Synthesis, spectral properties and photobehaviour of push-pull distyrylbenzene nitro-derivatives

Article abstract of DOI:10.1016/j.jphotochem.2012.06.009

Six novel asymmetric 2,5-distyryl-furan, 2,5-distyryl-thiophene and 2,6-distyrylpyridine derivatives, bearing an electron acceptor (p-nitro) group at one side and an electron donor (p-methoxy or p-dimethylamino) group at the other side, have been prepared. The experimental absorption properties have been measured and compared with the computed parameters. Theoretical and experimental results indicate that one conformational isomer (the compressed one) is largely prevalent in all compounds. The measured radiative and reactive relaxation properties of these donor/acceptor disubstituted compounds were also compared with those of the unsubstituted analogues previously investigated. The presence of the donor/acceptor groups leads to a significant increase of the charge transfer character of both the ground and the excited states and to strong red shifts of the absorption spectra, an effect that can be useful for potential applications in material science. The fluorescence/photoisomerization competition of the furan and thiophene derivatives was found to be rather similar to that of the unsubstituted analogues whereas significant changes were found for the pyridine derivatives where a drastic decrease of fluorescence, a sizable increase of photoisomerization and a predominance of radiationless deactivation to the ground state becomes operative.

Full text of DOI:10.1016/j.jphotochem.2012.06.009

Journal of Photochemistry and Photobiology A: Chemistry 244 (2012) 38–46
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis, spectral properties and photobehaviour of push–pull distyrylbenzene
nitro-derivatives
a
b
a,∗
a
b
b,∗∗
ˇ
ˇ
I. Kikasˇ , B. Carlotti , I. Skoric´ , M. Sindler-Kulyk , U. Mazzucato , A. Spalletti
a Department of Organic Chemistry, Faculty of Chemical Engineering and Technology, University of Zagreb, Maruli´cev trg 19, 10000 Zagreb, Croatia
b Dipartimento di Chimica and Centro di Eccellenza Materiali Innovativi Nanostrutturati (CEMIN), University of Perugia, 06123 Perugia, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Six novel asymmetric 2,5-distyryl-furan, 2,5-distyryl-thiophene and 2,6-distyrylpyridine derivatives,
bearing an electron acceptor (p-nitro) group at one side and an electron donor (p-methoxy or p-
dimethylamino) group at the other side, have been prepared. The experimental absorption properties
have been measured and compared with the computed parameters. Theoretical and experimental results
indicate that one conformational isomer (the compressed one) is largely prevalent in all compounds. The
measured radiative and reactive relaxation properties of these donor/acceptor disubstituted compounds
were also compared with those of the unsubstituted analogues previously investigated. The presence
of the donor/acceptor groups leads to a significant increase of the charge transfer character of both the
ground and the excited states and to strong red shifts of the absorption spectra, an effect that can be
useful for potential applications in material science. The fluorescence/photoisomerization competition
of the furan and thiophene derivatives was found to be rather similar to that of the unsubstituted ana-
logues whereas significant changes were found for the pyridine derivatives where a drastic decrease of
fluorescence, a sizable increase of photoisomerization and a predominance of radiationless deactivation
to the ground state becomes operative.
Received 30 March 2012
Received in revised form 12 June 2012
Accepted 16 June 2012
Available online 3 July 2012
Keywords:
Synthesis of distyrylbenzene analogues
Push–pull distyryl-furan, -thiophene and
-pyridine derivatives
Fluorescence/photoisomerization
competition
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
a strong CT character. Moreover, in the first molecule a peculiar,
very interesting, dependence of the intersystem crossing kinetic
We reported recently the synthesis and photophysical study
of conjugated systems bearing electron-donor/acceptor groups at
the extreme ends, connected by a spacer [1–7]. The design and
study of these push–pull (D-␲-A) systems have attracted our atten-
tion because of their potential application as fluorescent probes,
as dye laser systems and in optoelectronics [8,9], especially for
their hyperpolarizability properties which make them candidates
as components of non linear optical (NLO) materials [10–13].
A positive fluorosolvatochromism was observed for these com-
pounds caused by an increase of the dipole moment under
photoexcitation. The occurrence of intramolecular charge transfer
(ICT) from the locally excited (LE) singlet state to a singlet CT state,
characterized by an increased charge separation was reported. The
singlet CT state was found to be strongly stabilized in polar solvents
becoming in this condition the lowest (often poor) fluorescent state.
Of particular interest was the study of 2-(p-nitrostyryl),5-
styrylfuran and 2-(p-methylstyryl),3-(p-nitrostyryl)benzofuran
[1,2,4,5], where the nitro group and the central ␲-excessive furan
ring are responsible for the introduction of electronic states with
constant (kISC) on the dielectric constant (ε) of the solvent was
evidenced [14]. The kISC value was found very high in non-polar
solvents and sensitive to very small differences of dielectric
constant in the range of the low polarity solvents, reaching a lower
and constant value in more polar solvents.
On the line of these results our effort was directed to design
asymmetric distyrylbenzene analogues with heteroaromatic cen-
tral rings and strong push/pull groups at the ends with the aim to
induce a more efficient ICT.
This paper deals with the synthesis and photobehaviour of
the EE isomers of 2,5-distyryl-furan, 2,5-distyryl-thiophene and
2,6-distyrylpyridine derivatives with a strong electron-acceptor
(nitro) group and an electron-donor substituent (dimethyl-amino
or methoxy) of different strength at the opposite side. The com-
parison of the spectral properties and photobehaviour of the new
synthesized compounds with those of the unsubstituted analogues
previously studied [15–18] was particularly useful and led the role
of the central ring and the side push/pull groups to be highlighted.
2. Experimental
∗
2.1. Synthesis
Corresponding author. Tel.: +385 1 4597241; fax: +385 1 4597250.
∗∗
Corresponding author. Tel.: +39 075 5855575; fax: +39 075 5855598.
ˇ
E-mail addresses: iskoric@fkit.hr (I. Skoric´), faby@unipg.it (A. Spalletti).
The compounds investigated are shown in Scheme 1.
1010-6030/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotochem.2012.06.009

I. Kikasˇ et al. / Journal of Photochemistry and Photobiology A: Chemistry 244 (2012) 38–46
39
6-methyl-2-pyridinecarboxaldehyde, furan-2-carboxaldehyde and
thiophene-2-carboxaldehyde were obtained from a commercial
source. 2-Furyl- and 2-thienyl-triphenylphosphonium bromide
and p-methoxybenzyl- and p-nitrobenzyl-triphenylphosphonium
bromides were synthesized from the corresponding bromides and
triphenylphosphine in benzene solution.
Preparation of NNDSF and NNDST. To a stirred solution of
2-furyl- or 2-thienyl-triphenylphosphonium bromide (3 mmol)
and p-dimethylaminobenzaldehyde (3 mmol), in absolute ethanol
(50 mL) a solution of sodium ethoxide (69 mg, 3 mmol Na in 10 mL
ethanol) was added dropwise (Scheme 2). Stirring was continued
under a stream of nitrogen for 3 h at RT. After removal of the sol-
vent, the residue was worked up with water and toluene. The
toluene extracts were dried and concentrated. The pure cis- and
trans-p-dimethylamino-2-styrylfuran/thiophene were obtained in
the first fractions by column chromatography on silica gel using
petroleum ether/diethyl ether mixture as eluent and subjected
to formylation. Vilsmeier formylation was carried out from p-
dimethylamino-2-styrylfuran/thiophene (1.8 mmol) dissolved in
dry dimethylformamide (1.4 mL). After adjusting the temperature
at ∼10 ◦C, phosphorus oxychloride (2 mmol) was added and the
reaction mixture was allowed gradually to warm up to room tem-
perature and stirred for 24 h. The reaction mixture was decomposed
by the continuous addition (with cooling) of 10% NaOH solution and
the product was extracted with diethyl ether. The ether extracts
were washed with water and then dried with MgSO4. After removal
of the solvent, the crude reaction product of formyl derivatives,
as mixture of cis- and trans-isomers, was used in the next reac-
tion step. To a stirred solution of formyl derivatives (0.4 mmol)
and p-nitrobenzyl-triphenylphosphonium bromide (0.4 mmol) in
absolute ethanol or benzene (40 mL), respectively, sodium ethoxide
(9.0 mg, 0.4 mmol Na dissolved in 5 mL ethanol and evaporated to
obtain powder NaOEt for benzene solution) was added (Scheme 2).
Stirring was continued under a stream of nitrogen for 3 h at RT.
After removal of the solvent, the residue was worked up with
water and toluene and dried with MgSO4. The crude reaction prod-
uct was purified and the mixtures of four isomers of NNDSF and
NNDST were obtained by column chromatography on silica gel
using petroleum ether/diethyl ether mixture as eluent. The geo-
metrical isomers thus obtained were separated and purified by
HPLC.
Scheme
dimethylaminostyryl)furan
styryl)furan (NODSF),
styryl)thiophene (NNDST), EE-2-(p-nitrostyryl)-5-(p-methoxystyryl)thiophene
(NODST), EE-2-(p-nitrostyryl)-5-(p-N,N-dimethylaminostyryl)pyridine (NNDSP),
EE-2-(p-nitrostyryl)-5-(p-methoxystyryl)pyridine (NODSP).
1. Molecular
structures
(NNDSF),
of
EE-2-(p-nitrostyryl)-5-(p-N,N-
EE-2-(p-nitrostyryl)-5-(p-methoxy-
EE-2-(p-nitrostyryl)-5-(p-N,N-dimethylamino-
The 1H and 13C NMR spectra were recorded on a Bruker AV-600
Spectrometer at 300 and 600 MHz and 75 or 150 MHz, respectively.
All NMR spectra were measured in CDCl3 using tetramethylsilane
as reference. High-resolution mass spectra (HRMS) were obtained
on a matrix-assisted laser desorption/ionization time-of-flight
MALDI-TOF/TOF mass spectrometer (4800 Plus MALDI-TOF/TOF
analyzer, Applied Biosystems Inc., Foster City, CA, USA) equipped
with Nd:YAG laser operating at 355 nm with firing rate 200 Hz
in the positive ion reflector mode. 1600 shots per spectrum
were taken with mass range 100–1000 Da, focus mass 500 Da
and delay time 100 ns. Nicotinamide and azithromycin were
used for external mass calibration in positive ion mode. Each
spectrum was internally calibrated, providing measured mass
accuracy within 5 ppm of theoretical mass. The GC–MS analyses
were performed on
a Varian CP-3800 Gas Chromatograph-
Varian Saturn 2200 equipped with FactorFour Capillary Column
VF-5ms, 30 m × 0.25 mm ID; GC operating conditions for all
experiments: column temperature programmed from 110 ◦C to
300 ◦C (6 min isothermal) at a rate of 33 ◦C min−1; carrier gas:
helium; flow rate: 1 mL min−1; injector temperature: 300 ◦C;
volume injected: 5 ␮L. Melting points were obtained using an
Original Kofler Mikroheitztisch apparatus (Reichert, Wien) and
are uncorrected. Silica gel (Merck 0.063–0.2 mm) was used
for chromatographic purifications. Solvents were purified by
distillation. Boiling range of petroleum ether, used for chromato-
graphic separation, was 40–70 ◦C. p-Dimethylaminobenzaldehyde,
Yield for the mixture of four isomers of NNDSF: 50%.
E,E-NNDSF: Rf 0.86 (petroleum ether/diethyl ether 1:0.25); red
solid; m.p. 195 ◦C; 1H NMR (CDCl3, 600 MHz) ıH 8.20 (d, J = 8.8 Hz,
2H, Har), 7.58 (d, J = 8.8 Hz, 2H, Har), 7.40 (d, J = 8.7 Hz, 2H, Har),
7.12 (d, J = 16.0 Hz, 1H, Het), 7.10 (d, J = 16.0 Hz, 1H, Het), 7.00 (d,
J = 16.0 Hz, 1H, Het), 6.71 (d, J = 8.7 Hz, 1H, Har), 6.70 (d, J = 16.0 Hz,
1H, Het), 6.51 (d, J = 3.5 Hz, 1H, Hf), 6.33 (d, J = 3.5 Hz, 1H, Hf), 3.00 (s,
6H, CH3); 13C NMR (150 MHz, CDCl3) ıC 155.4 (s), 151.2 (s), 150.4 (s),
146.3 (s), 144.0 (s), 129.2 (s), 127.7 (2d), 127.5 (d), 126.4 (2d), 125.0
(d), 124.2 (2d), 123.4 (d), 120.2 (d), 114.3 (d), 112.4 (d), 111.7 (d),
POCl3
p-N(CH3)2C6H4CHO
CH2P+Ph3Br-
X = O, S
(H3C)2N
X
X
DMF
NaOEt/EtOH
X = O, S
p-NO2-C6H4-CH2P+Ph3Br-
CHO
(H3C)2N
X
(H3C)2N
X
NaOEt/EtOH
or benzene
NO2
X = O, S
NNDSF: X = O
NNDST: X = S
Scheme 2. Preparation of NNDSF and NNDST.

40
I. Kikasˇ et al. / Journal of Photochemistry and Photobiology A: Chemistry 244 (2012) 38–46
109.6 (d), 40.3 (q); HRMS (TOF ES+) m/z calculated for C22H20N2O3
360.1468; found 360.1470.
3H, OCH3); 13C NMR (75 MHz, CDCl3) ıC 159.7 (s), 154.6 (s), 151.6
(s), 146.4 (s), 143.8 (s), 129.5 (s), 128.3 (s), 128.3 (d), 127.8 (2d),
126.5 (2d), 124.2 (2d), 124.0 (d), 120.1 (d), 114.3 (2d), 114.1 (d),
113.9 (d), 110.7 (d), 55.4 (q); HRMS (TOF ES+) m/z calculated for
C21H17NO4 347.1163; found 347.1158.
Yield for the mixture of four isomers of NNDST: 35%.
E,E-NNDST: Rf 0.62 (petroleum ether/diethyl ether 1:0.18);
brown solid; m.p. 240 ◦C; 1H NMR (CDCl3, 600 MHz) ıH 8.22 (d,
8.8 Hz, 2H, Har), 7.58 (d, J = 8.8 Hz, 2H, Har), 7.40 (d, J = 8.7 Hz, 2H,
Har), 7.36 (d, J = 16.0 Hz, 1H, Het), 7.06 (d, J = 3.6 Hz, 1H, Het), 7.03 (d,
J = 16.0 Hz, 1H, Het), 6.93 (d, J = 3.6 Hz, 1H, Ht), 6.92 (d, J = 15.8 Hz,
1H, Het), 6.74 (d, J = 8.7 Hz, 2H, Har), 3.03 (s, 6H, CH3); 13C NMR
(150 MHz, CDCl3) ıC 154.2 (s), 151.2 (s), 145.9 (s), 139.2 (s), 136.5
(s), 129.7 (d), 129.0 (d), 127.2 (2d), 126.0 (d), 125.9 (2d), 125.0 (d),
124.3 (d), 123.7 (2d), 116.7 (d), 111.9 (2d), 39.8 (q), one singlet is not
seen; HRMS (TOF ES+) m/z calculated for C22H20N2O2S 376.1240;
found 376.1247.
Yield for the mixture of four isomers of NNDST: 30%.
E,E-NODST: Rf 0.27 (petroleum ether/diethyl ether 1:0.25);
orange solid; m.p. 203 ◦C; 1H NMR (CDCl3, 600 MHz) ıH 8.20
(d, J = 8.80 Hz, 2H, Har), 7.56 (d, J = 8.8 Hz, 2H, Har), 7.42 (d,
J = 8.4 Hz, 2H, Har), 7.33 (dd, J = 16.0 Hz, 1H, Het), 7.06 (dd,
J = 16.0 Hz, 1H, Het), 7.05 (d, J = 3.8 Hz, 1H, Ht), 6.95 (d, J = 3.8 Hz,
1H, Ht), 6.91 (d, J = 15.7 Hz, 1H, Het), 6.90 (d, J = 8.7 Hz, 2H,
Har), 6.89 (d, J = 15.7 Hz, 1H, Het), 3.84 (s, 3H, CH3); 13C NMR
(150 MHz, CDCl3) ıC 159.7 (s), 146.6 (s), 144.4 (s), 143.6 (s),
139.9 (s), 129.4 (s), 129.3 (2d), 127.8 (2d), 126.5 (2d), 126.4 (d),
126.3 (d), 125.3 (d), 124.2 (2d), 119.5 (d), 114.3 (2d), 55.4 (q);
HRMS (TOF ES+) m/z calculated for C21H17NO3S 363.0934; found
363.0930.
Preparation of NNDSP. To a stirred solution of p-nitrobenzyl-tri-
phenylphosphonium bromide (4 mmol) and 6-methyl-2-
pyridinecarboxaldehyde (4 mmol) in absolute ethanol (40 mL)
a solution of sodium ethoxide (92 mg, 4 mmol in 10 mL abs
ethanol) was added dropwise and stirred for 3 h at RT (Scheme 3).
After removal of solvent an extraction with toluene was carried
out. The extract was dried and concentrated. The crude reaction
product was purified and the mixture of cis- and trans-isomers
of styrylpyridine derivative was obtained by column chromatog-
raphy on silica gel using petroleum ether/diethyl ether mixture
as eluent. A solution of styrylpyridine derivative (1.1 mmol), N-
bromosuccinimide (1.8 mmol) and azobisisobutyronitrile (AIBN)
(30 mg) in CCl4 (9 mL) was stirred at RT and then irradiated
with a halogen lamp under reflux for 12 h. The reaction mix-
ture was filtered to remove succinimide and evaporated. The
residue was dissolved in benzene and triphenylphosphine (PPh3)
in benzene was added. After stirring over night at RT the pre-
cipitate was filtered off and used in next step after drying. To
a stirred solution of obtained phosphonium salt (0.45 mmol)
and p-dimethylaminobenzaldehyde (0.45 mmol) in benzene
sodium ethoxide was added (10.3 mg, 0.45 mmol Na dissolved
in 5 mL ethanol and evaporated to obtain powder NaOEt for
benzene solution) (Scheme 3). Stirring was continued for 4 h
at RT. After removal of the solvent, the residue was worked
up with water and toluene and dried with MgSO4. The crude
reaction product was chromatographed and the mixtures of
isomers of NNDSP were obtained on silica gel column using
petroleum ether/diethyl ether mixture as eluent. The geomet-
rical isomers thus obtained were separated and purified by
HPLC.
Preparation of NODSP. A solution of sodium ethoxide was
added dropwise (92 mg, 4 mmol Na in 10 mL ethanol) to
a
stirred solution of 6-methyl-2-pyridinecarboxaldehyde (4 mmol)
and p-methoxybenzyl–triphenylphosphonium bromide (4 mmol)
in absolute ethanol (50 mL) (Scheme 5). Stirring was contin-
ued under the stream of nitrogen for 1 day at RT. After removal
of the solvent, water was added to the residue, extracted
with benzene and the benzene extract dried with MgSO4. The
product was purified by column chromatography on silica gel
using petroleum ether/diethyl ether mixture as eluent. A sus-
pension of the obtained mixture of cis and trans isomers of
styrylpyridine derivative (3.3 mmol) in acetic anhydride (3 mL)
and p-nitrobenzaldehyde (3.3 mmol) was heated at 160 ◦C for 10 h
(Scheme 5). The solution is then concentrated to a small volume
at reduced pressure giving a crystalline solid. After filtration of
solid reaction mixture the product NODSP was purified by col-
umn chromatography on silica gel using diethyl ether as eluent. The
geometrical isomers thus obtained were separated and purified by
HPLC.
Yield for the mixture of four isomers of NODSP: 50%.
E,E-NODSP: Rf 0.10 (petroleum ether/diethyl ether 1:0.25);
yellow solid; m.p. 171 ◦C; 1H NMR (CDCl3, 300 MHz) ıH 8.25
(d, J = 8.8 Hz, 2H, Har), 7.78 (d, J = 16.1 Hz, 1H, Het), 7.73 (d,
J = 8.8 Hz, 2H, Har), 7.68 (d, J = 16.1 Hz, 1H, Het), 7.67 (t, J = 7.7 Hz,
1H, Hp), 7.56 (d, J = 8.8 Hz, 2H, Har), 7.32 (d, J = 16.1 Hz, 1H,
Het), 7.30 (dd, J = 7.7; 0.7 Hz, 1H, Hp), 7.26 (dd, J = 7.7; 0.7 Hz,
1H, Hp), 7.09 (d, J = 16.1 Hz, 1H, Het), 6.93 (d, J = 8.8 Hz, 2H,
Har), 3.85 (s, 3H, OCH3); 13C NMR (75 MHz, CDCl3) ıC 160.0
(s), 156.1 (s), 154.0 (s), 147.2 (s), 143.4 (s), 137.1 (d), 132.9
(d), 132.6 (d), 130.2 (d), 129.4 (s), 128.5 (2d), 127.5 (2d),
125.8 (d), 124.1 (2d), 121.2 (d), 121.0 (d), 114.2 (2d), 55.4 (q);
HRMS (TOF ES+) m/z calculated for C22H18N2O3 359.1389; found
359.1386.
Yield for the mixture of four isomers of NNDSP: 15%.
E,E-NNDSP: Rf 0.08 (petroleum ether/diethyl ether 1:1); brown
solid; 1H NMR (CDCl3, 300 MHz) ıH 8.27 (d, J = 8.7 Hz, 2H, Har), 7.84
(d, J = 16.0 Hz, 1H, Het), 7.81 (d, J = 16.3 Hz, 1H, Het), 7.79–7.75 (m,
3H), 7.67 (d, J = 16.0 Hz, 1H, Het), 7.54 (d, J = 8.6 Hz, 2H, Har), 7.39
(d, J = 7.8 Hz, 2H, Hp), 7.38 (d, J = 16.0 Hz, 1H, Het), 6.75 (d, J = 8.7 Hz,
2H, Har), 3.04 (s, 3H, CH3); HRMS (TOF ES+) m/z calculated for
C23H21N3O2 371.1634; found 371.1637.
Preparation of NODSF and NODST. The mixtures of four iso-
mers of NODSF and NODST were obtained after the third reaction
step (Scheme 4) by column chromatography on silica gel using
petroleum ether/diethyl ether mixture as eluent. The geometrical
isomers thus obtained were separated and purified by HPLC. The
complete procedure for the preparation of NODSF and NODST can
be found in Supporting material.
2.2. Spectral, photophysical and photochemical measurements
Absorption spectra were recorded on a Perkin-Elmer Lambda
800 and a Cary 4E Varian spectrophotometers. The experimen-
ꢀ
tal oscillator strength was derived by f = (4.39 × 10−9 ε(ꢀ¯) dꢀ¯)/n
∼
[19], considering the refraction index n 1. Fluorescence emission
=
Yield for the mixture of four isomers of NODSF: 40%.
spectra were measured with a Fluorolog-2 (Spex, F112AI) fluorime-
ter. Dilute solutions (absorbance smaller than 0.1 at the excitation
wavelength, ꢁexc) were used for fluorimetric steady-state measure-
ments. Fluorescence quantum yields (ꢂF, experimental error of ca.
10%, from three independent experiments) were determined by
use of tetracene in cyclohexane (CH) (ꢂF = 0.17 in air-equilibrated
solvent) [20] and 9,10-diphenylanthracene in CH (ꢂF = 0.90 in
E,E-NODSF: Rf 0.52 (petroleum ether/diethyl ether 1:0.43); red
solid; m.p. 169 ◦C; 1H NMR (CDCl3, 300 MHz) ıH 8.20 (d, J = 8.8 Hz,
2H, Har), 7.59 (d, J = 8.8 Hz, 2H, Har), 7.45 (d, J = 8.7 Hz, 2H, Har),
7.13 (d, J = 16.3 Hz, 1H, Het), 7.12 (d, J = 16.2 Hz, 1H, Het), 7.00 (d,
J = 16.3 Hz, 1H, Het), 6.91 (d, J = 8.7 Hz, 2H, Har), 6.76 (d, J = 16.2 Hz,
1H, Het), 6.51 (d, J = 3.5 Hz, 1H, Hf), 6.38 (d, J = 3.5 Hz, 1H, Hf), 3.84 (s,

I. Kikasˇ et al. / Journal of Photochemistry and Photobiology A: Chemistry 244 (2012) 38–46
41
p-NO2-C6H4-CH2P+Ph3Br-
NBS/AIBN
N
CH3
NaOEt/EtOH
PPh3/benzene
H3C
N
CHO
O2N
p-N(CH3)2C6H4CHO
N
CH2P+Ph3Br-
NaOEt/EtOH
N
O2N
N(CH3)2
O2N
NNDSP
Scheme 3. Preparation of NNDSP.
p-NO2-C6H4-CH2P+Ph3Br-
POCl3
DMF
CHO
CHO
O2N
X
X
NaOEt/EtOH
X = O, S
X = O, S
p-OCH3-C6H4-CH2P+Ph3Br-
O2N
O2N
X
X
X = O, S
OCH3
NaOEt/EtOH
or benzene
NODSF: X = O
NODST: X = S
Scheme 4. Preparation of NODST.
p-OCH3-C6H4-CH2P+Ph3Br-
NaOEt/EtOH
p-NO2C6H4CHO
/Ac2O
H3C
N
CHO
H3C
N
OCH3
N
O2N
OCH3
NODSP
Scheme 5. Preparation of NODSP.
de-aerated solvent) [21] as fluorimetric standards. Fluorescence
lifetimes were measured by an Edimburgh Instrument 199S spec-
trofluorimeter, using the single photon counting method with a
LED source centred at 461 nm, coupled with an interference filter
centred at 460 nm in the excitation line and a cut-off in emis-
sion at 500 nm (time resolution of the experimental set-up ca.
0.5 ns).
at below 8% to avoid the competition from the back photore-
action. The photoreaction quantum yields (ꢂISO) are averages of
at least two independent experiments with mean deviations of
15%. A larger uncertainty should be ascribed to the value of
NNDSP where the sample available allowed only one measure-
ment.
All the measurements were performed in deaerated solutions
by bubbling with nitrogen.
For photochemical measurements, a 500 W mercury lamp
coupled with an interferential filter at ꢁexc = 436 nm was used
for NODSF and NODST; a xenon lamp coupled with appropri-
ate interferential filters was used for NNDSF, NNDST, NODSP
and NNDSP. Potassium ferrioxalate in 0.1 N sulphuric acid was
used as actinometer. The photoreaction (solute concentration
2.3. Computational details
∼10−4 M) was monitored by HPLC using
a Waters appara-
Quantum-mechanical calculations were carried out using the
HyperChem computational package (version 7.5). Total energies
and dipole moments were obtained for geometries optimized by HF
ab initio (3-21-G level) method. The computed transition energies
and oscillator strengths were obtained by ZINDO/S at the optimized
geometries, the configuration interaction including 64 (8 × 8) sin-
gle excited configurations. The value of the excited state dipole
moment (ꢃ*) was estimated by the “next lowest” function in the
ZINDO/S method.
tus equipped with a Prontosil 200-3-C30 (4.6 mm × 250 mm;
3 ␮m) column at 40 ◦C and an UV detector. Water/acetonitrile
mixtures (10%/90% or 20%/80%) were used as eluents during
the first minutes of the separation procedure replaced then
by 100% acetonitrile. The monitoring wavelength was at the
isosbestic point between the EE isomer and the photoprod-
uct absorption, otherwise corrections for different absorption
coefficients were introduced. The conversion percentage was held

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I. Kikasˇ et al. / Journal of Photochemistry and Photobiology A: Chemistry 244 (2012) 38–46
Table 1
Spectral properties (absorption maxima wavelength, ꢁabs, and coefficient, εmax, fluorescence maxima wavelength, ꢁF, emission vibronic progression, ꢄꢀ¯F and Stokes shift,
ꢄꢀ¯S) of the investigated compounds in CH (those of the unsubstituted analogues are reported for comparison).a
Compound
ꢁabs (nm)
εmax (M−1 cm−1
)
ꢁF (nm)
ꢄꢀ¯F (cm−1
)
ꢄꢀ¯S (cm−1
)
NNDSF
NNDST
NNDSP
NODSF
NODST
NODSP
465, 505sh
35,550
33,750
536, 577, 620sh
536, 576, 620sh
472, 500
486, 519, 555
486, 518, 557
–
1330
1300
1190
1310
1270
–
4170
4380
7620
3410
3630
–
460, 505sh
336sh, 362
420sh, 441, 470sh
416sh, 436, 463sh
323, 365
29,750
27,920
41,940, 23,350
2,5-DSFb
2,5-DSTc
2,6-DSPd
363sh, 382, 400sh
374sh, 392, 415sh
291.5, 306sh
44,400
41,000
44,000
416, 442, 470
1400
1300
1390
3550
3820
6290
435, 461, 489sh
367sh, 379, 400sh
a
The underlined values and εmax refer to the main maxima.
From Ref. [17].
From Ref. [15].
From Ref. [18].
b
c
d
the cis peak predicted by calculations as peculiar of the compressed
rotamer [22].
NNDSF
NODSF
The dimethylamino group causes a further red shift of the spec-
tra if compared with the analogous methoxy-derivatives, the first
absorption band becoming a bell-shaped curve (see Fig. 1). In
NODSP, the double absorption band, characteristic of systems with
ethenic bridges in the ortho position with respect to the nitrogen
heteroatom of pyridine [18,23], is particularly evident.
The emission spectra of furan and thiophene-derivatives are
NNDST
NODST
well structured with a vibrational progression around 1300 cm−1
,
characteristic of ethene derivatives. The NNDSP is poorly fluores-
cent, while its methoxy analogue does not emit at all. Comparison
of the spectral properties of these push–pull systems bearing elec-
tron donor/acceptor groups with the unsubstituted (Table 1) and
monosubstituted [4] analogues shows a significant red shift, rea-
sonably assignable to the CT character of the first transitions (see
below).
In principle, these flexible molecules could exist at room tem-
perature as an equilibrium mixture of different conformers [24,25],
originating by rotation around the quasi-single bonds between the
central ring and the ethenic bridges (see the conformational equi-
librium of EE-NNDSF in Scheme 6, as an example). Table 2 reports
the dipole moments of the ground and excited states, the total
energy and the spectral properties computed for the compounds
investigated. The calculations were carried out for all the four
rotamers of NNDSF, as an example, and for the pyridyl-derivatives,
where a conformational equilibrium was expected on the basis of
the comparison with the unsubstituted analogue [18]. In the other
cases, only the results for elongated and compressed species are
reported.
NNDSP
NODSP
300
400
500
λ (nm)
600
700
800
Fig. 1. Normalized absorption and emission spectra of the investigated compounds
in CH.
In the furan and thiophene analogues, quantum-mechanics
calculations predicted a conformational equilibrium completely
shifted towards the compressed most stable conformer (see Table 2
and Scheme 6), as in the case of the unsubstituted 2,5-DSF and 2,5-
DST. This finding is in agreement with the absence of a wavelength
effect on the excitation and emission spectra and with the observed
mono-exponential decay (see below, Table 3).
Also for the pyridine-derivatives the compressed species was
found to be the most stable conformer, the other rotamers not
contributing to the spectral and photophysical properties owing
to their total energy (Etot in Table 2) higher (>2 kcal/mol) than
that of the compressed one. This behaviour is contrary to what
was reported for the unsubstituted 2,6-DSP, where at least two
conformers were predicted to exist in equilibrium at room temper-
ature and the photophysical and photochemical study allowed the
properties of two different species to be separated [18]. Unluckily,
the very scarce emission yield of these pyridine derivatives did not
allow the usual excitation wavelength effect on the conformational
3. Results and discussion
3.1. Spectral properties
The absorption and emission spectra of the investigated com-
pounds in CH are shown in Fig. 1 and the main spectral properties
are collected in Table 1 with those of the unsubstituted ana-
logues (2,5-distyrylfuran, 2,5-DSF, 2,5-distyrylthiophene, 2,5-DST,
and 2,6-distyrylpyridine, 2,6-DSP) for comparison purposes.
The absorption and emission spectra of the compounds having
the furan and thiophene as a central ring, are shifted to the red
with respect to the pyridine-derivatives, in agreement with their
highly conjugated structures. As previously reported for analogous
compounds, the presence of poorly aromatic furan or thiophene is
responsible for the spectral behaviour very similar to that of ␣,␻-
diphenyloctatetraenes [17,22] and for the appearance of a second
absorption band at higher energy, in the 300–400 nm range. In the
case of the unsubstituted analogues, the latter band was assigned to

I. Kikasˇ et al. / Journal of Photochemistry and Photobiology A: Chemistry 244 (2012) 38–46
43
Scheme 6. Conformational equilibrium of EE-NNDSF.
equilibria [24,25] to be investigated. This behaviour, contrary to
that of 2,6-DSP, suggests that the push–pull substituents stabilize
preferentially the more rigid and planar compressed rotamer.
It has to be noted that interactions between the heteroatom
of the central ring and the ethenic hydrogens can play a role
in the conformational equilibrium of these systems, at least in
the pyridine-derivatives [18,26]. The NMR spectrum carried out
in deuterated chloroform containing a small water percentage
shows four doublets at ı = 7.78, 7.68, 7.32 and 7.09 ppm with
coupling constants typical of ethenic hydrogens in a trans config-
uration (J = 16.1 Hz), assigned to H1, H3, H2 and H4, respectively
(see Scheme 7). In deuterated benzene the resonance of H1 and H3
occurs at lower frequencies (ı = 7.99 and 7.76 ppm, respectively),
while the signal of H2 and H4 shifts to higher frequencies (ı = 7.2 and
7.0 ppm, respectively), as expected because of the shielding effect
of the benzene ring. The shift to lower frequencies observed for H1
and H3 (0.21 and 0.08 ppm, respectively) in the non-protic solvent,
even if smaller than that found for the unsubstituted analogue (0.42
Table 2
Calculated spectral properties (transition wavelength, ꢁ, and oscillator strength, f), total energy (Etot) and dipole moment of the ground (ꢃ) and excited (ꢃ*) states for the
rotamers of the investigated compounds. The experimental absorption maximum (ꢁexp) and oscillator strength (fexp) are reported for comparison.
Compound
Conformer
ꢃ (D)
ꢃ* (D)
Etot (kcal/mol)
ꢁ (nm)
f
ꢁexp (nm)
fexp
NNDSF
Elongated(D)
Semicomp(C)
Semicomp(B)
Compressed(A)
10.1
9.86
9.23
9.01
24.89
24.37
23.96
22.96
−735,098.750
−735,101.313
−735,101.125
−735,103.813
441
352
289
0.94
0.45
0.61
S1
S2
S6
467
364,351
0.70
0.51
NODSF
NNDST
NODST
NNDSP
Elongated
Compressed
9.24
6.26
22.58
18.9
−723,117.688
−723,122.750
432
345
282
1.00
0.37
0.53
S1
S2
S6
441
327
0.61
0.22
Elongated
Compressed
10.61
10.14
24.91
23.73
−936,601.688
−936,605.563
428
347
283
1.32
0.46
0.35
S1
S2
S6
460
372
0.90
0.42
Elongated
Compressed
9.04
7.24
21.77
19.15
−924,620.689
−924,624.563
429
339
285
0.96
0.44
0.48
S1
S2
S6
435
344
270,245
0.57
0.18
Elongated
8.56
9.31
8.44
8.41
23.87
25.16
25.35
21.53
−746,362.500
−746,368.438
−746,368.189
−746,370.813
Semicomp(C)
Semicomp(B)
Compressed
374
323
313
286
0.83
0.49
0.33
0.59
S1
S2
S4
S7
362
340
240
NODSP
Elongated
9.55
9.57
5.89
6.03
24.0
−734,384.688
−734,387.500
−734,387.438
−734,389.938
Semicomp(C)
Semicomp(B)
Compressed
24.08
19.85
20.11
372
319
276
259
0.86
0.35
0.77
0.25
S1
S2
S7
S8
363
323
220
0.28
1.23
0.38

44
I. Kikasˇ et al. / Journal of Photochemistry and Photobiology A: Chemistry 244 (2012) 38–46
Table 3
Fluorescence properties and trans → cis photoisomerization quantum yields (ꢂISO) of the investigated compounds in CH. The parameters for the unsubstituted analogues are
reported for comparison.
Compound
ꢂF
ꢅF (ns)
kF (108 s−1
)
ꢂISO
NNDSF
NNDST
NNDSP
NODSF
NODST
NODSP
0.80
0.74
0.085
0.77
0.30
−
2.6
1.8
<0.5
2.2
0.87
−
3.1
4.2
>1.7
3.5
3.5
−
0.0025
0.0074
(∼0.20)a
0.028
0.20
0.26 (EZ), 0.072 (ZE)
2,5-DSFb
2,5-DSTc
2,6-DSPd
0.68
0.30
0.72/0.24
2.9
0.85
3.5/1.5
2.3
3.5
2.1/1.6
0.0003
0.03
0.075/0.14
a
The value in parentheses accounts for its larger uncertainty (see Section 2).
In toluene, from Ref. [17].
In toluene, from Ref. [15].
b
c
d
In MCH/3MP, from Ref. [18] (the two fluorescence and photoisomerization parameters refer to the two main conformers).
H4
H2
N
H3 H1
OCH3
NO2
Scheme 7. H-type bonds between the ethenic hydrogens and the central nitrogen
atom in the compressed rotamer of NODSP.
for 2,6-DSP on going from methanol to benzene [18]) indicates that
stabilizing intramolecular N· · ·H interactions in the latter solvent
favour the compressed rotamer whereas in more protic solvent
such stabilization is broken by intermolecular interactions with the
medium.
The computed dipole moments, already high for the ground
state, increase strongly under excitation indicating a further signif-
icant increase of the CT character. This was also confirmed by the
computed charge distribution of the main configurations involved
in the S0 → S1 transition of the all-trans isomers (the case of NODSF
is shown in Fig. 2 as an example, while the others can be found
in Supplementary Data) that shows a net shift of the charge den-
sity towards the double bond adjacent to the p-nitrophenyl moiety
under excitation (see also next section).
3.2. Photobehaviour
The fluorescence quantum yields (ꢂF) and lifetimes (ꢅF) together
with the emission kinetic constant (kF) and the geometrical pho-
toisomerization quantum yield (ꢂISO) in CH are collected in Table 3.
High fluorescence quantum yields were found in the case
of dimethylamino-substituted furan and thiophene derivatives
while they selectively photoisomerize towards the EZ isomer (see
Scheme 8) with very low yields. The fluorescence rate constants of
the order of 108 s−1 point to an allowed transition for the S1 → S0
emission.
Scheme 8. Geometrical isomers formed by irradiation of EE-NNDSF.
cases [27]. However, a contribution of ISC followed by rotation in
the triplet state cannot be ruled out at this stage.
The selective rotation of the double bond adjacent to the nitro-
phenyl group is hypothesized on the basis of the calculated spectra
of the EZ and ZE geometric isomers and confirmed by NMR mea-
surements [28]. In Fig. 3 the calculated spectra (vertical bars) for
the EE, EZ and ZE isomers of NODSF, as an example, are compared
with the experimental spectra of the reagent (EE) and the photo-
product detected by HPLC. This comparison led to the assignment
of the main photoproduct to the EZ structure.
The preferential photoisomerization of the double bond adja-
cent to the nitrophenyl group is in agreement with the localization
of the excitation energy in this moiety of the molecule as shown
in Fig. 2 for NODSF, by the calculated highest occupied and lowest
unoccupied molecular orbitals.
The replacement of the dimethylamino by the methoxy group
in the compounds with 5-membered central groups causes an
increase of reactivity of roughly one order of magnitude that leads
to a significant value for ꢂEE → EZ and an important decrease of ꢂF
and ꢅF in the case of NODST. The coupling between emission and
photoreaction could suggest a singlet diabatic mechanism for the
photoreaction that involves S1 → S0 internal conversion (IC) at the
perpendicular configuration (P, at about 90◦) and relaxation to the
1
1
ground-state isomers, 1E* → P* → P → ˛1Z + (1 − ˛)1E, where the
partitioning factor ˛ towards Z and E, is assumed to be ∼0.5 in most

I. Kikasˇ et al. / Journal of Photochemistry and Photobiology A: Chemistry 244 (2012) 38–46
45
Fig. 2. Molecular orbitals of NODSF as derived by ZINDO/S method. The HOMO-LUMO configuration contributes to the main electronic transition with the highest coefficient
(−0.566225).
The computed formation enthalpies indicate that only the most
stable compressed A rotamer is present in solution at room tem-
perature, even if these systems can exist, in principle, as a dynamic
equilibrium of different conformers.
The introduction of strong electron donor/acceptor groups in
conjugative (para) position of the side phenyl rings, leads to a strong
red shift of the absorption and emission spectra, mainly in the case
of dimethylamino-derivatives, indicating an increased CT charac-
ter of the excited singlet states, making these systems of potential
interest as active materials in organic solar cells.
The different nature of the first excited singlet state of these
compounds (also underlined by the large values of their computed
dipole moments) with respect to the unsubstituted analogues, does
not affect the emissive properties in the case of thiophene and furan
derivatives, where ꢂF remains high, at least in a non polar solvent.
On the other hand, in the pyridine-derivatives, the absorption
spectral shape does not change too much if compared with that
of 2,6-DSP but the photobehaviour is quite different with a dras-
tic reduction of fluorescence and an increase of the non-radiative
deactivation pathways.
NODSF
EE
photoproduct
1.0
0.5
0.0
f
250
300
350
400
450
500
550
600
(nm)
Fig. 3. Absorption spectrum of EE-NODSF and its photoproduct as derived by HPLC
diode-array instrument. The vertical bars refer to the calculated transitions for the
most stable rotamer of EE (full line), EZ (dashed) and ZE (dotted) isomers.
Acknowledgements
In the case of NODSP, internal rotation of the double bond adja-
cent to the methoxy-phenyl group, that leads to the ZE isomer, was
also observed, but with a lower yield (Table 3).
This work was supported by grants from the Ministry of Sci-
ence, Education and Sports of the Republic of Croatia (Grant No.:
125-0982933-2926) and Ministero per l’Università e la Ricerca Sci-
entifica e Tecnologica (Rome, Italy), the University of Perugia (PRIN
2008-8NTBKR) and the Fondazione Cassa di Risparmio di Perugia.
The authors thank Mr. D. Pannacci for his technical assistance in
HPLC measurements.
The excited state relaxation yields of the compounds with a cen-
tral furan group do not show important changes with respect to
the unsubstituted [17] and mono(nitro)-substituted [4] analogues
previously investigated (high emission and quite low isomeriza-
tion yields). The sizable decrease of fluorescence and increase of
reactivity of the thiophene compared to the furan derivatives, pre-
viously reported and probably due to ISC induced by the sulphur
heteroatom [15], was here maintained for the methoxy-substituted
NODST only, whereas the dimethylamino-substituted NNDST dis-
plays a behaviour similar to the furan analogue.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.jphotochem.
2012.06.009.
The compounds with the central pyridine ring, despite the
scarce amount of the sample of NNDSP available, that limited the
measurements, showed the strongest changes in the relaxation
properties. In fact, while the unsubstituted compound, 2,6-DSP,
where two rotamers are present in solution, shows prevalent emis-
sive and modest reactive decays [18], the largely prevalent rotamer
of the di-substituted compounds here investigated shows a strong
decrease of fluorescence accompanied by a significant increase of
photoreactivity.
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