Photobehavior of the geometrical isomers of two 1,4-distyrylbenzene analogues with side groups of different electron donor/acceptor character

Article abstract of DOI:10.1021/jp105383e

The photobehavior of two 1,4-distyrylbenzene analogues where the central benzene ring is asymmetrically substituted with a pyrid-4-ylethenyl group at one side and thien-2-ylethenyl or a p-nitrostyryl group at the other side, has been studied in two solvents at room temperature. The four geometrical isomers (EE, ZE, EZ, and ZZ) of each compound were separated by the combined use of HPLC and spectrophotometric techniques. The radiative/reactive competition in their excited state relaxation was particularly examined: the diabatic/adiabatic contributions were estimated and a reasonable interpretation of the photoisomerization mechanism was proposed. The role of the conformational isomers was also investigated by measured and computed spectral data. Since the different electron donor/acceptor character of the side groups of these molecules can induce charge transfer phenomena that can affect the relaxation pathways of their excited states, the photobehavior was compared in inert and polar solvents to clarify the role of the intramolecular charge transfer. The latter was found to affect markedly the relaxation properties and to induce interesting fluorosolvatochromic effects, particularly in the p-nitro derivative. The participation of the triplet state in the reaction mechanism of the latter was also investigated by flash photolysis and sensitized experiments.

Full text of DOI:10.1021/jp105383e

J. Phys. Chem. A 2010, 114, 10761–10768
10761
Photobehavior of the Geometrical Isomers of Two 1,4-Distyrylbenzene Analogues with Side
Groups of Different Electron Donor/Acceptor Character
S. Ciorba, G. Galiazzo,† U. Mazzucato, and A. Spalletti*
Dipartimento di Chimica, UniVersita` di Perugia, 06123 Perugia, Italy
ReceiVed: June 11, 2010; ReVised Manuscript ReceiVed: July 29, 2010
The photobehavior of two 1,4-distyrylbenzene analogues where the central benzene ring is asymmetrically
substituted with a pyrid-4-ylethenyl group at one side and thien-2-ylethenyl or a p-nitrostyryl group at the
other side, has been studied in two solvents at room temperature. The four geometrical isomers (EE, ZE, EZ,
and ZZ) of each compound were separated by the combined use of HPLC and spectrophotometric techniques.
The radiative/reactive competition in their excited state relaxation was particularly examined: the diabatic/
adiabatic contributions were estimated and a reasonable interpretation of the photoisomerization mechanism
was proposed. The role of the conformational isomers was also investigated by measured and computed
spectral data. Since the different electron donor/acceptor character of the side groups of these molecules can
induce charge transfer phenomena that can affect the relaxation pathways of their excited states, the
photobehavior was compared in inert and polar solvents to clarify the role of the intramolecular charge transfer.
The latter was found to affect markedly the relaxation properties and to induce interesting fluorosolvatochromic
effects, particularly in the p-nitro derivative. The participation of the triplet state in the reaction mechanism
of the latter was also investigated by flash photolysis and sensitized experiments.
1. Introduction
SCHEME 1: Compounds Investigated
Donor-acceptors R,ω-diarylpolyenes, particularly the sub-
stituted stilbenes, have been widely investigated as interesting
models for studies of intramolecular charge transfer (ICT) in
the excited state.1–4 The interest in these dipolar push-pull
molecules is especially due to their hyperpolarizability proper-
ties, which make them candidate components of NLO materials.5,6
In the framework of our long-term investigation of the
photobehavior of stilbene-like molecules, particularly the het-
eroanalogues of stilbene and distyrylbenzene (DStB) [for some
representative papers, see refs 7–16] two asymmetric 1,4-DStB
analogues were prepared, bearing two different arylethenyl
groups at the opposite side of the central benzene ring, namely
1-(pyrid-4-ylethenyl)-4-(thien-2-ylethenyl)benzene (1) and 1-(py-
rid-2-ylethenyl)-4-(p-nitrostyryl)benzene (2). The presence of
four geometrical isomers (EE, ZE, EZ, and ZZ) and various
conformational isomers (conformers or rotamers) of these
compounds makes their photobehavior rather unforeseeable.
Apart from the role of conformers in the photoreactivity, new
interesting aspects offered by asymmetric DStBs bearing two
different isomerizable double bonds, with respect to the
analogous compounds with one bond only, are the possible
selectivity in the production of the ZE and EZ isomers under
irradiation of both EE and ZZ and the possible occurrence of
the “one photon-two bonds” mechanism in the ZE f EZ and
ZZ f EE photoprocesses. Interesting examples of regioselec-
tivity have been reported for alkyl-substituted 1,3-distyrylben-
zene, attributed to intramolecular energy transfer in the excited
state,17 and for other series of compounds with two isomerizable
double bonds, for instance, the asymmetrically substituted 1,4-
diarylbutadienes, attributed to ICT in the excited state.18–20
This paper aims to describe the different relaxation properties
of the four geometrical isomers of the two compounds, in
particular, the fluorescence/reactivity competition, the photo-
isomerization mechanism, and the role of the triplet state in the
photobehavior. The effect of ICT on their behavior under
irradiation in the polar solvent was also investigated.
2. Experimental Section
2.1. Materials. The two 1,4-DStB analogues bearing a
4-pyridyl (4P) group at one side and a 2-thienyl (2T) or a
4-nitrophenyl (p-NO2Ph) substituent at the other side (Scheme
1), were synthesized by application of a 2-fold Wittig reaction
to difunctional reactants [p-xylylene-bis(triphenylphosphonium
bromide) and 2-thienyl-aldehyde or p-nitrobenzaldehyde, re-
spectively]. The four geometrical isomers contained in the
photoproduct mixture of both compounds (Scheme 2) were
separated, purified, and characterized by the combined use of
preparative HPLC (column: Alltima C18, 22 × 250 mm; 5 µm)
and spectrophotometric techniques. The formula was calculated
on the basis of the isotopic abundance in the LC/MS analysis
* To whom correspondence should be addressed. E-mail: faby@unipg.it.
† Retired from Padua University (address: via Crescini 39, 35126 Padova,
Italy).
10.1021/jp105383e  2010 American Chemical Society
Published on Web 09/21/2010

10762 J. Phys. Chem. A, Vol. 114, No. 40, 2010
Ciorba et al.
SCHEME 2: Geometrical Isomers of the Compounds Investigated
(see Supporting Information). The geometrical structure was
assigned on the basis of their experimental and computed
absorption spectra. The ZZ isomers were characterized also by
1H NMR spectrometry (see Supporting Information).
The solvents were cyclohexane (CH) and acetonitrile (AcN)
from Fluka (spectrophotometric grade) except when otherwise
specified.
2.3. Theoretical Calculations. These were performed for the
different isomers of the investigated compounds using the
HyperChem computational package (version 7.5). The heats of
formation and dihedral angles between the ethenic bridge and
the central aromatic ring and the bond lengths were obtained
for the geometries optimized by the PM3 method. The electronic
spectra (transition energy and oscillator strenght) were calculated
by ZINDO/S (9 × 9 single excited configurations interaction)
(see Supporting Information).
2.2. Techniques and Methods. A Perkin-Elmer Lambda 800
spectrophotometer was used for the absorption measurements.
The fluorescence spectra were measured by a Spex Fluorolog-2
F112AI spectrofluorometer. Dilute solutions (absorbance <0.1
at the excitation wavelength, λexc) were used for fluorometric
measurements. The emission and photoreaction quantum yields
were determined at λexc corresponding to the maximum of the
first absorption band (λmax), except when otherwise indicated.
The 9,10-diphenylanthracene in CH was used as a fluoro-
metric standard (φF ) 0.90 in deaerated solvent21). Fluorescence
lifetimes were measured by an Edinburgh Instrument 199S
spectrofluorometer, using the single photon counting method.
The triplet state was investigated by nanosecond laser flash
photolysis (λexc ) 355 nm) using a Continuum Surelite II Nd:
YAG laser. The φεT × EE-2 was determined by using
benzophenone in AcN as a standard (φT × εT ) 6500 dm3 mol-1
cm-1 at 520 nm). The molar absorption coefficient of the triplet
state was determined through a sensitization method by using
thioxanthen-9-one as a sensitizer (εT ) 26200 dm3 mol-1 cm-1
at 650 nm in AcN).22 The triplet state of EE-1 was produced
by using thioxanthen-9-one as a sensitizer.
1
2.4. NMR Measurements. The H NMR spectra of the ZZ
isomers were measured on a Bruker DRX 400 spectrometer in
deuterated dichloromethane, using tetramethylsilane as the
reference (see Supporting Information).
2.5. LC/MS Measurements. These were performed using
combined UPLC (Agilent 1290 Infinity with Ascentis Express
C18 column, 50 × 2.1 mm; 2.7 µm) and MS (Agilent 6540
Accurate Mass, Q-TOF) techniques. The formulas of the
compounds were derived on the basis of the isotopic abundances
(see Supporting Information).
3. Results and Discussion
3.1. Spectral and Photophysical Properties. The absorption
spectra of the four geometrical isomers of the two compounds
in CH are shown in Figures 1 and 2. The spectral and
photophysical parameters in CH and AcN are collected in Tables
1 and 2. The computed structural and spectral properties are
reported in Tables S1 and S2, Supporting Information.
For photochemical measurements, a 150 W high pressure
xenon lamp coupled with a monochromator and potassium
ferrioxalate in water as actinometer were used. The photoreac-
tion (solute concentration ∼10-4 M) was monitored by HPLC
using a Waters apparatus equipped with analytical Alltima C18
(4.6 × 250 mm; 5 µm) and Prontosil 200-3-C30 (4.6 × 250
mm; 3 µm) columns and UV detector. Water/acetonitrile
mixtures were used as eluents. The monitoring wavelength was
at the isosbestic point; otherwise corrections for different
absorption coefficients were introduced. The conversion per-
centage was held at below 8% to avoid the competition from
the back photoreaction. Sensitized experiments were carried out
using biacetyl in benzene or AcN as triplet donor.
All measurements were carried out at room temperature in
deaerated solutions by purging with nitrogen. The parameters
reported in the tables are averages of at least two independent
experiments with mean deviations of 10% for fluorescence
quantum yields and lifetimes and 15% for photoisomerization
quantum yields.
Figure 1. Absorption spectra of the EE (solid line), ZE (dash), EZ
(dot), and ZZ (dash-dot) isomers of compound 1 in CH.

Geometrical Isomers of 1,4-Distyrylbenzene Analogues
J. Phys. Chem. A, Vol. 114, No. 40, 2010 10763
are the most and the least stable isomers, respectively, while
ZE and EZ occupy an intermediate position.
Torsional isomerism about carbon-carbon single bonds can
characterize the behavior of these geometrical isomers, as is
well-known for the series of diarylethenes and DStBs.23,24 The
possible conformational isomers of EE-1 are shown in Scheme
3, as an example. The presence of conformers in fluorescent
molecules is usually investigated by excitation and by monitor-
ing wavelength (λexc and λem) effects on the fluorescence
emission and excitation spectra, respectively, and by λexc effects
on the fluorescence quantum yield and decay kinetics. However,
the wavelength effect on the spectra of the EE isomers of these
two compounds was on the order of magnitude of the experi-
mental error and the fluorescence decay remained practically
monoexponential (the exception of ZE-1 in AcN, where the
fluorescence excitation spectrum did not overlap well the
absorption spectrum and a λexc effect on the fluorescence yield
was found, will be discussed in sections 3.1.1 and 3.2.1). Taken
together, the results above indicate that one species prevails in
the conformational equilibrium or that the expected conforma-
tions, due to rotation around the single bonds of the ethene
carbons with the central ring or the side aryl groups, have similar
photophysical parameters.
Figure 2. Absorption spectra of the EE (solid line), ZE (dash), EZ
(dot), and ZZ (dash-dot) isomers of compound 2 in CH.
TABLE 1: Spectral and Photophysical Properties of the
Geometrical Isomers of 1 in Two Solvents at Room
Temperaturea
in CH
in AcN
Since the computed heats of formation and then the relative
abundances are very similar for the two main conformers
(elongated and compressed), the latter are expected to display
indistinguishable properties, as also indicated by the calculated
electronic spectrum (Tables S1 and S2, Supporting Information)
and as reported for the parent hydrocarbon (1,4-DStB)9,25 and
styrylpyridines.26
isomer
λ
abs/nm λF/nm
φF
τF/ns
λ
abs/nm λF/nm
φF
τF/ns
EE
ZE
EZ
ZZ
367
349
331
272
428
0.40
1.2
365
348
309
285
468
0.43
1.1
(428) (0.17) (1.2)
(468) (0.05)
a The main absorption maxima are reported; data in parentheses
refer to the fluorescence of the adiabatically formed EE isomer.
Measurements in AcN showed that the polar solvent has a
negligible effect on the absorption spectrum. More details on
the photophysical behavior are described below separately for
the two compounds.
TABLE 2: Spectral and Photophysical Properties of the
Geometrical Isomers of 2 in Two Solvents at Room
Temperaturea
3.1.1. Photophysical Properties of 1-(Pyrid-4-ylethenyl)-4-
(thien-2-ylethenyl)benzene (1). The fluorescence emission of
compound 1, observed under irradiation of EE and ZE only, is
substantial in both solvents examined (Figures 3 and 4 and Table
1). The spectra and lifetimes are identical, while the fluorescence
excitation spectra overlap the corresponding absorption spectra,
thus indicating that the φF value measured for EE refers to its
intrinsic emission, which represents the main relaxation pathway
of this isomer, whereas the apparent emission yield, measured
under irradiation of ZE, pertains to the emission from EE*,
produced by adiabatic ZE* f EE* photoisomerization (see
below).
in CH
in AcN
isomer
λ
abs/nm λF/nm
φF
τF/ns
λ
abs/nm λF/nm
φF
τF/ns
EE
ZE
EZ
ZZ
377
362
300
282
425 ∼5 × 10-5 <0.5
378
368
309
285
595 0.47
(580) (0.26) (2.12)
2.10
a The main absorption maxima are reported; data in parentheses
probably refer to the contributions of ZE* and of the adiabatically
1
1
formed EE*.
Figures 1 and 2 show a similarity in the spectra of the
geometrical isomers of the two compounds: the EE isomer
shows the first absorption band, the most intense and slightly
structured one, in the 320-400 nm region and two weaker bands
toward the blue, while ZZ displays three blue-shifted broad
bands of comparable intensity. While the EE and ZZ isomers
were recognized by absorption spectra and NMR data, respec-
tively, the characterization of ZE and EZ (with the cis double
bond adjacent to the pyridyl or thienyl group, respectively) was
based on the comparison between the computed and experi-
mental absorption spectra, which are in good agreement between
themselves. In particular, the ZE isomers show an intense
absorption band slightly blue-shifted and hypochromic with
respect to that of EE while the EZ isomers show a further blue
shift and an increase in intensity of a second band in the range
270-300 nm, similarly to the case of the ZZ isomers. The
computed oscillator strength of the first electronic transition for
the various isomers follows the same trend as the experimental
intensity. Moreover, the calculations indicate that EE and ZZ
As a matter of fact, an anomalous behavior was found for
ZE-1 in AcN where the fluorescence excitation spectrum did
not correspond to its absorption (Figure 4). The possible
presence of an EE impurity was ruled out since the excitation
spectrum was different from that of EE too. It has to be noted
that no emission from 1EE* was induced by excitation of ZE-1
below 300 nm while the apparent emission yield increased at
longer λexc. An explanation of this puzzling behavior will be
proposed below with the help of the photochemical findings.
The fluorescence lifetimes of EE in Table 1 lead to a rate
constant of 3.9 and 3.3 (×108 s-1) in CH and AcN, respectively,
values characteristic of allowed transitions. The large Stokes
shift observed in AcN (∼100 nm) reveals a larger dipole
moment of the fluorescent state in the polar solvent.2 The
presence of a π-deficient group (4P) and a π-excessive group
(2T) at the opposite sides of 1 is expected to induce a charge
transfer process from the Franck-Condon state reached by
absorption (locally excited precursor, LE) to an ICT state

10764 J. Phys. Chem. A, Vol. 114, No. 40, 2010
SCHEME 3: Possible Conformers of EE-1
Ciorba et al.
responsible for the emission in polar solvent. Dual emission,
found for push-pull stilbenes4 and indicative of the presence
of planar LE and twisted (TICT) excited states, was not observed
in the two solvents here examined.
3.1.2. Photophysical Properties of 1-(Pyrid-4-ylethenyl)-4-
(p-nitrostyryl)benzene (2). Perusal of Tables 1 and 2 shows that
the behavior of the p-nitro derivative in CH differs from that
described for the 2T derivative in section 3.1.1. The fluorescence
yield of EE is here at the detection limit, the nonradiative decay
being the main deactivation process in this solvent. The Stokes
shift of about 50 nm in CH is similar to that observed for 1.
The absorption spectrum is negligibly affected by the polar
solvent AcN, but a quite different fluorometric behavior was
observed (Figure 5). The red shift of the emission spectrum
(fluorosolvatochromism) is strongly increased (to about 220 nm)
and the fluorescence yield undergoes a huge increase to almost
50% with a lifetime of about 2 ns and kF ) 2.2 × 108 s-1
(allowed transition). This behavior of 2, where the acceptor
properties of the p-NO2Ph group strongly prevails, clearly points
to an important role of ICT in the relaxation processes even if
no dual fluorescence was observed, as found for 1. In fact, the
solvent relaxation is accompanied by efficient ICT; involvement
of twisting of the nitro group around the single bond with
formation of a twisted (TICT) state cannot be here excluded
(see below).
3.2. Photochemical Properties. The photoisomerization
quantum yields of the four isomers of the two compounds in
two solvents are shown in Tables 3 and 4.
As is well-known for stilbene and DStB analogues, cis-trans
photoisomerization in fluid solutions can occur by two competi-
tive mechanisms: the most common (reversible) diabatic mech-
anism involves S1 f S0 internal conversion (IC) or T1 f S0
intersystem crossing (ISC) at the perpendicular configuration
(P, at about 90°) and relaxation to the ground-state isomers,
1,3E* f 1,3P* f 1P f R1Z + (1 - R)1E, where the partitioning
Figure 3. Normalized absorption and emission spectra of EE-1 in CH
(solid) and AcN (dash).
Figure 4. Normalized absorption of ZE-1 (dotted) and excitation and
emission spectra of EE-1 (dashed) and ZE-1 (solid) in AcN.
Figure 5. Normalized absorption and fluorescence emission spectra
of EE-2 in CH (solid) and AcN (dash).

Geometrical Isomers of 1,4-Distyrylbenzene Analogues
J. Phys. Chem. A, Vol. 114, No. 40, 2010 10765
TABLE 3: Photoisomerization Quantum Yields of the
Geometrical Isomers (X) of 1 in Two Solvents
in CH
in AcN
X
φXfZE
φXfEZ
φXfEE
φXfZE
φXfEZ
φXfEE
EE
ZE
EZ
ZZ
0.030
0.19
0.068
0.015
0.16
0.26
0.29
0.080
0.028a
0.34a
0.27
0.040
0.155
0.038
0.22
0.062
a Values measured at λexc ) 332 nm (these parameters are
slightly affected by λexc; see below).
TABLE 4: Photoisomerization Quantum Yields of the
Geometrical Isomers (X) of 2 in Two Solvents
in CH
in AcN
X
φXfZE
φXfEZ
φXfEE
φXfZE
φXfEZ
φXfEE
EE
ZE
EZ
ZZ
e0.005
0.20
0.21
0.007
0.057
Figure 6. Transient absorption spectrum of EE-1 (peak at 500 nm)
sensitized by thioxanthen-9-one (peaks at 320 and 620 nm) in AcN.
0.84
0.50
0.42
0.475
0.49
0.13
0.046
0.098
0.165
0.042
can be deduced for this isomer. The T1 f Tn spectrum shown
in Figure 6 was obtained in the presence of thioxanthen-9-one
as triplet donor sensitizer. Its absorption maximum is at ∼500
nm, and the lifetime is ∼2.5 µs.
factor R is assumed to be ∼0.5 in most cases.27 It has to be
noted that the radiationless processes at the 1,3P* region are now
preferably discussed in terms of conical intersections.28–30 By
the less common, but not rare, adiabatic mechanism, the
isomerization takes place directly from one to the other excited
isomer in a unique (singlet or triplet) potential energy surface,
generally in the 1,3Z* f 1,3E* direction.31–34
The photochemical behavior of the two compounds investi-
gated shows some analogies. In both cases irradiation of the
EE isomer leads to a net regioselectivity with largely prevalent
rotation of the double bond adjacent to the thienyl and p-NO2Ph
groups (favored formation of EZ). The ZE isomers give
prevalently EE accompanied by a smaller but non-negligible
amount of ZE (“one photon-two bonds” mechanism). As to
the EZ isomer, it gives only EE with sizable yield. The ZZ
isomer is not formed under irradiation of the other isomers in
our mild conditions (small concentration, low light dose, and
short irradiation time), as common for DStBs,9,17,35 but photo-
converts to all the other isomers in both solvents with different
yields for the two compounds.
As to the isomers with one or both double bonds in cis
configuration, ZE and EZ practically isomerize to EE only, with
good yields, while ZZ gives all the other isomers with a
prevalence of ZE in both solvents. The small but non-negligible
ZE f EZ 2-fold isomerization yield points to the presence of
adiabatic pathways in the ZE f EE photoisomerization whose
ad
contribution (φ
) can be derived by
ZEfEE
φZEfEZ
ad
ZEfEE
ad
φ
) φ
× φG,EE
)
× φG,EE
(1)
ZEfEE*
φEEfEZ
where φG,EE is the quantum yield of the nonreactive deactivation
to the ground state (1 - φEEfEZ - φEEfZE). The value of 0.28
thus derived, compared with the measured value in Table 3
(0.26), indicates that the photoreaction occurs through an
adiabatic pathway only. The expected fluorescence yield of EE
ad
under irradiation of ZE is then given by the product φ
×
ZEfEE*
φF,EE ) 0.14, a derived value not far from that obtained
experimentally (0.17), which confirms that the adiabatic pathway
is operative in the singlet manifold. On the contrary, EZ
produces EE diabatically since no fluorescence of EE under
irradiation of EZ was observed.
The reactivity is not strongly affected by the solvent (with
the exception of EE-2) but tends to decrease in AcN, contrary
to what is generally reported for stilbenoid compounds.36
A
detailed description of the photoreaction mechanism is reported
separately for the two compounds.
As to ZZ, the observed 2-fold ZZ f EE process implies at
least one adiabatic step. Should the twisting of both double
bonds be adiabatic (ZZ f ZE* and ZZ f EZ*), the quantum
yield of the overall process could be derived by eqs 2 (through
ZE*) and 3 (through EZ*):
3.2.1. Photochemical Properties of 1. As said above, the
photoreaction of EE is highly regioselective since twisting of
the double bond adjacent to the thienyl group largely prevails.
Both the intramolecular energy transfer toward the latter bond,
as hypothesized for alkyl-substituted 1,3-DStB,17 and the
formation of an ICT state, as proposed for asymmetric
diarylbutadienes,3,18–20 could be responsible for this behavior.
It has to be noted that the computed bond lengths in the gas
phase (see Supporting Information) show a substantial increase
for the double bond adjacent to the thienyl group under
excitation (from 1.34 Å in the ground state to 1.43 Å in the
excited state) whereas the lengthening of the double bond
adjacent to the 4P group is much smaller (from 1.34 to 1.36
Å). This indicates that the excitation energy is mainly localized
at the 2T side, thus explaining the prevalent rotation of the
nearby double bond.
φZZfZE
ad
ZZfEE
ad
ZZfZE*
φ
) φ
× φZEfEE
)
)
× φZEfEE (2)
φG,ZE
φZZfEZ
ad
ZZfEE
ad
ZZfEZ*
φ
) φ
× φEZfEE
× φEZfEE (3)
φG,EZ
where φG,ZE and φG,EZ represent again the quantum yields of
the nonreactive deactivation of ZE* and EZ* to the ground state,
respectively. The sum of the two values thus derived (0.06 by
eq 2 and 0.016 by eq 3) is in good agreement with the
No triplet formation was detected by laser flash photolysis
under direct irradiation of EE and a diabatic singlet mechanism

10766 J. Phys. Chem. A, Vol. 114, No. 40, 2010
Ciorba et al.
TABLE 5: Fluorescence and Photoisomerization Quantum
Yields of the Symmetrical 4P and 2T Analogues of
Compound EE-1 in Two Solvents
compound
4P-π-4P
2T-π-2T
in MCH/3MPa in AcN in MCH/3MPb in AcN
φF
φEEfZE
0.21
0.32
0.17
0.30
0.24
0.05
0.085
0.15
a From ref 9. b From ref 13.
experimental ZZ f EE quantum yield (0.08), thus confirming
the reliability of the starting hypothesis.
An analogous treatment of the data for ZE in AcN shows
ad
that the adiabatic pathway (φ
) 0.11) does not account
ZEfEE
for the overall experimental photoisomerization yield measured
at λexc ) 332 nm (0.34), pointing to a substantial contribution
of a diabatic pathway. The decrease of the adiabatic contribution
on going from CH to AcN is in line with the reduction of the
Figure 7. Transient absorption spectrum of EE-2 in AcN.
1
fluorescence yield of EE* measured under excitation of ZE
and rather less for the 2T derivative, particularly in a nonpolar
solvent. It has to be noted that the S1 f T1 ISC is quite modest
even in the symmetric analogues: values of 0.02 and 0.03 in
deaerated nonpolar solvent have been reported for the 4P and
2T derivatives, respectively.12,13
The red shift of the fluorescence spectrum in a polar solvent
is small (7 and 4 nm for the 4P and 2T symmetric compounds,
respectively). The increased extent of the ICT transfer in the
asymmetric (A-π-D) compound 1 leads to an increase of the
fluorosolvatochromism on going from CH to AcN [∆λ(CH-AcN)
) 40 nm]. In any case, the photophysical and photochemical
properties of EE-1 show that the 4P-2T asymmetric substitution
affects modestly the photobehavior in nonpolar solvent, the most
evident effect being the large Stokes shift of fluorescence, the
high φF value, and the change of the photoisomerization
mechanism (from adiabatic to mixed adiabatic/diabatic) in the
polar solvent.
(Table 1) since the intrinsic φF,EE is the same in both solvents.
A λexc dependent reaction yield was also found. No sign of the
one photon-two bonds mechanism was observed with excitation
at 300 nm (φZEfEE ) 0.37) whereas the ZE f EZ quantum yield
slightly changes from 0.028 to 0.035 and φZEfEE decreases from
0.34 to 0.30 when λexc increases from 332 to 405 nm. These
results, in qualitative agreement with the anomalous excitation
spectrum in AcN, reported in section 3.1.1, could be tentatively
ascribed to the CT character of the excited state of this
asymmetric compound in a polar solvent. One could then think
that two excited states of different natures are implied in the
photoreaction, namely the upper LE state, inducing the formation
1
of P* (diabatic mechanism), and the lower ICT state, leading
1
adiabatically to EE*.
It is interesting to note that the adiabatic contribution to the
ZE f EE photoisomerization in AcN (0.12), calculated by the
fluorescence data
3.2.2. Photochemical Properties of 2. The trend of the
reactive relaxation of 2 in Table 4 does not turn off substantially
that of 1 but is much more affected by the solvent (with the
exception of EZ), as expected from the effect of AcN on the
emission yield (see above). Even in this case, irradiation of
the EE isomer does not lead to ZZ and the photoreaction is
highly photoselective with rotation of only the double bond
adjacent to the p-NO2Ph group with a yield of 20% in CH but
less than 1% in AcN. The computed map of the MOs of EE in
the S0 and S1 states (Figure S1, Supporting Information) clearly
shows that the excitation energy is largely localized in the double
bond adjacent to the p-NO2Ph group, thus explaining the
regioselectivity. As proposed for donor/acceptor diarylbuta-
dienes,20 the 1,3P* intermediate formed by twisting of this bond
is probably more stabilized than the one formed by twisting
the double bond adjacent to the 4P group, thus favoring the
diabatic photoisomerization.
The isomerization mechanism could change from singlet to
triplet for compound 2 since laser flash photolysis experiments
showed that the ISC of EE has an appreciable efficiency and
populates T1 with a yield of 0.30 in AcN (Figure 7), which
becomes more than doubled in CH (0.68, by assuming the same
absorption coefficient, εT, found in AcN). However, the reactiv-
ity of EE is relatively modest, indicating that the equilibrium
in the triplet manifold is shifted toward the coplanar transoid
form and does not favor the 3P* intermediate of diabatic
isomerization. This was confirmed by measurements of the
isomerization yield sensitized by biacetyl as a triplet donor,
which was 0.31 in benzene and 0.13 in AcN using a concentra-
φF,ZE
1
ad
ZEfEE
φ
)
× φG,EE
(4)
φF,EE
is not far from that derived through eq 1 (0.15), thus confirming
that the photoreaction occurs in the singlet manifold.
A mixed diabatic/adiabatic mechanism with comparable
contributions was found to be operative even for the ZZ isomer
in AcN (Table 4). From the yield of the “one photon-two
bonds” ZZ f EE process, the adiabatic contribution of the first
ZZ f ZE* step was estimated by a treatment similar to that
described by eq 1. The involvement of the diabatic contribution
in polar solvents is in agreement with the preferential stabiliza-
tion of the twisted 1P* with respect to the quasi-planar transoid
geometry, as previously reported for analogous compounds.37
It is interesting to compare the present results with those
reported for the symmetric pull-pull (A-π-A) and push-push
(D-π-D) analogues bearing two π-deficient (4P) or π-exces-
sive (2T) side groups9,13 where symmetrical CT from or toward
the central benzene ring takes place (Table 5). In the EE isomer
of the parent hydrocarbon with two phenyl groups, 1,4-DStB,
the largely prevalent relaxation pathway of S1 is the fluorescence
emission (φF ) 0.84 in nonpolar solvent) whereas the reactivity
is practically negligible (φEEfZE e 0.001).9 Both pyridyl and
thienyl groups cause a drastic decrease of the radiative relaxation
(particularly for the compound bearing the 2T groups) while
the reactive pathway increases substantially for the 4P derivative

Geometrical Isomers of 1,4-Distyrylbenzene Analogues
J. Phys. Chem. A, Vol. 114, No. 40, 2010 10767
tion of 4 × 10-4 M, sufficient to ensure complete energy transfer
TABLE 6: Quantum Yields of the Nonreactive Deactivation
to the Ground State of the Four Isomers of 1 and 2 and
Derived Diabatic/Adiabatic Contributions in the
Photoreactivity of the ZE and ZZ Isomers
from the sensitizer.
The yield of the EE f EZ process in the triplet manifold,
sens
estimated by the product φISC × φ
(0.21), accounts for the
EEfEZ
compound
solvent
1
2
overall experimental yield in CH (0.20). On the contrary, in
AcN this product would lead to a value higher (∼0.04) than
the very low experimental one (0.007), even if both the
production of the triplet state and its reactivity decreased in the
polar solvent.
CH
AcN
CH
AcN
φG,EE
φG,ZE
φG,EZ
0.78
0.67
0.71
0.73
0.28
0.83
0.63
0.73
0.68
0.15
0.19
0.040
0.074
0.15
0.80
0.99
0.47
0.51
0.66
0.42
0.055
0.13
0.13
0.03
0.50
0.44
0.84
φG,ZZ
ad
The photobehavior of EE-2 in AcN can be explained by the
well-known “three state model”.1,4 According to the latter, the
fastest process following the vertical excitation to a nonemitting
(or poorly emitting) LE state leads to the formation of the
emitting ICT (or TICT) state. This process competes with
twisting around the double bond that leads to the P* configu-
ration responsible for diabatic isomerization. The polar character
of the P* intermediate in D/A stilbenes is probably reduced in
comparison with the unsubstituted stilbenes, as reported for D/A
1,4-distyrylbutadienes.18 This would lead to a reduced stabiliza-
tion of P* with respect to the planar excited state reached by
absorption, 1EE*.38,39 The final result is that the ICT state, more
stabilized by the polar solvent than both LE and P*, emits
strongly and competes efficiently with the nonradiative relax-
ation through twisting of the double bond, thus leading to the
huge increase in the fluorescent pathway (47%) and the
practically absence of isomerization (<1%) in AcN. A “four
state model”, distinguishing between the ICT state formed by
LE and a conformationally relaxed and possibly twisted TICT
state,2 would be here too speculative in the absence of
information based on ultrafast time-resolved measurements.
φ
ZEfEE
φdZEfEE
ad
φ
ZZfEE
0.080
0.155
0.42
ad
φ
ZZfZE
φdZZfZE
0.046
spectrum does not perfectly overlap that of EE, revealing a
1
contribution of a proper emission from ZE* (see Table 2 and
Figure 8).
The adiabatic contribution to the ZE f EE yield can be
estimated by the following equation
ad
ZEfEE
app
T,EE
sens
G,EE
φ
) φ
× φ
(5)
app
3
where φ
is the production of EE* by exciting ZE (0.48 in
T,EE
sens
AcN) and φ
is the yield of the ground state recovery after
G,EE
sensitization of 3EE* (1 - φ
) 0.87 in AcN). Instead, the
sens
EEfEZ
adiabatic contribution to the ZZ f ZE yield can be derived by
φZZfEE
ad
ZZfZE
ad
ZZfZE*
Both double bonds of ZE isomerize, leading to EZ and EE
with a prevalent yield of the latter (more than 50%). As found
for 1, the EZ isomer gives EE only and ZZ gives all the other
isomers with a high yield of EE in CH. The polar solvent
changes significantly the reaction yields of ZZ, giving preferably
ZE, followed by EE and with minor production of EZ. Since a
contribution of the triplet mechanism can be operative also for
these isomers with cis double bonds, the behavior of the ZE
isomer was also investigated by laser flash photolysis. The
transient (T1 f Tn) absorption spectrum, measured under
irradiation of ZE, was found to be the same, with the same
lifetime, as that measured for the EE isomer with an apparent
yield of 0.92 in CH and 0.48 in AcN. This indicates that, within
φ
) φ
× φG,ZE
)
× φG,ZE
(6)
φZEfEE
The values thus obtained in AcN (0.42 and 0.13, from eqs 5
and 6, respectively, Table 6) are to be compared with the two
overall experimental yields of Table 4 (0.475 and 0.165). As a
matter of fact, differently from compound 1, in the case of the
nitro derivative the photoreactions of ZE and ZZ mainly proceed
through an adiabatic pathway also in a polar solvent, probably
because of the CT character of the excited states.
Since the properties of the symmetric p-NO2Ph derivative
are not available in the literature to our knowledge, a rough
comparison can be made with those of 1,4-bis(2,4-dinitrostyryl)-
benzene.40 The fluorescence yield of the latter is negligible
3
3
its lifetime, ZE* photoconverts adiabatically to EE* with a
practically unitary yield in CH [a contribution of 3EE* formation
through an adiabatic photoisomerization in the singlet manifold
and subsequent ISC (1ZE* f 1EE* f 3EE*) cannot be excluded
since 1EE* practically does not emit]. This is in agreement with
the 2-fold ZE f EZ isomerization yield (0.21) that is the same
as that of the EE f EZ direct process in CH (0.20). The ratio
φXfEE/φXfEZ is roughly 4 also under irradiation of the ZZ isomer,
thus suggesting that the production of EE occurs through two
1
1
1
adiabatic steps, that is, ZZ* f ZE* f EE*. Since φG,ZE is
zero in CH, no formation of ZE in the ground state is expected
from this adiabatic mechanism. Therefore, a diabatic process
is probably responsible for the small experimental ZZ f ZE
yield. The derived diabatic/adiabatic contributions for com-
pounds 1 and 2 are collected in Table 6.
In AcN the reactivity of the isomers decreases (with the
exception of EZ that shows the same value of 0.5, characteristic
of a diabatic process) in favor of the nonradiative and nonre-
active deactivation (in favor of fluorescence in the case of the
EE isomer). As regards ZE, the competition of an intrinsic
radiative deactivation is probably operative because the emission
Figure 8. Normalized fluorescence spectra in AcN measured under
irradiation of ZE-2 (solid) and EE-2 (dash).

10768 J. Phys. Chem. A, Vol. 114, No. 40, 2010
Ciorba et al.
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the symmetric dinitro derivative with the difference that the
nonreactive ICT state prevalently decays by the radiative
pathway in our case and nonradiatively for the symmetric
compound.40
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4. Conclusions
The described results allowed the fluorescence/photoisomer-
ization competition of the four geometrical isomers of two
asymmetric distyrylbenzene analogues with side aryl groups of
different donor/acceptor character to be evaluated in CH and
AcN. A selective photoconversion of the all-trans isomers was
found for both compounds in both solvents, largely favoring
the double bond closest to the donor group for compound 1
and in complete favor of the double bond adjacent to the
acceptor group for compound 2.
The role of conformational equilibria has been also estimated
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Supporting Information Available: Calculated electronic
spectra (transition energy and oscillator strength), dipole mo-
ments, bond lengths, heats of formation, dihedral angles between
the ethenic bridge and the side or central aromatic rings, and
map of the MOs involved in the first electronic transition of
the isomers of the p-NO2 derivative. LC/MS analysis data for
all geometrical isomers and characterization of the ZZ isomers
1
by H NMR. This material is available free of charge via the
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JP105383E