Photoisomerization of Trans Ortho-, Meta-, Para-Nitro Diarylbutadienes: A Case of Regioselectivity

Article abstract of DOI:10.1111/php.12504

A series of ortho-, meta- and para-substituted trans-nitro aryl (phenyl and pyridyl) butadienes have been synthesized and characterized. The effect of substitution and positional selectivity on their fluorescence and photoisomerization were systematically investigated. Among all dienes, meta- and para-nitro phenyl-substituted derivatives exhibit remarkable solvatochromic emission shifts due to intramolecular charge transfer. On the other hand, ortho derivatives undergo regioselective isomerization upon photoexcitation in contrast to inefficient isomerization of para and meta nitro-substituted dienes. Single crystal X-ray analysis revealed existence of intramolecular hydrogen bonding between the nitro group and the hydrogen of the proximal double bond. This restricts the rotation of the proximal double bond thereby allowing regioselective isomerization. The observations were also supported by NMR spectroscopic studies.

Full text of DOI:10.1111/php.12504

Photochemistry and Photobiology, 2015, 91: 1324–1331
Photoisomerization of Trans Ortho-, Meta-, Para-Nitro Diarylbutadienes:
A Case of Regioselectivity
Harsha Agnihotri, Mahalingavelar Paramasivam, Veerabhadraiah Palakollu and Sriram Kanvah*
Department of Chemistry, Indian Institute of Technology Gandhinagar, Ahmedabad, India
Received 4 May 2015, accepted 12 July 2015, DOI: 10.1111/php.12504
ABSTRACT
phenyl-substituted conjugated derivatives, push-pull-substituted
heteroaryl containing derivatives were also investigated (29). For
instance, replacing the CH group by nitrogen (N) in the phenyl
ring yields an isoelectronic heteroaryl species that can signifi-
cantly change the observed photophysical properties (30). Many
of these isoelectronic pyridyl stilbenes and dienes have been
synthesized and their excited state properties were investigated
(31–34). Mechanistic studies of isomerization of these diphenyl
or diarylpolyenes led to a wholesome debate over isomerization
mechanisms (6–9). In this report, we discuss the photoinduced
behavior (fluorescence and photoisomerization) of different posi-
tional [ortho (o-), meta (m-) or para (p-)] isomers of nitro
diarylbutadienes (Fig. 1). We expect that changing the position
of the nitro group will influence the electron delocalization and
in turn affect the photochemical and fluorescence properties. The
results reveal that in case of phenyl-substituted derivatives,
p- and m-substitution leads to fluorescence characterized by
intramolecular charge transfer (ICT), whereas o-derivatives
quench the fluorescence. Upon replacing phenyl with pyridine,
p-derivative leads to ICT, m-substitution results in quenching
with weak long wavelength bands, whereas no ICT characteris-
tics were seen for o-nitro substitution. Photochemical studies
indicate o-substitution has maximum impact on photoisomeriza-
tion while m- and p-derivatives exhibit inefficient isomerization
in both phenyl- and pyridyl-substituted dienes. The results are
detailed in the following sections.
A series of ortho-, meta- and para-substituted trans-nitro aryl
(phenyl and pyridyl) butadienes have been synthesized and
characterized. The effect of substitution and positional
selectivity on their fluorescence and photoisomerization were
systematically investigated. Among all dienes, meta- and
para-nitro phenyl-substituted derivatives exhibit remarkable
solvatochromic emission shifts due to intramolecular charge
transfer. On the other hand, ortho derivatives undergo
regioselective isomerization upon photoexcitation in contrast
to inefficient isomerization of para and meta nitro-substituted
dienes. Single crystal X-ray analysis revealed existence of
intramolecular hydrogen bonding between the nitro group
and the hydrogen of the proximal double bond. This restricts
the rotation of the proximal double bond thereby allow-
ing regioselective isomerization. The observations were also
supported by NMR spectroscopic studies.
INTRODUCTION
Photoinduced isomerization of the double bond plays a vital role
for many biologically relevant pigments such as the opsin family
of proteins (1–3). The energy level proximities of diphenylpolye-
nes in comparison to the retinylidene polyenes led to their use as
model compounds toward examination of photoisomerization
reactions (4) as well for mechanistic considerations (5–9).
Coupled with their biological relevance, the emission and photo-
switchable properties of such arylethylene derivatives have found
use for biological (10) and optical electronic applications
(11,12). In particular, suitably substituted p-conjugated deriva-
tives bearing different functional groups exhibiting intramolecular
charge transfer (ICT) have found applicability as functional
materials (13,14), as fluorescence probes (15–18), for optical
applications (19–21) and to understand substituent dependent
photochemical and photophysical processes (22–24). Common to
some of these known materials is presence of the nitro group as
an acceptor moiety despite it being accounted for efficient nonra-
diative transitions such as internal conversion and intersystem
crossing processes. The preference for electron withdrawing nitro
group is due to its profound influence on emission characteris-
tics, owed to the existence of ICT or twisted intramolecular
charge transfer states (25–27), and its influence on photochem-
istry due to involvement of triplet excited states (28). Aside from
MATERIALS AND METHODS
The reagents required for the synthesis of diene derivatives were obtained
from chemical vendors such as Sigma-Aldrich, Alfa Aesar, Acros, S.D Fine
and Spectrochem. The solvents needed for the synthesis and spectral stud-
ies were dried using established procedures. 1H and 13C NMR spectra were
carried out in CDCl3 with tetramethylsilane as internal standard using
500 MHz Bruker Avance spectrometer. Accurate mass analysis was per-
formed using Waters-Synapt G2S (ESI-QToF) mass spectrometer. Absorp-
tion spectra were recorded on Analytikjena, Specord 210 model UV–vis
spectrophotometer. High pressure liquid chromatography (HPLC) was per-
ꢀ
formed using Agilent 1260 infinity fitted with Kinetex 5l C18 100 A
(250 9 4.6 mm) column. Acetonitrile-water mixture (50–60%) with a flow
rate of 1.0 mL minꢀ1 is used as eluent for all the samples. The monitoring
wavelengths for detecting the photoproducts are 315 nm (for diene 6),
320 nm (for 3), 330 nm (for 2 & 5) and 370 nm (for 1 & 4). Steady-state
fluorescence studies were performed utilizing Horiba Jobin Yvon fluo-
rolog-3 spectrofluorimeter and relative fluorescence quantum yields were
measured using quinine sulfate (0.545 in 0.5 N H2SO4) as standard (35).
The concentrations used for fluorescence experiments are in the order of
10ꢀ5 M. Typically, the excitation wavelengths were set at the absorption
maxima (ka) of the compounds under investigation. For photochemical irra-
diation experiments, a sample (3 mL of ~10ꢀ5 M) in acetonitrile was
*Corresponding author email: kanvah@gmail.com (Sriram Kanvah)
© 2015 The American Society of Photobiology
1324

Photochemistry and Photobiology, 2015, 91 1325
added drop wise and stirring was continued for an additional
30 minutes. The propenal derivative was synthesized using
established literature procedures by a reaction of triphenylphos-
phoranylidene acetaldehyde and pyridine-4-carbaldehyde at room
temperature (38) (Scheme 1b). Progress of the reaction was mon-
itored using thin layer chromatography (TLC) 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 as a stationary phase
with 5–30% ethyl acetate in petroleum ether as eluent. Character-
ization spectra for all the synthesized products were given in the
supplementary information.
Figure 1. Structures of dienes investigated.
performed in a 1 cm path length quartz cuvette and irradiated using 125 W
medium pressure Hg vapor lamp. Ten percent copper sulfate pentahydrate
solution having 100% transmittance from 330 to 560 nm was chosen as the
cut-off solution filter. Progress of the irradiation was monitored using UV–
visible spectroscopy and HPLC. For characterization purposes, a higher
concentration of sample solutions (3 mg in 1 mL of CDCl3) was irradiated
for 2 hours using 125 W medium pressure Hg lamp. Quantum yields of
photoisomerization (фPI) were performed using potassium ferrioxalate acti-
nometry (36). Experimental details of quantum yield calculation are given
in the supporting information.
Absorption
Diene (1) containing a nitro group at the p-position absorbs (ka)
at ~377 nm in acetonitrile. In comparison, changing the position
of the nitro group to meta (2) results in significant blueshifted
(~47 nm) absorption. Furthermore, placing the nitro group in
ortho (3) results in a greater hypsochromic shift (~61 nm).
Dienes (4–6) also demonstrate a similar trend in their absorption
properties. Nitro substitution at m- and o-positions leads to sig-
nificant hypsochromic shifts, 37 nm for (5) and 52 nm for (6),
as compared to p-substituted (4) which has absorption maxima at
363 nm in acetonitrile. In general, substitution of an electron
withdrawing pyridyl ring in place of a phenyl ring yields a blue-
shifted absorption. The absorbance and fluorescence data of
dienes are listed in Table 1. The observed order of absorption,
p- > m- > o- is explained by mesomeric effects caused by
changes in position of the nitro group. Among the dienes investi-
gated, (1) exhibits the largest ka and o-substituted (6) the lowest
(Fig. 2a). These hypsochromic shifts may be ascribed to the non-
planar interaction between the nitro and conjugated diene system
hampering the delocalization. Upon changing the solvent polarity
from heptane to dioxane or acetonitrile varying amounts (1–
13 nm) of redshifts in absorption maxima were observed. The
absorption spectrum is generally structured in nonpolar heptane,
whereas structural features are less demarcated in polar solvents.
These structural and absorption shifts are a result of increased
solute–solvent interactions.
Single crystal X-ray analysis. The crystallographic data of (3) were col-
ꢀ
lected using Mo-Ka radiation (k = 0.71073 A) using SMART APEX
DUO diffractometer equipped with charge coupled device detector. Data
were collected at room temperature. The data integration and reduction
were carried out using SAINT-PLUS software. An empirical absorption
correction was applied to the collected reflections with SADABS. The
structure was solved by direct methods using SHELXS97 and refinement
was carried out by full-matrix least-squares technique using SHELXL97.
Anisotropic displacement parameters were calculated for all nonhydrogen
atoms.
RESULTS AND DISCUSSION
Synthesis
The dienes (1–6) utilized in this investigation were synthesized
by
using
Horner–Wadsworth–Emmons
reaction
(37)
(Scheme 1a). In
a
typical procedure, triethyl phosphite
(5.74 mmol) was added to the THF solution of o-, m- and p-ni-
trobenzyl bromide (2.3 mmol) and refluxed for 24 hours under
N2 atmosphere. The reaction mixture was first cooled to room
temperature and then to 0°C using an ice bath. Sodium hydride
(11.5 mmol) was added to this cooled mixture and allowed to
stir for 5 minutes. Aldehyde (2.3 mmol) [trans-cinnamaldehye
for (1–3) and E isomer of 3-pyridin-4-yl-propenal for (4–6)] was
Scheme 1. (a) Synthetic steps for the synthesis of substituted diarylbutadienes; (b) Synthesis of 3-pyridin-4-yl-propenal.

1326 Harsha Agnihotri et al.
Table 1. Absorption and fluorescence data of dienes (1–6) in homogeneous solvents.
(1)
(2)
(3)
Dienes
ka
kf
Фf*
ka
kf
Фf*
ka
kf
Фf*
Heptane
Dioxane
THF
CH3CN
DMF
372
374
378
377
385
427
462
486
560
560
0.005
0.006
0.020
0.114
0.171
331
332
332
330
333
375
521
541
–
0.004
0.002
0.001
0.001
0.001
316
320
319
316
320
356
365, 413
383
365
368
0.008
0.001
0.001
0.001
0.002
–
(4)
(5)
(6)
Heptane
Dioxane
THF
CH3CN
DMF
356
364
366
363
372
410
503
520
526
521
0.002
324
379
0.006
0.001
0.001
–
310
315
316
311
317
426
433
434
434
364
0.001
0.001
0.001
0.001
0.003
0.002
0.004
0.014
0.020
328
328
326
327
390, 503
431, 499
401, 572
384, 562
0.002
*Фf is determined using quinine sulfate (0.545 in 1N H2SO4 as a standard).
Figure 2. (a) UV–Vis absorption spectra of dienes (1–6) in acetonitrile; (b) Emission spectra of (4) in solvents of different polarity.
bond angle of (3) in the crystal were measured to be 38.1 and
Molecular geometry and crystal structure
ꢁ
ꢀ
119.7°, respectively (Fig. 3). The (C-HꢁꢁꢁO) distance of 2.236 A
The ortho-derivative (3) was crystallized in acetone and the crys-
tal structure was elucidated by single X-ray diffraction. The com-
pound crystallizes into the orthorhombic space group Pca21 with
one molecule in the asymmetric unit. The summary of the crystal
data is given in the Table S1 (ESI). The X-ray molecular struc-
ture is shown in Fig. 3. X-ray data reveal that the nitro-substi-
tuted phenyl group in (3) is slightly deviated from the plane of
in (3) indicates possibility of an intramolecular hydrogen bond
between the oxygen of the nitro group and hydrogen of the
proximal double bond.
Emission of dienes (1–6)
p-nitro-substituted diphenylbutadiene derivatives exhibit remark-
able solvatochromism that was attributed to the presence of
strong intramolecular charge transfer (ICT) states (25,39,40). As
the diene system with
a dihedral angle (C3-C4-C7-C8) of
ꢀ175.5°. The dihedral angle of (N1-O1-C3-C4) and \C-H ꢁ ꢁ ꢁ O
(a)
(b)
Crystal (3)
-175.5°
38.1°
∠
−
−
−
−
−
−
∠
∠
119.7°
2.236 A°
−
…
(
−
⋯
)
Figure 3. (a) Intramolecular hydrogen bonding interaction; (b) Major geometrical parameters of the ortho-substituted diene obtained from the X-ray
structure.

Photochemistry and Photobiology, 2015, 91 1327
Photoisomerization
shown in Table 1, solvent polarity changes from heptane to
acetonitrile result in significant a bathochromic emission shift
(17), ~133 nm, for (1) (Fig. S1). Similar to (1), diene (4) also
shows ICT characteristic emission (Fig. 2b). In both cases, the
charge transfer excited state is greatly stabilized by the solvent
resulting in a large redshifted emission. The greater sensitivity of
fluorescence to solvent is also partly due to presence of twisted
intramolecular charge transfer states (27).
Direct irradiation of diphenylbutadiene leads to formation of
corresponding cis (c) and trans (t) isomers (42,43). In the case
of unsymmetrically substituted diphenylbutadiene four isomers
are technically possible, trans-trans (tt), trans-cis (tc), cis-trans
(ct) and cis-cis (cc) isomers. But, it is established that suitably
substituted diarylbutadienes upon irradiation yields highly selec-
tive isomerization products (34,44). In certain substitutions, such
as para-substituted nitro diene (1), direct irradiation in acetoni-
trile leads to inefficient isomerization with about 2–4% formation
of cis isomer despite prolonged irradiation (23). Similarly irradia-
tion of (4) bearing an isoelectronic electron withdrawing pyridine
ring in place of the phenyl ring also resulted in inefficient pho-
toisomerization. Figure 5a displays the UV spectra of (4) at vari-
ous time intervals of irradiation in acetonitrile and this result in a
small decrease from 0.34 to about 0.30 absorbance units. The
HPLC peak profiles before and after irradiation indicate only one
predominant peak corresponding to the original tt isomer
(Figs. S3a,b). NMR of the photomixture reveals no changes to
the original NMR of tt spectra (Figs. 5b and S3c). Despite pres-
ence of different electron withdrawing strengths at opposing ends
(pyridine and nitrophenyl) the isomerization to the corresponding
cis-trans isomer is almost nonexistent. Changing the position of
the nitro group from para to meta (2 & 5) also do not yield any
detectable photoproducts.
Such inefficient isomerization is not reported in similarly sub-
stituted nitrostilbenes (45,46). The UV–vis spectra of dienes (2
and 5) at various time intervals of irradiation, HPLC spectral
details and NMR data of irradiated mixture are given in ESI
(Figs. S4 and S5). Interestingly, the positional change in the nitro
group to ortho (3 & 6) yields corresponding cis isomers. Absorp-
tion spectra of (3) & (6) indicate a gradual decrease in absor-
bance (Fig. 6a,b) with increase in time of irradiation. Clear
isosbestic point indicates the presence of an equilibrating isomers
and the observation was confirmed by HPLC peak profile of the
photoirradiated mixture (Fig. S6). In contrast to para- or meta-
substituted dienes, it is clear that o-substitution paves the path
for easy isomerization. It is likely that the double bond closer
to pyridine ring is the preferred site of isomerization (34) as
substantiated by NMR studies (vide infra).
In contrast to para-substituted dienes, o-nitro substitution
results in very weak emission behavior for (3) and (6). No ICT
characteristic behavior was observed (Fig. S1). In these cases,
the emission observed is merely due to locally excited state and
changing the solvent does not affect the emission properties. The
ortho-nitro group in (3) and (6) contribute to steric hindrance
and also involve in favorable H-bonding interactions with proxi-
mal vinylene hydrogens. The geometries obtained from X-ray
crystal data substantiate the presence of intramolecular C-HꢁꢁꢁO
interactions in the ground state (Fig. 3). In contrast to this
observation, both m-derivatives, despite the apparent loss in con-
jugation, show long wavelength intramolecular charge transfer
band (ICT). Diene (2) gives dominant emission band in solvents
of medium polarity such as THF and dioxane and quenched
emission in acetonitrile and DMF (Fig. 4a). This emission is
somewhat akin to that observed for p-substituted (1 and 4) where
significant solvatochromism was observed. But in the case of
(5), the CT band is very weak with emission bands at 500 nm in
dioxane and 575 nm for acetonitrile along with bands in the
higher energy region (Fig. S2). Sharp peaks at [~370 nm (2, 5)
~405 nm (4)] seen in emission spectra are due to Raman scatter-
ing peaks and are not characteristic of the compounds investi-
gated. (These peaks were unavoidable despite using drier
solvents and the intrinsic weak fluorescence of the dienes may
also be a contributory factor.) The polarity dependent quenching
of the emission is best illustrated using dioxane-acetonitrile bin-
ary mixture. Accordingly, as solvent polarity (% of acetonitrile)
is increased, gradual redshifted emission is observed with
concomitant quenching of fluorescence intensity (Fig. 4b).
Solvent polarity also affects the fluorescence quantum yield.
Fluorescence quantum yield (фf) is greater for medium polar sol-
vents and decreases for solvents of higher polarity and is a result
of competing radiationless decay pathways such as intersystem
crossing (41). In general, the fluorescence quantum yield for all
these investigated dienes is very low because of involvement of
n-p* transitions of the pyridyl ring or competing decay processes
due to nitro substitution.
To confirm this, trans-cis (tc) isomer of (3) was isolated and
characterized using absorption spectroscopy and NMR. UV spec-
tra of tc isomer shows a blueshift of ~9 nm, with respect to tt
(Fig. 6c), as a consequence of reduction in conjugation due to
Figure 4. (a) Normalized emission spectra of diene (2) in varying solvent polarity; (b) in acetonitrile (ACN)/dioxane binary mixture.

1328 Harsha Agnihotri et al.
Figure 5. (a) UV spectral changes of diene (4) upon irradiation; (b) Photomixture NMR of diene (4) after irradiation.
Figure 6. UV–vis absorption spectral changes upon irradiation of (a) diene (3) and (b) diene (6) in acetonitrile at various time intervals. (c) Compara-
tive UV spectra of pure trans (black line) and cis (red-line) isomers of diene-3. (d) 1H NMR spectra of cis-3 in CDCl3.
nonplanarity. The NMR spectra of pure tc isomer was also
obtained by preparative TLC (5% ethylacetate/petroleum ether)
and shown in Fig. 6d. The up-field shifting of chemical shift con-
firms that trans double bond far to nitro group converts to cis.
Detailed NMR analysis reveals coupling constants 11.5 Hz for Hd
clearly indicate that only one of the double bonds retains its trans
configuration. Table 2 gives comparative NMR values and their
coupling constants of both the dienes (3) and (6). The peak at d
6.77–6.74 of tt isomer merged with tc isomer. To observe a clear
pattern, time dependent irradiation of (3) in CDCl3 (3 mg/1 mL)
was performed followed by NMR characterization which reveals
0
0
& Hd protons and 15.5 Hz for Hc and Hc hydrogens. The values

Photochemistry and Photobiology, 2015, 91 1329
the emergence of new isomer peaks (Fig. S7). We have further
verified the formation of isomers using COSY spectral analysis.
COSY spectra (Fig. S8) of irradiated (3) showed three cross peaks
(A, B + B’ and C). COSY spectra of tc isomer showed two cross
peaks (B + C). The A and B cross peaks (squares) formed due to
the interaction between hydrogens of tt isomer. The B0 cross peak
display the interaction between hydrogens of trans configuration
(Hc and Hc0) of tc isomer. The C cross peak display the interac-
tion between hydrogens of cis configuration (Hd and Hd0) of tc
isomer.
Similar to (3), pyridine containing diene (6) also undergoes
facile isomerization and the position of isomerization is closer to
the pyridyl group. The coupling constant observed (for 6) was
J = 11.0–11.5 Hz indicating cis coupling between protons. Fig-
ure S9 shows irradiation dependent time progression NMR spec-
tra of (6) in CDCl3 and reveal formation of new isomer peaks.
The NMR of the photomixture, COSY spectra, mass spectral
data for (6) are given in supplementary information (Figs. S10
and S11). We were unable to isolate the pure-cis component of
(6) from the photomixture. The quantum yield of photoisomer-
ization (фPI) obtained for the ortho-substituted dienes is also
listed in Table 2. There is no significant trend observed upon
changing the solvent polarity on фPI.
double bond, and second the possible involvement of zwitteri-
onic intermediates. In the first case, the H-bond may impart
steric congestion and structural rigidity to the system and this H-
bonding decelerates the nonradiative processes to some extent
leading to an isomerized product. The C-HꢁꢁꢁO Interactions,
although relatively weak, play a decisive role in structural prop-
erties (47,48). Even though nitro group may experience steric
repulsion with the double bond (49), single X-ray data reveal
that the interaction between nitro group and hydrogen of the
precedent double bond is quite high which may be predominant
than the steric repulsion. As a consequence of this (C-HꢁꢁꢁO)
interaction, ortho derivatives are more stable and also enable free
rotation of the other double bond facilitating regioselective
isomerization.
Although the electrostatic interactions involving the vinylene
hydrogen and the nitro group may play a crucial role, we also
envisage involvement of highly polarized zwitterionic excited
states in imparting the regioselectivity (50–53) for ortho-isomers
(Scheme 2). In this pathway, presence of strong electron
withdrawing nitro group enables formation of zwitterionic canon-
ical cationic intermediates (3a). Zwitterionic (3a) may lead to
formation of a cyclical intermediate (3b) causing clear constraints
for the isomerization and dictating the isomerization product at
the phenyl (3) or pyridyl (6) side of the double bond. However,
our experiments using time dependent NMR of the dienes
(Figs. S7 and S9) under irradiation doesn’t detect formation of
Preferential isomerization in o-substituted derivatives can be
due to two reasons. First, possibility of intramolecular H-bond
formation between nitro group and hydrogen of the precedent
Table 2. Comparative NMR values and their coupling constants of dienes (3) and (6).
Diene
trans-trans (tt)
Photomixture
trans-cis (tc)
Solvent
фPI
3
d 7.18–7.15 (d, 1H, J = 14.0 Hz),
7.03–6.94 (m, 2H),
6.77–6.74 (d, 1H, J = 15.0 Hz).
d 7.18–7.15 (d, 14.5 Hz tt),
d 7.04–6.91 (m, tt, tc),
d 6.84– 6.82 (d, J = 11.5 Hz tc),
d 6.77– 6.74 (d, J = 15.5 Hz tt),
d 6.60–6.55 (t, 11.5 Hz tc)
d 7.20–7.15 (m, tt),
d 6.96–6.91 (m, J = 15.5 Hz),
6.84–6.82 (d, J = 11.5 Hz),
6.77–6.74 (d, J = 15.5 Hz),
6.60–6.55 (t, 11.5 Hz).
Dioxane
CH3CN
0.145
0.144
6
d 7.20–7.15 (m, 1H, J = 15.5 Hz),
6.96–6.91 (m, 1H, J = 15.5 Hz),
6.68–6.65 (d, 1H, J = 15.5 Hz)
d 7.10–7.04 (m, J = 15.5 Hz),
d 6.99–6.94 (d, J = 11.5 Hz),
d 6.68–6.65 (d, 15.5 Hz),
Dioxane
CH3CN
0.15
0.14
7.10–7.04 (m, J = 15.5 Hz, tc),
6.99–6.92 (m, tt, tc),
6.68–6.65 (d, J = 15.5 Hz, tt),
6.61–6.57 (t, J = 11.0 Hz, tc)
6.61–6.57 (t, J = 11.0 Hz).
Scheme 2. Plausible mechanistic pathway to explain regioselective isomerization of ortho derivatives.

1330 Harsha Agnihotri et al.
1
any cyclical intermediates. We assume that the time scale of their
formation may be too small to be detected by using NMR. Thus,
in these systems, interplay of electrostatic interactions through
highly polarized zwitterionic excited states and (C-HꢁꢁꢁO) interac-
tions directs the regioselective isomerization product.
Figure S10. H NMR spectra of diene (6) (a) before irradia-
tion and (b) after irradiation. COSY spectra (c) before irradiation
and (d) after irradiation.
Figure S11. LCMS chromatogram and mass spectra of diene
6 after irradiation.
Appendix S1. Procedure for Quantum yield measurement.
Appendix S2. checkCIF/PLATON report.
CONCLUSIONS
The fluorescence and photochemical behavior of the meta
isomers resemble that of the para isomers although significant
absorption changes are detected upon change in the position of
nitro substitution. In addition, o-nitro substitution induces greater
photochemical reactivity than para- and meta-substituted com-
pounds. X-ray crystal data analysis reveals possibility of
intramolecular hydrogen bonding between oxygen of ortho-sub-
stituted NO2 moiety with the proximal hydrogen of the vinylene
bond. NMR spectroscopic observations clearly show preferential
isomerization of the double bond that is in proximity to the
pyridyl or the unsubstituted phenyl ring in ortho-dienes.
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Acknowledgements—Authors gratefully acknowledge Council of
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Figure S1. Normalized emission spectra of dienes (1) (3) and
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Figure S2. Normalized emission spectra of diene (5) in sol-
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Figure S3. (a) HPLC peak profile of diene (4) before and (b)
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(c) H NMR spectra of diene (4) before irradiation.
1
Figure S4. (a) UV–Vis absorption spectrum upon irradiation
of diene (2) in acetonitrile at different time intervals. HPLC peak
1
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