Selective photoisomerization of methyl substituted nitro diphenylbutadienes

Article abstract of DOI:10.1016/j.jphotochem.2014.07.019

A series of p-nitro substituted trans-diphenylbutadienes is synthesized and their photophysical and photochemical properties are investigated. All the dienes have a very low quantum yield of fluorescence but exhibit remarkable solvatochromic emission shifts attributed to twisted intramolecular charge transfer. Photochemical irradiation of simple p-nitro substituted diphenylbutadienes reveals inefficient or no detectable photoisomerization. However, substituting a methyl group on the butadiene chain of p-nitro substituted diphenylbutadiene or replacing the nitro group with cyano group yields the corresponding trans-cis isomers. In the case of simple nitrodienes, strong intramolecular charge transfer character in the excited state aids dissipation of absorbed energy through non-photochemical and non-radiative channels. The steric effect caused by the presence of methyl group lowers the isomerization barrier in methyl substituted dienes leading to a regioselective isomerization.

Full text of DOI:10.1016/j.jphotochem.2014.07.019

Journal of Photochemistry and Photobiology A: Chemistry 293 (2014) 40–49
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Selective photoisomerization of methyl substituted nitro
diphenylbutadienes
Harsha Agnihotri, Veerabhadraiah Palakollu, Sriram Kanvah∗
Department of Chemistry, Indian Institute of Technology Gandhinagar, VGEC Campus, Chandkheda, Ahmedabad 382 424, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 April 2014
Received in revised form 17 July 2014
Accepted 23 July 2014
Available online 1 August 2014
A series of p-nitro substituted trans-diphenylbutadienes is synthesized and their photophysical and pho-
tochemical properties are investigated. All the dienes have a very low quantum yield of fluorescence but
exhibit remarkable solvatochromic emission shifts attributed to twisted intramolecular charge trans-
fer. Photochemical irradiation of simple p-nitro substituted diphenylbutadienes reveals inefficient or no
detectable photoisomerization. However, substituting a methyl group on the butadiene chain of p-nitro
substituted diphenylbutadiene or replacing the nitro group with cyano group yields the corresponding
trans–cis isomers. In the case of simple nitrodienes, strong intramolecular charge transfer character in the
excited state aids dissipation of absorbed energy through non-photochemical and non-radiative chan-
nels. The steric effect caused by the presence of methyl group lowers the isomerization barrier in methyl
substituted dienes leading to a regioselective isomerization.
Keywords:
Regioselective photoisomerization
Intramolecular charge transfer
Solvatochromism
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
detailed photochemical investigations utilizing diphenylbutadiene
were communicated, isomerization of substituted diphenylbu-
Light induced isomerization around a double bond is an impor-
tant step for many biologically important pigments such as the
opsin family of proteins [1,2]. To understand this highly rapid and
efficient trans–cis photoisomerization of all-trans retinal to 13-
cis retinal in bacteriorhodopsin [3] and 11-cis to all-trans retinal
in rhodopsin [2], diphenylpolyenes have been extensively inves-
tigated as model compounds. Their utility as model compounds
arose in part because of their energy level proximity with retinyl
polyenes and visual pigments [4]. Apart from the photochemical
processes, the emission and photoswitchable properties of such
arylethylene derivatives in solution and solid state have also found
utility in a variety of biological [5–9] and materials applications
[10–12]. Photoisomerization of 1,4-diphenylbutadiene leads to the
formation of the corresponding cis–trans isomers and trans–cis
isomers and isomerization is presumed to happen via perpen-
dicular intermediates [13]. Rigorous mechanistic investigations
of photoisomerization of diphenylbutadiene and other polyenes
revealed involvement of Bicycle Pedal (BP) and Hula twist (HT)
pathways [14–21]. Both BP and HT pathways were utilized to
explain the specificity of photoisomerization of the opsin proteins
despite the protein imposed volume restrictions [15]. Although
tadienes [22–24] has received little attention in comparison to
their fluorescence spectroscopic investigations [25–31]. In specific,
diphenylbutadienes substituted with donor or acceptor groups
yields one-photon-one bond isomerization of the C C bond lying
closer to the acceptor group [23,32]. Similarly photoisomerization
of trans,trans,trans p-methoxy-pꢀ-nitro substituted diphenylhexa-
triene derivative yielded a regioselective cis isomer [33]. Recent
investigations on similar butadiene containing systems reveal sig-
nificant influence of the methyl group or halogen group on the
photoinduced behavior [34,35] and efficient trans to cis isomer-
ization of dendrimeric diphenylbutadiene derivatives [36,37]. In
general, the observations reveal discrete one-bond rotations, bar-
rier to the isomerization and involvement of competing internal
conversion processes [13]. In the current work, we seek to exam-
ine the effect of methyl substituent and the nature of substituent
on the photoisomerization of a series of diphenylbutadiene deriva-
tives (Fig. 1) in acetonitrile at room temperature. Our results show
that substituting methyl group on the double bond and replacing
nitro group with a cyano group has a significant influence on the
photoisomerization of the diphenylbutadienes examined.
2. Experimental
∗
The reagents required for the synthesis of diphenylbutadiene
derivatives were obtained from Sigma-Aldrich, Alfa Aeser, Akcros
Corresponding author. Tel.: +91 11 7932419500.
E-mail address: kanvah@gmail.com (S. Kanvah).
http://dx.doi.org/10.1016/j.jphotochem.2014.07.019
1010-6030/© 2014 Elsevier B.V. All rights reserved.

H. Agnihotri et al. / Journal of Photochemistry and Photobiology A: Chemistry 293 (2014) 40–49
41
2.1. Synthesis
Diphenylbutadiene derivatives utilized in this investigation
were prepared by using Horner–Wadsworth–Emmons reaction
[23] (Scheme 1a) in about 35–60% yields. In a typical procedure,
triethyl phosphite (5.74 mmol) was added to the THF solution
of benzyl bromide (2.3 mmol) and refluxed for 24 h under a N2
atmosphere. The reaction mixture was cooled to room tempera-
ture and further cooled to 0 ◦C using an ice bath. Sodium hydride
(11.5 mmol) was added to this mixture and stirred for 5 min.
Aldehyde (2.3 mmol) was subsequently added drop wise to the
pale/dark pink colored solution and stirring was continued for
additional 30 min. In the case of methyl substituted aldehydes,
corresponding (E) geometric isomer of ␣-methyl cinnamaldehyde
(Aldrich) was used. Thin Layer Chromatography (TLC) was uti-
lized to monitor the progress of the reaction and the reaction
was quenched by adding water. The organic layer was extracted
using dichloromethane and concentrated under reduced pressure.
The crude reaction mixture so obtained was purified by column
chromatography using silica gel, 5% ethyl acetate in petroleum
ether. Starting materials, 4-methylcinnamaldehyde required for
the synthesis of (2) and nitro cinnamaldehyde required for syn-
thesis of (3) and (5), were obtained by DDQ oxidation of 4-allyl
toluene [40] (Scheme 1b) and by simple nitration using a nitrating
mixture generated by mixing KNO3 with H2SO4 of ␣-methyl-
cinnamaldehyde (scheme 1c), respectively. Characterization data
for all the synthesized dienes were given in the Supplementary
information.
Fig. 1. Structures of diphenylbutadienes investigated.
and S.D. Fine Chemicals India. The solvents needed for the syn-
thesis and spectral studies were dried using reported procedures.
1H and 13C NMR spectra were measured in CDCl3 with tetram-
ethylsilane as internal standard using a 500 MHz Bruker Avance
spectrometer. Absorption spectra were recorded on Analytikjena,
Specord 210 model UV–vis spectrophotometer. HPLC was done
˚
using Agilent 1260 infinity fitted with Kinetex 5 ␮m C18 100 A
(250 × 4.6 mm) column. 65% acetonitrile-water eluent conditions
at a rate of 1.0 mL/min are used for all the samples. The monitor-
ing wavelengths for detecting the photoproducts are 360 nm and
390 nm. Fluorescence studies were performed using fluorolog-3
spectrofluorimeter and fluorescence quantum yields were calcu-
lated using quinine sulfate as a standard [38]. For photochemical
irradiation experiments, a sample (3 mL of ∼10−5 M) in acetoni-
trile was taken in a 1 cm path length quartz cuvette and irradiated
using 125 W medium pressure Hg vapor lamp. 10% copper sul-
phate pentahydrate solution having 100% transmittance from 330
to 560 nm was used as the cut-off solution filter. Progress of the
photoisomerizationwas monitored using UV–vis. spectroscopy. For
characterization purposes, a higher concentration of sample solu-
tions (3 mg in 1 mL of CDCl3) was irradiated for 12–16 h using 125 W
medium pressure Hg lamp. Quantum yields of photoisomerization
(ꢀPI) were performed using potassium ferrioxalate actinometry
[39].
3. Results and discussion
A series of diphenylbutadiene derivatives containing nitro and
cyano substituents are synthesized and examined for their pho-
tochemical isomerization behavior. The synthesized dienes (1–6)
are listed in Fig. 1. Diene (1) has a p-substituted nitro group on
the aromatic ring. Diene (2) is a p,pꢀ disubstituted diphenylbu-
tadiene containing methyl and nitro groups para to each other.
Dienes (3)–(5) have a methyl substitution on the double bond. In
Diene (3), a methyl group is proximal to the aromatic nitro group
and in (4) the methyl group is placed distal to the nitro group.
Diene (5) is the same as (3) but have nitro groups on both sides
of the double bond in the aromatic ring. Through this arrangement,
Scheme 1. (a) Typical synthetic schemes for the synthesis of substituted diphenylbutadienes. (b) Synthesis of 4-methylcinnamaldehyde [40] and (c) nitration of aromatic
cinnamaldehydes.

42
H. Agnihotri et al. / Journal of Photochemistry and Photobiology A: Chemistry 293 (2014) 40–49
Fig. 2. (a) UV–vis absorption spectra of dienes (1–5) in acetonitrile. (b) UV–vis absorption spectra of diene (3) in different solvents.
we sought to increase the distance of methyl to nitro group and
understand the positioning effect of the methyl group on photo-
isomerization. Diene (6) bears a cyano group instead of a nitro
group.
solvents such as acetonitrile. These structural and absorption shifts
are ascribed to solute-solvent interactions. Table 1 lists the absorp-
tion and emission data of dienes (2) and (3).
All nitro substituted diphenylbutadienes exhibit remarkable
solvatochromism that was attributed to the presence of strong
intramolecular charge transfer (ICT) states [42,43]. As shown in
Table 1, solvent polarity changes from heptane to acetonitrile result
in significant bathochromic emission shifts. Shifts of 161 nm for (2)
and 142 nm for (3) were observed along with heavily quenched
emission in methanol (Fig. 3a and b). Diene (2) exhibits a weak
emission band near 600 nm while no such emission band was
observed for (3) in methanol. In polar solvents, the charge transfer
excited state is highly stabilized resulting in a bathochromic and a
broad emission spectrum contrary to heptane where a structured
emission spectrum is seen. The characteristic long wavelength ICT
band in the case of methanol is heavily quenched leading to emis-
sion from the locally excited state. This quenching in emission
is a consequence of solute and solvent intermolecular hydrogen
bonding [25]. The introduced methyl group decreases the con-
jugation in the butadiene chain. This induced non-planarity still
contributes to a significant solvatochromic shift (142 nm). Such
behavior is accounted due to the formation of strong intramolecular
charge transfer states [44]. Solvent polarity also affects the fluores-
cence quantum yield. Fluorescence quantum yield (ꢀf) is greater
for medium polar solvents and decreases with increase in sol-
vent polarity because of competing radiationless decay pathways
[26,45] such as internal conversion. It is known that nitroaromatic
3.1. Absorption and fluorescence of dienes (1)–(6)
Substitution of a strongly electron-withdrawing nitro group
leads to significant bathochromic absorption shift as compared to
the absorption (328 nm) of unsubstituted diphenylbutadiene [41].
The presence of a methyl group on p-position of the aromatic ring
(2) causes a slight red-shift of ꢁabs (+6 nm) as compared to (1) while
introduction of methyl group on the conjugated double bond (3 and
4) leads to hypsochromic absorption (up to 10 nm) shifts (Fig. 2a)-
Table 1. Presence of an additional nitro group on the aromatic ring
(5) has a weak influence (∼3 nm) on the ꢁabs as compared to mono-
substituted diene (1) but has a greater effect (∼at least 13 nm) as
compared to methyl substituted dienes (3 and 4). Enhanced reso-
nance contributes to a red-shift in the absorption while hindered
non-planar configuration induced by methyl substitution explains
the blue shifted absorbance in (3)–(5). Thus, among the dienes
investigated, diene (2) exhibits the largest ꢁabs. Slight red shifts
in absorption maxima are observed in all dienes upon increasing
solvent polarity from heptane to acetonitrile (Fig. 2b). All dienes
exhibits structured absorption spectrum in non-polar solvent hep-
tane while a smooth structure less spectrum was found in polar
Table 1
Absorption data of (1)–(6) in acetonitrile and emission data of (2) and (3) in varying solvents.
Absorption data of dienes (1–6) in acetonitrile
Diene
1
2
3
4
5
6
ꢁabs (nm)
377
383
367
370
380
343
Absorption and emission data of dienes 2 and 3
Diene 2
Diene 3
Solvents
*
*
ꢁabs
ꢁem
ꢀf
ꢁabs
ꢁem
ꢀf
Heptane
Dioxane
Tetrahydrofuran
Acetonitrile
Methanol
374
384
386
384
382
427
511
527
588
0.001
0.08
0.16
0.067
0.003
354
365
366
367
364
423
470
511
565
0.001
0.004
0.018
0.015
0.002
429, 600 (weak)
433, –
*
Quinine sulfate (0.545 in 1 N H2SO4 is used as a standard to measure).

H. Agnihotri et al. / Journal of Photochemistry and Photobiology A: Chemistry 293 (2014) 40–49
43
Fig. 3. Fluorescence emission spectra of (a) diene (2) and (b) diene (3) in solvents of different polarity. The excitation wavelengths are 380 nm for 2 and 360 nm for diene 3.
compounds are generally weakly fluorescent due to such non-
radiative decay processes such as intersystem crossing and internal
conversion [25]. The data for other dienes (1, 4 and 5) are provided
in Table-S1 have similar observations [26,46].
characteristic of many aromatic nitrocompounds [51]. The efficient
ISC from the charge transfer state competes with formation of the
transition state yielding low photoisomerization and fluorescence
quantum yields. The lower quantum yields of photoisomerization
(ꢀPI) of 0.078 (3), 0.098 (4) and 0.075 (5), obtained using ferrioxalate
actinometry, coupled with weaker fluorescence quantum yields
imply significant non-radiative processes. To assess the strength
of the electron-withdrawing group, (6) containing a moderately
deactivating electron withdrawing cyano group was synthesized.
In contrast to nitro containing dienes (1) and (2), upon irradiation
of (6), progressive photoisomerization was noted. UV absorption
spectral changes upon irradiation (Fig. S6a) and NMR of pure (Fig.
S6b), NMR of photo mixture (Fig. S6c) is shown in the supporting
information. Dienes (3)–(5) have methyl groups on the conjugated
double bond. The methyl group in (3) is located proximal and in
(4) the methyl group is distal to the aromatic nitro group. Upon
photoirradiation of the diene (3), a sharp decrease in absorption
intensity was noted (Fig. 5a). Similarly photoirradiated UV spectra
of diene 4 show a drop in absorption intensity with increase in time
of irradiation (Fig. 5b). Formation of isosbestic point indicates the
presence of an equilibrating isomer that is cis. HPLC spectra reveal
the presence of two components in the photoirradiated mixture for
the methyl substituted dienes investigated (Fig. 5c and d). When the
methyl group is placed on the aromatic ring (2) photostability of
the molecule persists. But once the methyl group is placed on the
double bond (3) or (4) molecules undergoes photoisomerization
yielding corresponding cis–trans (ct) isomers.
In (3), methyl substitution results in 100% isomerization site
preference for the double bond containing methyl group (Fig. 5a
and c). To confirm this observation, cis–trans (ct) isomer of (3) was
diligently isolated and the obtained NMR spectrum is shown in
Fig. 5e. The calculated coupling constants [J = 16.0 Hz for Hc and
Hd protons) ratifies our observation of the site of isomerization.
It is known that the presence of an electron-withdrawing group
could withdraw the electron density from the proximal double
bond and facilitate regioselective isomerization [23,24,32]. In this
case, electron withdrawing nitro group and steric effects of the
methyl group both complement each other. To test if this pref-
erence for methyl substituted double bond is certain and to rule
out the intervention by the nitro group, isomerization of (4) bear-
ing methyl group on the second double bond was investigated.
Even in this case, we observed 100% isomerization selectivity for
the methyl substituted bond (Fig. 5b and d) yielding trans–cis (tc)
3.2. Trans–cis photoisomerization
Direct photoisomerization of diphenylbutadiene leads to the
corresponding cis (c) and trans (t) isomers. Technically four differ-
ent isomers are possible, trans–trans (tt), trans–cis (tc), cis–trans
(ct) and cis–cis (cc). In the case of unsubstituted diphenylbuta-
diene tc and ct are indistinguishable [47,13]. (1) and (2) have H
and CH3 groups in the para position of the aromatic ring and
exert a feeble donating influence on the overall conjugation. Irra-
diation of (1), containing single nitro group in para-position, in
acetonitrile at room temperature led to highly inefficient photo-
chemical reaction despite prolonged irradiation. Fig. 4a displays
the UV spectra of (1) at various time intervals of irradiation in
acetonitrile. HPLC peak profiles after irradiation indicate only one
peak corresponding to the tt-isomer (Fig. 4b). Analysis of the NMR
spectra reveals no changes in spectral data of irradiated mixture
with respect to pure tt isomer (Fig. 4c and d). To ascertain our
result, time-dependent NMR study of (1) in CDCl3 was performed.
The corresponding spectra given in Supplementary information
[Fig. S2] reveals no changes to the initial (tt) isomer. However,
in glassy or polymer matrices, strong UV irradiation gave rise
to partial cis–trans isomerization involving only one of the two
double bonds [48] and gave different EPR spectra upon irradia-
tion at 355 nm or in the range of 420–455 nm. Following these
observations, a CDCl3 solution (3 mg in 1 mL) of (1) was irradiated
using a photoreactor fitted with either 360 nm or 420 nm lamps.
Characterization by NMR reveals minimal structural changes to
the initial tt spectra [Figs. S3 and S4]. The minor peaks observed
contribute to less than 3–4% of the overall isomer. Likewise (2)
containing a methyl group in the para position also does not
form any significant photoisomers upon photoirradiation (Figs.
S5a–d). No such non-detectable photoisomerization is reported
for similarly substituted trans-stilbene derivatives [49,50]. This
could imply that the energy absorbed is released back in non-
photochemical channels and non-radiative channels. In this case,
electron withdrawing nitro group promotes strongly polarized TICT
excited states (through rotation of Ar NO2 bonds) and increase
the internal conversion and intersystem crossing (ISC) processes

44
H. Agnihotri et al. / Journal of Photochemistry and Photobiology A: Chemistry 293 (2014) 40–49
Fig. 4. (a) UV–vis absorption spectral changes upon irradiation of diene (1) in acetonitrile at different time intervals. (b) HPLC peak profile of diene (1) after irradiation
obtained using 390 nm as detection wavelength. (c) 1H NMR spectra of diene (1) before irradiation and (d) 1H NMR spectra of diene (1) after irradiation.
isomer. Thus simple p-nitro substituent (for 1 and 2) increases
the molecular planarity of the butadiene backbone and enhances
rapid internal conversion [52] and intersystem crossing processes.
These ultrafast non-radiative processes may lead to a photore-
active excited singlet state that favors the decay back to the
trans-isomer or it may encounter a high potential energy barrier in
the singlet-excited state that prevents decay of this species to the
ground-state cis-isomer. On the contrary, presence of methyl sub-
stitution in (3)–(5) despite the presence of nitro moiety, provides a
pre-distortion through steric hindrance and creates no such energy
barrier in the singlet excited state and yield the corresponding
cis-isomers.
Thus, methyl substitution on the phenyl ring has little effect
on photochemistry, but the photoisomerization is significantly
enhanced when the methyl groups are substituted on butadi-
enyl carbon. This enhanced reactivity is primarily attributed to a
decrease of torsional barrier [43]. The geometric isomers of (3) and
(4) were identified using 1H NMR by careful calculation of coupling
constants of their cis and trans isomers. NMR spectra of pure trans,
irradiated mixture and isolated trans–cis (tc) isomer of 4 are given
in Supplementary information (Fig. S7a–c). In case of (4), analy-
sis of photo mixture NMR revealed two doublets at 7.47–7.44 ppm
(J = 15.5 Hz) and at 6.75–6.72 ppm (J = 16.5 Hz) that correspond to
tt-configuration and a singlet at 6.72 ppm that corresponds to tc.
Absence of cis-coupling constant in the data indicates that observed
changes are caused by the tc-isomer (Fig. S7c) resulting from the
methyl substituted double bond. To show clearly the impact of
methyl on the isomerization, (4) in CDCl3 (3 mg/1 mL) was irra-
diated at various time intervals (Fig. 6; Fig. S7f). The emergence of
additional peaks is due to the tc-isomer arising from the methyl-
substituted double bond. Table 2 gives comparative NMR values
and their coupling constants of dienes (3)–(5).
More importantly, site of photoisomerization migrates with the
positioning of the methyl group. To further emphasize the steric
influence of the methyl group on the photoisomerization, (5) con-
sisting of two nitro groups was synthesized. Despite having two
very intense electron-pulling groups on opposite ends of the chain,
diene (5) does not inhibit the photoisomerization of the methyl-
substituted double bond. This shows the prevalence of steric effects
(due to methyl) over the electronic effects associated with the nitro
group. A comparison of NMR spectra of pure trans (Fig. 7a) and
irradiated mixture (Fig. 7b) of (5) clearly indicates the formation
of another cis isomer (cis–trans). The UV and HPLC data of (5) are
shown in the supporting information (Fig. S9a and b).

H. Agnihotri et al. / Journal of Photochemistry and Photobiology A: Chemistry 293 (2014) 40–49
45
Fig. 5. UV–vis absorption spectral changes on irradiation of (a) diene (3) and (b) diene (4) in acetonitrile at various time intervals. HPLC spectra of (c) diene (3) and (d) diene
(4) after irradiation; HPLC data was obtained using 390 nm as detection wavelength (e) 1H NMR spectra of cis-3 in CDCl3.
Coupling constants ı 6.86–6.83 (d, J = 16.0 Hz ct) and 7.34–7.30
(d, J = 16.5 Hz ct) confirm that only the methyl attached double
bond converts to the cis configuration. This observation is also
confirmed by COSY spectra of photomixture of 5. The interac-
tion between trans-hydrogens of the corresponding isomers was
observed in COSY spectra. The cross peaks depicted in the COSY
spectra of pure trans of diene 5 (Fig. 8a) shows the interaction
between two trans hydrogens (Ha and Hb). The COSY spectra of
irradiated 5 (Fig. 8b) has two such cross peaks (squares), the
smaller one indicates interaction between Ha and Hb of trans–trans
isomer and larger one indicates interaction between Hc and Hd
of trans–cis isomer. COSY spectra of (3) and (4) are also given
in the Supplementary information (Figs. S8a and b and S7d
and e).

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H. Agnihotri et al. / Journal of Photochemistry and Photobiology A: Chemistry 293 (2014) 40–49
Fig. 6. 1H NMR spectra of diene (4) at (a) 0 (b) 30 and (c) 120 min of irradiation.

H. Agnihotri et al. / Journal of Photochemistry and Photobiology A: Chemistry 293 (2014) 40–49
47
Table 2
Comparative NMR values and their coupling constants of dienes (3)–(5).
Diene
3
Trans–trans (tt)
Photomixture
Trans-cis(tc) or Cis-trans (ct)
ı 7.00–6.97 (d, 1H, 16.0 Hz), ı 6.79–6.76
(d, 1H, J = 16.0 Hz), ı 6.67 (s, 1H)
ı 7.24–7.21 (17.0 Hz ct), ı 7.00–ı 6.97 (d, J = 16.0 Hz tt),
ı 6.84–ı 6.81 (d, J = 16.0 Hz ct), ı 6.79–ı 6.76 (d,
J = 16.0 Hz tt), ı 6.67 (s, tt), 6.55 (s ct)
7.24–7.20 (d, J = 16.0 Hz), 6.84–6.81 (d,
J = 16.0 Hz), 6.55 (s)
4
5
ı 7.15–7.12 (1H, d, J = 16.0 Hz), ı 6.78
(s,1H), ı 6.70–6.67 (1H, d, J = 16.0 Hz)
ı 7.47–7.44 (d, J = 15.5 Hz tc), ı 7.15–7.12 (d, J = 16.0 Hz
tt), ı 6.78 (s tt), ı 6.75–6.72 (d, J = 16.5 Hz tc), ı 6.72 (s
tc), ı 6.70–6.67 (d, J = 16.0 Hz tt)
ı 7.34–7.30 (d, J = 16.5 Hz ct), ı 7.14–7.11 (d, J = 16.0 Hz
tt), ı 6.79–6.76 (d, J = 15.5 Hz tt), ı 6.79 (s tt), ı
6.86–6.83 (d, J = 16.0 Hz ct), ı 6.71 (s ct)
ı 7.47–7.44 (d, J = 15.5 Hz), ı 6.75–6.72 (d,
J = 16.5 Hz), ı 6.72 (s)
ı 7.14–7.11 (d, 1H, 16.0 Hz), ı
6.81–6.78 (d, 1H, 15.5 Hz), ı 6.79 (s, 1H)
ı 7.34–7.30 (d, J = 16.5 Hz), ı 6.86–ı 6.83 (d,
J = 16.0 Hz), ı 6.71 (s)
Fig. 7. 1H NMR spectra of diene (5) (a) pure trans isomer before irradiation (b) after irradiation.
Fig. 8. COSY spectra of diene (5) (a) pure trans isomer before irradiation (b) after irradiation.

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4. Conclusions
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merization, Photochem. Photobiol. Sci. 5 (2006) 874–882.
Photoisomerization of nitro substituted diphenylbutadienes
lacking methyl group results in loss of absorbed energy as vibra-
tional, rotational or thermal channels and therefore leads to
non-detectable or extremely inefficient photoisomerization. How-
ever, a methyl group on the conjugated double bond that is either
proximal or distal to the nitro group enables facile isomerization.
In the case of dienes (1) and (2) electron withdrawing effect of
the nitro group dominates and in the case of dienes (3)–(5) steric
encumbrance of methyl has a significant influence. Presence of an
angular methyl group twists the structure out of plane reducing
the conjugation and minimizing the strong electron withdrawing
effects of the nitro group. This reduction in conjugation allows facile
trans–cis isomerization and the molecule exists in an isomeriza-
tion facilitating twisted-conformation owing to lowering of excited
state torsional barrier. Moreover, by changing the position of the
methyl group the site of isomerization is completely controlled.
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Acknowledgements
Financial grant from Council of Scientific and Industrial Research
(01(2487)/11/EMR-II) and Department of Science and Technology,
India (SR/S1/PC-024/2010) and support from IIT Gandhinagar is
greatly appreciated. We also thank reviewers for helpful sugges-
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Appendix A. Supplementary data
Supplementary material related to this article can be found,
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