Synthesis and photophysical properties of conjugated anthracene-based compounds

Article abstract of DOI:10.1016/j.jphotochem.2010.03.002

Four novel diarylanthracene derivatives (1, 5a, 5b, 6) have been synthesized by Suzuki and Sonogashira coupling reactions, characterized and their structure assigned by ¹H and ¹³C NMR spectroscopy. The spectral and photophysical properties of a series of symmetrically and asymmetrically disubstituted anthracene-based compounds (1-6) and of benzothiadiazole derivative 7 have been studied. All these photostable systems show high fluorescence emission and a batochromic shift of the absorption spectrum that increase with π-conjugation. Flash photolysis measurements were carried out on the poor fluorescent compound 4.

Full text of DOI:10.1016/j.jphotochem.2010.03.002

Journal of Photochemistry and Photobiology A: Chemistry 211 (2010) 162–169
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis and photophysical properties of conjugated
anthracene-based compounds
Assunta Marrocchia,b, Anna Spallettia,b,∗, Serena Ciorbaa,b, Mirko Seria,
Fausto Eliseia,b, Aldo Taticchia,b
a Dipartimento di Chimica, Università degli Studi di Perugia, Via Elce di Sotto, 8, 06123 Perugia, Italy
b Centro di Eccellenza Materiali Innovativi Nanostrutturati (CEMIN), Università degli Studi di Perugia, 06123 Perugia, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Four novel diarylanthracene derivatives (1, 5a, 5b, 6) have been synthesized by Suzuki and Sonogashira
coupling reactions, characterized and their structure assigned by 1H and 13C NMR spectroscopy. The
spectral and photophysical properties of a series of symmetrically and asymmetrically disubstituted
anthracene-based compounds (1–6) and of benzothiadiazole derivative 7 have been studied. All these
photostable systems show high fluorescence emission and a batochromic shift of the absorption spec-
trum that increase with ␲-conjugation. Flash photolysis measurements were carried out on the poor
fluorescent compound 4.
Received 9 December 2009
Received in revised form 1 March 2010
Accepted 6 March 2010
Available online 15 March 2010
Keywords:
Anthracene derivatives
Arylacetylenes
© 2010 Elsevier B.V. All rights reserved.
Synthesis
Spectral/photophysical properties
1. Introduction
luminescence quantum yield. Indeed, we demonstrated that
soluble one-dimensional ␲-conjugated anthracene derivatives
Organic materials characterized by an extended ␲-conjugation
are extensively investigated for their applications in organic and
molecular electronics [1]. Considerable efforts have been directed
towards the synthesis as well as the study of the photophysical
properties of different types of molecular and polymeric conjugated
systems, with the aim to exploit their potential as materials for
efficient, stable, and easy processable devices.
Soluble organic molecules are promising materials for electron-
ics since they can be deposited from solution, e.g. by a printing
process, and are therefore compatible with low cost roll-to-roll
fabrication technology [2]. Moreover, they offer advantages over
polymeric materials in terms of easy synthesis and purification, and
generally exhibit high charge carrier mobilities. Most important,
molecular materials do not suffer from the structural variability of
polymers, namely the control of regioregularity, molecular weight
and polydispersity.
with acetylenic triple bonds as conjugation bridges, are promising
candidates for organic semiconductors.
The present report deals with the synthesis and photophysi-
cal properties of the new anthracenes 1, 5a, 5b and 6 (Fig. 1), to
further elucidate architecture-electronic properties relationships
within this semiconductor class. The detailed study of the spectral
and photophysical properties of these compounds has been also
extended to the anthracenes 2–4 [3d–f] (Fig. 1), with the aim to
evidence the relationship between molecular structure and prop-
erties of the excited states in order to drive next syntheses towards
compounds more useful as active materials in optoelectronics.
In particular, the design of the rod-shaped derivatives 1–4,
involves the extension of the ␲-conjugation of anthracene unit
by introducing aryl groups at the 9- and 10-positions, that are
connected by saturated or multiple linkages. The addition of end
hexyloxy-chains to the backbone is expected to favour the ordered
arrangements of the molecules in their solid state [4], which facili-
tates the charge transport through their ␲–␲ orbital stacking. These
groups might also increase the material solubility, so that solution-
processable devices may be fabricated.
Recently [3] we described the synthesis of
a series of
anthracene-containing semiconductors, and their use as active
layer in OTFTs as well as photovoltaic devices. Anthracene deriva-
tives may exhibit large charge carrier mobility and excellent
stability. Moreover, they generally show very high photo-
Asymmetrically substituted systems 5a and 5b were also syn-
thesized, as analogues of compound 2 in which an electron acceptor
group replaces the alkoxy chains at one end of the molecule; though
investigated for decades, interest in push-pull chromophores, with
strong electron-donors and electron-acceptors connected by ␲-
conjugating spacers, is still growing, in view of their outstanding
∗
Corresponding author at: University of Perugia, Department of Chemistry, Via
Elce di Sotto, 8, 06123 Perugia, PG, Italy. Tel.: +39 75 5855575; fax: +39 75 5855598.
E-mail address: faby@unipg.it (A. Spalletti).
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.03.002

A. Marrocchi et al. / Journal of Photochemistry and Photobiology A: Chemistry 211 (2010) 162–169
163
Fig. 1. Investigated compounds.
opto-electronic properties, in particular their non-linear optical
behaviour, and their potential for application as advanced func-
tional materials in molecular devices [1].
The doubly end-capped dianthryl compound 6 has been also
considered, in order to study the effect of the terminal anthracene
moieties on the spectral properties.
Finally, the comparison of the spectral and photophysical prop-
erties of compounds 2 and 3 with those of 7, in which the anthryl
core has been replaced by a benzothiadiazole unit [3f], allows to
get information on the effects of fluorophore substitution.
A PerkinElmer Lambda 800 spectrophotometer was used for
the absorption measurements. The fluorescence spectra were
recorded by a Spex Fluorolog-2 F112AI spectrofluorimeter. Dilute
solutions (absorbance less than 0.1 at ꢀexc) were used for fluori-
metric measurements. 9,10-Diphenylanthracene and tetracene in
air-equilibrated cyclohexane were used as fluorimetric standards
(ꢁF = 0.73 [5] and 0.17 [6], respectively). Fluorescence lifetimes
were measured by an Edinburgh Instrument 199S spectrofluorime-
ter, equipped with a pulsed lamp, filled by N2 or H2, using the
∼
single photon counting method (time resolution 0.5 ns). All the
=
photophysical measurements were carried out at room temper-
ature in spectroscopic grade cyclohexane and chloroform, from
Fluka. When required, the solutions were de-aerated by purging
with pure nitrogen. The fluorescence quantum yields and lifetimes
are averages of at least three independent experiments with mean
deviation of ca. 10%.
2. Materials and methods (experimental)
2.1. General
For time-resolved laser flash photolysis measurements on the
nanosecond time scale, the third harmonic (ꢀexc = 355 nm) from a
Continuum Surelite Nd:YAG laser was used with the energy less
than 1mJ per pulse and 10 ns time resolution. Spectrophotometric
analysis was performed by using a 150 W Xenon source, a Baird-
Tatlock monochomator, a Hamamatsu R29 photomultiplier and a
Tektronix DSA 602 digital analyzer.
Melting points (uncorrected) were determined by a Büchi melt-
ing point apparatus. Chromatography was performed by Riedel
de Haën silica gel (230–400 mesh ASTM). Commercially available
reagents (Sigma–Aldrich Co.) were utilized without further purifi-
cation, unless otherwise noted. Petroleum ether as the 40–60 ◦C
boiling fraction was used. NMR spectra were recorded by a Varian
Associates VXR-400 multinuclear instrument (internal Me4Si).

164
A. Marrocchi et al. / Journal of Photochemistry and Photobiology A: Chemistry 211 (2010) 162–169
Scheme 1.
The ˚T × εT product, for compound 4, was determined by using
137.0, 148.5, 148.9; Anal. calcd. for C50
Found: C, 82.00; H, 9.09%.
H66O4: C, 82.15; H, 9.10.
anthracene in cyclohexane as a standard (εTT = 4.55 × 104 M−1 cm−1
at 432.5 nm, ˚T = 0.71) [7]. The molar absorption coefficient of the
triplet state (εTT) was determined through a sensitization method
by using anthracene as a sensitizer.
9-(3,4-Dihexyloxyphenylethynyl)-10-(p-nitrophenyl-
ethynyl)anthracene (5a). Prepared by coupling compound
12 and 1,2-dihexyloxy-4-ethynylbenzene (13a) (1/1.1) following
the above procedure. Reaction time: 18 h. Reaction temperature
50 ◦C; m.p.: 134–135 ◦C (ethyl acetate); 1H NMR (CDCl3) ı: 8.65
(m, 2H), 8.55 (m, 2H), 8.24 (d, J = 8.8 Hz, 2H), 7.80 (d, J = 8.7 Hz, 2H),
7.61 (m, 4H), 7.30 (d, 1H), 6.88 (d, 1H), 4.01 (m, 4H), 1.82 (m, 4H),
1.62–1.20 (m, 10H), 0.87 (s, 6H); 13C NMR (CDCl3) ı: 150.4, 148.9,
147.0, 132.4, 132.1, 130.3, 127.6, 127.3, 126.7, 125.4, 123.8, 120.6,
116,7, 116.2, 115.2, 113.3, 103.9, 100,0, 92.0, 84.8, 69.5, 69.2, 31.6,
29.2, 29.1, 25.7, 22.6, 14.0; UV–vis (CHCl3) [ꢀmax nm (log ε)] 278
(4.8), 315 (4.3), 474 (4.6). An. Calcd. For C42H41NO4: C, 80.87; H,
6.63; N, 2.25. Found: C, 80.99; H, 6.62; N, 2.23%.
The quantum-mechanical calculations were performed by using
the Hyper-Chem computational package (version 6.1): the heats
of formation and dipole moments were obtained for geometries
optimized by the PM3 method. The electronic spectra (transition
energies and oscillator strengths) were calculated by ZINDO/S at
the optimized geometries. Calculations of the configuration inter-
action included 1600 (40 × 40) single excited configurations. For 7
the starting conformations of minimal energy to be used for the
optimization procedure were obtained by Conformational Search
analysis (MM+ method).
9-(3,4-Didodecyloxyphenylethynyl)-10-(p-nitrophenyl-
ethynyl)anthracene (5b). Prepared by coupling compound 12
and 1,2-didodecyloxy-4-ethynylbenzene (13b) (1/1.1) following
the procedure described for reaction between 11 and 10. Reaction
time: 18 h. Reaction temperature 50 ◦C. Purified by crystallization
from toluene; 86% yield (red crystals); m.p. 125–126 ◦C (toluene);
1H NMR (CDCl3) ı 8.70 (m, 2H), 8.57 (m, 2H), 8.27 (d, J = 8.9 Hz, 2H),
7.85 (d, J = 8.9 Hz, 2H), 7.63 (m, 4H), 7.30 (m, 1H), 7.20 (m, 1H), 6.89
(m, 1H), 4.03 (m, 4H), 1.81 (m, 4H), 1.45 (m, 4H), 1.22 (m, 32H),
0.82 (m, 6H). 13C NMR (CDCl3) ı 150.4, 148.9, 147.0, 132.4, 132.1,
131.8, 130.3, 127.5, 127.3, 126.7, 125.4, 123.8, 120.6, 116.8, 116.2,
115.2, 113.3, 103.9, 100.0, 92.0, 84.8, 69.5, 69.2, 47.9, 31.9, 29.7,
29.6, 29.4, 29.34, 29.3, 29.2, 26.03, 26.0, 22.6, 14.1; UV–vis (CHCl3)
[ꢀmax nm (log ε)] 278 (4.8), 472 (4.64). An. Calcd. for C54H65NO4:
C, 81.88; H, 8.27; N, 1.77. Found: C, 82.09; H, 8.26; N, 1.79%.
3,3ꢀ-Dihexyloxy-4,4ꢀ-diethynylanthracene-1,1ꢀ-biphenyl (6).
Dry toluene (3 ml), 9-ethynyl-anthracene (16) (0.11 g, 0.52 mmol),
CuI (0.002 g, 0.010 mmol), (0.15 g, 0.25 mmol), Pd(Ph3)4 (0.012 g,
0.010 mmol) and i-Pr2NH (1.5 ml) were placed in a flask and
degassed with Ar. The mixture was heated at 55 ◦C for 18 h,
2.2. Synthesis
9,10-Di[3,4-dihexyloxyphenyl]anthracene (1). To a solution of
boroderivate 8 (0.404 g, 1 mmol) in dry toluene (3 ml) and under
Ar atmosphere, was added 9,10-dibromoanthracene (9) (0.134 g,
0.4 mmol), Aliquat (0.081 g, 0.2 mmol) and Pd(PPh3)4 (0.046 g,
0.04 mmol). To this mixture was added a 1 M Na2CO3 aqueous solu-
tion (17 ml) degassed for 2 h. The mixture was then stirred for 24 h
under reflux. The aqueous layer was extracted with CH2Cl2. The
organic layer was dried with Na2SO4, filtered and concentrated in
vacuo. The crude product was purified by column chromatogra-
phy on silica gel, eluting with petroleum ether/CH2Cl2 5:1 to afford
0.150 g of 1 (55% yield); recrystallization from EtOAc gave 1 as
white crystals; m.p. 159–160 ◦C (ethyl acetate); 1H NMR (CDCl3) ı
0.82–0.84 (m, 6H), 0.89–0.93 (m, 6H), 1.25–1.57 (m, 24H), 1.75–1.82
(m, 4H), 1.85–1.93 (m, 4H), 3.87–4.00 (m, 4H), 4.09–4.13 (m, 4H),
6.92–6.96 (m, 4H), 7.05–7.07 (m, 2H), 7.27–7.31 (m, 4H), 7.72–7.76
(m, 4H); 13C NMR (CDCl3) ı 14.0, 22.6, 22.7, 25.7, 25.8, 29.2, 29.4,
31.6, 31.7, 69.2, 69.3, 113.5, 116.8, 123.6, 124.9, 127.1, 130.1, 131.5,
Scheme 2.

A. Marrocchi et al. / Journal of Photochemistry and Photobiology A: Chemistry 211 (2010) 162–169
165
Scheme 3.
then the solvent was removed under reduced pressure and the
residue was purified by column chromatography on silica gel (hex-
ane/dichloromethane 85:15) to afford pure 6 as a yellow solid (62%
yield); m.p. 203–204 ◦C (toluene); 1H NMR (CDCl3) ı 0.80 (m, 6H);
1.25–1.40 (m, 12H); 1.60 (m, 4H); 2.06 (m, 4H); 4.23 (m, 4H); 7.18
(m, 2H), 7.25 (m, 2H), 7.46–7.60 (m, 8H), 7.77 (m, 2H), 7.98 (d, 2H,
J = 8.0 Hz), 8.38 (s, 2H), 8.78 (d, 2H, J = 8.8 Hz); 13C NMR (CDCl3) ı
165.3, 135.8, 131.5, 129.4, 128.6, 127.9, 126.7, 119.5, 96.7, 92.3,
68.9, 31.9, 28.8, 26.0, 23.2, 13.7. Anal. calcd. for C56H50O2: C, 89.09;
H, 6.68. Found: C, 88.95; H, 6.66%.
For details on the preparation of intermediate compounds 12
and 15 see Supporting Information.
3. Results and discussion
3.1. Synthesis
9,10-Diarylanthracene 1 was prepared by Suzuki cross-coupling
reaction [8] between boro-derivative 8 and the commercial 9,10-
dibromoanthracene (9) in the presence of the liquid phase-transfer
catalyst Aliquat 336 (tricaprylmethylammonium chloride, A336) in
two-phase solvent system (toluene/water) (Scheme 1). The boro-
derivative 8 was synthesized starting from catechol according to
previously described procedures [9].
Fig. 2. Normalized absorption (A) and fluorescence emission (B) spectra of the stud-
ied compounds in cyclohexane at room temperature.
The unsymmetrically substituted anthracene derivatives
5
were obtained by the repetitive Cu/Pd catalysed cross-coupling
Sonogashira reaction [10]. Coupling reaction of 9-bromo-10-
iodo-anthracene (10), prepared from 9,10-dibromoanthracene (9)
[11], with p-nitro-ethynylbenzene (11) led to bromoarylethynyl
anthracene 12 in high yield (90%) (Scheme 2). This compound was
then subjected to coupling with 3,4-dialkoxyethynylbenzenes 13a
and 13b to afford the diarylethynylanthracenes 5a and 5b, respec-
tively, in high yield (61 and 75%) (Scheme 2).
3,3ꢀ-Dihexyloxy-4,4ꢀ-diarylethynylanthracene-1,1ꢀ-biphenyl
(6) was synthesized by cross-coupling Sonogashira reaction
between 3,3ꢀ-dihexyloxy-4,4ꢀ-diiodo-1,1ꢀ-biphenyl (15), obtained
by the corresponding 4,4ꢀ-diiodo-3,3ꢀ-biphenol (14) [12], and
9-ethynyl-anthracene [13] (16) in good yield (62%) (Scheme 3).
The structures of all new compounds, i.e. 1, 5a, 5b, 6, 12 and 15
were unambiguously confirmed by 1H and 13C NMR spectroscopy
and elemental analysis. Compounds 2, 3, 4 and 7 have been re-
prepared according to previously reported procedures [3d–f].
3.2. Spectral behaviour
Normalized absorption and emission spectra of the investigated
compounds in cyclohexane are shown in Figs. 2 and 3 and spectral
properties are collected in Tables 1 and 2.
Fig. 3. Normalized absorption and fluorescence emission spectra of 4 in cyclohexane
at room temperature (the spectra of 2 are reported for comparison).

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Table 1
Absorption properties (absorption maxima wavelength, ꢀmabasx, and coefficient, εmax
of compounds 1–7 in chloroform at room temperaturea.
)
max
abs
Compound
ꢀ
(nm)
εmax (104 M−1 cm−1
)
1
2
3
4
292sh, 34sh, 359, 377, 398
306, 322sh, 457, 475sh
331, 349, 442sh, 464, 488
267, 317, 418
0.91, 0.45, 1.00, 1.65, 1.59
2.44, 2.61, 4.30, 3.98
5.76, 5.47, 5.32, 7.19, 6.53
9.50, 1.91, 2.36
5a
5b
6
266sh, 278, 315sh, 472, 494sh
266sh, 278, 315sh, 473, 494sh
300, 347, 393sh, 419, 444
358, 502
5.57, 6.30, 2.30, 3.91, 3.52
5.59, 6.33, 2.32, 3.96, 3.57
1.55, 1.63, 2.36, 4.85, 6.58
5.03, 3.72
7
a
The underlined wavelengths refer to the main maxima, and sh means shoulder.
Except for 1 (which does not absorb at wavelengths larger than
420 nm), the absorption spectra generally show an intense band
in the visible region, 2, 3, 5 and 7 having a second strong transi-
tion in the 290–400 nm range. Also the fluorescence spectra are
intense, being the radiative process the largely main deactivation
pathway, with the exception of the olefinic derivative 4 (see below).
The spectra are shifted towards the red on increasing the extended
conjugation through the system of aryl groups and triple bonds.
The asymmetrically substituted compounds (5), containing a nitro-
group, show absorption and emission spectra red-shifted of about
15/20 nm with respect to the symmetric analogous 2. The length
of the side aliphatic chain (–C6H13 or –C12H25) does not affect the
spectral behaviour of these compounds.
Fig. 4. Emission spectra of 7 in cyclohexane at room temperature as a function of
ꢀexc
.
significant blue shift of the conjugation absorption band and the
complete loss of the structure of the fluorescence spectrum that
becomes a very large bell-shaped band with a strong displacement
of the maximum towards the red. The blue shift of the absorp-
tion and the loss of resolution of both absorption and emission
spectra could be explained by the presence of sets of non-planar
structures caused by geometric distortions around the single bonds
between the anthryl group and the ethenic carbons, as previously
reported for the parent 9-styrylanthracene and analogous flexible
compounds [15,16].
The huge Stokes shift observed for 4 (see Table 2) is reminis-
cent of a large one reported for the styrylanthracene in particular
when the anthracene is length to the ethylenic bridge in 9 position
[15,17]. An increase of planarity between the anthryl group and the
two ethylenic bridges on excitation is probably responsible for this
behaviour [15].
The spectra of anthracene derivatives are more or less struc-
tured (with the exception of 4) with a vibronic progression of
−1
1000–1400 cm−1 for the absorption (ꢂꢃ˜abs) and 1300 cm for
∼
=
the emission (ꢂꢃ˜F), characteristic of a transition delocalized on
the anthracene and the adjacent phenyl-triple-bonds moiety (see
Scheme S1 in the Supporting Data) in the case of 2 and 3, as found in
the analogous unsubstituted 9,10-bis(phenylethynyl)anthracene
[14a] (see Table 2).
The first transition of 1 is mainly localized in the anthryl group
as evidenced by the typically structured absorption spectrum that
shows vibronic peaks of feature and relative intensities similar to
those of anthracene [14b], but red-shifted of about 20 nm, as con-
firmed by calculations (Scheme S2 in the Supporting Data).
In the case of 5a and 5b, the first absorption band is due to
an allowed transition with a partial charge-transfer character as
shown by calculations (Scheme S3).
The presence of two anthracene units in 6 does not cause a sig-
nificant red shift of the absorption which however shows a very
large absorption coefficient pointing to a spectral behaviour of 6 as
due to two separate chromophores.
Compound 7 showed the most red-shifted absorption spectrum
whose maximum well overlaps that of the solar spectrum.
The high absorption coefficients measured for these compounds
(particularly in the case of 3) and reported in Table 1 in chloroform,
make them very useful as solar absorbers in photovoltaic devices.
The comparison between 2 and 4 spectra (Fig. 3) show that
the replacement of two triple with two double bonds causes a
The experimental findings are nicely confirmed by calculations
(see next section).
A negligible dependence of the shape of the fluorescence exci-
tation and emission spectra on the emission wavelength (ꢀem
)
and excitation wavelength (ꢀexc), respectively, was found for all
the investigated compounds, together with an excitation spectrum
practically overlapping the absorption. This behaviour is probably
due to the presence of a prevalent conformer or to very similar spec-
tral properties of abundant species as discussed in Section 3.3. Only
in the case of 7 the bell-shaped fluorescence spectrum is slightly
affected by the excitation energy as shown in Fig. 4.
3.3. Quantum-mechanical calculations
The compounds 1–7 could exist, in principle, as mixtures of
rotamers originated by the free rotation around quasi-single bonds
Table 2
Spectral behaviour of compounds 1–7 in cyclohexane at room temperaturea.
max
abs
b
Compound
ꢀ
(nm)
ꢂꢃ˜abs (cm−1
)
ꢀmF ax (nm)
ꢂꢃ˜F (cm−1
)
Stokes shift (cm−1
)
1
2
3
4
339, 357, 375, 395
433sh, 454, 478sh
437sh, 460, 485
413
1350
1100
1100
421, 443sh, 467sh
488, 521, 556sh
497, 532, 561
615
1200
1300
1300
2910 (1560)
1530 (430)
1620 (500)
7950
1680 (680)
750
5a/5b
6
7
448sh, 469, 492sh
391sh,413, 439
505
1000
1400
509, 545, 586sh
454, 482.5, 509sh
600
1290
1300
3140
a
The underlined wavelengths refer to the main maxima, and sh means shoulder.
The values in parentheses refer to the Stokes shift calculated between the absorption and emission 0,0 transitions.
b

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167
Fig. 5. Conformational equilibrium of 2 and 4.
connecting aromatic groups and triple (or double) bonds [18,19].
The possible rotamers for the compounds 2 and 4 are reported in
Fig. 5, as an example.
Table S1 reports the heat of formation, dipole moment and spec-
tral properties calculated at the PM3 optimized geometries of the
rotamers of the investigated compounds.
All the compounds were found to show a rigid and almost planar
structure (neglecting the aliphatic chains) with the exception of
compounds 1, 4 and 7.
the absorption coefficient reported in Table 1. As an example, the
increase of the conjugation going from 2 to 3 causes an increase
of f from 1.19 to 1.84, parallel to the increase of εmax from 4.30 to
7.19 × 104 M−1 cm−1
.
Moreover, for the compound
1
having the lowest εmax
=
(1.65 × 104 M−1 cm−1) the smallest f ( 0.4/0.5) was calculated.
∼
3.4. Photophysical properties
The nitro-derivatives 5 exist as a unique conformer, the rota-
tion around the quasi-single bond producing identical species. For
1 and the other symmetric compounds containing the anthracene
group and triple bonds (2, 3, 6) the calculations showed that the
two possible conformers with the aliphatic chains on the same
(asymmetrical) or on the opposite (symmetrical) side with respect
to the molecular long axis (e.g., see compound 2 in Fig. 5), have
differentdipole moments but identical absorption spectra, in agree-
ment with the absence of wavelength effect on the excitation and
emission spectra. More complex is the case of 4 (Fig. 5) and even
more of 7, where several conformers are expected to be present
in the ground state. For 7 only the PM3 optimized and most sta-
ble conformation is reported in Fig. 6 and its relative calculated
parameters in Table S1 (see Supplementary Data) [20]. The con-
former with the planar central moiety with short intramolecular
S· · ·N contacts [20] (reported as the most stable in analogous com-
pounds on the base of crystallographic data) [21] is placed at
sizable higher energy by our calculations (PM3 method) indicat-
ing its negligible contribution to the conformational equilibrium
in solution at room temperature. The calculations reproduce quite
well the maxima of the experimental absorption spectra. Also the
calculated oscillator strength (f) is reasonably in agreement with
The fluorescence parameters of the compounds 1–7 are reported
in Table 3. These systems are efficient fluorophores, the high emis-
sion quantum yields being little affected by oxygen, as expected
by the relatively short fluorescence lifetimes (in the range of few
nanoseconds). The presence of the nitro-group does not affect the
photobehaviour of these molecules, the fluorescence being the
unique relaxation channel of their excited singlet state, contrary to
the case of other nitro-derivatives where the emission decreases in
favour of an efficient intersystem crossing (ISC) [22,23]. The emis-
Fig. 6. Most stable conformation of 7, as optimized by the PM3 method.

168
A. Marrocchi et al. / Journal of Photochemistry and Photobiology A: Chemistry 211 (2010) 162–169
Table 3
Strong steric interactions occur when anthracene is placed
between two single bonds, as in 1, or two double bonds, as in 4;
this leads to a blue shift of the absorption spectra of approximately
70 nm, as compared with the case where anthracene is adjacent
to two triple bonds (2). Indeed, the high absorption coefficient
observed for anthracene-containing arylacetylenes 2 and 3 make
them potentially useful solar absorbers in organic photovoltaic
devices.
The replacement of the anthracene with a dithienylbenzothiadi-
azole unit in 2 causes a further significant red-shift of the absorption
spectrum (more than 40 nm), pointing to the fact that compound 7
remains the most interesting for solar cells applications.
Fluorescence parameters of compounds 1–7 in cyclohexane at room temperature.
a
a
Compound
ꢁF
ꢁF
ꢁF/ꢁF
ꢄF (ns)
kF (108 s−1
)
1
2
3
4
5a
5b
6
0.89
0.99
0.86
0.29
1.00
0.99
1.0
0.70
0.91
0.80
0.26
0.93
0.93
0.88
0.73
1.3
1.1
1.1
1.1
1.1
1.1
1.1
1.0
5.9
3.0
2.0
3.6
3.0
2.9
1.8
4.6
1.5
3.1
4.1
0.72
3.3
3.3
5.6
1.6
7
0.76
a
In air-equilibrated solutions.
sion decay was mono-exponential for all compounds (ꢅ2 in the
range 0.92–1.04, see Figs. S1–S7), in agreement with the negligible
dependence of the fluorescence excitation and emission spectra on
ꢀem and ꢀexc, respectively, and the calculation results that point
to the presence of a prevalent species in the case of the olefinic
derivative and to very similar spectral and photophysical properties
of conformers of comparable abundance for the other acetylenic
compounds. The measured lifetime for 7 is probably a mean value
among quite similar lifetimes of several conformations.
Acknowledgments
The Authors thank MIUR (Ministero dell’Università e della
Ricerca), Rome, Italy, Fondazione Cassa di Risparmio and Regione
Umbria (POR FSE 2007-2013, Risorse CIPE), Perugia, Italy, for fund-
ings.
Appendix A. Supplementary data
All the compounds are photostable. No photoproducts were
found under prolonged irradiation by monochromatic light at the
absorption maximum. For compound 4, this photostability (the
disappearance yield, at ꢀexc = 436 nm, is around 10−4 in cyclohex-
ane) together with a reduced fluorescence quantum yield (see
Table 3), points to the presence of non-reactive, non-radiative
deactivation pathways of its excited states (S1 → S0 internal con-
version and/or S1 → T1 intersystem crossing, ISC, to a non-reactive
triplet state followed by ISC to the ground state), in analogy with
the parent 9-styrylanthracene [24]. The transient spectrum of 4
observed on direct laser excitation (ꢀmax = 490 nm, see Fig. S8), was
assigned to T1 → Tn absorption since it was quenched by oxygen
at nearly diffusion rate and generated by photosensitization using
anthracene as triplet donor. εTT = 4.8 × 104 M−1 cm−1 in cyclohex-
ane at ꢀmax = 490 nm was determined for 4, very similar to that
of anthracene. The measured relatively low triplet quantum yield
(ꢁT = 0.01) of 4 does not account for all the non-radiative decay
pointing to a prevalent deactivation of the singlet excited state
through internal conversion.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jphotochem.2010.03.002.
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