Synthesis and photobehaviour of donor-π-acceptor conjugated arylacetylenes

Article abstract of DOI:10.1016/j.jphotochem.2011.08.011

Two series of soluble and thermally stable asymmetric arylacetylene derivatives bearing a fluorene (Fl-Xs) or an anthracene (An-Xs) core unit and end-substituted with different electronically active groups (X = NO₂, CN, CHO and OR) were synthes

Full text of DOI:10.1016/j.jphotochem.2011.08.011

Journal of Photochemistry and Photobiology A: Chemistry 223 (2011) 140–148
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis and photobehaviour of donor-␲-acceptor conjugated arylacetylenes
Rebecca Flamini, Ivan Tomasi, Assunta Marrocchi∗, Benedetta Carlotti, Anna Spalletti∗∗
Dipartimento di Chimica and Centro di Eccellenza Materiali Innovativi Nanostrutturati (CEMIN), Università degli Studi di Perugia, Via Elce di Sotto, 8, 06123 Perugia, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 May 2011
Received in revised form 9 August 2011
Accepted 13 August 2011
Available online 22 August 2011
Two series of soluble and thermally stable asymmetric arylacetylene derivatives bearing a fluorene (Fl-
Xs) or an anthracene (An-Xs) core unit and end-substituted with different electronically active groups
(X = NO2, CN, CHO and OR) were synthesized and characterized. The spectral and photophysical behaviour
of these compounds was studied in two solvents of different polarity to evaluate them as candidates for
the role of active materials in optoelectronic devices. The presence of intramolecular charge transfer
states, strongly stabilized in the polar solvent, was found mainly in the nitro-anthryl and fluorenyl-
derivatives. A complete picture of the excited states nature and their deactivation channels was also
achieved by semiempirical quantum-mechanical calculations.
Keywords:
Anthracene and fluorene derivatives
Push–pull arylacetylenes
Synthesis
© 2011 Elsevier B.V. All rights reserved.
Spectral/photophysical properties
Intramolecular charge transfer states
1. Introduction
anthracene core unit and end-substituted with different electron-
ically active groups (Scheme 1).
During the last decades the interest in push–pull (D-␲-A)
systems, comprising an electron-releasing donor unit (D), a ␲-
conjugated bridge, and an electron-withdrawing acceptor unit (A)
has increased dramatically due to potential applications in sev-
eral fields such as telecomunication, organic electronics, optical
data storage and optical information. Moreover, these materials
have been used extensively for non-linear optic (NLO) applica-
tions. Because efficient intramolecular charge transfer may occur
from the donor unit to the acceptor unit upon excitation, such
chromophores are strong light-absorbing dyes possessing a broad
absorption window extending to the near-IR region. More impor-
tantly, the band-gap of D-␲-A chromophores can be simply
controlled by adjusting the electron donating strength of the donor
and the electron withdrawing strength of the acceptor units in the
dye. As a result, D-␲-A chromophores have been used as effective
photoactive materials in energy conversion devices [1–4].
In this context, linear conjugated organic molecules peripher-
ally functionalized with electron donating and electron accepting
species at the extremities are good candidates for the role of active
materials in optoelectronic devices [5–10].
Here we report the synthesis, the spectral characterization and
the photobehaviour of two series of soluble and thermally sta-
ble asymmetric arylacetylene derivatives comprising a fluorene or
Conjugated (poly)arylacetylenes have emerged as a promising
class of semiconducting materials [11–13]. Among their impor-
tant properties are their rigid shape-persistent structures, which
are highly controllable due to precise and efficient synthetic
approaches, their interesting photophysics, and structural diver-
sity.
Note that the fluorene-based compounds 1 have been designed
keeping in mind that ␲-conjugated fluorene-based semiconduc-
tors exhibit a great ability in displaying high-levels of photo- and
electroluminescence as well as useful electronic charge transport
properties [14,15]. Recently, a promising photovoltaic performance
has been achieved through incorporation of such compounds in
bulk heterojunction solar cells [16–18]. Additionally, we report
here anthracene-containing arylacetylene derivatives 2 and 3. It is
known that anthracene structure may enable large carrier mobil-
ities, greater solubility, and improved stability as compared to
pentacene [19,20], which has become a benchmark in the organic
opto-electronic field. Moreover, it is a good chromophore. For these
reasons we synthesized new arylacetylenes 2 as analogues of com-
pounds 1 in which an anthracene unit replaces the fluorene core.
The nitro-derivative 3 was also resynthesized [21] for comparison
purposes.
The comparison of the 1–3 spectral and photophysical
behaviour allowed to gain insights into the correlation between
molecular structure and excited state electronic properties. The
effects of the presence of different electronically active periph-
eral substituents on the photobehaviour of these systems have
been also discussed and related to the presence of intramolecular
charge transfer (ICT) states strongly stabilized by polar solvents.
∗
Corresponding author. Tel.: +39 075 5855547; fax: +39 075 5855560.
Corresponding author. Tel.: +39 075 5855575; fax: +39 075 5855598.
E-mail addresses: assunta@unipg.it (A. Marrocchi), faby@unipg.it (A. Spalletti).
∗∗
1010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2011.08.011

R. Flamini et al. / Journal of Photochemistry and Photobiology A: Chemistry 223 (2011) 140–148
141
C12H25
O
C12H25
C12H25
O
C12H25O
X
O
X
R
R
a: X= NO2, R=H
b: X= CHO, R=H
c: X= CN, R=H
Fl-X
a: X= CHO
b: X= CN
1
An-X
2
CHC(CN)2
c: X=
d: X= CN, R= C6H13
C6H13
C6H13
O
O
NO2
An-NO2
3
Scheme 1. Investigated compounds.
Furthermore, we performed semiempirical quantomechanical cal-
culations to estimate the dependence of the spectral properties,
heats of formation, and dipole moments of the ground and excited
states on the structure of the corresponding arylacetylene deriva-
tives. Finally, this study can be helpful to drive the synthesis
towards innovative functionalized materials with progressively
higher performance in optoelectronic devices.
by coupling of 6b with m,p-(dodecyloxy)-4-ethynylbenzene
(1:1.1 molar ratio), following the above procedure. The crude
product was purified by column chromatography (silica gel,
petroleum ether/dichloromethane 1.5:1). Yield: 71% (yellow
crystals); m.p.: 124–126 ◦C (ethyl acetate); 1H NMR (CDCl3) ı:
9.99 (s, 1H), 7.86–7.50 (m, 10H), 7.06 (m, 2H), 6.80 (d, J = 8.3 Hz,
1H), 3.95 (m, 6H), 1.77 (m, 4H), 1.49–1.22 (m, 36H), 0.82 (m,
6H); 13C NMR (CDCl3) ı: 149.8, 143.6, 142.0, 132.0, 130.9, 130.6,
129.6, 128.4, 128.0, 125.0, 120.8, 120.2, 120.1, 116.6, 115.3,
113.3, 69.3, 69.2, 36.6, 31.9, 29.6, 29.4,29.4, 29.2, 26.0, 22.7, 14.1.
An. Calcd. for C54H66O3: C, 84.99; H, 8.72. Found: C, 83.90; H,
8.71%. MS (high resolution, EI) m/z: M+ Calcd: 763.100, Found:
763.092.
2. Experimental
2.1. Synthesis
All reagents were purchased from Sigma–Aldrich Co. and used
without further purification unless otherwise noted. Anhydrous
THF was distilled from Na/benzophenone, and toluene was dis-
tilled from LiAlH4. Petroleum ether was used as the 40–60◦ boiling
fraction. Chromatography was performed on Riedel de Haën silica
gel (230–400 mesh ASTM). Melting points were determined on a
Büchi melting point apparatus and are uncorrected. NMR spectra
were recorded on a Varian Associates VXR-400 multinuclear spec-
trometer (internal SiMe4). Elemental analyses were performed on
a Fisons EA 1108 instrument. Mass spectrometry was performed
with a Thermo Finnegan model LCQ Advantage mass spectrometer.
Synthesis
of
p-[(2-{[m,p-didodecyloxyphenyl]ethynyl}-7-
fluorenyl)ethynyl]benzonitrile (1c). This compound was prepared
by coupling of 6c with m,p-(dodecyloxy)-4-ethynylbenzene (1:1.1
molar ratio), following the above procedure. The crude product
was purified by column chromatography (silica gel, petroleum
ether/dichloromethane 2.5:1). Yield: 37% (yellow crystals); m.p.:
124–125 ◦C (toluene); 1H NMR (CDCl3) ı: 7.68 (m, 4H), 7.58 (m,
6H), 7.05 (m, 2H), 6.80 (d, J = 8.3 Hz, 1H), 3.96 (m, 4H), 3.88 (s, 2H),
1.78 (m, 4H), 1.51 (s, 2H), 1.42 (m, 4H), 1,21 (m, 30H), 0.82 (m, 6H);
13C NMR (CDCl3) ı: 143.6, 140.6, 132.0, 132.0, 130.9, 130.5, 128.3,
128.0, 124.9, 122.5, 120.4, 120.2, 120.1, 116.7, 115.3, 113.3, 69.2,
69.1, 36.5, 31.9, 29.7, 29.6, 29.4, 29.3, 29.2, 26.0, 22.6, 14.1. An.
Calcd. for C54H65NO2: C, 85.33; H, 8.62; N, 1.84. Found: C, 86.90;
H, 8.63; N, 1.80%. MS (high resolution, EI) m/z: M+ Calcd: 760.099,
Found: 760.100.
Synthesis
nitrophenyl)ethynyl]-fluorene
of
2-{[m,p-didodecyloxyphenyl]ethynyl}-7-[(p-
(1a). Dry toluene (6 ml),
bromoderivative 6a (0.42 g, 1.1 mmol), CuI (0.04 g, 0.02 mmol),
Pd(PPh3)4 (0.02 g, 0.02 mmol) and diisopropylamine (3 ml) were
placed in a flask and degassed with argon at 0 ◦C. m,p-(Dodecyloxy)-
4-ethynylbenzene (0.47 g, 1 mmol) was then added and the mixture
was kept at 75 ◦C overnight. Next, the solvent was evaporated
to dryness, and the crude product was purified by column chro-
matography (silica gel, petroleum ether/dichloromethane 4:1).
Yield: 62% (yellow crystals); m.p.: 135–136 ◦C (toluene); 1H NMR
(CDCl3) ı: 8.19 (d, J = 9 Hz, 2H), 7.76–7.50 (m, 8H), 7.09–7.02 (m,
2H), 6.80 (d, J = 8.2 Hz, 1H), 3.95 (m, 6H), 1.77 (m, 4H), 1.51–1.22
(m, 36H), 0.82 (m, 6H); 13C NMR (CDCl3) ı: 149.7, 148.7, 146.8,
143.6, 142.3, 140.5, 132.2, 131.0, 130.6, 130.4, 128.4, 128.0,124.9,
123.7, 122.5, 120.2, 120.2, 116.5, 115.2, 113.1, 95.6, 90.4, 88.3,
87.9, 69.2, 69.1, 36.5, 31.9, 29.7, 29.6, 29.4, 29.4, 29.2, 26.0, 22.7,
14.1. An. Calcd. for C53H65NO4: C, 81.60; H, 8.40; N, 1.80. Found:
C, 82.90; H, 8.39; N, 1.78%. MS (high resolution, EI) m/z: M+ Calcd:
780.088, Found: 780. 089.
Synthesis
of
p-[(2-{[m,p-didodecyloxyphenyl]ethynyl}-9,9-
dihexyl-7-fluorenyl)ethynyl]benzonitrile (1d). This compound
was prepared by coupling of 6a with m,p-(dodecyloxy)-4-
ethynylbenzene (1:1.1 molar ratio), following the above procedure.
The crude product was purified by column chromatography (silica
gel, petroleum ether/dichloromethane 4:1). Yield: 84% (white
crystals); m.p.: 69–70 ◦C (hexane); 1H NMR (CDCl3) ␦: 7.60 (m,
6H), 7.47 (m, 4H), 7.08 (dd, J = 1.95, 1.92 Hz,1H), 7.02 (s, 1H), 6.79
(d, J = 8.3 Hz), 3.97 (m, 4H), 1.92 (m, 4H), 1.78 (m, 4H), 1.50 (s,
2H), 1.42 (m, 5H), 1.21 (m, 33H), 1.04 (m, 12H), 0.83 (m, 6H), 0.71
(m, 6H), 0.55 (m, 4H); 13C NMR (CDCl3) ı: 151.2, 151.1, 149.8,
148.7, 141.6, 140.0, 132.0, 132.0, 131.0, 130.6, 128.4, 126.0, 125.8,
124.9, 122.6, 120.6, 120.0, 120.0, 118.5, 116.6, 115.2, 113.2, 111.3,
95.0, 90.2, 88.6, 88.0, 69.2, 69.1, 55.2, 40.4, 31.9, 31.5, 29.7, 29.6,
29.4, 29.3, 29.2, 26.0, 23.7, 22.6, 22.6, 14.1, 14.0. An. Calcd. for
C66H89NO2: C, 85.38; H, 9.66; N, 1.51. Found: C, 86.52; H, 9.67;
Synthesis
of
p-[(2-{[m,p-didodecyloxyphenyl]ethynyl}-7-
fluorenyl)ethynyl]benzaldehyde (1b). This compound was prepared

142
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N, 1.37%. MS (high resolution, EI) m/z: M+ Calcd: 928.418, Found:
2.3. Spectral and photophysical experiments
928.419.
Synthesis
of
p-[(10-{[m,p-didodecyloxyphenyl]ethynyl}-9-
A Perkin-Elmer Lambda 800 spectrophotometer was used
anthryl)ethynyl]benzaldehyde (2a). This compound was prepared
by coupling of 8a with m,p-(dodecyloxy)-4-ethynylbenzene (1:1.1
molar ratio), following the procedure described for 1a. Reaction
temperature: 50 ◦C. The crude product was purified by column
chromatography (silica gel, petroleum ether/dichloromethane
3:1). Yield: 90% (orange crystals); m.p.: 118–119 ◦C (ethyl acetate);
1H NMR (CDCl3) ı: 10.00 (s, 1H), 8.59–8.66 (m, 4H), 7.60 (m, 4H),
7.30–7.35 (m, 2H), 6.88 (d, 2H, J = 8.4 Hz), 4.03 (m, 4H), 1.83 (m,
4H), 1.47 (m, 7H), 1.22 (s, 29 H), 0.83 (m, 6H); 13C NMR (CDCl3) ı:
191.2, 150.3, 148.9, 132.3, 132.0, 131.8, 129.7, 129.6, 127.4, 127.1,
126.9, 126.7, 125.3, 113.3, 103.6, 90.7, 69.4, 69.1, 31.9, 29.6, 29.4,
29.3, 29.1, 26.0, 22.7, 14.1. An. Calcd. for C55H66O3: C, 85.23; H,
8.58. Found: C, 85.90; H, 8.55%. MS (high resolution, EI) m/z: M+
Calcd: 775.111, Found: 775.112.
for the absorption measurements. The experimental oscilla-
ꢀ
tor strength was derived by f = (4.39 × 10−9/n) ε(ꢀ) dꢀ [22a],
neglecting the refraction index. The fluorescence spectra were
measured in dilute solutions (absorbance <0.1 at the excitation
wavelength, ꢁexc) by a Spex Fluorolog-2 F112AI spectrofluorimeter.
The emission quantum yields were determined at ꢁexc correspond-
ing to the maximum of the first absorption band (ꢁmax) using
tetracene and 9,10-diphenylanthracene in cyclohexane as fluori-
metric standards (ꢂF = 0.17 [22b] and ꢂF = 0.73 [23], respectively,
in aerated solvent).
The fluorescence lifetimes (ꢃF) were measured by a spectrofluo-
rometer based on the single photon counting technique, equipped
for anthracene derivatives with a LED source centered at 461 nm
using an interference filter centered at 460 nm in the excitation
line and a cut-off in emission at 500 nm. For fluorene derivatives
a pulsed lamp filled with H2 at 330 nm and a LED source centered
at 370 nm (using an interference filter centered at 370 nm in the
excitation line and a cut-off in emission at 396 nm) were used. The
resolution time of the experimental set-up is 0.2 and 0.5 ns when
the filled lamp and LED are used as a source, respectively.
All the photophysical measurements were carried out at room
temperature in spectroscopic grade cyclohexane (CH) and acetoni-
trile (AcN) or N–N dimethylformamide (DMF), from Sigma–Aldrich.
When required, the solutions were de-aerated by purging with pure
nitrogen. The parameters reported in the tables are averages of at
least three independent measurements with mean deviation of ca.
10%.
Synthesis
of
p-[(10-{[m,p-didodecyloxyphenyl]ethynyl}-9-
anthryl)ethynyl]benzonitrile (2b). This compound was prepared
by coupling 8b with m,p-(dodecyloxy)-4-ethynylbenzene (1:1.1
molar ratio), following the above procedure. Reaction temperature:
50 ◦C. The crude product was purified by column chromatography
(silica gel, petroleum ether/dichloromethane 4:1). Yield: 70% (red
crystals); m.p.: 144–145 ◦C (ethyl acetate); 1H NMR (CDCl3) ı: 8.65
(m, 2H), 8.57 (m, 2H), 7.80 (d, J = 8.6 Hz, 2H), 7.69 (d, J = 8.6 Hz, 2H),
7.61 (m, 4H), 7.30 (m, 1H), 7.20 (m, 1H), 6.88 (m, 1H), 4.32 (m, 4H),
1.82 (m, 4H), 1.46 (m, 4H), 1.22 (m, 32 H), 0.82 (m, 6H); 13C NMR
(CDCl3) ı: 150.4, 148.9, 132.3, 132.2, 132.0, 131.8, 128.3, 127.5,
127.2, 126.8, 126.7, 125.4, 120.3, 118.5, 116.8, 116.4, 115.2, 113.3,
111.6, 103.7, 100.16, 91.0, 84.8, 69.5, 69.2, 31.9, 29.6, 29.4, 29.3,
29.2, 26.0, 22.7, 14.1. An. Calcd. for C55H65NO2: C, 81.88; H, 8.27;
N, 1.77. Found: C, 82.29; H, 8.28; N, 1.75%. MS (high resolution, EI)
m/z: M+ Calcd: 772.110, Found: 772.110.
2.4. Theoretical calculations
Synthesis
of
{p-[(10-{[m,p-didodecyloxyphenyl]ethynyl}-9-
The theoretical calculations were performed by using the Hyper-
Chem computational package (version 7.52): the heats of formation
and dipole moments were obtained for geometries optimized
by the PM3 method. The electronic spectra (transition energies,
ꢁ, and oscillator strengths, f) were calculated by ZINDO/S at
the PM3 ground state optimized geometries. Calculations of the
configuration interaction included 169 (13 × 13) single excited con-
figurations. The dipole moment of the first singlet excited state was
estimated by ZINDO/S with the function “next lowest”. A Conforma-
tional Search analysis was performed in the case of two compounds
(see Section 3.3 and Scheme 8 and Table 2 in the Supplemen-
tary Data) changing the torsional dihedral angle between the X
group and the adjacent phenyl ring using PM3 as geometry opti-
mization method.
anthryl)ethynyl]benzylidene}malono nitrile (2c). A 50 ml-flask was
charged with aldehyde 2a (0.10 g, 0.13 mmol), malononitrile
(0.026 g, 0.39 mmol), ethanol (16 ml) and dry THF (4 ml). The reac-
tion mixture was kept under nitrogen and magnetically stirred at
45 ◦C for 72 h. The solvent was then removed in vacuo. The residue
was purified by column chromatography (silica gel, petroleum
ether/dichloromethane 2:1) to obtain compound 2c (50% yield)
as a red solid; m.p.: 142–143 ◦C (ethyl acetate); 1H NMR (CDCl3)
ı: 8,65 (m, 2H), 8.56 (m, 2H), 7.93 (d, 2H, J = 8.0 Hz), 7.82 (d, 2H,
J = 8.0 Hz), 7.72 (s, 1H), 7.58–7.65 (m, 4H), 7.30 (dd, 1H, J = 8.0,
2.0 Hz), 6.88 (m, 1 H), 4.03 (m, 4H), 1.81 (m, 4H), 1.45 (m, 6H), 1.21
(m, 30H), 0.82 (m, 6H); 13C NMR (CDCl3) ı: 158.2, 150.4, 148.9,
132.4, 132.3, 131.8, 130.7, 130.3, 127.5, 127.3, 126.7, 125.4, 120.7,
116.7, 116.3, 115.2, 113.7, 113.3, 112.6, 104.0, 101.1, 92.5, 84.8,
69.5, 69.1, 31.9, 29.6, 29.4, 29.3, 29.2, 29.1, 26.0, 22.6, 14.0. An.
Calcd. for C58H66N2O2: C, 84.63; H, 8.08; N, 3.40. Found: C, 85.10;
H, 8.05; N, 3.27%. MS (high resolution, EI) m/z: M+ Calcd: 823.157,
Found: 823.150.
3. Results and discussion
3.1. Synthesis
For details on the preparation of intermediate compounds 6a–d
and 8a and b see Supporting Information.
Schemes 2 and 3 detail the synthetic routes to the new
compounds 1a–d and 2a–c, while compound 3 was synthe-
sized as previously described [21]. The synthesis of compound 1
(Scheme 2) begins with the selective coupling between 2-bromo-7-
iodofluorenes 4 and the alkynes 5 in toluene/diisopropylamine. The
resulting bromo-derivatives 6 (71–85%) are then subjected to addi-
tional coupling with m,p-bis(dodecyloxy)-4-ethynyl benzene to
provide target compound 1 in good yield (37–84%). Pd(PPh3)4/CuI
catalyzed Sonogashira coupling of 9-bromo,10-iodo-anthracene
(7) with terminal alkynes 5a and b led to bromoarylethynyl
anthracenes 8 in high yield (84–90%, Scheme 3). These compounds
were then subjected to coupling with m,p-bis(dodecyloxy)-4-
ethynyl benzene to afford the diarylethynyl anthracenes 2a and
2.2. Thermal stability
Solutions of compounds were prepared in deuterated chlo-
roform and toluene and then transferred into NMR tubes. NMR
tubes were sealed and submersed in water baths maintained at
50 ◦C (chloroform) and at 80 ◦C (toluene). At predetermined times
NMR tubes were removed and their contents analyzed by 1H NMR
spectroscopy. No changes were observed in the spectra as to the
reference ones, for which NMR tubes were kept at room tempera-
ture.

R. Flamini et al. / Journal of Photochemistry and Photobiology A: Chemistry 223 (2011) 140–148
143
i
Br
+
Br
I
X
R
R
R
R
X
a: X= NO
5
a: R= H
4
2
a: X= NO2, R=H
b: X= CHO, R=H
c: X= CN, R=H
6
b: X= CHO
c: X= CN
b: R= C6H13
d: X= CN, R= C6H13
C12H25O
C12H25O
ii
X
R
R
a: X= NO2, R=H
b: X= CHO, R=H
c: X= CN, R=H
Fl-X
1
d: X= CN, R= C6H13
Scheme 2. Synthesis of fluorene-based compounds 1. (i) Pd(PPh3)4/CuI, diisopropylamine, PhMe, 50 ◦C, 71–85% and (ii) m,p-bis(dodecyloxy)-4-ethynyl benzene,
Pd(PPh3)4/CuI, diisopropylamine, PhMe, 75 ◦C, 37–84%.
b (75–99% yield). The synthesis of target compound 2c was accom-
plished by Knoevenagel condensation between aldehyde 2a and
malononitrile, and subsequent dehydration reaction.
absorption of all compounds shows a main bell-shaped band, not
influenced by the solvent polarity, centered around 370 nm in the
case of the fluorene derivatives. The replacement of the fluorene
central ring with an anthracene unit leads to an absorption spec-
trum that is a little structured and extended till 550 nm. In this
case the polar solvent produces a different intensity of the vibronic
peaks in the absorption spectra.
3.2. Spectral properties
In Figs. 1 and 2 the absorption and emission spectra of the fluo-
rene and anthracene derivatives, respectively, are reported in two
solvents of different polarity, namely CH and DMF (or AcN). The
The fluorescence spectra are structured in CH showing
a
very similar shape with a vibrational progression around
i
+
Br
I
Br
X
X
7
5a: X= CHO
8 a: X= CHO
b: X= CN
b: X= CN
C12H25
C12H25
O
ii
O
X
a: X= CHO
b: X= CN
An-X
2
C12H25
O
C12H25
C12H25
O
CN
CN
iii
C12H25O
O
CHO
An-CHC(CN)2
2c
An-CHO 2a
Scheme 3. Synthesis of anthracene-based compounds 2. (i) Pd(PPh3)4/CuI, diisopropylamine, PhMe, 50 ◦C, 84–90%; (ii) m,p-bis(dodecyloxy)-4-ethynyl benzene,
Pd(PPh3)4/CuI, diisopropylamine, PhMe, 50 ◦C, 75–99%; (iii) malononitrile, ethanol/THF, 45 ◦C, 44%.

144
R. Flamini et al. / Journal of Photochemistry and Photobiology A: Chemistry 223 (2011) 140–148
Table 1
Spectral properties of fluorene derivatives (Fl-Xs) in two solvents.a
X
Solvent
ꢁmabasx (nm)
ꢁmF ax (nm)
418, 440
ꢄꢀ˜S,max (cm−1
)
NO2
CH
DMF
337sh, 370
344sh, 370
3100
CHO
CN
CH
DMF
364, 384sh
364
397, 420
520
2280
8240
CH
DMF
364, 380sh
363, 378sh
392, 415
500
1960
7550
CH
DMF
369, 386sh
366, 379sh
396, 419
493
1850
7040
CN(R)
a
The underlined wavelengths refer to the maxima; sh means shoulder.
1300–1400 cm−1, for all the investigated compounds. They are
very sensitive to the solvent polarity, becoming red shifted (par-
ticularly in the case of the NO2-anthryl- and fluorene-derivatives)
bell shaped bands. The observed fluorosolvatochromism causes a
with the absorption spectrum and the mono-exponential decay
found for all the investigated compounds (see Section 3.4 and
Figs. S1–S3 of the Supplementary Data where typical excitation
spectra are compared with the absorption ones).
large increase of the Stokes shift (ꢄꢀ˜S,max, calculated by 1/ꢁmax
−
The calculated dipole moment of the ground state (ꢅ) was found
sizable for these push–pull systems (the biggest one being that of
the nitro-derivatives and the smallest that of An-CHO) and that of
the first excited singlet state (ꢅ* in Table 3) even higher, in agree-
ment with the positive solvatochromism observed for the studied
compounds and discussed in Section 3.2. ꢅ* is roughly twice as
high as ꢅ for the anthracene-derivatives, but it is even higher,
till to three times of ꢅ, in the case of the Fl-Xs. This fact is in
agreement with the large fluorosolvatochromism (high ꢄꢀ˜S,max),
particularly evident for all the fluorene-derivatives and only in
the nitro-anthracene compound which bears the strongest elec-
tron acceptor substituent. The different behaviour of the fluorene
and anthracene derivatives is illustrated in Fig. 3 where the molec-
ular orbitals mostly involved in the first electronic transition for
the nitro-derivatives are shown (MO for the other compounds
are reported in Schemes S1–S7 and Table S1 of the Supplemen-
tary Data). The fluorene unit favours transitions with high charge
transfer character (particularly in Fl-NO2, Fig. 3 and Schemes S1–S4)
while, in the presence of an anthracene unit, the HOMO → LUMO + 1
configuration, that contributes to the first transition with a high
coefficient (0.43 in Table S1 of Supplementary Data), reduces the
charge transfer character of S1. Moreover the first transition seems
to be strongly localized in the anthracene in the case of the other
antracene-derivatives (see Schemes S5–S7) in agreement with sim-
ilar Stokes shifts observed in non-polar and polar solvents (Table 2)
suggesting a scarce contribution of ICT to the excited state compo-
sition of these compounds.
abs
1/ꢁmF ax and reported in Table 1) on going from CH to DMF, as
reported for analogous push–pull systems [24,25]. The only excep-
tion to this behaviour is the compound with the dicyanoethenyl
group where the Stokes shift slightly decreases in the polar sol-
vent (Table 2). As a matter of fact, the so called “anomalous Stokes
shift”, calculated as the difference between the 0,0 transition in
absorption and emission in the two solvents (ꢄꢀ˜S,00 = 730 cm−1
and 1270 cm−1 in CH and DMF, respectively) showed the correct
trend: in fact it doubled on going from a non polar to a polar solvent.
The broad absorption band of An-CH C(CN)2 in CH becomes
narrower and little structured with a blue shifted maximum in the
polar solvent, the fluorescence spectrum loosing the structure and
shifting towards high energies. The broad absorption in non polar
solvent could be due to a superimposition of spectra belonging to
slightly different structures of these flexible molecules (see Section
3.3 and Refs. [26,27]) the polar solvent selecting only the most polar
ones. On the other hand, the structured emission of this compound
points to a relaxed emitting species.
3.3. Quantum-mechanical calculations
Most of these molecules are expected to exist as a mixture of
different conformers [28,29] (due to rotation around the quasi-
single bonds between the triple bonds and the aromatic rings)
where the position of the lateral aliphatic chains is changed (see
the conformations of Fl-CN in Scheme 4, as an example). The results
obtained by theoretical calculations on the different rotamers of
the investigated compounds are collected in Table 3. Generally, the
conformers resulted equally abundant but with very similar dipole
moments and UV–vis spectra, pointing to experimentally indistin-
guishable species. This fact was in agreement with no wavelength
effect on the emission spectrum, the overlapping of the excitation
The calculated spectral properties showed that S1 is reached
by an allowed transition (␲,␲* nature with partial charge transfer
character) for all the investigated compounds with a particularly
high oscillator strength in the case of the fluorene-derivatives and
resulted in good agreement with the experimental absorption spec-
tra and with the kF values (see below, Section 3.4).
Table 2
Spectral properties of anthracene derivatives (An-Xs) in two solvents.a
X
Solvent
ꢁmabasx (nm)
467, 490sh
ꢁmF ax (nm)
510, 546
ꢄꢀ˜S,max (cm−1
)
NO2
CH
AcN
1800
7840
464, 484
780
CHO
CH
DMF
439sh, 460, 480sh
463, 486
497, 530
550
1620
2390
CH
DMF
490, 526sh
459, 479
547, 590
2130
2010
CH C(CN)2
510sh, 530
CN
CH
DMF
436sh, 458, 477sh
460, 483
495, 528
537
1630
2080
a
The underlined wavelengths refer to the maxima; sh means shoulder.

R. Flamini et al. / Journal of Photochemistry and Photobiology A: Chemistry 223 (2011) 140–148
145
OR
CN
OR
CN
OR
OR
conformer A
conformer B
Scheme 4. Conformations of Fl-CN.
Table 3
Calculated spectral properties (transition wavelength, ꢁ, and oscillator strength, f)a, heats of formation (ꢄHf0) and dipole moment of the ground (ꢅ) and excited singlet (ꢅ*)
states for the conformers of anthracene and fluorene derivatives. The transition wavelength and oscillator strength as derived by integrating the first band of the experimental
spectrum in CHCl3, ꢁexp and fexp, respectively, are also reported for comparison.
Compound
Fl-NO2
Conformer
A
ꢅ (D)
ꢅ* (D)
ꢄHf0 (kcal/mol)
−1.436
ꢁ (nm)
f
ꢁexp
fexp
8.21
25.54
371
321
371
321
342
259
343
258
218
345
257
345
257
344
257
345
257
447
286
264
262
429
290
262
429
289
262
432
285
262
433
286
262
429
291
262
1.74
0.71
1.75
0.71
2.60
0.26
2.56
0.25
0.66
2.45
0.30
2.44
0.30
2.40
0.33
2.40
0.33
1.45
0.38
0.60
1.17
1.40
0.22
1.76
1.40
0.23
1.76
1.61
0.50
1.76
1.64
0.54
1.76
1.40
0.27
1.80
S1
S2
S1
S2
S1
S9
S1
S9
S15
S1
S9
S1
S9
S1
S9
S1
S9
Fl-NO2
B
A
B
7.30
4.18
4.79
24.03
11.41
11.32
−1.448
26.683
26.657
378
1.54
Fl-CHOb
Fl-CHOb
367
364
1.98
1.97
Fl-CN
A
B
A
B
3.77
4.28
3.51
4.50
7.81
9.55
9.93
42.608
42.674
Fl-CN
Fl-CN(R)
Fl-CN(R)
An-NO2
9.33
−18.990
−18.969
79.680
368
471
1.36
0.72
10.48
14.43
S1
S10
S13
S14
S1
S9
S13
S1
S9
S13
S1
S9
S13
S1
An-CHO
A
B
A
B
3.79
2.95
5.64
6.22
5.76
6.91
6.04
−10.437
−10.498
108.531
112.988
58.611
An-CHO
465
0.70
An-CH C(CN)2
An-CH = C(CN)2
An-CN
10.26
11.22
8.99
S9
S13
S1
S8
S12
462
0.61
a
The reported values refer to transitions with f > 0.2 and ꢁ > 250 nm.
For this compound the configuration interaction included 100 (10 × 10) single excited configurations.
b
A Conformational Search performed by the PM3 method, chang-
3.4. Photobehaviour
ing the torsion dihedral angle ϕ between the X group and the
adjacent phenyl ring (see Scheme S8 in the Supplementary Data)
confirmed that, in the case of An-CH C(CN)2, a set of differ-
ent conformations having very similar ground state energies but
slightly different transition wavelengths is expected to contribute
to the broadening of the absorption spectrum. The results obtained
for a prototype of An-CH C(CN)2 (where the long side chains
are replaced by two methoxy groups for calculation time prob-
lems, the spectral and physical properties of the system being not
affected by the different length of the side chains) are reported in
Table S2 of the Supporting Data together with those found for a
prototype of An-NO2 for comparison. The different conformations
show different dihedral angles ϕ and also different dipole moments
of the ground state (ꢅ). This fact could be responsible of the nar-
rower absorption band observed in DMF, the polar solvent strongly
stabilizing the molecules with higher ꢅ thus selecting a smaller set
of similar energy geometries.
The emission of the fluorene derivatives (Table 4) is character-
ized by short lifetimes (below 1 ns) that fall under the detection
limit of our apparatus in the case of Fl-NO2 and Fl-CHO. The
Table 4
Photophysical parameters of fluorene derivatives (Fl-Xs) in two solvents.
X
Solvent
ꢂF
ꢃF (ns)
kF (108 s−1
)
NO2
CH
DMF
3 × 10−4
<0.5
<1 × 10−4
CHO
CN
CH
DMF
0.37
0.002
0.2
<0.5
19
CH
DMF
1.0
0.17
0.8
0.7
13
2.4
CH
DMF
1.0
0.22
0.8
0.8
13
2.8
CN(R)

146
R. Flamini et al. / Journal of Photochemistry and Photobiology A: Chemistry 223 (2011) 140–148
X = NO2
CH
X=NO2
CH
AcN
DMF
X= CHO
CH
DMF
CH
DMF
X= CHO
X= CN
CH
X= CH=C(CN)2
CH
DMF
DMF
X= CN(R)
X= CN
CH
DMF
CH
DMF
300
400
500
600
700
λ (nm)
300
400
500
600
700
800
Fig. 1. Normalized absorption and emission spectra of Fl-Xs in two solvents.
λ (nm)
Fig. 2. Normalized absorption and emission spectra of An-Xs in two solvents.
fluorescence quantum yield goes from unity for the cyano-
derivatives to the undetectable value of Fl-NO2 in DMF. The kF
photo-oxidation or photo-degradation of the compounds was
observed during the experiments even under prolonged irradiation.
The reduction of fluorescence and the huge red shift of the emis-
sion spectra in DMF (see Section 3.2) could indicate the internal
conversion (IC) to the ground state as the main process deactivat-
ing S1 of these fluorene derivatives in polar solvents, as observed
in analogous compounds [33].
When the central fluorene ring is replaced by an anthracene
unit, the photobehaviour becomes quite different (Table 5). In fact,
the radiative decay is the largely prevalent deactivation channel of
9
values resulted high ( 10 s−1) in CH, in agreement with a full
∼
=
allowed transition, as suggested by theoretical calculations (Section
8
−1
)
∼
3.3), and decreased by almost an order of magnitude ( 2 × 10 s
=
in DMF pointing to a different nature of the emitting state in polar
solvents. This fact well agrees with the spectral results showing the
presence of an ICT state, strongly stabilized by the polar solvent,
that becomes the emitting state.
The low ꢂF value found for Fl-CHO, and particularly for Fl-NO2,
is expected [24,30,31] and probably due to an increased ISC rate
constant for the presence of 3n,␲* states located near the S1 of
␲,␲* character. In fact, preliminary results obtained by ns flash
photolysis showed the presence, in these two systems, of a triplet
state efficiently populated by direct excitation [32]. No sign of
S1 even in a polar solvent. The only exception is An-NO2 in AcN (see
Table 5) where ꢂF collapsed to a very low value.
The fluorescence lifetime is around 3 ns for all the anthryl-
derivatives in the two solvents (CH and DMF) giving a kF value of

R. Flamini et al. / Journal of Photochemistry and Photobiology A: Chemistry 223 (2011) 140–148
147
Fig. 3. Molecular orbitals mainly involved in the first electronic transition of the nitro-derivatives of the investigated compounds, as derived by ZINDO/S method using PM3
ground state optimized geometries.
∼3 × 108 s−1, lower than that of fluorene analogues, in agreement
The photophysical and spectral behaviour suggests that ICT is
not helped by the introduction of anthracene as a central ring,
the noticeable exception being An-NO2 due to the presence of the
strongest electron acceptor group.
As a matter of fact, a non-negligible reduction of ꢂF was observed
in the case of An-CH C(CN)2 on going from CH to DMF. This
fact would indicate the presence of a partial ICT character of the
excited singlet state in this system bearing two electron-drawing
CN groups, as suggested by the calculated MO involved in the first
transition (see Table S1 and Scheme S6 of the Supplementary Data).
with the oscillator strength calculated by ZINDO/S and derived by
the experimental absorption spectra (f and fexp, respectively, see
Table 3). In An-Xs, kF does not change on going from CH to DMF
(or AcN), indicating that the fluorescence originates from the same
state in both solvents.
Table 5
Photophysical properties of anthracene derivatives (An-Xs) in two solvents.
X
Solvent
ꢂF
ꢃF (ns)
kF (108 s−1
)
NO2
CH
AcN
0.98
2.8
3.5
3 × 10−4
4. Conclusions
CHO
CH
DMF
0.90
0.80
2.7
2.7
3.3
3.0
The present study showed a peculiar photobehaviour of these
push–pull systems depending on the nature of the electron accep-
tor group in the side benzene and/or on the different central ring
(namely anthracene or fluorene) as summarized in the following
items:
CH
DMF
0.94
0.58
2.5
2.8
3.8
2.1
CH C(CN)2
CN
CH
DMF
0.84
0.77
3.0
2.9
2.8
2.7

148
R. Flamini et al. / Journal of Photochemistry and Photobiology A: Chemistry 223 (2011) 140–148
- The presence of the anthracene unit in the central moiety of the
structures leads to an absorption spectrum placed at longer wave-
lenghts (more than 100 nm) that makes these anthryl derivatives
very interesting for solar energy storage.
- The fluorene unit favours transitions with high charge transfer
character while the absorption seems to be strongly localized
in the anthracene derivatives, as evidenced by the spectral
behaviour in non-polar and polar solvents and the strong reduc-
tion of the fluorescence in favour to IC in the polar solvent in the
case of Fl-Xs only. The calculated MO for the two series of com-
pounds nicely confirmed the presence of ICT states and localized
transitions in the case of fluorene- and anthracene-derivatives,
respectively.
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Acknowledgments
The authors thank MIUR (Ministero dell’Università e della
Ricerca, Rome, Italy) and the University of Perugia (PRIN 2008),
and Fondazione Cassa di Risparmio and Regione Umbria (POR FSE
2007-2013, Risorse CIPE, Perugia, Italy), for fundings. The authors
are also grateful to Dr. Luca Bianchi and Dr. Mirko Seri for their
contribution to the experimental part of this work.
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