Synthesis, spectroscopic characterization and photophysics of new functionalized 2,3-distyrylfurans: Substituent and solvent effects on their photobehavior

Article abstract of DOI:10.1016/j.jphotochem.2010.02.013

Two new groups of p-chloro-, p-methoxy- and p-nitro-substituted derivatives of the trans,trans-isomers of 2,3-distyrylfuran have been synthesized by a sequence of reactions according to known methods. These new conjugated heterocyclic compounds along with the parent one and its methyl-derivative have been characterized spectrally, photophysically and photochemically. These compounds, except for the nitro-derivatives, display high fluorescence quantum yields in the non-polar n-hexane (Φ_F=0.76-0.98). In ethanol of high polarity, the lifetime of their singlet excited state increases from 1.31-2.59ns to 2.01-4.77ns, significantly enhancing the quantum yield of their photochemical reaction from 0.02-0.24 to 0.25-0.46, and diminishing their fluorescence efficiency (Φ_F=0.26-0.64). Deviating from the other compounds studied, the quantum yields for both the fluorescence and the photolysis of the nitro-derivatives are rather low (Φ_F<10^-4, Φ=0.025-0.042 in ethanol), probably due to an efficient intersystem crossing leading to a non-reactive triplet state. Accordingly, their singlet-state lifetime is also relatively short (τ_F<0.3ns).

Full text of DOI:10.1016/j.jphotochem.2010.02.013

Journal of Photochemistry and Photobiology A: Chemistry 211 (2010) 152–161
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis, spectroscopic characterization and photophysics of new functionalized
2,3-distyrylfurans: Substituent and solvent effects on their photobehavior
a,∗
a
b
a
b,∗∗
ˇ
ˇ
Irena Skoric´ , Ilijana Kikasˇ , Margit Kovács , Marija Sindler-Kulyk , Ottó Horváth
a Department of Organic Chemistry, Faculty of Chemical Engineering and Technology, University of Zagreb, Maruli´cev trg 19, 10000 Zagreb, Croatia
b Department of General and Inorganic Chemistry, Institute of Chemistry, Faculty of Engineering, University of Pannonia, P.O.B. 158, Veszprém H-8201, Hungary
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new groups of p-chloro-, p-methoxy- and p-nitro-substituted derivatives of the trans,trans-isomers
of 2,3-distyrylfuran have been synthesized by a sequence of reactions according to known methods.
These new conjugated heterocyclic compounds along with the parent one and its methyl-derivative have
been characterized spectrally, photophysically and photochemically. These compounds, except for the
nitro-derivatives, display high fluorescence quantum yields in the non-polar n-hexane (˚F = 0.76–0.98).
In ethanol of high polarity, the lifetime of their singlet excited state increases from 1.31–2.59 ns to
2.01–4.77 ns, significantly enhancing the quantum yield of their photochemical reaction from 0.02–0.24
to 0.25–0.46, and diminishing their fluorescence efficiency (˚F = 0.26–0.64). Deviating from the other
compounds studied, the quantum yields for both the fluorescence and the photolysis of the nitro-
derivatives are rather low (˚F < 10−4, ˚ = 0.025–0.042 in ethanol), probably due to an efficient intersystem
crossing leading to a non-reactive triplet state. Accordingly, their singlet-state lifetime is also relatively
short (ꢀF < 0.3 ns).
Received 14 January 2010
Received in revised form 16 February 2010
Accepted 20 February 2010
Available online 6 March 2010
Keywords:
Distyrylfuran
Fluorescence
Oxygen heterocycles
Photophysics
Synthesis
Solvent effect
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
furan derivatives proved to be applicable in electrophotographic
photoreceptors as charge transporting agents [24–26]. The spe-
Numerous types of heteropolycyclic compounds with conju-
gated bond structure play important roles in both natural and
artificial systems due to their peculiar photochemical and pho-
tophysical properties [1,2]. While these kinds of molecules with
nitrogen heterocycles such as porphyrins and diimines are involved
in visible light-driven photoinduced processes predominantly as
metal complexes [3–10], those with oxygen heterocycles undergo
various photochemical reactions upon UV irradiation [11–15].
Furan derivatives are of great importance due to their role in
organic chemistry as building blocks in various synthetic routes
and because they are involved in numerous natural compounds
[16,17]. Unsaturated furan and benzofuran derivatives contain-
ing furan rings connected through vinylene links are the starting
materials for photochemical reactions [18–21]. These types of com-
pounds offer potential applications as fluorescent probes in biology
and medicine, in environmental and analytical sciences as well as
in optoelectronics [22,23]. Besides, some 2,3-distyryl-substituted
cial photoinduced properties of these molecules have drawn our
attention to their design, synthesis and study.
Recently, synthesis, photophysical and photochemical study of
some new furan and benzofuran derivatives have been realized
[21,27–30]. In the case of 2,3-distyryl benzofurans 1 (Scheme 1),
hexatriene systems with the central double bond incorporated into
the furan ring, both the characteristic absorption and the emission
bands proved to be significantly influenced by the substituent on
the phenyl group connected to the furan ring through vinylene links
[28]. Except for the nitro-substituted compound, they displayed
efficient fluorescence with rather short lifetimes. However, sol-
vent polarity significantly affected the photophysical properties of
the nitro-derivative, while the corresponding behavior of the other
compounds studied was slightly or moderately solvent-dependent.
UV irradiation of these molecules did not result in trans–cis iso-
merization nor an intramolecular cycloaddition reaction. Instead,
intermediary formation of polymeric cyclobutane structures was
suggested on the basis of stilbene-like end-products. Conformation
study of these benzofuran derivatives indicated that the conforma-
tions are not planar and those rotational isomers are energetically
preferred in which the distance of the two vinyl links, and thus
that of the phenyl groups, are larger [30]. This result confirms the
lack or very low efficiency of photoinduced trans–cis isomerization
and intramolecular cycloaddition reactions. Very similar photo-
∗
Corresponding author. Tel.: +385 1 4597 241; fax: +385 1 4597 250.
Corresponding author. Tel.: +36 88 624 159; fax: +36 88 624 548.
E-mail addresses: iskoric@fkit.hr (I. Skoric´), ilijana.kikas@gmail.com (I. Kikasˇ),
∗∗
ˇ
makovacs@almos.uni-pannon.hu (M. Kovács), marija.sindler@fkit.hr
ˇ
(M. Sindler-Kulyk), otto@vegic.uni-pannon.hu (O. Horváth).
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.02.013

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I. Skori´c et al. / Journal of Photochemistry and Photobiology A: Chemistry 211 (2010) 152–161
153
Scheme 1.
chemical behavior was observed for 2,3-distyrylfuran derivatives
(2 (R = H), 3 (R = CH3)) [21].
tion served as a light source. All the new compounds were identified
and proved by GC–MS system getting chromatograms and mass
spectra. 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 i.d.; GC operating condi-
tions 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 Orig-
inal Kofler Mikroheitztisch apparatus (Reichert, Wien) and are
uncorrected. Elemental analyses were carried out on PerkinElmer,
Series II, CHNS Analyzer 2400. Silica gel (Merck 0.063–0.2 mm)
was used for chromatographic purifications. Thin-layer chromatog-
raphy (TLC) was performed on Merck precoated silica gel 60
F254 plates. Solvents were purified by distillation. Boiling range
of petroleum ether, used for chromatographic separation, was
40–70 ◦C. Furan-3-carboxaldehyde was obtained from a commer-
cial source (Aldrich). Benzyltriphenylphosphonium bromide, and
p-chloro-, p-methoxy- and p-nitrobenzyltriphenylphosphonium
bromides were synthesized from the corresponding bromides and
triphenylphosphine in benzene solution.
Detailed photophysical and conformation studies were also
made on unsubstituted 2,3- and 2,5-di(phenylethenyl)furan
derivatives [27]. Similarly to the benzofuran derivatives 1, for 2,3-
distyrylfuran derivative 2 (R = H) the non-planar conformations
are energetically favorable, and in the most stable rotamer the
distance of the phenyl groups is the longest possible. The latter
theoretical results are confirmed by the X-ray crystal structure
of methyl-substituted 2,3-distyrylfuran derivative 3 (R = CH3) [20].
However, for 2,5-distyrylfurans the planar conformations are pre-
ferred, and in the most stable rotamer the distance of the phenyl
groups is the shortest possible. Since irradiation of these organic
fluorophores resulted in a photoisomerization equilibrium totally
shifted towards the trans,trans-isomers of these distyrylfuran com-
pounds, only these isomers were studied from the viewpoints of
conformation and photoinduced behavior [27]. Their absorption
and emission spectra display a slight solvent-dependence, while
their fluorescence lifetimes and quantum yields are significantly
affected by the solvent polarity–non-polar solvents increase these
parameters.
Deviating from the corresponding benzofuran compounds 1, the
substituted derivatives 2 and 3 (R =/ H, CH3) have not been pre-
pared yet, although the study of the photoinduced properties of
these fluorophores and the comparison with those of the previous
molecules may reveal general tendencies and correlations of their
characteristics, promoting their potential applicability in biological
and analytical sciences.
Thus, as a natural continuation of our studies regarding 2,3-
distyrylbenzofurans 1 and unsubstituted 2,3-distyrylfuran 2 and/or
3 (R = H), in this work we publish the synthesis of two new groups of
p-chloro-, p-methoxy- and p-nitro-substituted derivatives 2 and 3
(having a substituent in the p-position on one of the benzene rings),
their spectral characterization, and the detailed investigation of
the photophysical behavior of these compounds. For comparison,
the corresponding measurements were also carried out with the
unsubstituted compound 2 (R = H) and the p-methyl-substituted
derivative 3 (R = CH3) [21].
2.2. Preparation of 2a–c and 3a–c
The new functionalized 2,3-distyrylfurans 2a–c and 3a–c were
prepared via three-step synthesis according to known methods
(Schemes 2 and 3). In the first step by Wittig reaction from
furan-3-carboxaldehyde and corresponding aryltriphenylphos-
phonium bromides (R = H, Cl, OCH3, NO2), the 3-styryl derivatives
4 [20] and 6a–c were synthesized, and formylated using N,N-
dimethylformamide and POCl3 producing the corresponding
formyl derivatives 5 and 7a–c. These products were the starting
compounds for the second Wittig reaction with the other respective
triphenylphosphonium salts resulting in 2,3-divinylfuran deriva-
tives 2a–c and 3a–c as mixtures of configurational isomers.
The trans,trans-isomers 2a–c and 3a–c were isolated combining
thin-layer and column chromatography and/or photochemical iso-
merization (Schemes 2 and 3). Related derivatives trans,trans-2d
and trans,trans-3d, parent one and its methyl-derivative, have been
already characterized spectrally earlier [20,27], and here their pho-
tophysically and photochemically characterization is broadened for
comparison with the new compounds.
2. Experimental
2.1. General
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 tetramethylsi-
lane as reference. UV spectra were measured on a Varian Cary
50 UV/VIS Spectrophotometer and a SPECORD S-100 diode array
spectrophotometer. For the measurement of fluorescence and exci-
tation spectra a PerkinElmer LS50B spectrofluorimeter was applied.
Luminescence lifetime measurements were performed using a
laser flash photolysis system previously described [31]. A Quantel
Brilliant Nd:YAG laser yielding 355-nm pulses of about 5 ns dura-
2.2.1. Preparation of 2a–c
Compounds 2a–c were prepared in three steps by two Wit-
tig reactions and Vilsmeier formylation (Scheme 2). In the first
step 3-styrylfuran
4 was prepared by Wittig reaction from
benzyltriphenylphosphonium bromide and freshly distilled furan-
3-carboxaldehyde. To a stirred solution of benzyltriphenylphos-
phonium bromide (7.85 g, 18.1 mmol) and furan-3-carboxaldehyde
(1.74 g, 18.1 mmol) in absolute ethanol (100 mL) a solution of
sodium ethoxide (0.442 g, 19.2 mmol in 20 mL ethanol) was added
dropwise. Stirring was continued under a stream of nitrogen for

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I. Skori´c et al. / Journal of Photochemistry and Photobiology A: Chemistry 211 (2010) 152–161
Scheme 2.
3 h at RT. After removal of the solvent, the residue was worked
up with water and benzene. The benzene extracts were dried and
concentrated. The crude reaction mixture was purified by column
chromatography on silica gel using petroleum ether as eluent.
Vilsmeier formylation was carried out from 3-styrylfuran 4 (0.99 g,
5.8 mmol) dissolved in dimethylformamide (1 mL, 18.8 mmol).
After adjusting the temperature at ∼12 ◦C, phosphorus-oxychloride
(0.89 g, 5.8 mmol) was added and the reaction mixture was allowed
gradually to warm up to room temperature and stirred for 24 h.
The reaction mixture was decomposed by the continuous addition
(with cooling) of sodium hydroxide solution (10%) and the product
was worked up with diethylether. The diethylether extracts were
washed with water. After removal of the solvent, the crude reaction
mixture of 2-formyl-3-styrylfuran 5 (as a mixture of cis- and trans-
isomer) was used in a Wittig reaction to prepare 2,3-distyrylfuran
derivatives 2a–c. To a stirred solution of formyl derivative 5
(0.106 g, 0.52 mmol) and the phosphonium salts (0.52 mmol in
50 mL of: absolute ethanol, R = Cl, OCH3 or benzene, R = NO2),
sodium ethoxide (0.014 g, 0.61 mmol in 10 mL ethanol) was added.
Stirring was continued under a stream of nitrogen for 1 day at RT.
After removal of the solvent, the residue was worked up with water
and benzene. The benzene extracts were dried and concentrated.
The crude reaction mixture was purified and the mixture of 4 iso-
mers of 2a–c was isolated by column chromatography on silica
gel using petroleum ether/diethylether (0–5%) as eluent. After the
photochemical isomerization of the photomixtures of 2a and 2c
the trans,trans-isomers of 2a and 2c were isolated as the main iso-
mers. A mixture of isomers of 2a and 2c in benzene (∼4.0 × 10−3 M)
was purged with argon for 15 min and irradiated at 350 nm in a
Rayonet reactor in a quartz tube. After 20 min the reaction mix-
ture of 2a contained 90% of the trans,trans-isomer. The solvent was
removed in vacuum and the oily residue chromatographed on sil-
ica gel using petroleum ether to isolate pure trans,trans-2a in the
last fractions. Reaction mixture of 2c was completely isomerised to
trans,trans-isomer of 2c after 90 min of irradiation. The photochem-
ical isomerization was followed by NMR measurements with time.
Isomers of 2b were separated by repeated thin-layer chromatogra-
phy using petroleum ether/diethylether (3–7%) mixture as eluent.
Characterization data of the new compounds trans,trans-2a–c are
given below.
trans,trans-2-[2-(4-Chlorophenyl)ethenyl]-3-(2-
phenylethenyl)furan (trans,trans-2a): Yield 66%; Rf 0.37 (petroleum
ether/diethylether 10:0.1); yellow crystals; mp 129 ◦C; UV (EtOH)
ꢁmax (log ε) 384 (4.32, sh), 364 (4.47), 347 (4.37, sh), 284 (4.28),
276 (4.27, sh) nm; 1H NMR (CDCl3, 600 MHz) ıH 7.51 (d, J = 8.0, 2H),
7.44 (d, J = 8.40 Hz, 2H), 7.38 (d, J = 1.7 Hz, 1H), 7.36 (t, J = 8.0 Hz, 2H),
7.32 (d, J = 8.4 Hz, 2H), 7.25–7.28 (m, 1H), 7.11 (d, J = 15.8 Hz, 1H,
Het), 7.07 (d, J = 15.8 Hz, 1H, Het), 7.03 (d, J = 15.8 Hz, 1H, Het), 6.86
(d, J = 15.8 Hz, 1H, Het), 6.69 (d, J = 1.7 Hz, 1H); 13C NMR (75 MHz,
CDCl3) ıC 149.79 (s), 142.5 (d), 137.31 (s), 135.62 (s), 134.84 (s),
133.23 (s), 129.24 (d), 128.91 (2d), 128.72 (2d), 127.59 (d), 127.57
(2d), 126.27 (2d), 126.07 (d), 117.72 (d), 114.30 (d), 108.90 (d); MS
m/z (ESI) (%) 307 (M+, 100%); Anal. Calcd for C20H15ClO: C, 78.30;
H, 4.93. Found: C, 78.21; H, 5.07.
trans,trans-2-[2-(4-Methoxyphenyl)ethenyl]-3-(2-
phenylethenyl)furan (trans,trans-2b): Yield 80%; Rf 0.50
Scheme 3.

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155
(petroleum ether/diethylether 10:0.4); yellow–orange crys-
tals; mp 158 ◦C; UV (EtOH) ꢁmax (log ε) 388 (4.15, sh), 366 (4.34),
352 (4.28, sh), 284 (4.16), 275 (4.13, sh) nm; 1H NMR (CDCl3,
300 MHz) ıH 7.44–7.53 (m, 5H, Har), 7.32–7.39 (m, 3H, 2Har, H5f),
7.12 (d, J = 15.9 Hz, 1H, Het), 7.06 (d, J = 16.2 Hz, 1H, Het), 6.97 (d,
J = 15.9 Hz, 1H, Het), 6.90 (d, J = 8.9 Hz, 2H, Har), 6.82 (d, J = 16.2 Hz,
1H, Het), 6.67 (d, J = 1.9 Hz, 1H, H4f), 3.83 (s, 3H, OCH3); 13C NMR
(75 MHz, CDCl3) ıC: 159.45 (s), 150.48 (s), 142.19 (d, C5f), 137.52
(s), 129.93(s), 128.69 (2d), 128.51 (d), 127.74 (2d), 127.39 (d),
127.25 (d), 126.20 (2d), 121.28 (s), 118.08 (d), 114.23 (2d), 111.95
(d), 108.76 (d, C4f), 55.35(q, OCH3); MS (ESI) m/z (%) 303 (M+,
100%); Anal. Calcd for C21H18O2: C, 83.42; H, 6.00. Found: C, 83.21;
H, 6.26.
reactor in a quartz tube. After 40 min the reaction mixture con-
tained 80% of the trans,trans-3a which was isolated by thin-layer
chromatography using petroleum ether/diethylether (0–5%) mix-
ture as eluent. The photochemical isomerization was followed by
NMR measurements. Isomers of 3b,c were separated by TLC using
petroleum ether/diethylether (1.5–2.5%) as eluent. Characteriza-
tion data of the new compounds 6a–c, 7a–c and trans,trans-3a–c
are given below.
3-(p-Chlorostyryl)furan (6a). Yield 12.6%; according to 1H NMR
spectroscopy, a mixture of 55% cis- and 45% trans-isomer.
cis-6a: Rf 0.27 (petroleum ether); colorless oil; UV (EtOH) ꢁmax
(log ε) 276 (4.08) nm; 1H NMR (CDCl3, 600 MHz) ıH 7.36–7.37 (m,
1H, H5f), 7.24–7.31 (m, 5H, 4Har, H2f), 6.47 (d, J = 11.7 Hz, 1H, Het),
6.39 (d, J = 11,7 Hz, 1H, Het), 6.11 (dd, J = 1.6; 0.5 Hz, 1H, H4f); 13C
NMR (150 MHz, CDCl3) ıC 142.24 (d, C2f/5f), 141.73 (d, C2f/5f), 135.80
(s), 132.38 (s), 129.59 (2d), 127.96 (2d), 127.61 (d), 121.51 (s),
120.29 (d), 109.58 (d, C4f); MS m/z (EI) 204/206 (M+, 100%).
trans-6a: Rf 0.15 (petroleum ether); white crystals; mp 94 ◦C;
UV (EtOH) ꢁmax (log ε) 317 (4.00, sh), 293 (4.26), 285 (4.25, sh) nm;
1H NMR (CDCl3, 300 MHz) ıH 7.53 (bs, 1H, H2f/5f), 7.40–7.42 (m, 1H,
H2f/5f), 7.37 (d, J = 8.6 Hz, 2H, Har), 7.29 (d, J = 8.6 Hz, 2H, Har), 6.94
(d, J = 16.2 Hz, 1H, Het), 6.75 (d, J = 16.2 Hz, 1H, Het), 6.63–6.65 (m,
1H, H4f); 13C NMR (150 MHz, CDCl3) ıC 143.33 (d, C2f/5f), 140.68
(d, C2f/5f), 135.38 (s), 132.35 (s), 128.30 (2d), 126.77 (2d), 126.60
(d), 123.82 (s), 118.56 (d), 106.79 (d, C4f); MS m/z (EI) 204/206 (M+,
100%).
trans,trans-2-[2-(4-Nitrophenyl)ethenyl]-3-(2-
phenylethenyl)furan (trans,trans-2c): Yield 31%; Rf 0.34 (petroleum
ether/diethylether 10:0.5); orange crystals; mp 195–196 ◦C; UV
(EtOH) ꢁmax (log ε) 415 (4.31), 311 (4.19), 283 (4.17), 284 (4.28) nm;
1H NMR (CDCl3, 600 MHz) ıH 8.24 (d, J = 8.8 Hz, 2H, Har), 7.65 (d,
J = 8.8 Hz, 2H, Har), 7.55 (d, J = 8.0 Hz, 2H, Har), 7.46 (d, J = 2.0 Hz, 1H,
H5f), 7.40 (t, J = 7.7 Hz, 2H, Har), 7.25–7.34 (m, 2H, Har, Het), 7.15 (d,
J = 16.2 Hz, 1H, Het), 7.13 (d, J = 16.2 Hz, 1H, Het), 6.39 (d, J = 16.2 Hz,
1H, Het), 6.76 (d, J = 2.0 Hz, 1H, H4f); 13C NMR (75 MHz, CDCl3) ıC
148.64 (s), 143.20 (d, C5f), 143.17 (s), 136.52 (s), 129.89 (d), 128.29
(2d), 127.43 (d), 126.18 (2d), 125.90 (2d), 124.08 (d), 123.74 (2d),
117.15 (d), 116.73 (d), 108.71 (d, C4f); too small quantity to see
two remaining singlets; MS (ESI) m/z (%) 318 (M+, 100%); Anal.
Calcd for C20H15NO3: C, 75.70; H, 4.76. Found: C, 75.44; H, 4.99.
3-(p-Methoxystyryl)furan (6b). Yield 54.2%; according to 1H NMR
spectroscopy, a mixture of 47% cis- and 53% trans-isomer.
cis-6b: Rf 0.30 (petroleum ether/diethylether 10:0.3); yellow oil;
UV (EtOH) ꢁmax (log ε) 278 (3.02) nm; 1H NMR (CDCl3, 600 MHz)
ıH 7.36 (bs, 1H, H2f/5f), 7.26 (d, J = 8.6 Hz, 2H, Har), 7.24–7.25 (m, 1H,
H2f/5f), 6.84 (d, J = 8.6 Hz, 2H, Har), 6.48 (d, J = 11.8 Hz, 1H, Het), 6.30
(d, J = 11.8 Hz, 1H, Het), 6.18 (d, J = 1.5 Hz, 1H, H4f), 3.81 (s, 3H, OCH3);
13C NMR (150 MHz, CDCl3) ıC 155.24 (s), 141.33 (d, C2f/5f), 133.73
(d, C2f/5f), 125.80 (s), 123.38 (s), 124.57 (d), 123.38 (2d), 114.82 (d),
112.33 (s), 111.88 (2d), 105.96 (d, C4f), 51.97 (q); MS m/z (EI) 200
(M+, 100%).
trans-6b: Rf 0.20 (petroleum ether/diethylether 10:0.3); white
crystals; mp 101 ◦C; UV (EtOH) ꢁmax (log ε) 293 (4.04), 286 (4.05,
sh) nm; 1H NMR (CDCl3, 600 MHz) ıH 7.49 (bs, 1H, H2f/5f), 7.38–7.39
(m, 1H, H2f/5f), 7.38 (d, J = 8.6 Hz, 2H, Har), 6.87 (d, J = 8.6 Hz, 2H, Har),
6.83 (d, J = 16.2 Hz, 1H, Het), 6.76 (d, J = 16.2 Hz, 1H, Het), 6.64 (d,
J = 2.0 Hz, 1H, H4f), 3.81 (s, 3H, OCH3); 13C NMR (150 MHz, CDCl3)
ıC 158.57 (s), 143.10 (d, C2f/5f), 138.84 (d, C2f/5f), 128.68 (s), 127.47
(d), 126.78 (2d), 115.84 (d), 113.58 (s), 113.60 (2d), 106.86 (d, C4f),
54.81 (q); MS m/z (EI) 200 (M+, 100%).
2.2.2. Preparation of 3a–c
Compounds 3a–c were prepared in three steps by a Wittig
reaction and Vilsmeier formylation (Scheme 3). In the first step 3-
styrylfurans 6a–c were prepared by Wittig reaction. To a stirred
solution of furan-3-carbaldehyde (0.5 g, 5.21 mmol) and the phos-
phonium salts (5.21 mmol in 50 mL of: absolute ethanol, R = Cl,
OCH3 or benzene, R = NO2), sodium ethoxide (0.15 g, 6.5 mmol in
20 mL ethanol) was added. Stirring was continued under stream of
nitrogen for 1 day at RT. After removal of the solvent the residue
was worked up with water and benzene. The benzene extracts
were dried and concentrated. The crude reaction mixture was puri-
fied and the cis- and trans-isomers were separated by TLC using
petroleum ether/diethylether (0–10%) mixture as eluent. Vilsmeier
formylation was carried out from 3-styrylfurans 6a–c (2.68 mmol)
dissolved in dimethylformamide (1 mL, 18.8 mmol). After adjust-
ing the temperature at ∼11 ◦C, phosphorus-oxychloride (0.41 g,
2.68 mmol) was added and the reaction mixture was allowed grad-
ually to worm up to room temperature and stirred for 24 h. The
reaction mixture was decomposed by the continuous addition
(with cooling) of 10% sodium hydroxide solution and the prod-
uct was worked up with diethylether. The diethylether extracts
were washed with water. After removal of the solvent, the crude
reaction mixtures of 2-formyl-3-styrylfuran 7a–c (as a mixture
of cis- and trans-isomer) were used in a Wittig reaction to pre-
pare 2,3-distyrylfuran derivatives 3a–c. To a stirred solution of
benzyltriphenylphosphonium salt (0.113 g, 0.26 mmol) and formyl
derivatives 7a–c (0.26 mmol) in absolute ethanol (50 mL) a solu-
tion of sodium ethoxide (0.013 g, 0.57 mmol in 10 mL ethanol) was
added dropwise. Stirring was continued under a stream of nitro-
gen for 1 day at RT. After removal of the solvent, the residue was
worked up with water and benzene. The benzene extracts were
dried and concentrated. The crude reaction mixture was purified
and the 4 isomers of 3a-c were isolated by column chromatography
on silica gel using petroleum ether/diethylether (0–10%) mixture
as eluent. After the photochemical isomerization of the mixture
of four isomers of 3a the trans,trans-3a was isolated as the main
isomer. A mixture of isomers of 3a in benzene (∼3.5 × 10−3 M) was
purged with argon for 15 min and irradiated at 350 nm in a Rayonet
3-(p-Nitrostyryl)furan (6c). Yield 64.8%; according to 1H NMR
spectroscopy, a mixture of 33% cis- and 67% trans-isomer.
cis-6c: Rf 0.55 (petroleum ether/diethylether 10:0.2); yellow oil;
UV (EtOH) ꢁmax (log ε) 285 (4.11) nm; 1H NMR (CDCl3, 600 MHz)
ıH 8.15 (d, J = 8.8 Hz, 2H, Har), 7.49 (d, J = 8.8 Hz, 2H, Har), 7.40
(d, J = 0.6 Hz, 1H, H2f), 7.27 (dd, J = 1.5; 0.6 Hz, 1H, H5f), 6.56 (d,
J = 12.1 Hz, 1H, Het), 6.52 (d, J = 12.1 Hz, 1H, Het), 6.09 (d, J = 1.5 Hz,
1H, H4f); MS m/z (EI) 215 (M+, 100%).
trans-6c: Rf 0.27 (petroleum ether/diethylether 10:0.2); orange
crystals; mp 117 ◦C; UV (EtOH) ꢁmax (log ε) 325 (4.07) nm; 1H NMR
(CDCl3, 600 MHz) ıH 8.20 (d, J = 8.8 Hz, 1H, Har), 7.62 (s, 1H, H2f), 7.57
(d, J = 8.8 Hz, 1H, Har), 7.46 (d, J = 1.4 Hz, 1H, H5f), 7.16 (d, J = 16.2 Hz,
1H, Het), 6.85 (d, J = 16.2 Hz, 1H, Het), 6.69 (d, J = 1.4 Hz, 1H, H4f); 13
C
NMR (150 MHz, CDCl3) ıC 144.33 (d, C2f/5f), 144.08 (s), 142.54 (d,
2f/5f), 126.57 (2d), 126.17 (d), 124.28 (2d), 124.14 (s), 123.26 (d),
C
107.31 (d, C4f), 1 singlet is overlapped; MS m/z (EI) 215 (M+, 100%).
2-Formyl-3-(p-chlorostyryl)furan (7a). Yield 47%; according to 1H
NMR spectroscopy, a mixture of 57% cis- and 43% trans-isomer.
cis-7a: Rf 0.37 (petroleum ether/diethylether 10:1); yellow oil;
UV (EtOH) ꢁmax (log ε) 305 (4.13), 261 (3.88) nm; 1H NMR (CDCl3,

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156
600 MHz) ıH 9.83 (s, 1H, CHO), 7.44 (d, J = 1.8 Hz, 1H, H5f), 7.29 (d,
J = 8.5 Hz, 2H, Har), 7.22 (d, J = 8.5 Hz, 2H, Har), 6.93 (d, J = 12.1 Hz,
J = 7.3 Hz, 2H, Har), 7.36 (d, J = 1.9 Hz, 1H, H5f), 7.26 (t, J = 7.3 Hz,
1H, Har), 7.11 (d, J = 15.8 Hz, 1H, Het), 7.08 (d, J = 15.8 Hz, 1H, Het),
7.00 (d, J = 16.0 Hz, 1H, Het), 6.91 (d, J = 8.6 Hz, 2H, Har), 6.81 (d,
J = 16.0 Hz, 1H, Het), 6.67 (d, J = 1.9 Hz, 1H, H4f), 3.84 (s, 3H, OCH3);
13C NMR (75 MHz, CDCl3) ıC 13C NMR (75 MHz, CDCl3) ıC 150.11
(s), 144.15 (d, C5f), 143.09 (s), 137.25 (s), 129.18 (d), 128.77 (2d),
127.97 (d), 125.79 (2d), 124.99 (2d), 124.38 (d), 122.94 (2d), 118.36
(d), 117.77 (d), 107.97 (d, C4f); two singlets are missing; MS m/z
(ESI) 302 (M+, 100%); Anal. Calcd for C21H18O2: C, 83.42; H, 6.00.
Found: C, 83.18; H, 6.35.
1H, Het), 6.82 (d, J = 12.1 Hz, 1H, Het), 6.22 (d, J = 1.8 Hz, 1H, H4f); 13
C
NMR (75 MHz, CDCl3) ıC 178.45 (d, CHO), 148.57 (s, C2f), 146.92
(d, C5f), 134.99 (s), 134.15 (d), 133.79 (s), 130.12 (2d), 128.71 (2d),
118.92 (d), 117.03 (s), 112.78 (d, C4f); MS m/z (EI) 232/234 (M+,
100%).
trans-7a: Rf 0.24 (petroleum ether/diethylether 10:1); white
crystals; mp 78 ◦C; UV (EtOH) ꢁmax (log ε) 331 (4.35), 265 (4.31) nm;
1H NMR (CDCl3, 300 MHz) ıH 9.92 (s, 1H, CHO), 7.59 (d, J = 16.2 Hz,
1H, Het), 7.60 (d, J = 1.7 Hz, 1H, H5f), 7.48 (d, J = 8.5 Hz, 2H, Har), 7.35
(d, J = 8.5 Hz, 2H, Har), 7.08 (d, J = 16.2 Hz, 1H, Het), 6.85 (d, J = 1.7 Hz,
1H, H4f); 13C NMR (75 MHz, CDCl3) ıC 178.89 (d, CHO), 154.11 (s),
147.52 (d, C5f), 134.73 (s), 133.79 (d), 133.0 (s), 129.03 (2d), 128.16
(2d), 119.0 (s), 117.40 (d), 109.65 (d, C4f); MS m/z (EI) 232/234 (M+,
100%).
trans,trans-2-(2-Phenylethenyl)-3-[2-(4-
nitrophenyl)ethenyl]furan (trans,trans-3c): Yield 50%; Rf 0.20
(petroleum ether/diethylether 10:0.2); orange crystals; mp 213 ◦C;
UV (EtOH) ꢁmax (log ε) 403 (4.33), 324 (4.34); 1H NMR (CDCl3,
300 MHz) ıH 8.21 (d, J = 8.8 Hz, 2H, Har), 7.61 (d, J = 8.8 Hz, 2H,
Har), 7.54 (d, J = 7.8 Hz, 2H, Har), 7.24–7.43 (m, 5H, 3Har, Het, H5f),
2-Formyl-3-(p-methoxystyryl)furan (7b). Yield 30%; according to
1H NMR spectroscopy, a mixture of 26% cis- and 74% trans-isomer.
cis-7b: Rf 0.15 (petroleum ether/diethylether 10:1); white crys-
tals; mp 68 ◦C; UV (EtOH) ꢁmax (ε) 313 (4.15), 271 (3.93) nm; 1H
NMR (CDCl3, 600 MHz) ıH 9.81 (s, 1H, CHO), 7.44 (d, J = 1.8 Hz, 1H,
H5f), 7.23 (d, J = 8.7 Hz, 2H, Har), 6.84 (d, J = 8.7 Hz, 2H, Har), 6.83 (d,
J = 11.9 Hz, 1H, Het), 6.79 (d, J = 11.9 Hz, 1H, Het), 6.31 (d, J = 1.8 Hz,
1H, H4f), 3.82 (s, 3H, OCH3); MS m/z (EI) 228 (M+, 100%).
trans-7b: Rf 0.08 (petroleum ether/diethylether 10:1); yellow
crystals; mp 93 ◦C; UV (EtOH) ꢁmax (log ε) 340 (4.32), 277 (4.30) nm;
1H NMR (CDCl3, 600 MHz) ıH 9.91 (s, 1H, CHO), 7.57 (d, J = 1.7 Hz,
1H, H5f), 7.49 (d, J = 8.5 Hz, 2H, Har), 7.46 (d, J = 16.2 Hz, 1H, Het), 7.09
(d, J = 16.2 Hz, 1H, Het), 6.91 (d, J = 8.5 Hz, 2H, Har), 6.83 (d, J = 1.7 Hz,
1H, H4f), 3.82 (s, 3H, OCH3); 13C NMR (75 MHz, CDCl3) ıC 179.86 (d,
CHO), 150.69 (s), 147.93 (d, C5f), 134.46 (d), 128.48 (s), 127.99 (2d),
114.03 (s), 113.53 (2d), 109.21 (d), 54.90 (d), too small quantity to
see one remaining singlet; MS m/z (EI) 228 (M+, 100%).
7.18 (d, J = 16.0 Hz, 1H, Het), 7.09 (d, J = 16.0 Hz, 1H, Het), 6.86 (d,
J = 15.9 Hz, 1H, Het), 6.70 (d, J = 1.6 Hz, 1H, H4f); 13C NMR (75 MHz,
CDCl3) ıC 151.60 (s), 146.55 (s), 143.97 (s), 142.93 (d, C5f), 136.71
(s), 129.04 (d), 128.82 (2d), 128.16 (d), 126.63 (2d), 126.51 (2d),
126.23 (d), 124.21 (2d), 122.49 (d), 121.33 (s), 113.25 (d), 108.58
(d, C4f); (ESI) 317 (M+, 100%); Anal. Calcd for C20H15NO3: C, 75.70;
H, 4.76. Found: C, 75.87; H, 4.49.
2.3. Photochemical and photophysical experiments
Typically, the irradiations were carried out with 3.5-cm3 solu-
tions in 1-cm cells at room temperature. HPLC grade ethanol and
n-hexane were used as solvents in these experiments. During the
photolyses the reaction mixtures were continuously homogenized
by magnetic stirring. For photochemical reactions under contin-
uous irradiation at 366 nm, an AMKO LTI photolysis equipment,
containing a 200 W Xe–Hg-lamp and a monochromator was used
[32]. Incident light intensity was determined with a thermopile
calibrated by ferrioxalate actinometry [33,34]. Quantum yield mea-
surements were carried out with samples of nearly 100% light
absorption. Following the change of the absorption spectrum of
the solution photolyzed, the quantum yield for the photochemi-
cal transformation of the starting material was determined from
the initial rate calculated from the absorbance vs. time plot at a
characteristic wavelength where the products did not absorb.
Emission spectra were measured on a PerkinElmer LS50B spec-
trofluorimeter. Ru(bpy)3Cl2 [35], quinine bisulphate (in 0.5 M
H2SO4), and 9,10-diphenyl-anthracene [36] were utilized as refer-
ences for determination of the fluorescence quantum yields. Each
2,3-distyrylfuran derivative studied was excited at the wavelength
of its absorption maximum. Luminescence spectra were corrected
for the detector sensitivity.
2-Formyl-3-(p-nitrostyryl)furan (7c). Yield 61%; according to 1
NMR spectroscopy, a mixture of 27% cis- and 73% trans-isomer.
H
cis-7c: Rf 0.40 (petroleum ether/diethylether 10:0.5); mp
82–83 ◦C; UV (EtOH) ꢁmax (ε) 322 (4.19) nm; 1H NMR (CDCl3,
600 MHz) ıH 9.86 (s, 1H, CHO), 8.18 (d, J = 8.7 Hz, 2H, Har), 7.47 (d,
J = 1.8 Hz, 1H, H5f), 7.46 (d, J = 8.7 Hz, 2H, Har), 7.11 (d, J = 12.2 Hz,
1H, Het), 6.88 (d, J = 12.2 Hz, 1H, Het), 6.16 (d, J = 1.8 Hz, 1H, H4f); MS
m/z (EI) 243 (M+, 100%).
trans-7c: Rf 0.35 (petroleum ether/diethylether 10:0.5); mp
122 ◦C; UV (EtOH) ꢁmax (log ε) 345 (4.41) nm; 1H NMR (CDCl3,
600 MHz) ıH 9.95 (s, 1H, CHO), 8.24 (d, J = 8.7 Hz, 2H, Har), 7.82 (d,
J = 16.3 Hz, 1H, Het), 7.68 (d, J = 8.7 Hz, 2H, Har), 7.63 (d, J = 1.6 Hz,
1H, H5f), 7.18 (d, J = 16.3 Hz, 1H, Het), 6.89 (d, J = 1.9 Hz, 1H, H4f); MS
m/z (EI) 243 (M+, 100%).
trans,trans-2-(2-Phenylethenyl)-3-[2-(4-
Fluorescence lifetimes were measured by a kinetic laser system
utilizing a Quantel Brilliant Nd:YAG laser with a Tektronix DPO
4034 digital oscilloscope. Since the fluorescence lifetimes of the
compounds studied are comparable with the laser half-width, a
deconvolution method was applied for their determination [37].
To compare the photochemical behavior of 2a–c and 3a–c with
that of the analogous 2d and 3d [21], solutions of trans,trans-2a–c
and trans,trans-3a–c in deuterated benzene (e.g. 7 × 10−2 M) were
irradiated in NMR tubes in a Rayonet RPR-100 photochemical reac-
tor at 366 nm and the reaction course was followed by H NMR until
the complete conversion.
chlorophenyl)ethenyl]furan (trans,trans-3a): Yield 57%; Rf
0.40 (petroleum ether/diethylether 10:0.4); yellow crystals;
mp 130–131 ◦C; UV (EtOH) ꢁmax (log ε) 362 (4.23), 347 (4.20,
sh), 327(4.12, sh), 284 (4.22), 277 (4.20) nm; 1H NMR (CDCl3,
300 MHz) ıH 7.52 (d, J = 7.5 Hz, 2H, Har), 7.23–7.45 (m, 8H, 6Har
,
H5f), 7.06–7.13 (m, 3H, Har), 6.77 (d, J = 16.1 Hz, 1H, Het), 6.66 (d,
J = 1.8 Hz, 1H, H4f); 13C NMR (75 MHz, CDCl3) ıC 150.36 (s), 142.63
(d, C5f), 137.00 (s), 135.94 (s), 133.00 (s), 128.87 (2d), 128.76 (2d),
127.83 (d), 127.52 (d), 127.39 (2d), 126.49 (2d), 121.83 (s), 118.52
(d), 113.67 (d), 108.72 (d, C4f); MS m/z (ESI) 307/305 (M+, 100%);
Anal. Calcd for C20H15ClO: C, 78.30; H, 4.93. Found: C, 78.55; H,
4.67.
trans,trans-2-(2-Phenylethenyl)-3-[2-(4-
3. Results and discussion
methoxyphenyl)ethenyl]furan (trans,trans-3b): Yield 52%; Rf
0.60 (petroleum ether/diethylether 10:0.6); yellow crystals; mp
187–188 ◦C; UV (EtOH) ꢁmax (log ε) 380 (4.38, sh), 366 (4.50), 347
(4.42, sh), 286 (4.39), 278 (4.33, sh); 1H NMR (CDCl3, 300 MHz)
ıH 7.53 (d, J = 7.3 Hz, 2H, Har), 7.45 (d, J = 8.6 Hz, 2H, Har), 7.37 (t,
For the sake of comparability with the previous results [21,28]
and due to their thermodynamic stability in the photoisomer-
ization equilibrium, exclusively the trans,trans-isomers of the
distyrylfuran compounds 2a–d and 3a–d were photophysically

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157
the similarity of the photochemical behavior to that of analogous
compounds.
Since these photochemical results implied that the 2,3-
distyrylfurans 2a–d and 3a–d do not give interesting heteropoly-
cyclic compounds (Scheme 4) as intramolecular cycloaddition
products, further intention was the study of substituent and solvent
effects on their photoactivity.
The quantum yields of the photoinduced disappearence of the
starting molecules in both solvents are summarized in Table 1.
As the data show a very strong solvent effect can be observed
in the case of the unsubstituted 2,3-distyrylfuran (2d). In the
non-polar n-hexane the quantum yield is a bit more than one
order of magnitude lower than in the polar ethanol. This phe-
nomenon indicates that the transition state of the reaction reached
upon excitation is more polar than the initial (ground) state. An
increase in polarity of solvent will stabilize the transition state rel-
ative to the initial state and, thus, lead to an increase in reaction
rate, i.e. quantum yield [38]. A similarly strong effect is found for
the methyl-substituted derivative (3d), suggesting that a simple
electron-donating group without lone pairs does not influence the
effect of the solvent polarity.
Scheme 4. Possible intramolecular photochemical reactions of the ethylenic bonds
of 2a–d and 3a–d.
and photochemically studied in this work. Thus, in the following,
the codes 2a–d and 3a–d refer to the corresponding trans,trans-
isomers.
3.1. Photochemical experiments
For the chloro- (2a and 3a) and methoxy-substituted (2b and
3b) derivatives, the quantum yields in ethanol are as similarly high
as for the unsubstituted and methyl-substituted compounds. In
all these cases this phenomenon can be accounted for the phenyl
groups connected to the styryl links, promoting their reactivity
for intermolecular [2 + 2]-photocycloaddition which results in the
intermediate cyclobutane structures suggested. In ethanol the pho-
tolysis quantum yields are appreciably higher for the isomers with
the substituent on the styryl group of position 3 than for the
corresponding ones of position 2. Besides, a less strong but still sig-
nificant solvent effect of the same type is also shown in these cases,
no matter on which phenyl group are these substituents. This mod-
erate decrease in the solvent effect may be attributed to the fact
that both the chlorine and the oxygen atoms directly connected
to the aromatic ring possess lone pairs, which can be involved in
the conjugation of the whole bond system. Hence they can slightly
increase the stability of the excited state in the non-polar solvent
compared to the case of the unsubstituted and methyl-substituted
compounds.
The nitro-substituted derivatives (2c and 3c) display much less
efficiencies for the photolysis, besides the effects of the solvent
polarity in these cases are opposite to those of the other com-
pounds studied. Both phenomena can be attributed to the strong
electron-withdrawing effect of the nitro group. This substituent
in para position significantly decreases the electron density on
the styryl link, diminishing the chance for the intermolecular
[2 + 2]-photocycloaddition, which is apparently the key step of the
photolysis. Thus, the corresponding quantum yields in ethanol are
one order of magnitude lower than those for the other compounds
studied. The opposite solvent effect indicates that in the case of
the nitro-derivatives another way of decay of the excited state
becomes more efficient in ethanol than in n-hexane in competi-
tion with the photochemical reaction. Since, as it is shown later
(see in Section 3.2.2), also the corresponding fluorescence quan-
tum yields are lower in the solvent of higher polarity, an efficient
S1 → T1 intersystem crossing leading to a non-reactive may be the
competitive way of decay. This conclusion is in accordance with the
results published on the nitro-derivative of the analogous benzo-
furan compound [28,30].
Irradiation experiments with each new substituted 2,3-
distyrylfuran derivative (2a–c and 3a–c) were carried out in both
ethanol and n-hexane in order to get some information regarding
the effect of the solvent polarity. For comparison, solutions of one
methyl-substituted (3d) and the unsubstituted 2,3-distyrylfuran
derivatives (2d), as known compounds [21], were also irradi-
ated to study how the substituents influence the efficiency of the
photolysis. The systems were argon-saturated for avoiding the pho-
toinduced oxidation of the excited molecules. The concentration of
the starting compounds was in the order of 10−4 M. The excitation
wavelength of 366 nm was applied for each photolysis.
Regardless of the substituent, upon irradiation of each
compound the longest-wavelength absorption band gradually dis-
appeared (see Figs. 1–4), while the band (or the double band of the
nitro-derivatives) at about 270 nm significantly decreased and in
some cases slightly shifted. These spectral changes indicate that
the long-range conjugation between the two phenyl groups via
the hexatriene link has been ceased by the photolysis, and the
electronic system of the aromatic rings became isolated in the
case of all derivatives. This phenomenon also suggests that nei-
ther trans–cis isomerization nor an intramolecular cycloaddition
reaction plays a considerable role in the photolysis of these com-
pounds. Instead, as it was observed previously in the case of the
photolysis of the distyrylfuran [21] and benzofuran derivatives [28],
stilbene-like end-products may form via intermediate cyclobutane
structures. The NMR spectra of all photomixtures after irradiation
of 2a–c and 3a–c did not show any characteristic signal either for
the aliphatic protons of hypothetic bicyclic structures (Scheme 4)
or for cyclobutane dimeric structures. No intramolecular het-
eropolycyclic cycloaddition products were detected, confirming
Table 1
Quantum yields for the photolysis of 2,3-distyrylfuran derivatives in argon-
saturated n-hexane and ethanol (ꢁir = 366 nm).a.
R (code)
˚ (in n-hexane)
˚ (in ethanol)
Cl (2a)
0.09
0.14
0.07
0.02
0.24
0.07
0.16
0.03
0.30
0.25
0.025
0.38
0.46
0.33
0.042
0.35
OCH3 (2b)
NO2 (2c)
H (2d)
3.2. Photophysics
Cl (3a)
OCH3 (3b)
NO2 (3c)
CH3 (3d)
3.2.1. UV–vis absorption spectra
Figs. 5 and 6 display the UV–vis absorption and fluorescence
emission spectra of 3c and 3d (as examples) in ethanol and n-
a
Standard deviation: 7%.

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158
Fig. 1. Spectral change during the irradiation of 2a in n-hexane (a) after 0, 2, 4, 8, 16, 32, 64 min, and in ethanol (b) after 0, 1, 2, 4, 8, 16 min (ꢁir = 366 nm, ꢂ = 1 cm).
Fig. 2. Spectral change during the irradiation of 2c in n-hexane (a) after 0, 4, 8, 16, 32, 64 min, and in ethanol (b) after 0, 8, 16, 32, 64, 128 min (ꢁir = 366 nm, ꢂ = 1 cm).
Fig. 3. Spectral change during the irradiation of 2d in n-hexane (a) after 0, 2, 4, 8, 16, 32, 64 min, and in ethanol (b) after 0, 1, 2, 4, 8 min (ꢁir = 366 nm, ꢂ = 1 cm).
Fig. 4. Spectral change during the irradiation of 3b in n-hexane (a) after 0, 1, 2, 4, 8, 16, 32, 64 min, and in ethanol (b) after 0, 1, 2, 4, 8, 16 min (ꢁir = 366 nm, ꢂ = 1 cm).

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Table 2
Spectral behavior of 2,3-distyrylfuran derivatives in n-hexane at room temperature.
R (code)
ꢁmabasx/nma (log ε)
ꢃꢄ˜abs/cm−1
ꢁmF ax/nm
ꢃꢄ˜F /cm−1
ꢃꢄ˜S,max/cm−1
ꢃꢄ˜S,00/cm−1
Cl (2a)
344sh, 362 (4.65), 382sh
346sh, 364 (4.34), 387sh
386, 405 (4.51), 429
1450
1630
1380
1550
1670
1310
1470
1620
400, 425, 450
401, 426, 450
451, 477, 508
398, 422, 445
397, 421, 446
403, 427, 451
447, 470, 500
398, 422, 446
1470
1460
1210
1430
1440
1390
1090
1430
4090
4000
3730
4240
4020
4210
3720
4240
1180
902
1140
1260
920
1500
1160
1190
OCH3 (2b)
NO2 (2c)
H (2d)
343sh, 358 (4.48), 379sh
344sh, 360 (4.23), 383sh
345sh, 362 (4.46), 380sh
382, 400 (4.45), 425
Cl (3a)
OCH3 (3b)
NO2 (3c)
CH3 (3d)
343sh, 358 (4.61), 380sh
a
The main maximum is underlined.
Table 3
Spectral behavior of 2,3-distyrylfuran derivatives in ethanol at room temperature.
R (code)
ꢁmabasx/nma (log ε)
ꢃꢄ˜abs/cm−1
ꢁmF ax/nm
ꢃꢄ˜F /cm−1
ꢃꢄ˜S,max/cm−1
ꢃꢄ˜S,00/cm−1
Cl (2a)
347sh, 362 (4.47), 382sh
349sh, 364 (4.34), 387sh
414 (4.31)
1450
1630
–
1540
1580
1450
–
405, 429, 454
407, 431, 452
571
404, 426, 452
403, 426, 452
412, 434, 458
567
1380
1370
–
1280
1340
1230
–
4310
4270
6640
4380
4150
4580
7180
4380
1490
1270
–
1560
1230
1900
–
OCH3 (2b)
NO2 (2c)
H (2d)
342sh, 359 (4.51), 380sh
327sh, 347sh, 362 (4.23), 384sh
345sh,362 (4.50), 382sh
403 (4.33)
Cl (3a)
OCH3 (3b)
NO2 (3c)
CH3 (3d)
344sh, 359 (4.51), 380sh
1540
403, 426, 453
1340
1500
a
The main maximum is underlined.
hexane at room temperature and Tables 2 and 3 summarize the
characteristic spectral data (maxima wavelengths and vibronic
progressions of the absorption and emission spectra and the
Stokes-shifts) of the compounds studied in ethanol and n-hexane,
respectively. Stokes-shifts were calculated for both the maxima
(ꢃꢄ˜S,max) and the 0–0 transition (ꢅꢄ˜S,00).
For the derivatives with R = H, Cl, and OCH3 (on either styryl
group) as well as CH3 on styryl group in position 3, band absorp-
tion maxima are found in the range of 358–364 nm, and assigned
to the S0 → S1 transition of essentially ␲ → ␲* nature. Compared
to the corresponding benzofuran derivates (2-(4ꢀ-methyl-styryl),3-
(4ꢀ-R-styryl)benzofuran) [30], this band is 12–13 nm blue-shifted.
This phenomenon can be attributed to the less extended conjugated
␲ bond system in the case of the furan derivatives. However, the
effects of the substituents are the same regarding the shift com-
pared to the unsubstituted derivative. Thus, very slight (2–6 nm)
red shifts were observed for the Cl and OCH3 derivatives of both
types of compounds (2a, 3a and 2b, 3b) in n-hexane. The sub-
stiuents on the styryl group in position 3 result in smaller shifts
(2 nm for Cl (3a) and 4 nm for OCH3 (3b)) than on the styryl group
in position 2 (4 nm for Cl (2a) and 6 nm for OCH3 (2b)). The methyl
substituent on the furan compound (3d) does not cause any shift.
In ethanol the same tendencies can be observed, and the solvent
polarity does not affect the position of the main absorption band of
these furan derivatives.
Fig. 5. UV–vis absorption and fluorescence emission spectra of 3c in n-hexane
(dashed line) and ethanol (solid line).
In both solvents this rather strong band shows a reduced vibra-
tional structure with maxima corresponding to the transition from
the vibrational level ꢄ = 0 of the ground state to the ꢄꢀ = 2 level of the
first excited singlet state (S1). Its vibronic progression depends on
the substituents in the range of 1450–1670 cm−1. This electronic
excitation probably causes an appreciable change in the molec-
ular geometry as indicated by the relatively large Stokes-shifts
(ꢃꢄ˜S,max). Beside the principal absorption band, the data of which
are summarized in Tables 2 and 3, a second absorption band of
similar intensity is displayed by these distyrylfuran compounds at
shorter wavelengths. This band of two maxima, assigned to the
S0 → S2 transition (of also ␲ → ␲* nature), shows more significant
blue shifts with respect to that observed for the corresponding
benzofuran derivatives (20–23 nm) than the principal band does.
This phenomenon suggests that the stabilization effect of the more
extended conjugated ␲ bond system in the case of the benzofuran
Fig. 6. UV–vis absorption and fluorescence emission spectra of 3d in n-hexane
(dashed line) and ethanol (solid line).

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160
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Table 4
derivatives compared to that of the corresponding furan com-
pounds is stronger for the second singlet excited state (S2) than
for the first one (S1).
Fluorimetric parameters of 2,3-distyrylfuran derivatives in n-hexane and ethanol.
R (code)
n-Hexane
Ethanol
Deviating from the previously discussed substituents, NO2 (on
either styryl group, i.e., 2c, 3c) results in significant red shifts
(32–41 nm) in both solvents, compared to the absorption bands of
the unsubstituted compound. Similar phenomenon was observed
with the corresponding benzofuran derivative [30]. This effect may
be the consequence of the more than doubled dipole moment of
the nitro-derivatives in this excited state than in the ground state.
The significantly larger dipole moments of the nitro compounds in
the excited state are in accordance with the appreciable red shifts
(3–9 nm) of the main band in ethanol compared to its position
in the non-polar n-hexane, deviating from the other derivatives.
The excited state of larger dipole moment is more strongly stabi-
lized in polar solvent [39]. While in n-hexane the main band shows
a vibrational structure with much better resolution than in the
case of the other substituents, in ethanol this character completely
vanishes due to the strong interaction with the solvent of high
polarity (Fig. 5). Hence, in ethanol the Stokes-shift for the 0–0 tran-
sition (ꢃꢄ˜S,00) cannot be calculated (Table 3). Similarly to the other
derivatives studied, also in this case, the second, strong absorption
band displays two maxima and more significant blue shifts com-
pared to that observed for the corresponding benzofuran derivative
(16–33 nm) than the principal band does.
a
a
˚
F
ꢀF/nsb
˚
F
ꢀF/nsb
Cl (2a)
0.91
0.86
0.03
0.98
0.76
0.93
0.016
0.89
1.79
1.31
<0.3
2.59
2.10
1.79
<0.3
2.05
0.44
0.26
3.61
2.06
<0.3
3.48
2.01
4.77
<0.3
2.44
OCH3 (2b)
NO2 (2c)
H (2d)
<10−4
0.64
Cl (3a)
0.27
OCH3 (3b)
NO2 (3c)
CH3 (3d)
0.63
<10−4
0.59
a
Standard deviation: 7%.
4%.
b
geometry upon excitation is much stronger in ethanol than in non-
polar solvent. This is in accordance with the observations of the
analogous benzofuran derivative, in the case of which an appre-
ciable increase (more than doubling) of the dipole moment was
calculated for the S0 → S2 transition [30]. Thus, the stabilization
effect of ethanol for excited state of the nitro compounds is very
strong, resulting in increased Stokes-shifts compared to the other
derivatives.
Table 4 summarizes the quantum yield and lifetime data for
the fluorescence of the compounds studied in both solvents.
Their fluorescence intensities display monoexponential decays.
This indicates that the all the competitive ways of decay of the
excitation state are of first order kinetics. Due to the rather short flu-
orescence lifetimes, practically no emission quenching by oxygen
was observed in these cases.
Except for the nitro-derivatives, the fluorescent emission quan-
tum yields in n-hexane are rather high, in several cases close
to one. This indicates that the main relaxation route of the S1
excited state in non-polar solvent is fluorescence. Generally, the
sum of the quantum yields of the photolysis and the fluorescence
for these compounds is about one, thus these two ways of decay
used up the total energy of the excited state. In the case of the
nitro-derivatives both the fluorescence and the photolysis quan-
tum yields are rather low, and the emission lifetimes are much
shorter than those observed for the other compounds studied.
These phenomena indicate that another way of decay of the excited
state is more efficient for the nitro-substituted compounds (2c and
3c). This is probably an intersystem crossing (S1 → T1) leading to a
non-reactive triplet state. Similar phenomena were observed in the
case of analogous benzofuran compound, and the formation of the
triplet state was proved by the efficient formation singlet oxygen
[28,30].
In ethanol the fluorescence quantum yields are significantly
lower than in n-hexane (Table 4), indicating that the efficient sta-
bilization of the polar excited state increases the probability of
the photochemical conversion in competition with the emission
as it was observed in the photolysis quantum yields (Table 1). This
explanation is supported by the longer lifetimes of the emitting
excited states in ethanol than in n-hexane. While even in ethanol,
in several cases, the sum of the quantum yields of fluorescence and
photolysis is close to one, in similar number of instants it is much
below one, indicating that a third route, e.g., internal conversion or
intersystem crossing can play a significant role in the decay of the
excited state in this solvent.
3.2.2. Fluorescence emission spectra and photophysics
The comparison of the emission and the absorption spectra of
the compounds studied (see Figs. 5 and 6 and Tables 2 and 3)
reveals some interesting features. Generally, for these compounds
the vibrational structure of the fluorescence band is much better
than that of the absorption bands. This kind of spectral behav-
ior indicates a larger rigidity and planarity of the geometry of the
emitting state (S1) compared to that of the ground state (S0). The
fluorescence spectra do not depend on the excitation wavelength,
and the excitation spectrum well overlaps with the absorption
spectrum for all the compounds studied. The maximum emis-
sion band corresponds to the transition from the vibrational level
ꢄ = 0 of the first excited singlet state (S1) to the ꢄꢀ = 2 level of the
ground state. The progression of the vibronic bands in n-hexane
is about 1390–1470 cm−1, slightly depending on the substituent
and its position. The only exception is the nitro group, in the case
of which ꢃꢄ˜F = 1210 cm−1 for position 2 and 1090 cm−1 for posi-
tion 3. In ethanol no vibrational structure can be observed for the
nitro-derivatives, similarly to the absorption bands, due to the high
polarity of the solvent, while for the other compounds studied the
vibronic progressions (1230–1380 cm−1) are smaller than the cor-
responding ones in n-hexane. No such solvent effect was observed
on the vibronic progressions of the main absorption bands.
Compared to the unsubstituted compound, methyl group does
not result in any shift of the main fluorescence emission band, nei-
ther in ethanol nor in n-hexane. The Cl and OCH3 substituents (2a,
3a and 2b, 3b) cause a slight red shift in both ethanol (3–8 nm)
and n-hexane (3–5 nm). However, the measure of the red shift for
the nitro-derivatives (2c and 3c) is rather large; 48 and 55 nm in
n-hexane, while 141 and 145 nm in ethanol for position 3 and 2,
respectively. This dramatic effect of the NO2 group can be attributed
to its strong electron-withdrawing property and the lack of the
non-bonding electron pair on nitrogen, changing the geometry and
polarity in both the ground and the excited state.
4. Conclusion
In ethanol the main emission band shows a slight red shift with
respect to its wavelength in n-hexane for all the compounds stud-
ied with the exception of the nitro-derivatives, in the case of which
a very strong red shift can be observed. The latter phenomenon is
in accordance with the slight red shift of the corresponding absorp-
tion band, indicating that for the nitro-derivatives the change in the
The new 2,3-distyrylfuran derivatives, trans,trans-2a–c and
trans,trans-3a–c, along with the previously synthesized trans,trans-
2d and trans,trans-3d compounds, display interesting photoin-
duced properties, which promote their application as, e.g., potential
fluorescence probes. Their fluorescence quantum yields are very

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I. Skori´c et al. / Journal of Photochemistry and Photobiology A: Chemistry 211 (2010) 152–161
161
sensitive to the solvent polarity, indicating that the emitting excited
states are of higher dipole moment compared to the corresponding
ground states. Hence, in non-polar n-hexane the fluorescence quan-
tum yields in most cases are rather high, for some compounds close
to one. The polar ethanol stabilizes these excited states, increas-
ing their lifetime, thus promoting their decay via photochemical
reaction of significant efficiency. The sum of the fluorescence and
photolysis quantum yields is about one for some compounds in
both solvents, indicating that these two ways of decay use up the
energy of their excited state.
Deviating from the other compounds studied, the quantum
yields for both the fluorescence and the photolysis of the nitro-
derivatives 2c and 3c are rather low, probably due to an efficient
intersystem crossing (S1 → T1) leading to a non-reactive triplet
state. The strong electron-withdrawing effect of the nitro group
also results in significant red shifts in both the absorption and the
emission bands compared to those of the unsubstituted compound.
Due to the effect of the solvent polarity, even stronger red shifts of
the fluorescence bands are caused by the nitro group in ethanol.
This is manifested in the large Stokes-shifts indicating a significant
change in the molecular geometry upon excitation.
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Photobiol. Sci. 6 (2007) 444–453.
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Acknowledgements
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[21] I. Skoric´, I. Flegar, Z. Marinic´, M. Sindler-Kulyk, Synthesis of the novel conju-
gated ␻,␻ꢀ-diaryl/heteroaryl hexatriene system with the central double bond
in a heteroaromatic ring: photochemical transformations of 2,3-divinylfuran
derivatives, Tetrahedron 62 (2006) 7396–7407.
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) also in the frame of the Hungarian–Croatian
Intergovernmental S&T Cooperation Program for 2009–2010
jointly financed by the Hungarian National Office of Research and
Technology (OMFB-01247/2009). I. K. is grateful for the support
by the Croatian–Hungarian ERASMUS student mobility program.
Many thanks are due to Dr. Lajos Fodor (Institute of Chemistry,
University of Pannonia) for the professional assistance in the life-
[22] J.R. Lakowicz, Principle of Fluorescence Spectroscopy, third ed., Springer, Sin-
gapore, 2006.
[23] B. Valeur, Molecular Fluorescence – Principles and Applications, Wiley–VCH,
Weinheim, 2002.
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furans. 10. Regiospecific synthesis of 3,4-disubstituted furans, Pure Appl. Chem.
68 (1996) 723–726.
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Murakami, Synthetic studies of indoles and related compounds. 32. An inter-
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(1993) 1739–1742.
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time measurements, to Mr. Zeljko Marinic´ (NMR Center, Institute
[26] V. Vuligonda, M.E. Garst, A.S. Chandraratna, Stereoselective synthesis and
receptor activity of conformationally defined retinoid X receptor selective lig-
ands, Bioorg. Med. Chem. Lett. 9 (1999) 589–594.
Rudjer Bosˇkovic´) for the considerable help in NMR spectroscopy,
and to Andrijana Mesˇcˇic´ (Department of Organic Chemistry, Uni-
versity of Zagreb) for the valuable contribution to the preparative
work.
ˇ
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[27] I. Baraldi, E. Benassi, S. Ciorba, M. Sindler-Kulyk, I. Skoric´, A. Spalletti,
Structures, spectra and photophysics of new organic fluorophores: 2,3-and
2,5-di(phenylethenyl)furan, Chem. Phys. 353 (2008) 163–169.
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hexatriene derivatives with the central double bond in the benzofuran ring and
their photochemical and photophysical properties, J. Photochem. Photobiol. A
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[29] I. Skoric´, M. Smehil, Z. Marinic´, K. Molcˇanov, B. Kojic´-Prodic´, M. Sindler-
Kulyk, Photochemistry of ␻-(o-vinylphenyl)-␻’-(phenyl/2-furyl) butadienes:
new approach to 4-substituted benzobicyclo[3.2.1]octadienes, J. Photochem.
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