Optical properties of dibenzo[d,d′]thieno[3,2-b;4,5-b′] dithiophene monocrystals: The effect of intermolecular interactions

Article abstract of DOI:10.1021/jp1052826

The polarized UV-visible absorption spectra of dibenzo[d,d′]thieno[3, 2-b;4,5-b′]dithiophene single crystals are reported and interpreted to definitively attribute the observed bands and their polarizations. The results provide information on the intermolecular interactions and on the aggregation in the condensed phase, which can be of either herringbone- or H-type, depending on the electronic transition taken into considerations, with consequences on the order and polarization of the absorption bands. A relatively easy method is also discussed to obtain information on the structural/morphological properties of different types of samples, including thin films, which have been recently proposed for high-performance organic film-effect transistors for their high ionization potential and photostability.

Full text of DOI:10.1021/jp1052826

J. Phys. Chem. A 2011, 115, 225–231
225
Optical Properties of Dibenzo[d,d′]thieno[3,2-b;4,5-b′]dithiophene Monocrystals: The Effect
of Intermolecular Interactions
Laura Alessandrini,† Daniele Braga,†,‡ Abdelhafid Jaafari,§ Luciano Miozzo,‡ Stefano Mora,†
Leonardo Silvestri,† Silvia Tavazzi,* and Abderrahim Yassar‡
Dipartimento di Scienza dei Materiali, UniVersita` degli Studi di Milano Bicocca, Via Cozzi 53,
I-20125 Milano, Italy, ITODYS, UniVersite´ Paris Diderot-Paris 7, 15 rue Jean de Ba¨ıf,
75205 Paris Cedex, France, and Faculte´ des Sciences et Techniques de Settat, UniVersite´ Hassan 1er, Maroc
ReceiVed: June 8, 2010; ReVised Manuscript ReceiVed: September 22, 2010
The polarized UV-visible absorption spectra of dibenzo[d,d′]thieno[3,2-b;4,5-b′]dithiophene single crystals
are reported and interpreted to definitively attribute the observed bands and their polarizations. The results
provide information on the intermolecular interactions and on the aggregation in the condensed phase, which
can be of either herringbone- or H-type, depending on the electronic transition taken into considerations,
with consequences on the order and polarization of the absorption bands. A relatively easy method is also
discussed to obtain information on the structural/morphological properties of different types of samples,
including thin films, which have been recently proposed for high-performance organic film-effect transistors
for their high ionization potential and photostability.
Introduction
Neckers and co-workers reported an isomer-specific synthesis
of two thieno bis(benzothiophenes): thieno[2,3-f:5,4-f]bis[1]-
benzothiophene and thieno[3,2-f:4,5-f]bis[1]benzothiophene along
with detailed characterization of these materials. These isomers
are isoelectronic with DBTDT and pentacene and composed of
well-defined alternating benzene and thiophene moieties.11 Zhu
and co-workers12 reported the synthesis and the characterization
of the linearly fused pentathienoacene in which all the carbocy-
clic rings of pentacene are replaced with thiophene systems.
Oligoacene-based materials represent the most studied and
promising class of molecular organic semiconductors, used as
active elements in new generations of plastic and organic
electronic devices.1-3 Pentacene, the major representative of this
group, shows a hole mobility of up to few tens of cm2 V-1 s-1
and has become the benchmark for organic electronics.4 Despite
its potential, pentacene and other higher acenes suffer from low
environmental stability. Pentacene decomposes rapidly because
it has an electron-rich central ring, which is subject to a
Diels-Alder reaction with oxygen.5 To circumvent the instabil-
ity of pentacene, heteroacenes in which a carbocyclic ring or
rings are replaced by a heteroaromatic group have recently
emerged as a new class of organic semiconductors, and their
organic field effect transistors (OFETs) are reported to show
both high field effect mobility and good stability.6 The improved
stability is attributed to relatively low-lying higher occupied
molecular orbital (HOMO) level, although heteroacene mol-
ecules contain highly π-extended fused aromatic rings like
acene.
One of the earliest reports of a high-mobility heteroacene
described the synthesis and characterization of anthra-
dithiophene.5 This material exhibited high air stability and good
device properties in the vacuum-deposited thin film form. Thus,
the anthradithiophene and alkyl derivatives combine a penta-
cene-like intrinsic mobility with greater solubility and oxidative
stability. Gao and co-workers recently reported high field effect
mobility on OFETs of dibenzo[d,d′]thieno[3,2-b;4,5-b′]dithiophene
(C16H8S3, DBTDT) (1),7 that was first synthesized sixty years
ago,8 proposing this material as valid alternative to currently
used p-type organic thin-film OFETs of pentacene.9 In contrast
to pentacene, DBTDT is much more stable against oxidation,
being photoinduced decomposition the main limit of pentacene.5,10
Despite the extended π-system, the spectroscopic character-
ization of thieno[f,f′]bis[1]benzothiophenes, DBTBT, pen-
tathienacene and structurally related dibenzo[b,b′] thieno[2,3-
f:5,4-f′] heterocycles indicates that they possess lower HOMO
energy levels and larger band gaps than pentacene. The structural
and electronic properties of the single DBTDT molecule have
been widely discussed by Osuna et al.,13 together with other
thiophene- and selenophene-based heteroacenes studied by
UV-visible-NIR spectroscopy. The conjugation properties of
these systems are discussed as deduced from Raman spectros-
copy. Density functional theory calculations have been per-
formed to identify the vibrations associated with the Raman
features as well as to assess information regarding the minimum-
energy molecular structure and vertical one-electron excitations
giving rise to the main optical absorption bands. As discussed
by the authors,13 the stabilization of the HOMO and destabiliza-
tion of the lowest unoccupied molecular orbital (LUMO) lead
to a broader energy gap than that of the corresponding nonfused
oligothienyl counterpart. The authors discuss the significant
contribution of the sulfur atoms to molecular orbitals, concluding
that these heteroacenes cannot be described simply as rigidified
R-oligoheteroaromatic systems. Also Okamoto et al.14 report a
UV-vis photophysical characterization and attribute the ob-
served absorption bands of DBTDT to vibronic transitions of
the single molecule. Interestingly, these semiconductors show
charge carrier mobilities comparable to those of pentacene, but
have a molecular gap comparable with anthracene.
* To whom correspondence should be addressed. Tel.: +390264485035.
Fax: +30264485400. E-mail: silvia.tavazzi@unimib.it.
† Universita` degli Studi di Milano Bicocca.
‡ Universite´ Paris Diderot-Paris 7.
Despite their interesting properties as semiconductors, reports
concerning the analysis of their optical properties are scarce.
§ Universite´ Hassan 1er.
10.1021/jp1052826  2011 American Chemical Society
Published on Web 12/17/2010

226 J. Phys. Chem. A, Vol. 115, No. 3, 2011
Alessandrini et al.
SCHEME 1: Synthesis of
Dibenzo[d,d′]thieno[3,2-b;4,5-b′]dithiophene (1)
Figure 1. DBTDT molecular structure.
In the crystalline form, the arrangement of the DBTDT
molecules has been reported by Okamoto et al.14 No detailed
studies are reported on the optical properties of the monocrystals.
The UV-vis absorption of vacuum-deposited thin film is
reported by Gao et al.7 The absorption band at lowest energy
of the film is found to be red shifted with respect to the band
at lowest energy of the DBTDT solution. The authors tentatively
attribute this effect to strong intermolecular interactions. How-
ever, the lack of a detailed analysis of the optical properties of
the monocrystals did not allow the proper assignment of the
optical bands.
In this paper, the polarized UV-vis absorption spectra of
DBTDT single crystals are reported. The observed bands and
their polarizations are definitively attributed. The results provide
information on the intermolecular interactions and aggregation
in the condensed phase and suggest a simple method to deduce
information about the structural/morphological properties of
different types of DBTDT samples, such as polycrystalline thin
films, which are of special interest for device applications.
Experimental Section
The synthesis of DBTDT has been known for a long time.15,16
An interesting route with improved yield has been recently
proposed by Gao et al..7 We followed a slightly different
approach, which is shown schematically in Scheme 1. The key
step of the synthesis is the ring closure reaction of the
appropriate 3,3′-dibromo-2,2′-bi(benzo[b]thiophene) (2) with
sulfur. The building block 3,3′-dibromo-2,2′-bi(benzo[b]thiophene)
was prepared from 2,3-dibromobenzo[h]thiophene (3). The
incorporation of the sulphure bridge to produce dibenzo-
[d,d′]thieno[3,2-b;4,5-b′]dithiophene (1) was smoothly accom-
plished by in situ bromo-lithium exchange and reaction with
bis(phenyl sulfonyl) sulphide. The yield of the reaction is 43%.
Single crystals of DBTDT were grown by the floating-drop
technique of dissolving 5-6 mg of sublimated product in 3 mL
of toluene.17 The solution was filtered (size of the pores 0.2
µm), placed on the surface of water, and kept in air during
solvent evaporation. Crystals grow as platelets with one acces-
sible face for optical investigation. Single crystals were selected
under crossed polarizations using an optical polarizing transmis-
sion microscope Olympus SZX12. The thickness of the crystals
placed upon quartz plates was measured by atomic force
microscopy (Nanoscope V Multi-Mode Veeco).
The electronic optical transitions of the isolated molecule in
vacuum and the corresponding direction of polarization were
calculated by means of the Gaussian 03 software, using as input
parameters the molecular geometries obtained from the crystal-
lographic data.18 Absorbance measurements in the UV-visible
spectral range were performed at room temperature using a
Perkin-Elmer Lambda 900 spectrometer, equipped with Glan-
Taylor calcite polarizers to vary the polarization direction of
the incoming light.
Figure 2. Images taken under the optical microscope of a DBTDT
crystal with either crossed (a,b) or parallel (c) polarizer and analyzer.
the literature.14 It is orthorhombic with cell axes a ) 7.933 Å,
b ) 26.589 Å, and c ) 5.906 Å and four molecules per unit
cell. Figures 2, 3, and 4 show images taken under the optical
microscope of DBTDT crystals. In all of the figures, the images
of panels a and b were taken with crossed polarizer and analyzer,
either (a) with the sample oriented to give extinction of the
transmitted light or (b) for a generic orientation of the sample.
The extinction is found when the electric field of the incident
light is along a unit-cell axis because for orthorhombic systems
the unit-cell axes are principal axes.19 The complete extinction
of the transmitted light in panels a indicates that the crystals
are single. The extinction was also found by rotating the crystal
of 90° around the normal to the surface (not shown here), as
expected.19 On the contrary, for a generic orientation of the
sample the emerging electric field is rotated with respect to
the incident electric field and the angle of rotation depends on
the wavelength of light and on the crystal thickness (birefrin-
gence).19 For this reason, the crystals are clearly distinguishable
in panels b with respect to the dark background. Their uniform
color and brilliance indicate that the thickness of each crystal
Results and Discussion
DBTDT is a conjugated molecule (Figure 1) with planar
conformation. The crystal structure of DBTDT is taken from

DBTDT Crystal Optics
J. Phys. Chem. A, Vol. 115, No. 3, 2011 227
Figure 4. Images taken under the optical microscope of a DBTDT
crystal with either crossed (a,b) or parallel (c) polarizer and analyzer.
(Inset of c) Image under the atomic force microscope of a border of
the crystal placed on a quartz substrate. The three lines indicate the
three directions where the thickness profile was extracted.
substrate was found (51 ( 4) nm. This is one of the thinnest
crystals that we could grow. As can be seen in Figure 4b,c, it
presents few small regions where the thickness is higher (up to
few hundreds of nanometers as measured by atomic force
microscopy) due to the superposition of other small crystallites.
These small crystallites are often misaligned with respect to
the main crystal, as can be seen in Figure 4a, which shows few
regions where the extinction of the transmitted light is not
perfect.
Polarized absorption spectra were taken at normal incidence
on the most developed face of DBTDT crystals. Figure 5 shows
the spectra taken on the crystal shown in Figure 4 in the region
where it is single and thin (estimated thickness equal to (51 (
4) nm). The sample was masked thus probing an area of about
0.7-0.8 mm2. The reported spectra were measured with two
orthogonal polarizations corresponding to the extremes low- and
high-energy shift of the peaks between about 3.4 and 3.8 eV
(the spectra taken with intermediate polarizations are shown in
the inset in a restricted energy range). These two polarizations
also correspond to the two directions of the electric field that
give extinction of the transmitted light under crossed polarizer
and analyzer. The spectra are dominated by two orthogonally
polarized vibronic series with peaks at 3.45 and 3.61 eV and a
shoulder at about 3.78 eV in one case and at 3.46 and 3.62 eV
Figure 3. Images taken under the optical microscope of a DBTDT
crystal with either crossed (a,b) or parallel (c) polarizer and analyzer.
is uniform in the investigated area. Finally, panels c show the
images taken with parallel polarizer and analyzer. The crystals
appear slightly colored (yellow due to the absorption of the
complementary blue light). In particular, we selected the
configuration corresponding to the incident electric field along
one principal axis, the one where the absorption is larger. For
the opposite case (parallel polarizer and analyzer and the incident
electric field along the principal axis giving the minimum
absorption, not shown here), the image of the crystal could be
detected but the color is clearer and the crystal is less
distinguishable with respect to the background due to its weaker
absorption. Finally, the inset of Figure 4c shows the image of
the corresponding sample under the atomic force microscope.
Three lines on the image indicate the directions where the
thickness profile was measured. A step was found, and the
thickness of the sample with respect to the level of the quartz

228 J. Phys. Chem. A, Vol. 115, No. 3, 2011
Alessandrini et al.
Figure 7. View of the molecular packing along the b crystal axis.14
the corresponding direction of polarization of the isolated
molecule in vacuum by means of the Gaussian 03 software,
using as input parameters the molecular geometries obtained
from the crystallographic data.14,18 Since our aim was the
attribution of the polarized bands in the crystal, we did not
optimize the molecular geometry. The obtained components of
the transition moments along the inertial molecular axes (L, M,
N, where L and N are the longest and shortest axes, respectively)
are reported in Table 1, together with the available data taken
from the literature for comparison.13 The agreement with the
results reported by Osuna et al.13 is satisfactory. Spectra of
DBTDT in solution have been reported by Osuna et al.13 and
the corresponding peaks of the vibronic progression have been
detected at 3.62, 3.79, 3.96 eV. Also Okamoto et al.14 report a
UV-visible photophysical characterization in solution and the
maximum in the absorption spectrum is reported at 344 nm (3.60
eV). The optical band observed in solution at lowest energy is
attributed to the transition calculated at lowest energy, which
is also the strongest one.
Figure 5. Absorbance spectra of the DBTDT single crystal shown in
Figure 4 taken at normal incidence with two orthogonal polarizations
corresponding to the extremes high-energy shift (E//c) and low-energy
shift (E//a) of the peaks between about 3.4 and 3.8 eV. (Inset) Spectra
taken on the same crystal with intermediate polarizations. The labels
indicate the corresponding angle of the electric field to the c axis.
In principle, in the crystalline form each molecular electronic
transition is predicted to give rise to four excitonic transitions.18
We can express crystal excitonic states with a given wave vector
k in terms of the delocalized wave functions Ψk,R ) N-1/2 Σn
exp(ikn)|n, R〉, where |n, R〉 indicates a state in which the
electronic excitation resides on molecule R in cell n while all
other molecules in the crystal are unexcited and N is the total
number of cells. At k ) 0, symmetry dictates that the four
excitonic eigenstates are 1/2 (Ψk,1 + Ψk,2 + Ψk,3 + Ψk,4), 1/2
(Ψk,1 - Ψk,2 - Ψk,3 + Ψk,4), 1/2 (Ψk,1 - Ψk,2 + Ψk,3 - Ψk,4),
and 1/2 (Ψk,1 + Ψk,2 - Ψk,3 - Ψk,4). The unit cell dipole
moments of the excitonic transitions at k ) 0 are given in Table
1. In the crystal, the L molecular axes of all the four molecules
in the unit cell are parallel to b, so that L polarized molecular
transitions give rise to a pure H aggregate and only one allowed
b polarized excitonic transition.18 On the contrary, for all the
molecular M polarized transitions, the arrangement in the crystal
is of herringbone type. The M molecular axes of the four
molecules in the unit cell lie in the ac plane and form an angle
of absolute value 27° with the c axis. As a consequence, each
M polarized molecular transition gives rise to two allowed
excitonic transitions polarized along the a and c axes with
significant projections along both the axes. The intensity ratio
between the transitions is predicted to be 3.7. The comparison
between the experimental results and the predictions of the
calculations allows deducing that the accessible face (the most
developed one of the crystal) is the ac face, which is shown in
Figure 8 as taken from ref 14. Indeed, two different excitonic
transitions are detected in the spectra taken at normal incidence,
which are slightly shifted and orthogonally polarized. These
bands are therefore attributed to the excitonic transitions
(showing a vibronic progression) originating from the second
Figure 6. Absorbance spectra of the DBTDT single crystal shown in
Figure 4 taken at different angles of incidence from 0 to 60° on the
accessible face (ac) with p (ab) polarized light using the plane (ab)
formed by the normal to the surface and the direction of polarization
labeled as E//a in Figure 2 as plane of incidence.
and a shoulder at about 3.78 eV in the other case. The stronger
intensity is observed in the latter case. In Figure 5, the two
polarization directions are labeled E//a and E//c, since the
comparison with the theoretical prediction (see below) allows
deducing the corresponding polarization with respect to the axes
of the crystal unit cell.
Figure 6 shows the spectra taken at oblique incidence at the
angles of incidence indicated by the labels with p-polarized light
(polarized in the plane of incidence) using, as plane of incidence,
the plane formed by the normal to the surface and the direction
of polarization labeled as E//a in Figure 5. Besides the replicas
in the low-energy portion of the spectrum, at oblique incidence
an emerging broad band is detected centered at about 4.07 eV,
together with a further shoulder at about 4.6 eV and a structure
at about 4.8 eV. The maximum intensity for these bands is
detected for the highest angle of incidence, it decreases when
decreasing the angle of incidence, and it disappears at normal
incidence.
To interpret the obtained data on the single crystal, we have
calculated, as a first step, the electronic optical transitions and

DBTDT Crystal Optics
J. Phys. Chem. A, Vol. 115, No. 3, 2011 229
TABLE 1: Energies (Em) and Oscillator Strength (f) of the Electronic Transitions Calculated by Osuna et al.13 for the Single
DBTDT Molecule and Energies and Components (in Debyes) along the Inertial Molecular Axes L, M, and N of the Electronic
Transition Moments Calculated in This Worka
molecular electronic transitions
crystal excitonic transition
Em (eV)13
f13
Em (eV)
L
M
N
b
a
c
3.77
3.89
0.69
0.07
3.86
3.95
6.72
0
0
2.10
0
0
13.44
0
0
1.93
0
0
0
2.11
0
0
0
1.04
3.73
0
0
4.28
0.03
4.35
4.69
1.05
0
0
1.12
0
0
0
4.51
5.24
0
0
0
1.97
0
0
4.84
4.99
2.25
2.62
0
0
0
0
a For each molecular transition, the unit cell transition dipole moments (in Debyes) of the corresponding excitonic components at k ) 0
(which are polarized along one of the unit cell axes) are also given.
To confirm the attribution of the transitions, we computed the
relevant Davydov splittings using the Davydov theory of molecular
excitons.20 Excitonic bands at a given k can be found by
diagonalizing the 4 × 4 resonance-interaction matrix defined as20
′
˜
J (k) )
n exp(ikn)JRꢀ(n)
(1)
∑
Rꢀ
where JRꢀ(n) is the interaction energy between a molecule of type
R in cell 0 ) (0, 0, 0) and a molecule of type ꢀ in cell n. The
primed summation indicates that self-interaction must be excluded,
that is, if R ) ꢀ then n * (0, 0, 0). Molecular interactions inside
the crystal are often approximated to be interactions between point-
dipoles located at the center of mass of each molecule. Such an
approximation fails for molecules close to each other compared to
their dimensions. To improve our estimate of such interactions,
we used the transition charge distribution method, which consists
in approximating the delocalized molecular dipole with a distribu-
tion of point charges located at atomic positions.23,24 Such an
approach gives a better accuracy for nearest neighbors interactions
and, at the same time, allows the use of Ewald’s method20,25 to
compute the sum in eq 1 for an infinite crystal, since for large |n|
molecules are well described by point-dipoles. Transition charge
distributions have been found as follows. From calculations based
on time-dependent density-functional theory, we found that about
64% of the dipole moment of the first molecular transition comes
from the HOMO/LUMO transition. So, we expressed the HOMO
and LUMO wave functions as a sum of localized atomic orbitals
and wrote the HOMO/LUMO transition dipole moment as the total
dipole moment of a distribution of point-charges localized on
atomic sites. To account for contributions from other orbitals, we
finally normalized the dipole moment of the so obtained transition
charge distribution to the computed transition dipole moment. The
same procedure has been adopted for the second molecular
transition, whose dipole moment is provided for more than 66%
by the HOMO-1/LUMO transition. The transition charge distribu-
tions for the first two molecular transitions are reported in Figure
9.
Figure 8. Schematic diagram showing the modifications from the
single molecule to the crystal of the electronic transitions al lowest
energy. The labels indicate the orientation of the corresponding
transition moments with respect to the L, M, N molecular axes and
with respect to the a, b, c crystal axes.
M-polarized molecular electronic transition (calculated at 3.95
eV in this work and at 3.89 by Osuna et al.).13 The labels E//c
and E//a in Figure 5 are derived from this attribution. The
Davydov splitting between the corresponding replicas of the a
and c progressions is found to be about 0.01 eV. Such a low
value indicates that for this electronic transition the intermo-
lecular interactions are relatively weak. At higher energy in
Figure 5, broad and weaker bands are detected at about 4.21
and 5.07 eV and 4.21 and 5.08 eV for the two polarizations.
These bands are attributed to other excitonic transitions in the
ac plane of M molecular origin. For example, the bands at about
4.2 eV are attributed to the c and a excitonic transitions deriving
from the molecular one calculated at 4.69 eV (the c-polarized
band is more intense than the corresponding a-polarized band,
as expected).
When varying the angle of incidence, the emerging band at
4.07 eV is attributed to the b polarized excitonic transition
originating from the strongest transition of the single molecule
at the lowest energy. This molecular transition has been
calculated at 3.86 eV in this work and at 3.77 eV in ref 13 and
has been observed at about 3.6 eV in the solution spectra.13,14
The blue shift observed in the crystal spectra is explained by
the arrangement of the corresponding L-molecular transition
moments forming a pure H aggregate. We underline that
polarized spectroscopy on monocrystals shows that there is no
correspondence between the replicas observed at lowest energy
in the solid state (originating from the second molecular
transition) and the strong electronic transition at lowest energy
of the single molecule. The diagram in Figure 8 schematically
shows the modification of the first and second electronic states
from the single molecule to the crystal.
We also included isotropic screening, so that the interaction
energy between two molecules inside the crystal is finally given
by20
qi,Rqj,ꢀ
1
JRꢀ(n) )
(2)
∑
4πε0εs
δr0iR,njꢀ
i,j
where εs is the static dielectric constant, chosen to be isotropic,
qi,R is the transition charge located on the ith atom of the Rth

230 J. Phys. Chem. A, Vol. 115, No. 3, 2011
Alessandrini et al.
The experimental results and the interpretation based on
theoretical considerations suggest a simple method to deduce
information about the structural/morphological properties of
different types of DBTDT samples, such as polycrystalline thin
films. For example, Gao et al.7 report the absorption spectrum
of a thin film. The observed progression at lowest energy is
clearly observed, while no experimental evidence is found of
the excitonic transition at about 4.07 eV. Since the measurements
were performed at normal incidence on the film, this clearly
indicates that the plane of the films deposited on quartz is the
ac plane. This information was obtained by the authors7 by
means of X-ray diffraction measurements on the films deposited
on octadecyl-trichlorosilane-treated SiO2/Si substrates. Our
analysis of the optical properties of the monocrystals provides
a simple method to deduce information on the crystallinity and,
in particular, on the orientation of the molecules and of the unit
cell axes with respect to the substrate. For applications in thin-
film transistors, this is fundamental: the orientation must be
favorable for achieving high mobility because the stacking
direction is consistent with the direction of current flow. A
similar approach was used also in the case of oligothiophenes
films.21 Optical absorption technique at oblique incidence has
been used to experimentally recognize the macroscopic order
and structural features of samples of quaterthiophene from single
crystals to polycrystalline or twinned samples.
Figure 9. Transition charge distributions for the first two molecular
transitions indicated by balls of increasing size. The arrows indicate
the direction of the resulting transition dipole moment.
molecule, and δr0iR,njꢀ is the distance between the ith atom of
the Rth molecule in cell 0 and the jth atom of the ꢀth molecule
in cell n. In the present work, we used a screening dielectric
constant εs ) 3, which is in agreement with the values
commonly adopted for similar molecules such as pentacene.22,26,27
The sum in eq 1 was computed for an infinite three-dimensional
crystal in two steps: first we used the point dipole-point dipole
approximation and made use of the Ewald’s procedure to obtain
convergence of the sum and then we replaced point dipole-
point dipole interactions with the more accurate expression of
eq 2 whenever the distance between the centers of mass of two
molecules was less than a cutoff distance rmax. In our calculations
we used rmax ) 100 Å, which was enough to achieve
convergence of the energy bands.
Conclusions
In this work, we have performed the organic synthesis and
the growth of DBTDT single crystals. From the UV-visible
polarized absorption analysis at normal and oblique incidence
it was possible to definitely attribute the observed bands and
their polarizations, on the basis of the molecular and crystal
geometry. We underline that, in the crystal, the molecular
arrangement gives rise to either a pure H aggregate or a
herringbone aggregate, for the electronic molecular transition
at lower energy or the following one, respectively. The different
arrangement and the different strength of the intermolecular
interactions cause different Davydov splitting. In particular, the
excitonic transitions at lowest energy in the crystal originate
from the second electronic transition of the molecule (arranged
in a herringbone configuration), while the one at lowest energy
of the single molecule gives rise to a blue shifted and relatively
strong excitonic state of pure H-type at higher energy. The
largest crystal face was attributed on the basis of the spectro-
scopic results. Taking into consideration the origin of the
different optical bands, which is provided here, the same simple
approach is proposed to determine the exposed face and the
structural/morphological properties of different types of DBTDT
samples, such as polycrystalline thin films, important for device
applications.
It is well-known that in three-dimensional crystals the energies
of the excitonic bands are nonanalytic functions of k as kf0.20
For this reason, to compare our predictions with the experimental
DS
˜
˜
results, we computed JRꢀ ) limkf0 JRꢀ(k//b), that is, we
considered incident light coming along a direction perpendicular
to the crystal exposed face. By diagonalizing JDRꢀS we could finally
˜
find the purely excitonic energies as probed by absorption at
normal incidence. Numerical calculations based on the method
described above predicted in the case of the first molecular
transition was found to be a Davydov splitting of 0.44 eV
between the dark exciton at lowest energy and the allowed
b-polarized upper exciton, while for the second molecular
transition the Davydov splitting between the two allowed
excitons was found to be 0.002 eV, thus confirming the weak
intermolecular interaction of the second M-polarized molecular
electronic transition. This suggests that the lowest excitonic
transition (dark) coming from the first molecular transition is
expected to be about at the same energy as the excitonic
transitions detected in the normal-incidence spectra, which
comes from the second molecular M transition.
We stress that the theoretical approach that we adopted takes
full account of all the crystal symmetries, including of course
the translational symmetry, from which all the qualitative
features of the excitonic absorption spectra arise. In addition,
the numerical calculations described above provide a fairly
accurate description of the dispersion of the purely excitonic
bands and allow a quantitative comparison with experi-
ments.20-24,27 We are aware that a more accurate estimate of
the molecular interactions can be obtained by quantum me-
chanical calculations involving several molecules. Nevertheless
such a refinement would be relevant only for nearest-neighbor
or next-nearest-neighbor molecules and would only slightly
correct the Davydov splitting values without affecting the main
conclusions of the present work.
Acknowledgment. We are pleased to thank Marcello Cam-
pione for assistance in the crystal growth, Silvia Trabattoni for
assistance in atomic force microscopy, and Fondazione Cariplo
for financial support.
Supporting Information Available: Description of the
organic synthesis and characterization. This material is available
free of charge via the Internet at http://pubs.acs.org.
References and Notes
(1) Anthony, J. E. Chem. ReV. 2006, 106, 5028–5048.
(2) Anthony, J. E. Angew. Chem. Int. Ed. 2008, 47, 452–483.
(3) Bendikov, M.; Wudl Perepichka, D. F. Chem. ReV. 2004, 104,
4891–4945.

DBTDT Crystal Optics
J. Phys. Chem. A, Vol. 115, No. 3, 2011 231
(4) Jurchescu, O. D.; Baas, J. M.; Palstra, T. T. Appl. Phys. Lett. 2004,
84, 3061–3063.
(5) Maliakal, A.; Raghavachari, K.; Katz, H.; Chandross, E.; Siegrist,
T. Chem. Mater. 2004, 16, 4980–4986.
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C. Farkas, O.; Malick, D. K.; D. Rabuck, A.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, V. D.; Fox, J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.;
Chen, W.; Wong, M. W.; Gonzalez,; Pople, J. A. Gaussian 03, Revision
D.02; Gaussian, Inc.: Wallingford, CT, 2004.
(19) Born, M. Wolf, E. Principle of Optics; Cambridge University Press:
New York, 1999.
(20) Davydov, A. S. Theory of Molecular Excitons; Plenum: New York,
1971.
(21) (a) Spearman, P.; Borghesi, A.; Campione, M.; Laicini, M.; Moret,
M.; Tavazzi, S. J. Chem. Phys. 2005, 122, 014706. (b) Raimondo, L.;
Campione, M.; Laicini, M.; Moret, M.; Sassella, A.; Spearman, P.; Tavazzi,
S. Appl. Surf. Sci. 2006, 253, 271–274. (c) Tavazzi, S.; Laicini, M.;
Raimondo, L.; Spearman, P.; Borghesi, A.; Papagni, A.; Trabattoni, S. Appl.
Surf, Sci. 2006, 253, 296–299. (d) Tavazzi, S.; Campione, M.; Laicini, M.;
Raimondo, L.; Borghesi, A.; Spearman, P. J. Chem. Phys. 2006, 124,
194710.
(22) Silvestri, L.; Tavazzi, S.; Spearman, P.; Raimondo, L.; Spano, F. C.
J. Chem. Phys. 2009, 130, 234701.
(23) Scholz, R.; Kobitski, A.; Kampen, T.; Schreiber, M.; Zahn, D.;
Jungnickel, G.; Elstner, M.; Sternberg, M.; Frauenheim, T. Phys. ReV. B
2000, 61, 13659.
(24) Vragovic, I.; Scholz, R. Phys. ReV. B 2003, 68, 155202.
(25) Philpott, M. R.; Lee, J. W. J. Chem. Phys. 1973, 58, 595.
(26) Kang, H. S.; Lee, J. W.; Kim, M. K.; Joo, J.; Ko, J. M.; Lee, J. Y.
J. Appl. Phys. 2006, 100, 064508.
(27) Garnier, F.; Yassar, A.; Hajlaoui, R.; Horowitz, G.; Deloffre, F.;
Servet, B.; Ries, S.; Alnot, P. J. Am. Chem. Soc. 1993, 115, 8716–8721.
(6) Li, X.-C.; Sirringhaus, H.; Garnier, F.; Holmes, A. B.; Moratti,
S. C.; Feeder, N.; Clegg, W.; Teat, S.; Friend, R. H. J. Am. Chem. Soc.
1998, 120, 2206–2207.
(7) Gao, J.; Li, R.; Li, L.; Meng, Q.; Jiang, H.; Li, H.; Hu, W. AdV.
Mater. 2007, 19, 3008–3011.
(8) Szperl, L. Rocz. Chem. 1938, 18, 804–811.
(9) (a) Klauk, H.; Halik, M.; Zschieschang, U.; Schmid, G.; Radlik,
W.; Weber, W. J. Appl. Phys. 2002, 92, 5259–5263. (b) Sheraw, C. D.;
Zhou, L.; Huang, J. R.; Gundlach, D. J.; Jackson, T. N.; Kane, M. G.; Hill,
I. G.; Hammond, M. S.; Campi, J.; Greening, B. K.; Francl, J.; West, J.
Appl. Phys. Lett. 2002, 80, 1088–1090. (c) Gundlach, M. S.; Lin, Y. Y.;
Jackson, T. N.; Nelson, S. F.; Schlom, D. G. IEEE Electron DeVice Lett.
1997, 18, 87–89.
(10) (a) Yamada, M.; Ikemoto, I.; Kuroda, H. Bull. Chem. Soc. Jpn.
1988, 61, 1057–1062. (b) Coppo, P.; Yeates, S. G. AdV. Mater. 2005, 17,
3001–3005. (c) Meng, H.; Bendikov, M.; Mitchell, G.; Helgeson, R.; Wuld,
F.; Bao, Z.; Siegrist, T.; Kloc, C.; Chen, C. H. AdV. Mater. 2003, 15, 1090–
1093.
(11) Wex, B.; Kaafarani, B. R.; Kirschbaum, K.; Neckers, D. C. J. Org.
Chem. 2005, 70, 4502–4505.
(12) Xiao, K.; Liu, Y.; Qi, T.; Zhang, W.; Wang, F.; Gao, J.; Qiu, W.;
Ma, Y.; Cui, G.; Chen, S.; Zhan, X.; Yu, G.; Qin, J.; Hu, W.; Zhu, D.
J. Am. Chem. Soc. 2005, 127, 13281–13286.
(13) Osuna, R. M.; Ortiz, R. P.; Okamoto, T.; Suzuki, Y.; Yamaguchi,
S.; Hernandez, V.; Navarrete, J. T. L. J. Phys. Chem. B 2007, 111, 7488–
7496.
(14) Okamoto, T.; Kudoh, K.; Wakamiya, A.; Yamaguchi, S. Org. Lett.
2005, 7, 5301–5304.
(15) Szperl, L. Rocz. Chem. 1938, 18, 804.
(16) Schroth, W.; Hintzsche, E.; Jordan, H.; Jende, T.; Spitzner, R.;
Thondorf, I. Tetrahedron 1997, 53, 7509.
(17) Campione, M.; Ruggerone, R.; Tavazzi, S.; Moret, M. J. Mater.
Chem. 2005, 15, 2437–2443.
(18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
JP1052826