UV-vis absorption spectra of 1,4-dialkoxy-2,5-bis[2-(thien-2-yl)ethenyl]benzenes

Article abstract of DOI:10.1016/j.saa.2008.10.005

Complex theoretical and experimental studies and quantum-chemical calculations were applied to study the UV-vis spectroscopic features of the novel compounds: three stereoisomers of 1,4-diethoxy-2,5-bis[2-(5-methylthien-2-yl)ethenyl]benzene (A-C) and E,E isomer of 1,4-diisopropoxy-2,5-bis[2-(thien-2-yl)ethenyl]benzene (D). These structures are the derivatives of 2,5-bis[2-(thien-2-yl)ethenyl]benzene, and belong to a group of thienyl-PPV family that are able to polymerize due to the presence of π-conjugated bonding system. It was established that such compounds during electropolymerization are strongly dependent on their stereochemistry and on the eventual presence of substituents in α-positions of the tiophene ring. We have obtained a good agreement between the theoretically simulated optical within a framework of TDDFT approach and experimentally measured data. Influence of PMMA polymer matrices on the UV-vis spectra is explored. It is shown that a red wavelength spectral shift is observed only for D compounds and agreement between calculated and experimental spectral data is sufficiently good. This may indicate on different influence of local polymer matrix field on the spectral behaviors of the chromophores with different stereochemistry.

Full text of DOI:10.1016/j.saa.2008.10.005

Spectrochimica Acta Part A 72 (2009) 394–398
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
UV–vis absorption spectra of 1,4-dialkoxy-2,5-bis
[2-(thien-2-yl)ethenyl]benzenes
I. Fuks-Janczareka, Ali H. Reshakb, W. Kuz´nikc, I.V. Kitykc,∗, R. Gaban´ skic,
M. Łapkowskic,d, R. Motykac, J. Suwin´ skic
a Institute of Physics, J. Długosz University of Cze˛stochowa, Al. Armii Krajowej 13/15, Poland
b Institute of Physical Biology-South Bohemia University, Institute of System Biology and Ecology-Academy of Sciences – Nove Hrady 37333, Czech Republic
c Faculty of Chemistry, Silesian University of Technology, ul.Strzody 9, Gliwice, Poland
d Centre of Polymer and Carbon Materials, Polish Academy of Science, 34 M. Sklodowska-Curie Str. 41-819 Zabrze, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Complex theoretical and experimental studies and quantum-chemical calculations were applied to study
the UV–vis spectroscopic features of the novel compounds: three stereoisomers of 1,4-diethoxy-2,5-
bis[2-(5-methylthien-2-yl)ethenyl]benzene (A–C) and E,E isomer of 1,4-diisopropoxy-2,5-bis[2-(thien-
2-yl)ethenyl]benzene (D). These structures are the derivatives of 2,5-bis[2-(thien-2-yl)ethenyl]benzene,
and belong to a group of thienyl–PPV family that are able to polymerize due to the presence of ␲-
conjugated bonding system. It was established that such compounds during electropolymerization are
strongly dependent on their stereochemistry and on the eventual presence of substituents in ␣-positions
of the tiophene ring. We have obtained a good agreement between the theoretically simulated optical
within a framework of TDDFT approach and experimentally measured data. Influence of PMMA polymer
matrices on the UV–vis spectra is explored. It is shown that a red wavelength spectral shift is observed
only for D compounds and agreement between calculated and experimental spectral data is sufficiently
good. This may indicate on different influence of local polymer matrix field on the spectral behaviors of
the chromophores with different stereochemistry.
Received 9 August 2007
Received in revised form 11 August 2008
Accepted 9 October 2008
Keywords:
Absorption spectra
Quantum-chemical simulations
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
In the present work we study UV–vis spectral features
of three stereoisomers: 1,4-diethoxy-2,5-bis[2-(5-methylthien-2-
The research of benzene derivatives is of great importance due
to possible promising nonlinear optical properties. One of the
principal directions of search and design of novel nonlinear opti-
cal materials consists in finding of the practical ways to enhance
their nonlinear optical susceptibilities [1]. Particularly it is crucial
during the search of new transparent materials possessing very
low losses and large nonlinear optical response. Particular interest
presents chemical modifications of the ␲-conjugated chromophore
by appropriate substitution by other chemical complexes.
At the same time the organic conjugated materials are of
great interest for application as active components in photovoltaic
devices [2]. Advantages of using organic materials in these devices
are: low cost, facile processing, thin size, flexibility of properties
and shape [3]; however one of the drawbacks of organic materials
is their low chemical stability. The knowledge of UV–vis spectro-
scopical features plays here a principal role.
yl)ethenyl]benzene (A–C) and E,E isomer of 1,4-diisopropoxy-
2,5-bis[2-(thien-2-yl)ethenyl]benzene (D) belonging to so called
thienyl PPV derivatives. They were synthesized in the course of
our research on synthesis and electropolymerization of that type
of compounds. A behavior of that type of compounds during elec-
tropolymerization strongly depends on their stereochemistry [5]
and on the eventual presence of substituents in ␣-positions of the
tiophene ring. When the ␣-positions are free the polymerization
occurs readily to afford linear polymers; when the ␣-positions are
blocked by, e.g. alkyl groups like in A–C, a mechanism of the reac-
tion changes and at higher potentials the double bonds are involved
in electropolymerization [6]. The ␣-blocked isomers are also more
stable while irradiation. A type of alkoxy groups in benzene core is
of secondary importance for the process; it influences the stability
of stereoisomers, solubility of monomers and polymers.
In this work spectroscopic and theoretical calculations are
applied to study the properties of the molecules as shown in Fig. 1.
These structures are derivatives of 2,5-bis[2-(thien-2-yl)ethenyl]
benzene, thus belonging to a group of thienyl–PPV family that are
able to polymerize due to the presence of conjugated bonding sys-
∗
Corresponding author. Tel.: +48 601 50 42 68.
E-mail address: Iwan.Kityk@polsl.pl (I.V. Kityk).
1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2008.10.005

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395
Fig. 1. Chemical formula of studied compounds1,4-dialkoxy-2,5-bis[2-(thien-2-yl)ethenyl]benzenes (A–D), where (1) A; (2) B; (3) C; (4) D.
tem [4–8]. These types of molecules, which are in fact short-chain
models of the corresponding polymers, have properties similar to
their high molecular weight analogs [9].
Boetius HMK apparatus. NMR spectra were taken in CDCl3 with
TMS as an internal reference by Varian XL-300 at 300 MHz
for 1H and at 75.5 MHz for 13C. EA results were obtained
using Perkin-Elmer CHN automatic analyzer. For the separa-
tion of stereoisomers the Waters chromatograph with Prep.
Nova-Pak® column HR C18 (19 mm × 300 mm) was applied.
Methanol (5 ml/min) was used as the eluent at 520 psi of the
pressure and at the temperature 26 ◦C; detection UV at 220 nm.
2. Experimental
General
scheme
of
1,4-dialkoxy-2,5-bis[2-(thien-2-yl)
ethenyl]benzenes synthesis is presented below:
All solvents have been dried and distilled before use. Other
commercial substances and reagents were applied without purifi-
cation. Melting points (not corrected) were determined on
MS spectra were obtained from HPLC–MS Integrity Systems with
Termabeam Mass Detector (EI, 70 eV); samples (in methanol

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Table 1
molecules 1–4 were accomplished by using a semi-empirical
quantum-chemical method: AM1. The geometry relaxation’s opti-
mization performed by the Steepest Descent algorithm after the
molecular geometry optimization. The values of the total energy
minima were varied at around 4 eV/atom.
Theoretically calculated and experimentally measured principal spectral maxima
positions for the investigated chromophore.
Chromophore
Experimental
peak (nm)
Theoretical
peak (nm) ADF
Experimental spectral peak
positions in nm shifted in
PMMA matrices
Simultaneously we have used the ADF program for molecular
structure optimization. We obtained the optimization geometry
with basis set: DZ (double zeta) because we find the molecular
geometry for molecules with the lowest energy. The basis set cho-
sen will apply to all atoms in our molecules. The ADF program
uses Slater-type orbitals. There are a number of basis sets that can
be freely downloaded. These basis sets are optimized for use in
the zeroth-order regular approximated (ZORA) relativistic equa-
tion, which is an excellent approximation to the fully relativistic
Dirac equation, especially in the valence region, which is impor-
tant in quantum-chemical calculations. ZORA STO basis sets that
can be downloaded freely: DZ fc (double-zeta frozen core), DZ ae
(double-zeta all electron), TZP fc (triple-zeta frozen core 1 polar-
ization function), TZP ae (triple-zeta all electron 1 polarization
function), TZ2P fc (triple-zeta frozen core 2 polarization functions),
TZ2P ae (triple-zeta all electron 2 polarization functions), and QZ4P
ae (quadruple-zeta all electron 4 polarization functions). Seven dif-
ferent types of Slater-type basis sets for the elements H (Z = 1) up
to E118 (Z = 118), ranging from a double-zeta valence quality up to a
quadruple-zeta valence quality, are tested in their performance in
neutral atomic and diatomic oxide calculations. The exponents of
the Slater-type functions are optimized for use in (scalar relativis-
tic) ZORA equations.
A
B
A1 = 400
A2 = 495
Aꢀ1 = 355
Aꢀ2 = 490
B1ꢀ = 235
B2ꢀ = 287
B3ꢀ = 350
B4ꢀ = 440
C1ꢀ = 230
C2ꢀ = 285
C3ꢀ = 355
C4ꢀ = 500
Dꢀ1 = 230
Dꢀ2 = 275
Dꢀ3 = 340
Dꢀ4 = 520
Aꢀ1 = 362
Aꢀ2 = 494
B1 = 225
B2 = 350
B3 = 400
B4 = 445
B1ꢀ = 228
B2ꢀ = 282
B3ꢀ = 353
B4ꢀ = 439
C1ꢀ = 237
C2ꢀ = 278
C3ꢀ = 352
C4ꢀ = 489
Dꢀ1ꢀ = 280
Dꢀ2ꢀ = 390
Dꢀ3ꢀ = 418
Dꢀ4ꢀ = 562
C
C1 = 240
C2 = 270
C3 = 348
C4 = 470
D
D1 = 280
D2 = 370
D3 = 400
Dꢀ4 = 540
solutions) were introduced to the spectrometer using direct inlet.
X-ray diffraction data were collected on KUMA KM4 four-circle
diffractometer at 295 K. Cell parameters, reflection collection and
their reductions were done using KUMA KM4 software [10]. The
structures were determined by means of direct methods and
refined by the full-matrix least squares technique [11].
The structure of (E,E)-1,4-diisopropoxy-2,5-bis[2-(thien-2-
yl)ethenyl]benzene in the crystalline state was additionally
monitored by X-ray measurements (CCDC no. 27369510).
Table 1 contains more important data concerning crystal,
diffractometer, measurement conditions and refinement. The tio-
phene rings in the crystal are disordered over two orientations like
in other similar compounds [12] (Fig. 2).
The double-zeta basis set is very convenient because it allows us
to treat each orbital separately when we conduct the Hartree–Fock
calculation. These descriptions are meant to give an indication of
the quality. This gives us a more accurate representation of each
orbital. In order to do this, each atomic orbital is expressed as the
sum of two Slater-type orbitals (STOs). The two equations are the
same except for the value of ␰ (zeta) [13]. The zeta value accounts
for how diffuse (large) the orbital is. Then the two STOs are added
in some proportion.
3. Results and discussion
All the quantum-chemical computations were done using Ams-
terdam Density Functional package (ADF) and HyperChem package.
The first step we found was the geometry optimization by molec-
ular mechanics force filed (MM+), which is the most general
and frequently used method for molecular mechanics calcula-
tions developed principally for organic molecules. The geometry
optimization and the calculation of optical spectra for separated
In this case, each STO represents a different sized orbital because
the zetas are different. The ‘d’ accounts for the percentage of the sec-
ond STO to add in. The linear combination then gives us the atomic
orbital. Since each of the two equations is the same, the symmetry
remains constant. After we used the Single Point added Excita-
tions (Allowed only), this method determined the SCF solution and
properties running the current geometry.
Fig. 3 presents the principal structures for the four studied
molecules 1–4 with highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO). In all the figures
the HOMO orbital is indicated as full areas and the LUMO is marked
as lined areas. The hydrogen atoms was drawn as white spheres and
the carbon atoms as black spheres, each others atoms are noted by
red (oxygen) and yellow (sulfur).
The UV–vis spectra were recorded by Ocean Optics HR4000CG-
UV–NIR spectrometers (wavelength range 200–1100 nm, spectral
resolution 1 nm FWHM (full width at half maximum), 3648-
element linear silicon CCD array) and the Ocean Optics DT-MINI-
2-GS light source. The power output of light source was 3.8 W
for deuterium lamp (200–410 nm) and 1.2 W for tungsten halogen
lamp (360–2000 nm). Absorption was calculated automatically by
the software by the following equation:
ꢀ
ꢁ
Sꢀ − Dꢀ
Rꢀ − Dꢀ
Aꢀ = −log10
Fig. 2. Molecular structure of (E,E)-1,4-diisopropoxy-2,5-bis[2-(thien-2-yl)ethe-
nyl]benzene showing 50% probability displacement ellipsoids obtained by ORTEP 3.
A dominant conformation (82% probability) in the crystal is indicated by numbered
atom symbols without the letter “b”.
The light source, sample chamber and the spectrometer were
connected together by the 400 ␮m diameter fiber optics with col-
limating lenses.

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397
Fig. 3. 3D visualization of examined molecules. Plain surfaces, HOMO levels and transparent surfaces, LUMO levels.
When this equation is evaluated for each pixel of the detector,
the absorbance spectrum is produced, where S is the sample inten-
sity at wavelength ꢀ, D is the dark intensity at wavelength ꢀ, and R
is the reference intensity at wavelength ꢀ.
calculated using Gaussian model (1) with the half-bandwidth
ꢁ1/2,I = 2800 cm−1 [15].
It is necessary to emphasize that following their chemical for-
mula one can expect substantial second-order nonlinear optical
effects, similar to those existed for the stilbenes [16].
The calculated optical spectra were consequently performed for
isolated chromophore molecule as well as for the chromophore
incorporated into the PMMA polymer matrices. However, we
will also give theoretically simulated spectra obtained within a
framework of time-dependent DFT method (TDDFT) using ADF
package. We have found a good agreement with the experimental
spectra taking into account only the singly excited configuration
interactions. The criteria regarding the excitation energies were
chosen to include seven occupied and seven unoccupied orbital
for both methods. Our quantum-chemical calculations have been
performed for isolated molecules in vacuum. The experimental
absorption spectra in the THF solution were determined by the
Gaussian approximation, theoretical absorption spectra were cal-
culated using SWizard program [14]. Absorption profiles were
In Fig. 4 are presented the experimental absorption spectra
obtained in the THF solution with spectral resolution 1 nm. One
can clearly see the existence of fourth principal spectral maxima.
The obtained spectra are in a principal agreement with the the-
oretically calculated data obtained within a framework of TDDFT
approach (see Fig. 5). For the convenience of readers all the data
are presented in the table. One of the principal factors is the behav-
ior of the chromophore in the polymer matrices, in particularly in
the traditional PMMA matrices. Our earlier experience has shown
[17] that the surrounding polymer matrices usually enhance polar-
izability of the investigated chromophore and favor an enhanced
spectral shift of principal spectral maxima. To explore this influence
in our case we have carried out the quantum-chemical simulations

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4. Conclusions
Complex experimental and theoretical investigations of
UV–vis spectra for 1,4-diethoxy-2,5-bis[2-(5-methylthien-2-yl)
ethenyl]benzene (A–C) and E,E isomer of 1,4-diisopropoxy-
2,5-bis[2-(thien-2-yl)ethenyl]benzene (D) were done. We have
established a good agreement between the calculated and exper-
imentally measured optical spectra and spectral positions of
principal spectral maxima. It is crucial that incorporation of the
investigated chromophore into the PMMA matrices has an obvious
red wavelength spectral shift only for the chromophore D (up to
40–50 nm). At the same time this spectral shift is vanishingly low
for the chromophore molecule A–C. It may reflect a crucial influ-
ence of chromophore stereochemistry factor during incorporation
of such chromophore in the polymer matrices.
Acknowledgments
This research was partially financed by the grants provided by
the Ministry of Science and Higher Education (project no. 3 T09A
097 28). For the author Ali H. Reshak this work was supported by the
institutional research concept of the Institute of Physical Biology,
UFB (no. MSM6007665808), and the Institute of System Biology and
Ecology, ASCR (no. AVOZ60870520).
Fig. 4. Experimental absorption spectra of the investigated molecule.
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Fig. 5. Theoretically simulated absorption spectra obtained by the ADF program.
of the chromophore spectra embedded into the PMMA matrices.
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data gathered in the table show that only for the chromophore D
we obtain obvious red spectral shifts. The principal spectral max-
ima are shifted from 20 nm up to 50 nm to the longer wavelengths.
At the same time this shift is vanishingly small for the A–C chro-
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