Theoretical and experimental study of donor-bridge-acceptor system: model 2-[5-(9H-fluoren-9-ylidenemethyl)thiophen-2-yl]-1,3,4-oxadiazole derivatives

Article abstract of DOI:10.1007/s00706-016-1843-2

Abstract: The theoretical study of the titled model acceptor-bridge-donor molecular series based on 9H-fluoren-9-ylidenemethyl and 1,3,4-oxadiazole heterocycles is presented. The subunits are linked through a thienyl moiety, and the second α-site of oxadiazole was modified. The quantum chemical model indicated that the electron-donating properties of 9H-fluoren-9-ylidenemethyl were significantly supported or suppressed by the terminal substitution of 1,3,4-oxadiazole which affected the possible semiconductivity. Finally, the possible synthesis of selected molecules is described, and the prepared derivatives were fully characterized by ¹H NMR, ¹³C NMR, IR, and elemental analysis together with optical and electrochemical measurements. Graphical abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s00706-016-1843-2

Monatsh Chem
DOI 10.1007/s00706-016-1843-2
ORIGINAL PAPER
Theoretical and experimental study of donor-bridge-acceptor
system: model 2-[5-(9H-fluoren-9-ylidenemethyl)thiophen-2-yl]-
1,3,4-oxadiazole derivatives
1
2
•
Martin Michalık Anita Andicsova Eckstein Erika Kozma3
•
•
´
´
Francesco Galeotti3 Vladimır Lukes Pavel Hrdlovic Daniel Vegh
4
´
ˇ1
ˇ2
´
•
•
•
Received: 2 June 2016 / Accepted: 1 September 2016
Ó Springer-Verlag Wien 2016
Abstract The theoretical study of the titled model accep-
tor-bridge-donor molecular series based on 9H-fluoren-9-
ylidenemethyl and 1,3,4-oxadiazole heterocycles is pre-
sented. The subunits are linked through a thienyl moiety,
and the second a-site of oxadiazole was modified. The
quantum chemical model indicated that the electron-do-
nating properties of 9H-fluoren-9-ylidenemethyl were
significantly supported or suppressed by the terminal sub-
stitution of 1,3,4-oxadiazole which affected the possible
semiconductivity. Finally, the possible synthesis of selec-
ted molecules is described, and the prepared derivatives
Graphical abstract
1
were fully characterized by H NMR, 13C NMR, IR, and
Keywords Heterocycles Á Absorption spectra Á
Electronic structure Á Computational chemistry
elemental analysis together with optical and electrochem-
ical measurements.
Introduction
Conjugated organic molecules with suitable physical–
chemical properties are attractive modern materials for
academic and industrial applications. The small molecular
units in comparison with their polymer counterparts have the
advantage of a better synthetic reproducibility, higher purity,
and no batch-to-batch modification. In the last years, organic
molecule-based materials became a key factor in the devel-
opment of many fields, such as organic solar cells [1–3],
electroluminescent devices [4], memory devices, sensors,
transistors, batteries, photoreceptors, and displays [5]. Very
often, the modern material research of novel organic mole-
cules suitable for the application in electronics is connected
with the tuning of the optical and electrochemical properties,
e.g., using the combination of chromophoric moieties.
Synthetic strategies for the optimization of material prop-
erties could involve the addition of an electron donor
(D) and/or acceptor (A) substituents in the specific positions
Electronic supplementary material The online version of this
article (doi:10.1007/s00706-016-1843-2) contains supplementary
material, which is available to authorized users.
´
& Martin Michalık
martin.michalik@stuba.sk
1
Department of Chemical Physics, Slovak University of
´
Technology, Radlinskeho 9, SK-812 37 Bratislava, Slovak
Republic
2
3
4
´
´
Polymer Institute, Slovak Academy of Sciences, Dubravska
Cesta 9, 845 41 Bratislava, Slovak Republic
Istituto per lo Studio delle Macromolecole, Consiglio
Nazionale delle Ricerche, Via Corti 12, 20133 Milan, Italy
Department of Organic Chemistry, Slovak University of
´
Technology, Radlinskeho 9, SK-812 37 Bratislava, Slovak
Republic
123

´
M. Michalık et al.
of the molecule which modulate the energy gaps between the
frontier molecular orbitals, i.e., highest occupied (HOMO)
and lowest unoccupied (LUMO) molecular orbitals. Energy
gap between the HOMO and the LUMO is closely related to
the solid-state bandgap, although the latter is usually lower.
From the macroscopic point of view, the electric semicon-
ductivity in the organic materials is also dependent on the
position of Fermi energy level and the energy bandgap
between the valence and conduction bands. In general,
electric conductivity increases with the reduction of the
energy bandgap [6].
systems and their notation are presented in Fig. 1. The partial
aims of this study are: (1) to investigate systematically the
role of substitution on the electronic structure using the
densityfunctionaltheory; (2)tosuggest thepossiblesynthetic
route; and (3) to apply it for the synthesis of selected mole-
cules. Finally, the theoretical data will be compared with the
absorption spectra and electrochemical measurements of
synthesized molecules. Based on the molecular orbital anal-
ysis, we will specify which studied derivatives with minimal
corresponding energy gap prefer p-type (low-lying HOMO)
or n-type semiconductivities (high-lying LUMO).
Fluorene and thiophene derivatives [7, 8] usually form
the representative components in several oligomers or
polymers for the construction of light-emitting materials
[9]. Among these molecules, the fluorene units are often
added through single bonds at 2,7 sites [10]. The alkyl
chains connected to the C9 fluorene atom improves the
solubility and the incorporation into the polymer matrix.
Besides, the presence of 1,3,4-oxadiazole units can support
the charge-transport properties as well as the thermal sta-
bility [11]. In addition, the oxadiazole compounds possess
a broad biological activity [12] and are utilized in many
pharmaceuticals [13, 14] and agrochemicals [15]. From the
chemical point of view, the 1,3,4-oxadiazole linked
through a thienyl moiety to the 9H-fluoren-9-yliden-
emethyl unit represents the unique combination which was
not investigated and published in the literature yet. With
thienyl group as linker between these chromophoric pair
conjugation, length is extended what could play an
important role in electronic devices [16]. It is plausible to
suppose that the terminal substitution of 1,3,4-oxadiazole is
able to modify the electron structure and electron-donating
properties of 9H-fluoren-9-ylidenemethyl unit. These
modifications using the end-capping group method are now
frequently used in a novel material development [17, 18].
Herein, we report the theoretical and subsequent experi-
mental study of novel representative derivatives of 2-[5-(9H-
fluoren-9-ylidenemethyl)thiophen-2-yl]-1,3,4-oxadiazole.
The chemical structures of the acceptor-bridge-donor
Results and discussion
Theoretical geometries and electronic structure
Aromatic rings of the studied compounds are connected by
single bonds which lead to numerous possible conformations
with respect to the spatial setting. In the case of three five-
membered heterocyclic unit connection, up to four confor-
mations can be considered. As it is presented for parental
compounds (Y = H) in Table 1S and Fig. 1S, all-anti con-
formations are slightly energetically favored for FLOX0c
and FLOX1c. At the B3LYP/6-31?G(d,p) level of the
theory, the minimal Gibbs free-energy difference of
2.5 kJ mol-1 between all-anti and all-syn conformations at
298 K is found for FLOX1c. The minimal steric repulsion
and the global minima on the potential energy surface were
also theoretically predicted for unsubstituted all-anti olig-
othiophenes [19]. On the other hand, the presence of methyl
group at the alpha site of benzofuran prefers the anti-syn
conformation by 10.7 kJ mol-1. The B3LYP calculation for
the benzofuran derivative without the methyl group revealed
the lower energy difference of 3.1 kJ mol-1. In the case of
FLOX2c and FLOX3c, the energetically favored confor-
mation depicted in Fig. 1Sa will be denoted as all-anti.
Due to the large variability of possible orientations of
dihedral angles between the aromatic rings, in the
Fig. 1 Chemical structure and
notation of studied FLOX
molecules according to the
added –R group. The
substituents –Y are a –N(CH3)2,
b –NH2, c –H, d –CF3,
and e –NO2
123

Theoretical and experimental study of donor-bridge-acceptor system: model 2-[5-(9H-fluoren-9-…
preliminary screening, all-anti orientation of side chain
rings was selected for latter 23 derivatives (Y = H).
The top views on the optimized gas-phase B3LYP all-
anti geometries of selected FLOX molecules are presented
in Fig. 2S. These geometries are practically planar. The
maximal distortion of 10° was indicated between the
thiophene plane and the ylidene carbon atoms. Only in the
case of FLOX3Y molecules, the mutual orientation of
naphthyl moieties and 1,3,4-oxadiazole was 30°.
The gas-phase B3LYP energies of frontier orbitals and the
corresponding energy gaps for FLOX series in all-anti con-
formations are presented in Table 1 which also show the
influence ofringterminationand -Ysubstituentonthe HOMO
and LUMO energies. For example, the higher HOMO energy
exhibits the compounds containing strong electron-donating –
N(CH3)2 or –NH2 groups. On the other hand, the presence of
the strong electron-withdrawing nitro group decreases the
HOMO energy by about 0.45 eV with respect to the parental
compounds end-capped with hydrogen atom, i.e., Y = H.
The substituent effect can also be expressed by the
correlation of HOMO energies on the Hammett substituent
constants. The published values for para substituent effect
are -0.83 for –N(CH3)2, -0.66 for –NH2, 0.00 for –H,
0.54 for –CF3, and 0.78 for NO2 [20]. The data presented in
Fig. 3S show the linear properties for all five molecular
series. All regression coefficients are better than 0.96. The
minimal substituent effect was found for the FLOX5
derivatives having the slope of -0.28 eV. The highest
sensitivity to the substituent change exhibits the FLOX0
series which has the shortest p-conjunction length. The
corresponding slope is -0.50 eV. The slopes of the rest
molecular series range from -0.43 to -0.34 eV.
Table 1 Theoretical gas-phase B3LYP/6-31?G(d,p) HOMO (EHOMO) and LUMO (ELUMO) energies, energy gaps (Egap = ELUMO–EHOMO), and
electric dipole moments (l) of studied molecules in all-anti conformation
Derivative
EHOMO/eV
ELUMO/eV
Egap/eV
l/D
FLOX0a
FLOX0b
FLOX0c
FLOX0d
FLOX0e
FLOX1a
FLOX1b
FLOX1c
FLOX1d
FLOX1e
FLOX2a
FLOX2b
FLOX2c
FLOX2d
FLOX2e
FLOX3a
FLOX3b
FLOX3c
FLOX3d
FLOX3e
FLOX4a
FLOX4b
FLOX4c
FLOX4d
FLOX4e
FLOX5a
FLOX5b
FLOX5c
FLOX5d
FLOX5e
-5.50
-5.65
-5.90
-6.20
-6.35
-5.30
-5.47
-5.75
-5.93
-6.05
-5.40
-5.55
-5.77
-5.92
-6.01
-5.54
-5.48
-5.74
-5.90
-6.07
-5.26
-5.46
-5.74
-5.90
-5.98
-5.43
-5.64
-5.75
-5.89
-5.96
-2.54
-2.61
-2.79
-2.98
-3.66
-2.57
-2.63
-2.78
-2.98
-3.54
-2.57
-2.62
-2.76
-2.94
-3.35
-2.66
-2.63
-2.75
-2.92
-3.27
-2.66
-2.69
-2.75
-2.94
-3.19
-2.72
-2.75
-2.80
-2.93
-3.04
2.97
3.04
3.10
3.22
2.70
2.73
2.84
2.98
2.95
2.51
2.83
2.93
3.02
2.99
2.66
2.88
2.86
2.99
2.98
2.80
2.60
2.77
2.99
2.95
2.79
2.71
2.90
2.95
2.96
2.92
5.23
4.61
3.88
5.69
8.18
6.72
5.90
4.40
5.86
8.30
7.03
5.91
3.90
4.19
6.31
5.06
5.40
4.07
4.78
7.05
6.13
5.62
5.00
7.12
9.43
6.62
5.96
5.00
4.74
6.03
123

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M. Michalık et al.
Interestingly, the addition of –CF3 and –NO2 groups is
able to shift LUMO energies under -3.3 eV. The intro-
duction of the substituent to the ring also modulates the
energy gap. For example, the lowest energy gap of 2.51 eV
is predicted for the nitrothiophene containing FLOX1e
molecule. As it is indicated in Table 1, the energy gaps of
compounds containing electron-donating dimethylamino
group are changed from 2.60 to 2.97 eV. Contrary to the
HOMO energies, worse linear correlation of LUMO ener-
gies with Hammett constants was found (see Fig. 4S). The
regression coefficients range from 0.89 to 0.96.
The influence of the terminal electron-donating and
electron-withdrawing substitutions of end-capped aromatic
rings on the electronic structure can be estimated from the
electron density distribution or the shape of frontier
molecular orbitals. As it is presented in Fig. 2 for the
FLOX2a and FLOX2b molecules, the HOMO orbitals are
not dominantly delocalized over 9H-fluoren-9-yliden-
emethyl unit.
On the other hand, the presence of strong electron-
withdrawing groups –CF3 and –NO2 supports the uniform
delocalization of HOMO electron over the 9H-fluoren-9-
HOMO
LUMO
FLOX2a
FLOX2b
FLOX2c
FLOX2d
FLOX2e
Fig. 2 Influence of Y substitution on the shape of the B3LYP/6-31?G(d,p) frontier molecular orbitals for FLOX2Y molecules in all-anti
conformations (isovalue = 0.02 bohr-3/2
)
123

Theoretical and experimental study of donor-bridge-acceptor system: model 2-[5-(9H-fluoren-9-…
ylidenemethyl unit, thienyl C–C bonds neighbouring the
sulfur atom and C–N bonds of oxadiazole.
disubstituted-1,3,4-oxadiazoles, by the Huisgen reaction of
compound 4 with the corresponding acyl chloride. The
purification of the crude products by silica gel column
chromatography using hexane/ethylacetate (9/1) mixture as
eluent gives FLOX derivatives in 52–72% yields (Table 2).
The comparison of melting point shows that the highest
thermal stability supports the connection of 1,3,4-oxadiazole
with 3-methylbenzofuran-2-yl. From synthetic reasons, the
methyl group is present in b-site of benzofuran in the case of
FLOX4 and FLOX5 derivatives.
In the case of LUMOs, the substitution has the opposite
role (see also Fig. 5S for other derivatives). It seems that the
electron-donating properties of 9H-fluoren-9-ylidenemethyl
unit are significantly supported or suppressed by the terminal
Y group. Although the electric conductivity is affected by
the mutual orientation of neighboring molecule, the electron
charge distribution over one molecule is also important. The
p-type semiconductivity is connected with the positive
charge transport which prefers the molecular parts with the
excess of p-electron charge density. In addition, the n-type
semiconductivity is based on the negative charge shifting
along the electron-deficient parts.
The experimental UV–Vis absorption spectra of studied
molecules in dichloromethane solution are depicted in
Fig. 3, showing the maximum absorption wavelengths
(kmax) ranging from 390 to 404 nm.
As it was mentioned by Blouin et al. [21], the ideal
HOMO energy level should range between -5.2 and
-5.8 eV, while as for ideal donor, LUMO energy should
be in the range -3.7 and -4.0 eV. With respect to our
predictions, the investigated molecular series could
potentially satisfy p-type semiconductivity criterion.
Finally, the presence of substituent has the direct on
influence on the electric microscopic properties. The cal-
culated gas-phase electric dipole moments for all-anti
unsubstituted molecules reached the minimal values from
3.88 to 5.00 D. The maximal values of 6.03 to 9.43 D are
observed mostly for NO2 substitution. In the case of
diphenylamine substituent, the values are from 5.06 to 7.03
D. Interestingly, the dependence of evaluated dipole
moments on the Hammett substituent constant has the
parabolic shape (see Fig. 6S). As Table 2S illustrates for
FLOXYc molecules, all-syn conformations show minimal
dipole moments. Therefore, the Boltzmann weighted
dipole moments are lower than all-anti ones.
In general, all compounds have similar absorption fea-
tures characterized by two broad main bands. The first
intensive band occurs between 350 and 470 nm, and the
second, less intensive absorption band is observed in the
range of 280–350 nm. The TD-CAM-B3LYP//B3LYP/6-
31?G(d,p) calculation for the gas phase showed that the
lowest energy experimental band belongs to the electronic
S0 ? S1 transition. This transition has a predominant p À
pà character which is mostly connected with the electron
excitation from HOMO to LUMO, as it demonstrated for
FLOX2a and FLOX2c in Figs. 7S and 8S. All theoretical
optical transition energies for FLOX molecules in all-anti
conformation are collected in Table 3S. Substitution effect
on the S0 ? S1 transition energies is predicted to be
minimal, and this transition has the strongest oscillator
strength compared to the first ten calculated ones. For
example, the energies for FLOX2Y molecules are between
3.06 and 3.12 eV (405 and 398 nm). Furthermore, theo-
retical vertical optical transitions S0 ? S2 and S0 ? S3
shown in Fig. 9S with weak oscillator strength probably
form the second experimental band. The molecular orbital
analysis for FLOX2a and FLOX2c indicates that these
transitions are connected with the HOMO-1 and
HOMO-2 (Figs. 7S and 8S). Interestingly, the shapes of
HOMO-2 for FLOX2a and HOMO-1 for FLOX2c are
comparable, i.e., the lobes are delocalized over the fluorene
unit. The experimental absorption spectra measured in
methanol exhibit a very small solvent effect, and the uni-
form blueshift is ca. 3 nm. Moreover, the investigated
molecules exhibit insignificant fluorescence. As shown in
Table 4S, similar shifts are also predicted by theoretical
calculations, including the solvent effect for methanol,
dichloromethane, and water. The mutual comparison of
Boltzmann weighted transition energies shows that the
influence of solvent with respect to the gas phase is ca.
20 nm. Next, the electrochemical properties of prepared
FLOX molecules were investigated to estimate frontier
orbital energies. The dependences of oxidation potentials
E0ox vs standard calomel electrode (SCE) obtained from the
Synthesis and experimental characterization
of selected molecules
The following experimental part presents the possible
synthetic route, as illustrated in the Scheme 1. These multi-
step reactions with relatively high yields were used for
preparation for all parental derivatives except for FLOX0c.
With respect to the theoretical predictions and the reagents’
availability, FLOX2a was also prepared.
As a general synthetic procedure, the thiophene ring is
connected with the fluorene chromophore through the double
bonded carbon atom in a phase transfer reaction, using
aqueous NaOH and toluene as a solvent, yielding the fluo-
renyl-thiophene derivative 1. The formyl derivate 2 was
prepared by means of Vilsmeier–Haack formulation (POCl3/
DMF) in 91% yield. Compound 3 was obtained in 80% yield
and then treated with NaN3/Et3NÁHCl in toluene at 180 °C
affording the tetrazole derivative 4 in 92% yield. Finally, the
last step of the synthetic procedure yields the asymmetrically
123

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M. Michalık et al.
Scheme 1
HOMO energies are from the interval -5.34 eV for
FLOX2a to -5.67 eV for FLOX1c.
cyclic voltammetry measurements are presented for
FLOX2a in Fig. 4 and for the rest in Fig. 10S. Using
empirical formulas for frontier molecular orbitals in eV
and optical gap Eogpt [22]:
Considering the Boltzmann distribution at 300 K and
solvent effects, we theoretically obtained the following
B3LYP(methanol) values: -5.69 eV for FLOX1c;
-5.71 eV for FLOX2c; -5.29 eV for FLOX2a;
-5.68 eV for FLOX3c; and -5.65 eV for FLOX4c. The
theoretical values for gas-phase are shifted to more nega-
tive values by about 0.05–0.11 eV (see Table 5S). In the
case of LUMO energies, the higher discrepancy between
the theoretical and experimental values is observed. This
might be due to the larger uncertainty in the determination
of the experimental LUMO energy from the combination
of spectroscopic and electrochemical data.
EHOMO ¼ ÀeðEo1x=2ÀE0ðFcþ=FcÞ þ 4:80Þ
ELUMO ¼ EHOMOÀEgopt
ð1Þ
ð2Þ
the energy levels of HOMOs and LUMOs were calculated
and are collected in Table 3. For all compounds, except
FLOX2a, the halfway oxidation potential (E1ox/2) was found
1/2
in 1.43–1.51 eV range. In the latter case, E value was
ox
less positive due to better donating properties of the
dimethylamine group. The obtained electrochemical
123

Theoretical and experimental study of donor-bridge-acceptor system: model 2-[5-(9H-fluoren-9-…
Table 2 Yields and melting points of selected FLOXs
Entry
Product
Yield / % m.p. / °C
FLOX1c
60
52
63
72
70
177-180
150-152
168-169
186-188
228-230
FLOX2a
FLOX2c
FLOX3c
FLOX4c/5c
Fig. 4 Cyclic voltammetry of FLOX2a measured in dichloro-
methane solution. Inset displays the broader potential window
energies, and dipole moments. Finally, the quantum
chemical model predicted the derivatives with –N(CH3)2 as
one of the most promising organic material with p-type
semiconductivity.
Furthermore, the route for relatively high-yield synthesis
was suggested, and five selected FLOX molecules were
synthesized and chemically analyzed. The highest melting
point exhibits the molecule containing 3-methylbenzofuran-
2-yl, while the presence of phenyl group results in the lowest
melting points. Experimental electrochemical and spectro-
scopic data supported the relevance of the used quantum
chemical models. Results of our work can be further used for
the synthesis of a broad range of small conjugated molecules
with controlled electron acceptor-bridge-donor effect.
Fig. 3 UV–Vis absorption spectra of prepared FLOX molecules in
dichloromethane solution at 10-5 mol dm-3
Experiment
Conclusion
All starting materials and substrates are commercially
available (Sigma-Aldrich) and were used without further
purification. Melting points were recorded on an elec-
trothermal-9100 apparatus. All products were characterized
by spectral data, including 1H NMR, 13C NMR, and FT-IR
spectroscopy. The NMR spectra were obtained using
VARIAN VXR 300 spectrometer. The infrared spectra
(KBr) were determined on a Perkin-Elmer 1600 FT-IR
system. The elemental analyses were performed by means
of Carlo Erba Elemental Analyzer 1108. Silica gel 60
(230–400 mesh, Merck) was used for column chromatog-
raphy. Reactions and the collected fraction samples were
monitored by TLC (Merck 60 F254 silica gel). Visualiza-
tion was made with UV light.
In this paper, the systematic quantum chemical study of
novel electron acceptor-bridge-donor systems derived from
the
2-[5-(9H-fluoren-9-ylidenemethyl)thiophen-2-yl]-
1,3,4-oxadiazole was presented. On the selected molecules,
we tried to theoretically demonstrate a way of energy-gap
setting and possible activation of relevant molecule part
which may play role in the electric semiconductivity or
optical processes. The calculations indicated that the
electron-donating properties of 9H-fluoren-9-yliden-
emethyl unit are significantly supported by the electron-
withdrawing Y groups or suppressed by the Y substitution
at the side of 1,3,4-oxadiazole. Next, the Hammett-type
dependencies were illustrated for the HOMO, LUMO
123

´
M. Michalık et al.
Table 3 Electrochemical characteristics and experimentally determined energies of frontier molecular orbitals for FLOXs
Sample
E1o/x2/V
Epox/V
EHOMO(exp)/eV
ELUMO(exp)/
eV
Eogpt/eV
FLOX1c
FLOX2c
FLOX2a
FLOX3c
FLOX4c
1.51
1.47
1.18
1.43
1.47
1.64
1.60
1.41
1.52
1.57
-5.67
-5.63
-5.34
-5.59
-5.63
-3.08
-3.00
-2.87
-3.01
-3.00
2.64
2.72
2.53
2.64
2.76
E1o/x2—halfway oxidation potential—and Epox—peak reduction potential—were determined by cyclic voltammetry in 0.1 M TBAPF6 in CH2Cl2 vs
SCE, Eogpt experimental optical energy gap
2-(9H-Fluoren-9-ylidenemethyl)thiophene (1)
163–164 °C; 1H NMR (300 MHz, DMSO-d6): d = 7.25 (t,
1H, J = 1.12 Hz, 7.82 Hz), 7.35 (t, 1H, J = 1.25 Hz,
7.44 Hz), 7.42 (t, 2H, J = 0.96 Hz, 7.45 Hz), 7.62 (d, 1H,
J = 3.92 Hz), 7.83–7.89 (m, 4H), 7.96 (d, 1H,
J = 7.41 Hz), 8.06 (d, 1H, J = 3.92 Hz) ppm; 13C NMR
(75 MHz, DMSO-d6): d = 108.9, 114.1 (CN), 117.4,
119.9, 120.4, 121.2, 124.1, 127.3, 127.4, 129.2, 129.7,
130.1, 134.6, 138.4, 138.4, 138.5, 139.3, 141.1, 145.9 ppm;
To the solution of 10.0 g fluorene (0.06 mol) and 6.7 g
thiophene-2-carbaldehyde (0.06 mol) in 84 cm3 toluene, an
aqueous solution of NaOH (56 g/84 cm3 water) and 3.5 g
n-tetrabutylammonium bromide (0.01 mol) were added.
The resulting heterogeneous mixture was vigorously stirred
at room temperature for 3 h. After the reaction, the water
layer was separated, and the organic layer was washed with
20 cm3 HCl (10%), 20 cm3 water, 30 cm3 brine, and dried
over anhydrous Na2SO4. The isolated crude product was
recrystallized from ethanol which afforded 8.1 g (85%) 1.
Yellow crystals; m.p.: 71–72 °C ([8] 72 °C); NMR, and IR
spectra were found to be identical with the ones described
in [8].
~
IR (KBr): m = 539, 722, 780, 798, 864, 875, 933, 1055,
1239, 1291, 1352, 1442, 2219 (CN), 3091 cm-1; and UV–
Vis (dichloromethane, c = 1 9 10-5 mol dm-3): kmax
234, 261, 364 nm.
=
5-[5-(9H-Fluoren-9-ylidenemethyl)thiophen-2-yl]-1H-te-
trazole (4, C19H12N4S)
The mixture of 500 mg nitrile 3 (1.75 mmol), 340 mg
NaN3 (5.25 mmol), and 722 mg triethylamine hydrochlo-
ride (5.25 mmol) in 5.5 cm3 toluene was stirred at 100 °C
for 18 h. Subsequently, reaction mixture was cooled down
and extracted with water. To the aqueous layer, HCl (36%)
was added dropwise to salt out the produced tetrazole.
After filtration, the product was dried under reduced
pressure and isolated in 92% yield (520 mg). Yellow
5-(9H-Fluoren-9-ylidenemethyl)thiophene-2-carbaldehyde
(2, C19H12OS)
Phosphorus oxychloride (6.7 g, 44 mmol) was added
dropwise under an argon atmosphere to 3.2 g N,N-
dimethylformamide (44 mmol) at 5 °C. The solution was
stirred for 30 min at room temperature. Subsequently,
3.5 g of 1 (22 mmol) in 5 cm3 N,N-dimethylformamide
was added at room temperature. The reaction mixture was
stirred at 80 °C for 3 h. The solution was poured to 50 cm3
of ice water and stirred with sodium carbonate to pH 7. The
solution was filtered and washed by water. The crude
product was recrystallized (ethanol), and we isolated the
product 2 in 5.7 g (91%) yield. Yellow crystals; m.p.:
114 °C ([23] 113–115 °C); UV–Vis (dichloromethane,
c = 1 9 10-5 mol dm-3): kmax = 238, 261, 366 nm.
1
powder; m.p.: 230 °C; H NMR (300 MHz, DMSO-d6):
d = 7.29 (t, 1H, J = 7.59 Hz), 7.35 (t, 1H, J = 7.47 Hz),
7.39–7.43 (m, 2H), 7.68 (d, 1H, J = 3.81 Hz), 7.84 (d, 1H,
J = 7.5 Hz), 7.88–7.94 (m, 2H), 7.94 (s, 1H), 7.98 (d, 1H,
J = 7.22 Hz), 8.13 (d, 1H, J = 7.85 Hz) ppm; 13C NMR
(75 MHz, DMSO-d6): d = 118.9, 119.9, 120.4, 121.0,
124.0, 126.8, 127.2, 127.3, 128.7, 129.4, 129.6, 131.6,
134.9, 136.6, 138.2, 138.8, 140.9, 141.8, 151.2 ppm; and
5-(9H-Fluoren-9-ylidenemethyl)thiophene-2-carbonitrile
(3, C19H11NS)
~
IR (KBr): m = 721, 778, 873, 1049, 1145, 1445, 1454,
1597, 1620, 2740, 2784, 2862, 2913, 3052, 3087 cm-1
.
A solution of 1 g carbaldehyde 2 (3.5 mmol) and 0.29 g
NH2OH•HCl (4.2 mmol) in 5.5 cm3 N-methyl-2-pyrroli-
done was heated at 110–115 °C. After 8 h, the mixture was
poured into 200 cm3 water and extracted with EtOAc
(2 9 50 cm3). The combined organic layer was dried
(Na2SO4), and the solvent was evaporated in vacuo. The
separation of crude product by column chromatography
(toluene) afforded 0.8 g (80%) 3. Yellow crystals; m.p.:
Oxadiazoles synthesis
The solution of 200 mg tetrazole 4 (0.61 mmol), corre-
sponding to aromatic 2-carbonyl chloride (0.86 mmol,
1.4 eq.) in 2 cm3 pyridine, was heated at 115 °C under
argon atmosphere. The progress of the reaction was mon-
itored by TLC. After 5 h, the reaction mixture was poured
123

Theoretical and experimental study of donor-bridge-acceptor system: model 2-[5-(9H-fluoren-9-…
into 70 cm3 water. The aqueous layer was filtered. The
crude product was recrystallized and purified by column
chromatography (hexane: EtOAc, 9:1).
137.3, 139.0, 139.1, 139.3, 141.6, 144.2, 160.1, 164.1 ppm;
~
and IR (KBr): m = 725, 740, 751, 768, 802, 823, 857, 1008,
1034, 1070, 1220, 1249, 1362, 1398, 1446, 1506, 1526,
1584, 3025 cm-1
.
2-[5-(9H-Fluoren-9-ylidenemethyl)thiophen-2-yl]-5-(thio-
phen-2-yl)-1,3,4-oxadiazole (FLOX1c, C24H14N2OS2)
Yield: 150 mg (60%); yellow powder; m.p.: 177–180 °C;
1H NMR (300 MHz, CDCl3): d = 7.18–7.20 (m, 1H),
7.27–7.39 (m, 4H), 7.44 (dd, 1H, J = 3.9 Hz), 7.51 (s, 1H),
7.58 (dd, 1H, J = 5.1 Hz), 7.65–7.73 (m, 3H), 7.79–7.82
(m, 2H), 8.05 (d, 1H, J = 7.8 Hz) ppm; 13C NMR
(75 MHz, CDCl3): d = 116.7, 119.4, 119.7, 120.2, 124.3,
124.4, 125.3, 126.8, 126.9, 128.0, 128.7, 129.2, 129.7,
129.9, 129.9, 130.2, 135.3, 138.4, 138.7, 138.9, 141.3,
2-[5-(9H-Fluoren-9-ylidenemethyl)thiophen-2-yl]-5-(3-
methylbenzofuran-2-yl)-1,3,4-oxadiazole
(FLOX4c,
C29H18N2O2S)
1
Yield: 70%; yellow powder; m.p.: 228–230 °C; H NMR
(300 MHz, CDCl3): d = 2.72 (s, 3H), 7.18–7.23 (m, 1H),
7.30–7.42 (m, 4H), 7.44–7.50 (m, 2H), 7.55 (s, 1H), 7.60
(d, 1H, J = 6.9 Hz), 7.66–7.77 (m, 4H), 7.91 (d, 1H,
J = 3.9 Hz), 8.05 (d, 1H, J = 7.8 Hz) ppm; 13C NMR
(75 MHz, CDCl3): d = 9.6, 110.1, 111.5, 116.9, 119.2,
121.0, 123.3, 124.1, 124.6, 124.9, 127.5, 128.2, 128.4,
128.7, 128.9, 129.4, 130.1, 130.2, 136.8, 137.4, 137.5,
140.2, 142.1, 151.3, 154.1, 157.5, 161.2 ppm; and IR
~
143.7, 159.6, 160.1 ppm; and IR (KBr): m = 706, 722, 775,
823, 855, 1024, 1035, 1062, 1353, 1444, 1485, 1574, 1617,
3074, 3104 cm-1
.
~
(KBr): m = 718, 736, 769, 828, 875, 1029, 1074, 1138,
1266, 1342, 1358, 1380, 1404, 1442, 1454, 1485, 1578,
2-[5-(9H-Fluoren-9-ylidenemethyl)thiophen-2-yl]-5-phe-
nyl-1,3,4-oxadiazole (FLOX2a, C26H16N2OS)
1616, 1634, 2922, 3073 cm-1
.
Yield: 155 mg (63%); yellow powder; m.p.: 168–169 °C;
1H NMR (300 MHz, CDCl3): d = 7.17–7.20 (m, 1H), 7.25
(s, 1H), 7.32-7.42 (m, 3H), 7.45–7.53 (m, 4H), 7.54–7.68
(m, 3H), 7.71 (s, 1H), 7.75 (d, 1H, J = 8.4 Hz), 7.87(d, 1H,
J = 3.9 Hz), 8.07 (d, 1H, J = 7.8 Hz) ppm; 13C NMR
(75 MHz, CDCl3): d = 116.9, 119.7, 119.9, 120.4, 123.5,
124.5, 126.9, 127.1, 127.2, 128.4, 128.8, 129.1, 129.2,
129.4, 130.1, 130.5, 131.8, 133.7, 134.5, 135.7, 139.2,
Optical spectroscopy and electrochemistry
The optical and electrochemical properties of synthesized
FLOXs were determined by UV–Vis absorption and cyclic
voltammetry measurements. The UV–VIS absorption
spectra were taken on a UV 1650PC spectrometer (Shi-
madzu, Japan). Dichloromethane (CH2Cl2) was used for
UV spectroscopy (Merck). The solution was measured in
1 cm cuvette. Fluorescence spectra were recorded on a
Perkin–Elmer MPF-4 spectrofluorimeter (Norfolk, Con-
necticut USA). The cyclic voltammetry measurements
were performed in solution, under nitrogen atmosphere
with a computer-controlled electrochemical workstation
(Amel 2053) in a three electrode single-compartment cells
using platinum electrodes and SCE as the standard elec-
trode, with ferrocene/ferricinium (Fc/Fc?) redox couple as
internal standard, with a tetrabutylammonium tetrafluo-
roborate solution (0.1 M) in dichloromethane at a scan rate
of 50 mV s-1. The electric potential for ferrocene oxida-
tion versus the standard hydrogen electrode E(Fc?/Fc) was
0.64 V.
~
141.6, 144.1, 162.3, 171.9 ppm; and IR (KBr): m = 686,
717, 729, 776, 857, 1025, 1066, 1274, 1352, 1447, 1486,
1547, 1582, 3051 cm-1
.
4-[5-[5-(9H-Fluoren-9-ylidenemethyl)thiophen-2-yl]-
1,3,4-oxadiazol-2-yl]-N,N-dimethylaniline
(FLOX2c,
C28H21N3OS)
Yield: 141 mg (52%); yellow powder; m.p.: 150–152 °C;
1H NMR (300 MHz, CDCl3): d = 3.06 (s, 6H), 7.01 (s,
1H), 7.05-7.10 (m, 2H), 7.36–7.39 (m, 4H), 7.44 (dd, 1H,
J = 3.9 Hz), 7.61–7.63 (m, 3H), 7.72–7.73 (m, 3H), 8.28
(d, 1H, J = 7.8 Hz) ppm; and 13C NMR (75 MHz,
CDCl3): d = 41.3, 116.7, 119.4, 119.7, 121.0, 124.1,
124.4, 127.5, 128.4, 128.5, 128.7, 130.1, 136.8, 137.4,
137.5, 137.6, 140.2, 142.1, 155.3, 161.2, 164.5 ppm.
2-[5-(9H-Fluoren-9-ylidenemethyl)thiophen-2-yl]-5-(naph-
thalen-2-yl)-1,3,4-oxadiazole (FLOX3c, C30H18N2OS)
Yield: 199 mg (72%); yellow powder; m.p.: 186–188 °C;
1H NMR (300 MHz, CDCl3): d = 7.21 (t, 1H,
J = 5.7 Hz), 7.31–7.42 (m, 3H), 7.50–7.51 (m, 1H), 7.57
(s, 1H), 7.61–7.65 (m, 2H), 7.70–7.77 (m, 3H), 7.93 (d, 1H,
J = 3.0 Hz), 7.96 (d, 3H, J = 6.0 Hz), 8.05–8.09 (m, 2H),
8.28 (d, 1H, J = 5.5 Hz) ppm; 13C NMR (75 MHz,
CDCl3): d = 116.9, 119.8, 120.0, 120.2, 120.5, 124.6,
124.9, 125.9, 126.2, 126.8, 127.1, 127.2, 128.2, 128.4,
128.7, 128.9, 129.4, 130.0, 130.2, 132.7, 133.8, 135.7,
Quantum chemical calculations
All quantum chemical calculations were done using the
Gaussian 09 program package [24]. The optimal electronic
ground-state geometries were found at the density func-
tional level of theory employing Becke’s three parameter
hybrid functional using the Lee, Yang, and Parr correlation
functions (B3LYP) [25, 26]. For the all-anti conformations,
the initial setting of dihedral angle between thiophene and
oxadiazole rings was ?150°, and the dihedral angle
123

´
M. Michalık et al.
3. Kozma E, Catellani M (2013) Dyes Pigm 98:160
between oxadiazole and R group was -150°. Similar
alternation of mutual aromatic plane orientations was used
in the starting geometries of all-syn, syn-anti, and anti-syn
conformations. The obtained structures were confirmed to
be optimal by the standard normal mode analysis. On the
basis of optimized B3LYP geometries, the vertical transi-
tion energies and oscillator strengths between the initial
and final electronic states were computed by the time-de-
pendent (TD)-DFT method [27]. These optical properties
were calculated using the CAM-B3LYP (Coulomb-atten-
uating method applied to B3LYP) functional [28]. The
6-31?G(d,p) basis set was used [29, 30]. The energy cutoff
4. Fernandez-Hernandes JM, Beltran JI, Lemaur V, Galvez-Lopez
MD, Chien CH, Polo F, Orselli E, Frohlich R, Cornil J, De Cola L
(2013) Inorg Chem 52:1812
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(2007) Synth Met 157:770
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Photochem Photobiol Sci 12:1210
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used for all geometry optimizations was 4 9 10-3
-
kJ mol-1, and the final root mean square energy gradient
-1. The influence of solvents
˚
A
was below 0.04 kJ mol-1
was approximated by the continuum solvation model based
on the quantum mechanical charge density, SMD [31] of a
solute molecule interacting with a continuum description of
the solvent. The visualization of the obtained theoretical
results was done by the Molekel program package [32] and
Gimp [33]. For the sake of comparison, we have evaluated
the Boltzmann weighted quantities (Y) for dipole moments,
vertical transition energies, and frontier orbital energies.
The weighting overall conformers were performed
19. Bouzzine SM, Bouzakraoui S, Bouachrine M, Hamidi M (2005) J
Mol Struct (Theochem) 726:271
20. Hammett LP (1937) J Am Chem Soc 59:96
21. Blouin N, Michaud A, Gendron D, Wakim S, Blair E, Neagu-
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Chem Soc 105:6555
according to Eq. (3):
X
Y ¼
wiYi
ð3Þ
i
ˇ
ˇ´
´
´
ˇ´ˇ
23. Perasınova L, Stefko M, Vegh D, Kozısek J (2006) Acta Crys-
tallogr Sect E: Struct Rep Online 62:o3972
where Yi represents the calculated quantity for conformer
i. The population of conformer i (wi) with relative energy
DEi was calculated as
24. Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA,
Cheeseman JR, Scalmani G, Barone V, Mennucci B, Petersson
GA, Nakatsuji H, Caricato M, Li X, Hratchian HP, Izmaylov AF,
Bloino J, Zheng G, Sonnenberg JL, Hada M, Ehara M, Toyota K,
Fukuda R, Hasegawa J, Ishida M, Nakajima T, Honda Y, Kitao
O, Nakai H, Vreven T, Montgomery JA Jr, Peralta JE, Ogliaro F,
Bearpark M, Heyd JJ, Brothers E, Kudin KN, Staroverov VN,
Kobayashi R, Normand J, Raghavachari K, Rendell A, Burant JC,
Iyengar SS, Tomasi J, Cossi M, Rega N, Millam JM, Klene M,
Knox JE, Cross JB, Bakken V, Adamo C, Jaramillo J, Gomperts
R, Stratmann RE, Yazyev O, Austin AJ, Cammi R, Pomelli C,
Ochterski JW, Martin RL, Morokuma K, Zakrzewski VG, Voth
GA, Salvador P, Dannenberg JJ, Dapprich S, Daniels AD, Farkas
expðÀDEi=kTÞ
P
wi ¼
ð4Þ
i expðÀDEi=kTÞ
where k is the Boltzmann constant, T is the thermodynamic
temperature, and the relative energy DEi was obtained with
respect to the energy of the most stable conformer.
Acknowledgements This work has been supported by the Slovak
Grant agency VEGA 1/0594/16, 1/0829/14, 1/0601/15, 2/0142/14 and
APVV-0038-11, APVV-15-0079, and NANOSILK CNR-SAS Joint
¨
O, Foresman JB, Ortiz JV, Cioslowski J, Fox DJ (2009) Gaussian
ˇ
´
09, Revision D.01. Gaussian, Inc, Wallingford
25. Lee C, Yang W, Parr RG (1988) Phys Rev B 37:785
26. Becke AD (1993) J Chem Phys 98:5648
27. Furche F, Ahlrichs R (2002) J Chem Phys 117:7433
28. Yanai T, Tew DP, Handy NC (2004) Chem Phys Lett 393:51
29. Hehre WJ, Ditchfield R, Pople JA (1972) J Chem Phys 56:2257
30. Rassolov V, Ratner M, Pople J, Redfern P, Curtiss LJ (2001)
Comput Chem 22:996
Research Project. The authors also thank Marcela Kimlickova (SAS-
PI) for the UV analysis. We are grateful to the HPC center at the
Slovak University of Technology in Bratislava, which is a part of the
Slovak Infrastructure of High-Performance Computing (SIVVP Pro-
ject, ITMS code 26230120002, funded by the European Region
Development Funds, ERDF) for the computational time and resources
made available.
31. Marenich AV, Cramer CJ, Truhlar DG (2009) J Phys Chem B
113:6378
32. Varetto U (2009) Molekel 5.4.0.8. Swiss National Supercom-
puting Centre, Lugano
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