Synthesis and characterization of a class of donor-acceptor conjugated molecules: Experiments and theoretical calculations

Article abstract of DOI:10.1021/jp909635g

A class of conjugated molecules containing donor (thiophene) and acceptor (malononitrile) is synthesized by Knoevenagel condensation reaction between 2-(2,6-dimethyl-4H-pyran-4-ylidene) malononitrile and thiophene carbaldehyde containing two and three thiophene units. The resulting molecules are characterized by ¹H and ¹³C NMR. We have performed UV-vis absorption, fluorescence, and cyclic voltammetry measurements on these materials. The spectroscopic and electrochemical measurements proved beyond doubt that these materials possess low excitation gap and are suitable for being an active material in various electronic devices. We have also performed electronic structure calculations using density functional theory (DFT) and INDO/SCI methods to characterize the ground and excited states of this class of molecules. These donor-acceptor molecules show a strong charge transfer character that increases with the increase in the number of thiophene rings coupled to the malononitrile acceptor moiety. We have also calculated the π-coherence length, Stokes shift, and effect of solvents on excited states for this class of molecules. Our theoretical values agree well with experimental results.

Full text of DOI:10.1021/jp909635g

J. Phys. Chem. A 2010, 114, 4647–4654
4647
Synthesis and Characterization of a Class of Donor-Acceptor Conjugated Molecules:
Experiments and Theoretical Calculations
S. Mukhopadhyay,† Raja Bhaskar Kanth,† S. Ramasesha,* and Satish Patil*
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India
ReceiVed: October 8, 2009; ReVised Manuscript ReceiVed: February 11, 2010
A class of conjugated molecules containing donor (thiophene) and acceptor (malononitrile) is synthesized by
Knoevenagel condensation reaction between 2-(2,6-dimethyl-4H-pyran-4-ylidene) malononitrile and thiophene
carbaldehyde containing two and three thiophene units. The resulting molecules are characterized by 1H and
13C NMR. We have performed UV-vis absorption, fluorescence, and cyclic voltammetry measurements on
these materials. The spectroscopic and electrochemical measurements proved beyond doubt that these materials
possess low excitation gap and are suitable for being an active material in various electronic devices. We
have also performed electronic structure calculations using density functional theory (DFT) and INDO/SCI
methods to characterize the ground and excited states of this class of molecules. These donor-acceptor
molecules show a strong charge transfer character that increases with the increase in the number of thiophene
rings coupled to the malononitrile acceptor moiety. We have also calculated the π-coherence length, Stoke’s
shift, and effect of solvents on excited states for this class of molecules. Our theoretical values agree well
with experimental results.
I. Introduction
From the past two decades, considerable efforts have been
directed toward the improvement of organic electronic materials.
Low band gap organic materials are best suited for application
in organic electronics. Lowering of the excitation gap can be
achieved by choosing the appropriate donor and acceptor groups
in the donor-acceptor-donor (D-A-D) conjugated molecules.
Nowadays, these materials have become attractive because of
their tunable electronic properties, low cost of synthesis, ease
of processability, and flexible device structures. Many kinds of
organic dyes such as thiophenes, carbazoles, squarines, ben-
zothiadiazoles, and diphenyl-dithienyl-thienopyrazines are used
as a donor or an acceptor group. A detailed understanding of
the underlying electronic structure of these materials as well as
a study of their response to external fields is required to exploit
them systematically for technological applications.
Figure 1. Schematic representation of the oligo-thiophene (D-A-D)
molecules, where n indicates the number of thiophene units present in
it. In this article, n is varied between 4 and 14.
lecular charge transfer states.4 In addition to these useful
electronic properties, the D-A-D systems usually possess high
lattice energies.5 This leads to the generation of highly ordered
crystalline films having enhanced device performance.6 In this
article, we report a novel synthesis and characterization of a
class of D-A-D systems (Figure 1) with thiophene as a donor
and malononitrile derivative as an acceptor.
This article is organized as follows. In section II, we present
the synthesis and characterization of the D-A-D systems shown
in Figure 1; we name these molecules as oligo-thiophenes and
are characterized by the number of thiophene units present in
conjugation with the malononitrile acceptor. In section III, we
present the absorption and fluorescence spectra of these systems
in various solvents. Furthermore, in this section, we present the
cyclic voltammetry (CV) measurements to characterize various
energy levels of the synthesized oligo-thiophene molecules. In
section IV, we turn our attention to the theoretical calculations
of ground and excited-state geometries of these systems and
characterize the excited states by calculating various properties.
We do not restrict our theoretical calculations only to the
synthesized small oligo-thiophenes but also calculated the
effective conjugation length in this system, which is shown to
be larger than that in the usual polymeric systems. We
summarize our results in the final section.
One of the important classes of molecules that satisfies the
above criterion comprises a donor group (D) and an acceptor
group (A) separated by a conjugated backbone (D-π-A). The
π-electron density in these chromophores is highly polarized
because of the presence of both an electron-donating (donor)
and an electron-withdrawing (acceptor) group, thus creating a
dipole moment in the ground state. This is why: this class of
molecules is aptly named dipolar systems. Because such charge
imbalance is a characteristic feature of this class of compounds,
large nonlinear optical responses,1 high polaronic mobilities,2
and low singlet gaps are demonstrated by the devices containing
such chromophores as active material. We can also slightly
modify these dipolar systems to create quadrupolar (D-A-D,
A-D-A)3 or more generally multipolar systems.4 Similar to their
dipolar counterpart, they exhibit large charge displacement,
strong nonlinear optical responses,4 and low-energy intramo-
* To whom correspondence should be addressed. Tel: +91-80- 22932651.
Fax: +91-80-23601310. E-mail: satish@sscu.iisc.ernet.in (S.P.); ramasesh@
sscu.iisc.ernet.in (S.R.).
† These authors contributed equally to this work.
10.1021/jp909635g  2010 American Chemical Society
Published on Web 03/12/2010

4648 J. Phys. Chem. A, Vol. 114, No. 13, 2010
Mukhopadhyay et al.
Figure 2. Synthetic scheme for the preparation of thiophene carbaldehydes with varying number of thiophene units. The thiophene carbaldehydes
thus synthesized are used as precursors for preparation of oligo-thiophene dyes.
Figure 3. Synthetic scheme for the preparation of oligo-thiophene molecules. For 4-oligo-thiophene molecule, n is equal to 4, whereas for 6-oligo-
thiophene, it is 6.
II. Experimental Details
maintained at ∼1 × 10-6 M. The UV-visible spectra of these
oligomers were obtained with Perkin-Elmer (Lambda 35)
spectrometer. All measurements were undertaken at room
temperature.
First, we focus on the preparation of thiophene carbaldehydes
(compounds 4 and 6 in Figures 2 and 3), which are used as
precursor for the synthesis of oligo-thiophenes. The alkyl
thiophene derivatives are added to cooled DMF solution
containing POCl3, which is previously stirred for about 30-45
min. The mixture thus formed is heated at 70-75 °C for 6 h.
The solution is brought to room temperature, which is followed
by neutralization by sodium hydroxide (NaOH) and extraction
by ethylacetate. Organic layer is washed by brine solution and
dried over anhydrous sodium sulfate. The solution of thiophene
carbaldehyde thus prepared is concentrated in vacuum and
purified by column chromatography. A brief outline of the
synthetic procedure is summarized in Figure 2. The details of
the preparation, characterization, and yields of intermediate steps
are presented in the Supporting Information.
Now, we turn our attention to the synthesis of thiophene
oligomers shown in Figure 1 by Knoevenagel condensation
between 2-(2,6-dimethyl-4H-pyran-4-ylidene) malononitrile (com-
pound 7 in Figure 3) and respective thiophene carbaldehydes
(compounds 4 and 6 in Figures 2 and 3). Both the reactants
along with piperidine are mixed in a round-bottomed flask fitted
with a reflux condenser. The whole reaction mixture is kept
under an inert argon atmosphere. The reaction mixture is
refluxed for about 24 h and then filtered, which is followed by
washing in acetonitrile and drying in vacuum. The schematic
for this reaction is presented in Figure 3. Further experimental
details are also presented in the Supporting Information. In this
article, we report the preparation and characterization of two
oligo-thiophene molecules with four and six thiophene units.
The general features of the absorption spectra consist of a
broadband in the visible range and a less intense band in form
of a shoulder near the UV region. On the contrary, the
fluorescence spectra consist of a single broad peak that is red-
shifted compared with absorption maxima. The absence of
multiband structure in the fluorescence spectra can be attributed
to the fact that fluorescence mainly occurs from the lowest
singlet excited state7 in accordance with Kasha’s rule. It is
evident from the above spectra that both the absorption and
fluorescence maxima get red shifted as we increase the number
of thiophene units in conjugation with the malononitrile
acceptor. Such a red shift is generally associated with the
increase in the conjugation length8 and subsequent increase in
the donating capability of the thiophene units.9 We also recorded
UV-visible absorption as well as fluorescence spectra of both
dyes in various solvents other than toluene.
Although the absorption spectra roughly remain the same in
all of these solvents, the fluorescence maximum shows a
nominal red shift as we increase the solvent polarity. The related
photophysical data are presented in Table 1, and the corre-
sponding absorption and fluorescence spectra are presented in
the Supporting Information. The energy gaps estimated from
the absorption band edges are also presented in the same Table.
It is evident from Table 1 that the photoluminescence of these
oligo-thiophene dyes is not highly solvent sensitive.5 A nominal
bathochromic shift (∼40 nm) of the emission maxima is
observed with increasing solvent polarity, which saturates in
highly polar solvents like DMF. Such saturation effect is highly
prominent in 6-oligo-thiophene dye.
III. Spectroscopic and Electrochemical Analysis
Here we turn our attention to the photophysical properties of
the prepared oligo-thiophenes. Both UV-visible absorption and
fluorescence spectra presented in Figure 4a,b are recorded using
toluene as solvent and the oligo-thiophene concentration is
The electrochemical behavior of the oligomers was investi-
gated by CV.10 CV measurements are performed on a Solartron
SI1287 potentiostat. The electrochemical analyzer consists of

Donor-Acceptor Conjugated Molecules
J. Phys. Chem. A, Vol. 114, No. 13, 2010 4649
Figure 4. (a) UV-visible absorption and (b) fluorescence spectra of oligo-thiophene dyes (∼1 × 10-6 M). All spectra are recorded with Perkin
Elmer spectrometers at room temperature in toluene.
TABLE 1: Absorption and Fluorescence Maxima in
Different Solventsa
dielectric absorption fluorescence energy
constants maxima λab maxima λem
gap
(eV)
system
solvent of solvents
(nm)
(nm)
4 oligo-
thiophene THF
toluene
2.4
7.5
38.0
2.4
7.5
38.0
476
572
609
637
601
640
649
2.23
2.13
DMF
toluene
6 oligo-
thiophene THF
490
DMF
a Energy gaps of 4 and 6 oligo-thiophenes estimated from absorption
edges are also presented here.
three electrode cells with Pt disk as working electrode, Pt wire
as counter electrode, and Ag/AgCl (saturated KCl) as reference
electrode. The scan rate is fixed at 50 mV/s. Anhydrous tetra-
butylammonium perchlorate (Bu4NClO4) in nitrogen-saturated
DMF solvent is used as supporting electrolyte. Oligo-thiophene
dye (1 × 10-5 M) is mixed with the supporting electrolyte, and
both cathodic and anodic sweeps are performed to obtain the
redox potentials.
Figure 5. Cyclic voltammograms of oligo-thiophene dyes (4-oligo-
thiophene and 6-oligo-thiophene) with platinum disk as working
electrode in the presence of Bu4NClO4 (0.10 M) as supporting
electrolyte in DMF with Ag/AgCl (saturated KCl) as reference
electrode. The scan rate is fixed at 50 mV/s. Cyclic voltammogram of
only 0.10 M Bu4NClO4 in DMF without the oligo-thiophene dye is
also shown. All measurements are performed in ambient temperature.
The voltammograms obtained from CV measurements are
presented in Figure 5. As evident from these voltammograms,
these oligo-thiophenes do not show good reversible behavior.
On anodic sweep, the oxidation peaks are found to be 1.22 and
1.20 V for 4-oligo-thiophene and 6-oligo-thiophene, respec-
tively. The corresponding rereduction peaks are observed at 0.91
and 1.01 V. In contrast, the cathodic sweep shows reduction
peaks at -1.10 and -0.96 V for 4-oligo-thiophene and 6-oligo-
thiophene, respectively. The corresponding reoxidation peaks
are observed at -0.83 and -0.66 V. The reduction peak around
-0.40 V and its corresponding reoxidation peak at -0.30 V is
attributed to the solution containing only secondary electrolyte
(Bu4NClO4) as solute and DMF as solvent. This is confirmed
from the voltammogram of the 0.10 M solution of the secondary
electrolyte in DMF in the absence of oligo-thiophene dye. From
these potentials of oxidation and reduction processes we can
estimate the HOMO and LUMO energies and their correspond-
ing HOMO-LUMO gaps. For calculating the HOMO and
LUMO energy levels of oligo-thiophenes, we have considered
the potentials corresponding to the peak position of the current
in the oxidation and reduction processes.10 The HOMO and
LUMO energy levels for 4-oligo-thiophenes are -5.87
and -3.55 eV, and those of 6-oligo-thiophene are at -5.85 and
-3.69 eV. The HOMO-LUMO gaps calculated from these
values are 2.32 and 2.16 eV for 4-oligo-thiophene and 6-oligo-
thiophene, respectively. The HOMO-LUMO gaps thus calcu-
lated compare well with those obtained from spectroscopic
measurements. We will see in the next section that although
the HOMO and LUMO energies differ significantly from the
gas-phase calculated values, the HOMO-LUMO gaps obtained
from both CV and spectroscopic measurements compare well
with theoretical values.
IV. Theoretical Investigation
Here we focus on quantum theoretical investigation of oligo-
thiophene dyes, which include calculations of ground and excited-
state geometries and properties such as oscillator strengths and
permanent dipole moments. We do not restrict ourselves only
to the oligo-thiophenes synthesized by us but perform calcula-
tions on larger oligo-thiophene molecules to generalize the
experimental results presented in previous sections. Our theo-
retical calculations are not only restricted to the gas phase but
also incorporate the effects of different solvents on excited states
of these oligo-thiophenes.
The theoretical investigations are carried out by employing
the Gaussian 03 package11 and Zerner’s intermediate neglect
of differential overlap (ZINDO) program.12 First, we illustrate

4650 J. Phys. Chem. A, Vol. 114, No. 13, 2010
Mukhopadhyay et al.
TABLE 2: Molecular Orbital Energies of Various Oligo-Thiophene Molecules
no. of thiophene units HOMO-2 (eV) HOMO-1 (eV) HOMO (eV) LUMO (eV) LUMO+1 (eV) LUMO+2 (eV) GAPS (eV)
4
6
8
10
12
14
-6.0073
-5.7645
-5.6312
-5.5327
-5.4492
-5.3553
-5.4726
-5.2611
-5.1357
-5.0364
-4.9879
-5.0177
-5.4002
-5.2598
-5.1177
-5.0198
-4.9493
-4.8987
-2.7662
-2.8152
-2.8456
-2.8661
-2.8859
-2.8974
-2.4549
-2.5422
-2.5961
-2.6307
-2.6598
-2.6748
-1.4182
-1.7471
-1.9588
-2.1020
-2.2111
-2.2560
2.6340
2.4446
2.2721
2.1537
2.0633
2.0013
the computational details of the gas-phase quantum theoretical
calculations. Optimized ground-state geometries of oligo-
thiophene dyes in the ground state are obtained using density
functional theory (DFT) method. In these calculations, we
include the hybrid functional, which incorporates both Hartree-
Fock and DFT exchange correlations. This hybrid functional,
usually described in literature as the B3LYP functional, is an
improvement over the LDA exchange correlation.13 Furthermore,
to perform this geometry optimization, we use split valence basis
set along with polarizable functions for heavier atoms (6-31g*).
For smaller oligomers, we also optimize the geometry by adding
diffuse functions (6-31+g*) to the previously mentioned basis
set. In the Supporting Information, we present selected structural
parameters of 6-oligo-thiophene dye obtained by optimizing the
geometry using both B3LYP/6-31g* and B3LYP/6-31+g*
methods.
The optimized geometries obtained from both these methods
are quite similar; bond lengths and bond angles obtained differ
only in the third decimal place and the dihedral angles differ
by 4-6° (Table 1, Supporting Information). We have computed
the excited-state energies and other properties in both geometries
by the ZINDO single configuration interaction (SCI) method
keeping 33 highest occupied molecular orbitals (HOMOs) and
33 LUMOs. The results obtained for the two geometries
compare quite well with each other (Table 2, Supporting
Information). We therefore conclude that the density functional
method with 6-31g* as basis set and B3LYP as hybrid potential,
followed by ZINDO SCI, is sufficient to obtain consistent
prediction of properties such as excitation energies, oscillator
strengths from ground state, and dipole moments of low-lying
states for this class of D-A-D systems. These sets of methods
have also been applied by other workers with reasonable success
to obtain various excited-state properties for dipolar and
multipolar14 molecules.
Now we focus our attention on the calculation of excitation
energies, oscillator strengths, and dipole moments of ground
and excited states for various oligo-thiophenes, the largest of
which contains 14 thiophene units. First, we optimize the
ground-state geometry of all oligo-thiophenes by B3LYP/6-31g*
method. The density functional calculation provides us with MO
energies that are listed in Table 2. It is evident from Table 2
that the two of the HOMOs are nearly degenerate, whereas the
LUMO consists of a pair of closely spaced energy levels. The
energy difference between LUMO and LUMO+1 decreases
with the increase in the number of thiophene units. This
difference can be attributed to the slight difference in orientation
of the thiophene units in either arm of the D-A-D systems. The
nature of HOMO and LUMO can be well understood from the
electron density distribution, which is presented in Figure 6a,b.
The HOMO-1 and HOMO correspond to charge density being
localized on one arm or the other whereas LUMO and
LUMO+1 are akin to resonance structures.
phenomenon that plays a dominant role in the low-energy
photophysics of these D-A-D systems. In Figure 7, we show
schematically the dominant electron configurations in the ground
and first two excited states.
Now, we turn our attention to the calculation of excited-state
properties by ZINDO SCI method. We choose to work with
SCI method because the low-lying excited states obtained so
compare reasonably well with experiments because SCI incor-
porates electron correlation effects in the excited states to the
same extent as in ground state. Moreover, this method has the
advantage of being size-consistent, whereas the other restricted
CI methods are not.15
We present the excitation gaps, oscillator strengths from the
ground state, and permanent dipole moments in the ground and
low-lying excited states of different thiophene oligomers in
Table 3. The first and the second excited states are found to be
nearly degenerate.
The first excited state is dominated by the HOMO to LUMO
excitations (Figure 7), whereas the excitations involving HOMO
and LUMO+1 dominate the second excited state. The energy
difference between the first and the second excited state is
slightly higher than that obtained from the DFT calculations
because SCI incorporates to some extent the effects of electron
correlations in the excited states.
The gap is 2.60 eV in the oligomer with 4 thiophene units,
which reduces to 2.18 eV in the oligomer with 14 thiophene
units. However, the gap in the largest oligomer shown here (14-
oligo-thiophene) is very nearly the same as that in the oligomer
with 12 thiophene units. In Figure 8, we plot different excited
state energies as a function of the inverse of number of thiophene
units (N). This clearly indicates that the effective coherence
length for π-conjugation in these systems saturates for 14-oligo-
thiophene molecule. This coherence length is much longer
compared with polythiophenes without the acceptor moiety,
which only spans about six oligomer units.16 We present the
oscillator strengths of the low-lying excited states for these oligo-
thiophenes. It is noted that for all oligomers, the lowest excited
state is transitionally coupled to the ground state.
This theoretical fact matches well with our experimental
findings. As shown from Table 3, the oscillator strength of the
lowest excited state for 6-oligo-thiophene is higher compared
with that of the 4-oligo-thiophene molecule. On the contrary,
the oscillator strength of the second excited state for both the
oligo-thiophenes is roughly same. The slight difference between
the calculated and the observed absorption spectrum, in the 375
nm region, can be attributed to Franck-Condon factors and the
solvent effects. As presented in Figure 9, the oscillator strengths
per thiophene unit of low-lying excited states show convergence
behavior similar to that of excited-state energies.
Now we turn our attention to the estimation of permanent
dipole moments in ground and low-lying excited states of these
oligo-thiophenes. Although the absolute values of dipole mo-
ments are quite large in the ground and excited states, the
difference between them varies from 1.4 to 2.8 D. The variation
of the dipole moments in ground and low-lying excited states
As seen from Figure 6b, the electron density in the LUMO
level is primarily localized on the acceptor (malononitrile) unit.
This clearly indicates to an intramolecular charge transfer

Donor-Acceptor Conjugated Molecules
J. Phys. Chem. A, Vol. 114, No. 13, 2010 4651
Figure 6. (a) Electron density plots of three HOMOs for 14-oligo-thiophene molecule. The carbon atoms are marked in gray, sulfur in yellow,
nitrogen in blue, oxygen in red, and hydrogen in white. The positive and negative lobes of electron density are marked in brown and green,
respectively. (b) Electron density plots of three LUMOs for 14-oligo-thiophene molecule. Color coding is same as in part a.

4652 J. Phys. Chem. A, Vol. 114, No. 13, 2010
Mukhopadhyay et al.
Figure 7. Energy level diagram depicting the contributions of various excitations to ground and the degenerate first and second excited states.
Proper linear combination of the microstates shown in the Figure need to be constructed to obtain the corresponding singlets.
TABLE 3: Vertical Singlet Excitation Energies, Oscillator Strengths, and Permanent Dipole Moments of Various
Oligo-Thiophene Molecules Calculated by ZINDO SCI Methoda
gaps (eV) (oscillator strength)
magnitude of permanent dipole moment (debye units)
no. of thiophene units
1
2
3
4
G
1
2
3
4
4
6
8
10
12
14
2.60 (1.60) 2.76 (1.20) 3.63 (0.25) 3.79 (0.21)
2.38 (2.07) 2.56 (1.24) 3.36 (0.16) 3.41 (0.22)
2.30 (2.68) 2.45 (1.11) 3.14 (0.16) 3.19 (0.34)
2.24 (3.13) 2.37 (1.22) 2.95 (0.16) 3.01 (0.18)
2.20 (3.66) 2.31 (1.17) 2.81 (0.23) 2.87 (0.15)
2.18 (3.88) 2.30 (1.21) 2.71 (0.27) 2.85 (0.10)
12.79
17.01
13.37
15.63
12.80
13.01
15.51
19.69
15.42
17.39
14.32
14.42
13.62
18.32
14.43
16.60
13.62
13.87
11.65
16.64
14.86
17.33
14.43
14.51
15.62
19.73
14.34
16.28
13.30
13.87
a Oscillator strengths are from the ground state (G) to the low-lying excited states labeled 1 through 4.
Figure 9. Convergence behavior of oscillator strengths per thiophene
unit connecting low-lying excited sates and ground state with the
number of thiophene units.
Figure 8. Convergence of low-lying excited state energies as an inverse
function of number of thiophene units (N).
is presented in Figure 10. The permanent dipole moments show
convergence at 14 thiophene units for all states. Before
converging to the limiting value (at 14-oligo-thiophene), the
dipole moments in all the states show an oscillatory behavior,
which can be attributed to different orientation of thiophene
units from one oligomer to the other by about 3 to 4°. The small
difference in dipole moments between ground and first excited
state for this class of compounds results in nominal solvato-
chromic shifts, as observed experimentally.
calculations of Stoke’s shift17 and predicting the fluorescence
spectra for oligo-thiophenes in the presence of solvents of
various dielectric constants.18 For this purpose, we focus only
on the molecules synthesized in our laboratory. To calculate
the Stoke’s shift, we obtain the optimized geometry of the first
excited state by the following procedure. First, we obtain the
first excited state under the Born-Oppenheimer approximation
by SCI method. In this geometry, we calculate the matrix of
force constants (Hessian matrix) along with the displacement
vector. These force constants specify the curvature of the surface
Until now, we have calculated vertical excited-state energies
and properties in the gaseous phase. Here we focus on the

Donor-Acceptor Conjugated Molecules
J. Phys. Chem. A, Vol. 114, No. 13, 2010 4653
Figure 10. Variation of the magnitude of permanent dipole moments (|µ|) in ground and low-lying excited states of oligo-thiophene molecules
with different number of thiophene units.
TABLE 4: Singlet Excitation Energies, Oscillator Strengths and Permanent Dipole Moments of Thiophene Oligomers
Containing Four and Six Thiophene Units in Optimized Excited State Geometries Obtained As Described in the Texta
gaps (eV) (oscillator strength)
magnitude of permanent dipole moment (debye units)
systems
1
2
3
4
G
1
2
3
4
4-oligo-thiophene
6-oligo-thiophene
2.33 (1.45)
2.15 (1.76)
3.12 (1.38)
3.00 (1.50)
3.61 (0.15)
3.31 (0.14)
3.84 (0.005)
3.70 (0.26)
13.03
17.06
15.01
19.60
14.34
18.68
11.99
17.96
9.04
17.71
a Oscillator strengths are from the ground state (G) to the low-lying excited states (labeled 1 through 4).
TABLE 5: Comparison of Fluorescence Wavelength
Obtained from SCRF-SCI Calculation with Experimental
Results for Four and Six Oligo-Thiophene Molecules
at that electronic state and provides information useful for
determining the displacement at the next step. In the displaced
geometry, we again do a Hartree-Fock Gaussian (HFG)
calculation, followed by an SCI calculation. The whole proce-
dure is repeated until convergence is achieved for the excited
state geometry. We perform the frequency calculation on the
optimized geometry obtained from the previous self-consistent
calculation to ensure that the geometry obtained from the SCI
calculation is at the global minimum of the first electronic
excited state. The absence of any imaginary frequencies prove
beyond doubt that the geometry obtained from this self-
consistent SCI calculation is at the global minimum of the
prescribed electronic state.
calculated
wavelength
(nm)
experimental
wavelength
(nm)
systems
solvents
4-oligo-thiophene
Toluene
THF
DMF
toluene
THF
DMF
551
580
607
602
648
680
572
609
637
601
640
649
6-oligo-thiophene
fixed by calculating the greatest internuclear distance and by
adding the van der Waals radii of the atoms involved. The gaps
thus obtained for both four and six oligo-thiophene molecules
are presented in Table 5.
Here we present the results obtained from the ZINDO SCI
calculation performed on the optimized excited-state geometry.
Comparing the energy gaps from Table 4 with those from Table
3, we find that the Stoke’s shifts in both these oligo-thiophenes
are about ∼0.25 to 0.27 eV. Besides, the near degeneracy of
first and second excited states is lifted, and the oscillator strength
decreases for the first excited state, whereas it increases for the
second excited state.
Now we focus our attention to the effect of solvent polarity
on the energies of the excited states. To compare our theoretical
results with the observed fluorescence spectra, we should
incorporate the effect of solvents on the theoretically calculated
excitation spectra. For this reason, we perform the self-consistent
reaction field calculation along with SCI method (SCRF-SCI)19
to take into account the effect of solvents on the excitation
energies. In this method, the solvent is considered to be a
continuum of uniform dielectric constant represented by the
reaction field. The solute molecules are made to occupy a fixed
cavity of radius a0 within the solvent field. The radius a0 is
Our theoretical fluorescence wavelengths match reasonably
well with that of experimental values for solvents of low
polarity, but the SCRF calculation gives a poorer estimate for
highly polar solvents, as seen in case of 6-oligo-thiophene
molecule. The experimentally observed red shift in the fluo-
rescence spectra of 6-oligo-thiophene molecule is not significant
as we increase the solvent polarity from 7.5 (THF) to 38.0
(DMF). Such a saturation effect is not well reproduced in the
SCRF-SCI results tabulated in Table 5. Failure of the SCRF
model for high-polarity solvent can be attributed to various
factors like structural deformation of solute molecules, polariza-
tion of neighboring solvent molecules, nonuniform dielectric
constant, and nonspherical cavity of the solute molecules within
the solvent dielectric.

4654 J. Phys. Chem. A, Vol. 114, No. 13, 2010
Mukhopadhyay et al.
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V. Summary
A class of new conjugated molecules containing donor
(thiophene chain) and acceptor (malononitrile) moiety have been
successfully synthesized. Their molecular structures are con-
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1
firmed by H NMR and 13C NMR. By increasing the number
of thiophene units in the synthesized molecules, the excitation
energy gap shifts toward the red region and finally saturates
around 2.0 eV. From theoretical studies, we also observe a
strong intramolecular charge transfer phenomenon between the
thiophene donor and malononitrile acceptor in this class of
D-A-D systems. The charge transfer character increases with
the increase in the thiophene units and finally saturates in the
oligomer with 14 thiophene units in conjugation with malono-
nitrile. The saturation of gaps and other properties like
permanent dipole moments and the oscillator strengths are
arrived at from our theoretical calculations using DFT and
ZINDO SCI. We have also calculated the Stoke’s shift and the
effect of solvents on the excited-state energies. Our theoretical
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Acknowledgment. We thank the Department of Science and
Technology for supporting this work through the project SR/
S2/CMP-24/2003. We sincerely thank Snehangshu Patra for
helping us in performing the CV measurements. We also thank
Prof. N. Munichandraiah for useful discussions and comments
regarding the CV measurements.
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Supporting Information Available: Experimental proce-
dures including the synthesis and NMR analysis, absorption and
fluorescence spectra of 4-oligomer and 6-oligomer, and struc-
tural elucidation and geometry optimization of 6-oligo-thiophene
molecule. This material is available free of charge via the
Internet at http://pubs.acs.org.
References and Notes
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