, Bourhill G, Brauchle C. Angew. Chem. Int.
Ed. Engl. 1996; 35: 644–646.
Copyright # 2005 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 43–52