Crystal structure and optical properties of n-pyrrole end-capped thiophene/phenyl co-oligomer: Strong h-type excitonic coupling and emission self-waveguiding

Article abstract of DOI:10.1021/cg100113h

In view of their exploitation in working devices, mainly for their emission properties, the interest in crystals of thiophene/phenylene co-oligomers has recently increased. Here, we describe the strategies for the organic synthesis and crystal growth of a

Full text of DOI:10.1021/cg100113h

DOI: 10.1021/cg100113h
Crystal Structure and Optical Properties of N-Pyrrole End-Capped
Thiophene/Phenyl Co-Oligomer: Strong H-type Excitonic Coupling and
Emission Self-Waveguiding
2010, Vol. 10
2342–2349
Silvia Tavazzi,*,† Luciano Miozzo,†,‡ Leonardo Silvestri,† Stefano Mora,† Peter Spearman,†
Massimo Moret,† Silvia Rizzato,§ Daniele Braga,†,‡ Abdou Karim Diagne Diaw,#,‡
Diariatou Gningue-Sall,# Jean-Jacques Aaron, ,‡ and Abderrahim Yassar‡
†Universitaꢀ degli Studi di Milano Bicocca, Dipartimento di Scienza dei Materiali, Via Cozzi 53,
I-20125 Milano, Italy, ‡ITODYS, Universiteꢁ Paris Diderot-Paris 7, 15 rue Jean de Baif,
§
75205 Paris Cedex, France, Universita degli Studi di Milano, Dipartimento di Chimica Strutturale e
ꢀ
Stereochimica Inorganica and Facolta di Farmacia, Via Venezian 21, I-20133 Milano, Italy,
ꢀ
#Laboratoire de Chimie Physique Organique et d’Analyse Instrumentale (LCPOAI),
Deꢁpartement de Chimie, Faculteꢁ des Sciences et Techniques, Universiteꢁ Cheikh Anta Diop de Dakar;
BP 5005, Dakar-Fann, Senegal, and UniversiteꢁParis-Est, Laboratoire Geꢁomateꢁriaux et Environnement
(LGE), Ba^timent IFI, 5 Bd. Descartes, Champs-sur-Marne, 77454 Marne-la-Valleꢁe, Cedex 2, France
Received January 25, 2010; Revised Manuscript Received March 8, 2010
ABSTRACT: In view of their exploitation in working devices, mainly for their emission properties, the interest in crystals of
thiophene/phenylene co-oligomers has recently increased. Here, we describe the strategies for the organic synthesis and crystal
growth of a novel oligothiophene-based co-oligomer (thiophene/phenyl co-oligomer end-capped with pyrrole) designed to
explore the effect of an electron-rich aromatic ring, such as pyrrole, to end-cap oligothiophenes. The growth by the melted
material is also discussed and suggested for implementation in real devices. The UV-visible optical characterization is provided,
and the results (absorption and emission excitonic transitions, H-type aggregation, self-waveguiding of the emitted light) are
interpreted on the basis of the crystallographic data in the regime of strong intermolecular coupling.
Introduction
interaction are often found in such functionalized materials.
As a consequence of intermolecular interactions and excitonic
splitting, depending on the molecular arrangement the exci-
tonic state at lowest energy can be optically allowed (J aggre-
gates) or optically forbidden (for a perfect H aggregate). In the
latter case, emission can be strongly influenced by the presence
of nearby states of different momentum in the Brillouin zone,
which can be thermally populated and may dominate the spec-
trum, affecting also the polarization of the emitted light.19,20
Starting from the building block of the constituent mole-
cule, by means of selective functionalization the primary
molecular properties and the crystal packing can be modified
or improved upon by introducing pendant groups onto the
conjugated backbone. For example, the strategy of introdu-
cing different terminal groups tocap oligothiophenes has been
found to fulfill the requirements needed to obtain active mate-
rials with enhanced emission.21 Oligothiophenes terminated
with phenyl or biphenyl groups exhibit ASE.7 Oligothio-
phenes with terminal groups bearing diarylamino functional
groups have been reported to be potential hole transporting
materials.22 Pyridyl and quinolyl as end groups have been
used to modulate crystal structures of oligothiophenes.23
Different crystal packing motifs were found in crystals of
quinolyl end-capped oligothiophenes. For quinolyl end-
capped bithiophene, the donor-acceptor interaction between
quinolyl and thiophene units becomes the dominating factor
to determine the crystal packing motif and two-dimensional
(2D) face-to-face π-π slip stacking was observed. For the
longer molecule, quinolyl end-capped terthiophene, the weak
Organic semiconductors based on conjugated oligomers
and polymers represent a relatively new class of electronic
materials that is revolutionizing important technological ap-
plications including large-area electronics, owing to their com-
patibility with low-temperature processing, the simplicity of
thin-film device fabrication, and the tunability of their electro-
nic properties.1 The richness of the synthetic organic chemistry
allows the fabrication of π-conjugated materials with a degree
of control unattainable with conventional inorganic semicon-
ductors. They are widely exploited in electroluminescent
devices, and furthermore, in some cases, they show amplified
spontaneous emission (ASE). Gain-narrowing has been de-
monstrated in a few materials, such as oligothiophenes, oligo-
(p-phenylene), oligo(p-phenylene vinylene), trans-1,4-distyryl-
benzene, cyano derivatives, fluorene/phenylene co-oligomers,
hydroxy-substituted tetraphenyl-imidazole, and 1,1,4,4-tetra-
phenyl-1,3-butadiene.2-14
In view of the exploitation of ASE and solid-state lasing,
crystals of high quality are more advantageous compared to
polycrystalline and amorphous thin films that contain defects
which may well act as emission traps. Compared to amorphous
films commonly utilized in polymer optoelectronics, crystalline
materials also exhibit well-defined luminescence polarization
states and interesting effects related to the anisotropic pro-
pagation of light. For example, self-waveguiding can occur
as a consequence of particular molecular arrangements, whose
features deserve to be investigated in depth.2,7,9,15-18 Para-
llel molecular stacking, herringbone, and face-to-face π-π
C-H N hydrogen bonds between the dimer molecules and
3 3 3
the CH/π weak interactions force the molecules to form a
sandwich-herringbone crystal structure. The introduction of
*To whom correspondence should be addressed. Tel. þ390264485035.
Fax: þ390264485400. E-mail: silvia.tavazzi@unimib.it.
r
pubs.acs.org/crystal
Published on Web 03/29/2010
2010 American Chemical Society

Article
Crystal Growth & Design, Vol. 10, No. 5, 2010 2343
planar fluorene moieties as the end groups of bithiophene
facilitates the π-π stacking of the crystals, resulting in the
formation of new materials that exhibit extremely high field-
effect mobility.24 Oligothiophenes end-capped by carbazole,
diarylboryl, and other groups capable of charge transport
have also been documented.25-27 An elegant approach to
enhance and improve the properties of conjugated materials
involves the introduction of properly designed co-oligomers.
The thiophene/phenylene co-oligomers prepared by Hotta
et al.28,29 are formed by thiophenes and phenyl units joined
together in the same conjugated backbone. Crystals of thio-
phene/phenylene co-oligomers exhibit intriguing properties
such as photopumped laser oscillation, self-waveguided
photoemission, and spectrally narrowed emission under
photoexcitation.30-33 Moreover, charge transport experi-
ments pointed out that some of these co-oligomers in thin-
film-transistor configurations show higher charge mobilities
compared to those of their parent oligothiophenes.34,35 Thus,
the interest in crystals of thiophene/phenylene co-oligomers is
now increasing, and thiophene/phenylene co-oligomers con-
stitute a new specific family of organic semiconductors.
Crystal structure and physical properties of these co-oligo-
mers depend on the total number of thiophene and phenyl
rings and their mutual arrangement. The molecular shape
(either planar or twisted adjacent rings) adopted in the solid
state depends on the molecular design, and, in this context, the
molecules adopting a nonstraight (e.g., bent or zigzag) shape
exhibit particularly interesting features. For instance, several
crystals of such geometrically distorted molecules display
ASE at room temperature.36
In this work, we have designed and synthesized a novel
oligothiophene-based material, R-ω-N-pyrrole capped thio-
phene/phenyl co-oligomer (thiophene-phenyl-pyrrole, TPP),
with an electron-rich aromatic ring such as pyrrole to end-cap
oligothiophene. We describe the strategies for the organic
synthesis and crystal growth, including the description of the
growth process from the melted material which is suggested
for the implementation in devices. From the UV-visible
optical characterization, a relatively high oscillator strength
compared to standard oligothiophenes is found for the lowest
electronic molecular transition, which is polarized along the
long molecular axis. The structural and optical characteriza-
tion indicates that the crystals show a H-type aggregation of
the corresponding transition moments with strong intermo-
lecular coupling, giving a large excitonic splitting of about 1.2
eVand a strong and almost free excitonic statepolarized along
the normal to the surface. The intense orange emission is
dominated by transitions polarized along the normal to the
exposed face and is self-waveguided. It is attributed to transi-
tions involving excitonic states of nonzero momentum in the
Brillouin zone.
Table 1. Crystal Data for TPP
compound
5
formula
C28H20N2S2
448.58
monoclinic
P21
formula weight
crystal system
space group
Z
2
˚
a (A)
5.7692(10)
7.4252(12)
24.716(4)
91.760(2)
1058.3(3)
1.408
˚
b (A)
˚
c (A)
β (°)
volume (A )
3
˚
calc density (g cm-3
)
μ (mm-1
)
0.272
max/min transm factor
temperature (K)
0.979, 0.923
293(2)
29.49
θ
max (deg)
no. of measured reflns
no. of unique reflns, Rint
no. of obsd reflns (I > 2σ(I))
parameters
completeness to θ, %
absorption correction
GOF (all)
13585
5407 (0.0283)
4257
290
95.1
empirical
1.019
0.0432
0.1026
0.242, -0.179
R1 (I > 2σ(I))a
wR2 (all)a
ΔFmax,min (e A
-3
˚
)
a Rint = Σ |Fo2 - Fo2(mean)|/Σ [Fo2], R1 = Σ ||Fo| - |(1/k)Fc||/Σ |Fo |,
wR2 = [Σ w(Fo - (1/k)Fc2)2/ Σ w|Fo |2]1/2, GOF = [Σw(Fo - (1/
2
2
2
k)Fc2)2/(no - nv)]1/2
.
betweentwo quartzsubstrates from the melted material ina hot-stage
Linkam THMS600, equipped with a temperature controller Linkam
CI94 and coupled to a fluorescence microscope Olympus BX51.
These techniques offered single crystals with large flat surfaces, also
suitable for fabrication of optoelectronic devices. Images of the
samples were taken by an optical polarizing microscope Olympus
SZX12 and a fluorescence microscope Olympus BX51.
Crystal data for TPP are listed in Table 1. The data collection
ꢁ
˚
was performed with Mo-KR (λ = 0.71073 A) on a Bruker SMART
CCD area-detector diffractometer. An empirical absorption correc-
tion was applied.37 The structure was solved by direct methods
(SIR200238) and refined by full-matrix least-squares on F2
(SHELXL-97).39 Anisotropic thermal parameters were assigned to
all non-hydrogen atoms. The hydrogen atoms were placed in geome-
trically calculated positions and thereafter refined using a riding
model. Racemic twinning was refined giving two components of
0.46(6) and 0.54(6). All the structure diagrams were performed using
the ORTEPIII40 program.
Absorbance measurements 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. Photoluminescence (PL) emission spectra were taken
at room temperature using a homemade apparatus equipped with
CCD detector.
Results and Discussion
Organic Synthesis. In order to gain access to TPP (5),
possessing a longer conjugated chain with a well-defined
substitution pattern, it is necessary to initially introduce the
desired functional group into smaller building blocks, and
then to use this precursor to extend the conjugation length.
This was achieved by using N-(4-bromo-phenyl)-pyrrole (3),
as the building block, and the Stille procedure as cross-
coupling reaction to extend the conjugation length. Scheme 1
outlines the synthetic pathway to TPP. The synthesis of 3 was
performed according to the modified Clauson-Kaas method
by condensation between p-bromoaniline (1) and 2,5-di-
methoxytetrahydrofuran (2) in glacial acetic acid under
reflux.41 Double palladium-catalyzed Stille cross-coupling
Experimental Section
Experimental procedures for the synthesis of TPP are reported in
the Supporting Information (compound 5).
The electronic optical transitions of the isolated molecule in a
vacuum were calculated by means of the Gaussian 03 software using
as input parameters the molecular geometries obtained from the
crystallographic data.
Single crystals of TPP were grown in a horizontal furnace by
physical vapor transport (PVT) of the sublimated raw material under
a stream of argon (flow rate 30 mL/min) overnight. A controlled
smooth temperature gradient ensured compound sublimation in the
furnace hottest region(275 °C) and its crystallization atawell-defined
position close to the end of the furnace. Crystals were also grown

2344 Crystal Growth & Design, Vol. 10, No. 5, 2010
Tavazzi et al.
Figure 1. Two views of molecule 5. ADPs are drawn at the 30% probability level. Hydrogen atoms were given arbitrary radii. Dihedral angles
between least-squares ring planes are thienyl(S1)-phenyl(S2) 7.4°, thienyl(S1)-phenyl(C5) 7.2°, phenyl(C5)-pyrrole(N1) 5.0°, thienyl(S2)-
phenyl(C19) 8.0°, phenyl(C5)-pyrrole(N1) 3.0°.
Scheme 1. Synthetic Pathway to TPP
reaction of 3 with 5,50-bis(trimethylstannyl)-2,20-bithio-
phene (4) was performed in (N,N)-dimethyl-formamide
(DMF) in the presence of dichlorobis(triphenylphosphine)-
palladium(II) as catalyst. The target product was obtained in
a yield of 61%. Further details on the organic synthesis are
reported in the Supporting Information.
temperature polymorphs calculated for both 4T and 6T.42-45
The analogy between 5 and the low or high temperature
polymorphs of 4T and 6T is loose when considering the
tilting angle of the molecule with respect the plane compris-
ing the herringbone packing, that is 11.6° for 5 (calculated
with respect to the vector connecting the centroids of the
pyrrole rings) and 24.8, 24.1, 31.8, 33.4° for 4T-LT, 6T-LT,
4T-HT, 6T-HT, respectively. On the contrary, other co-
oligomers previously reported and containing oligothienyl
cores of different sizes and with terminal phenyl of biphenyl
groups display tilting angles in the range 1.2-21.0°.28
Molecular and Crystal Structure of TPP. The molecular
structure of 5, depicted in Figure 1, shows an evident bent
shape for this co-oligomer. The molecule displays an approxi-
mate noncrystallographic C2 molecular symmetry with the
2-fold axis bisecting the C1-C15 bond (see Figure 1 and the
pattern of bond lengths and angles reported in the CIF
material). The nonplanarity of molecule 5 is associated with
the slight out-of-plane bending of adjacent functional groups
coupled to nonzero inter-ring torsion angles. As a first exam-
ple, the central thienyl groups show the antiplanar confor-
mation usually observed in oligothiophenes with torsions
about the central C1-C15 bond of -0.3(3)° (S1-C1-C15-
C16) and -0.1(4)° (S2-C15-C1-C2), while the dihedral
angle between the two least-squares planes amounts to 7.4°
and is responsible of the molecule bending around the
molecular center of mass. On moving toward the molecule’s
periphery, the torsion angles of adjacent thienyl/phenyl and
phenyl/pyrrolyl rings deviate more and range from about 1
to 6°, while the dihedral angles between ring least-squares
planes range from 3 to 8°. The overall effect of these nonzero
torsions and dihedral angles is the distorted shape assumed
by the molecule. This intramolecular feature coupled to the
solid state packing gives to our system the interesting phy-
sical properties discussed later. The crystal structure com-
prises a herringbone packing of bent molecules in the ab
plane, the herringbone angle calculated over the least-
squares molecular planes being 53.8° (Figure 2 top). This
value is closer to that observed for the high temperature
polymorphs of quater- (4T) and sexi-thiophene (6T) (55.5
and 53.4°, respectively) than the value of 63.1° for the low
Finally, because of the close packing of molecules within
the herringbone ab plane the major part of the crystal energy
is contained in these molecular layers. Hence, the corre-
sponding {001} faces have the lowest surface energy, and this
in turn causes the crystals to grow with very large {001}
faces.46 A peculiar feature of the {001} faces is their slight
corrugation on the molecular scale. Indeed, differently from
4T and 6T which display flat faces for crystallographic planes
corresponding to the herringbone layers, crystals of 5 exhibit
alternate rows of molecules parallel to [100] differing by ca.
˚
1.9 A in height due to a significant slipping along the main
molecular axis (Figure 2 bottom).
Optical Properties and Modeling. Figure 3a shows the
absorption spectrum of a 10-6 M TPP solution in DMF.
The main band is centered at about 3.1 eV, followed by a
weaker band at about 4.0 eV. The calculated electronic
transitions for the isolated molecules are reported in
Figure 3b, and they are found at 2.95, 4.18, and 4.52 eV.
Since the same molecular geometry was assumed for both
ground and excited states, the calculated energies are only
indicative, but the order and polarization of the transitions
are provided, which is of interest for the following analysis
and comprehension of crystal spectra. The measured band
centered at about 3.1 eV is attributed to the lowest strong
calculated transition and a satisfactory agreement between

Article
Crystal Growth & Design, Vol. 10, No. 5, 2010 2345
Figure 3. (a) Absorption spectrum of a 10-6 M TPP solution in
DMF. (b) Calculated electronic transitions for the TPP isolated
molecule. Inset: calculated components along the molecular inertial
axes L, M, and N of the transition moment of the electronic
transitions (Em: calculated energy of the molecular electronic
transitions).
at 4.2 eV or to a superposition of the ones at about 4.2 and
4.5 eV. The lowest transition dominates the spectrum and its
oscillator strength is quite high (12.8 D) compared to other
π-conjugated molecules. For example, the lowest electronic
transition of two of the most studied oligothiophenes, 4T and
6T, have transition moments of about 10.6 and 10.4 D.47-49
In the inset of Figure 3b, the calculated projections of the
molecular electronic transitions on the L, M, and N axes are
reported, the axes being defined as the inertial axes of the
single molecule (L is the longest one, N is the shortest one).
The results clearly indicate the strong polarization of the
molecular transition at the lowest energy along the L axis.
The measured absorption spectra taken in different con-
figurations of a TPP single crystal grown by PVT are repor-
ted in Figure 4. At normal incidence, the reported spectra are
measured with two orthogonal polarizations corresponding
to the minimum and maximum absorption in the considered
spectral range. These two polarizations also correspond to
the two directions of the electric field which give extinction of
the transmitted light under crossed polarizer and analyzer.
The variable-angle measurements (at the angles of incidence
indicated by the labels) were taken with p polarized light
(incident electric filed in the plane of incidence) using, as the
plane of incidence, the plane formed by the normal to the
surface and the direction of polarization of the maximum
absorption measured at normal incidence.
Figure 2. Top: view of the herringbone packing found for crystals
of molecule 5. Bottom: details of the corrugated surface structure
for the {001} crystal form.
the experimental and calculated data is found. The weak
measured band at 4.0 eV is attributable to the calculated one

2346 Crystal Growth & Design, Vol. 10, No. 5, 2010
Tavazzi et al.
Table 2. Projections along the Crystallographic Axes of the Two
Excitonic Transition Dipole Momentsa
ac
Em (eV)
b
a
c*
2.95
4.18
4.52
-0.18
0.47
-0.02
2.68
0.47
-1.23
-17.84
1.13
4.77
a Em: energy of the corresponding molecular electronic transition.
to the ac polarized exciton transition originating from the L
molecular one, which is expected to be the strongest peak in
the spectra. It reveals several structures (the lowest one at
2.73 eV, followed by shoulders at 2.97, 3.18 eV and by a peak
centered at about 3.52 eV before the saturation region),
which are easily detected for the highest angles of incidence
and are attributed to vibronic replicas. The corresponding b
excitonic component is not clearly observed in the spectra of
Figure 4, but it is recognizable in the spectrum (inset) taken in
the same configuration on a thicker crystal with replicas of
decreasing intensity at 2.65, 2.84, and 3.03 eV. For this thick
sample, the variable-angle spectra showing the ac series are
omitted since they are affected by absorbance saturation. On
the basis of the calculated transition moments, the intensity
ratio between the ac polarized and b polarized excitonic
transitions is predicted to be on the order of 104. At normal
incidence, the intensity ratio between the component along a
of the ac exciton and the b exciton is predicted to be about
200. Since the computed molecular transition dipole is nearly
perpendicular to the b axis, the predicted ratio is only a rough
estimate: a change as small as 1° degree in the molecular
dipole inclination can produce a reduction of a factor 10 in
the estimated intensity ratio. In any case, these low values
explain the difficulty to detect the b excitonic transition for
the thinner sample in Figure 4. The results of the optical
characterization also indicate that the accessible face is the ab
face (and the spectra in Figure 4 were taken with ac as the
plane of incidence). Indeed, a strong component is measured
along the normal to the surface and the calculations clearly
indicate that the strongest excitonic band is expected to have a
large component along c*. These results are in agreement with
the results of the structural characterization. Moreover, the b
axis is a material principal axis and ac is a principal plane, thus
explaining the extinction of the transmitted light under
crossed polarizer and analyzer.50 The energy separation be-
tween the lowest b polarized band (2.65 eV) and the lowest ac
polarized band (2.73 eV as measured at the angle of incidence
of 30°) is 0.08 eV. The energy difference between the lowest
b-polarized peak (the most intense replica of the progression)
and the center of the ac polarized band measured at oblique
incidence for positive angles is deduced to be about 1.2 eV.
Therefore, TPP satisfies the conditions of the strong excitonic
coupling regime: the ac exciton is almost free, it possesses
nearly all the oscillator strength of the isolated molecules, and
it is considerably blue-shifted with respect to the molecular
electronic transition. The ac polarized replicas in the spectral
region between 2.7 and 3.6 eV are built on the lower b pola-
rized excitonic band and get their oscillator strength through
an intermolecular Herzberg-Teller coupling to the nearly free
ac polarized exciton.19,20,48,49,52 On the contrary, the b polar-
ized exciton is very weakly allowed, and it is more strongly
coupled to phonons, showing a vibronic progression.
Figure 4. Continuous lines: absorbance spectra of a TPP single
crystal grown by PVT taken at different angles of incidence from
þ30° to -40° on the accessible face (ab) with p (ac) polarized light
using the plane formed by the normal to the surface and the
direction of polarization of maximum absorption measured at
normal incidence (ac plane) as plane of incidence. The portion of
the spectrum between about 3.6 and 3.9 eV measured at þ30° is
affected by saturation. Dashed line with diamonds: spectrum taken
at normal incidence with orthogonal polarization (b polarizatiopn).
Inset: absorbance spectrum taken at normal incidence on a thicker
crystal with b polarization.
The spectra are dominated by a main broad band. When
varying the angle of incidence and using the selected plane of
incidence, the intensity and shape of this band strongly
depend on the angle of incidence. The maximum intensity
is detected for the highest positive angle of incidence (the
center of the band can be tentatively deduced to be 3.75 eV),
it decreases when decreasing the angle of incidence down to
zero at about -15°, and finally it again emerges slightly
shifted at higher energy (the maximum is found at about 3.85
eV). This indicates that the corresponding transition mo-
ment has a strong component along the normal to the surface
and shows directional dispersion.50,51 Considering that the
band disappears at about -15°, the inclination of the transi-
tion moment to the normal to the surface can be approxi-
mately estimated from the experiment to be on the order of
-10° (taking into consideration the effect of refraction at the
air/crystal interface).
In order to interpret these results, we consider that, in the
crystalline monoclinic form, each molecular electronic tran-
sition is predicted to give rise to two excitonic transitions,
which are b and ac polarized (with intensities depending on
the sum and difference of the transition moments of the two
molecules in the unit cell). Considering the orientation of the
molecules obtained from the structural characterization, the
components of the excitonic transitions at the center of
the Brillouin zone (k = 0) in the crystal were deduced and
are provided in Table 2.
The lowest L polarized molecular transition (at 2.95 eV)
gives rise to a very weak b component and a strong ac
component, mainly polarized along c* (the angle between
the ac transition moment and the c* axis is calculated to be
about 7°). Indeed, the arrangement of the corresponding
molecular transition moments in the unit cell gives rise to an
almost pure H aggregate with parallel molecular transition
moments. The main band observed in Figure 4 is attributed
Finally, in Figure 5 we report the optical spectra at oblique
incidence with p polarized light using, as the plane of

Article
Crystal Growth & Design, Vol. 10, No. 5, 2010 2347
Figure 6. PL spectrum of a TPP crystal grown by PVT. Inset: Image
of a TPP crystal under the fluorescence microscope. The orange line
on the right is the border of the sample, and the other lines are cracks
on its surface.
Figure 5. Absorbance spectra of a TPP single crystal taken at
different angles of incidence from 0° to 60° on the accessible face
(ab) with p (bc) polarized light, using the plane formed by the normal
to the surface and the direction of polarization of minimum
absorption measured at normal incidence as plane of incidence (bc).
distribution method and including isotropic screening48 in-
dicate that the minimum of the lower excitonic band is not at
the center of the Brillouin zone, and it is about 0.02 eV lower
in energy than the k = 0 point. As a consequence, the emitt-
ing state with k = 0 is predicted to have less than half the
population of the lowest energy excitonic state (with k ¼ 0) at
room temperature, confirming the conclusion that 0-0 emi-
ssion is expected to be very weak. On the contrary, replicas
are expected to be much stronger and mainly ac polarized.
Figure 6 shows the PL emission spectrum taken on a TPP
crystal grown by PVT. A broad band is observed whose
maximum is detected at 2.13 eV, with a very weak shoulder at
about 2.41 eV. The inset shows an image taken under the
fluorescence microscope. The photoluminescence is emitted
from the borders of the sample (or from some cracks on its
surface), while the crystal surface is dark. This indicates that
the emitting transition moment is mainly polarized along the
normal to the surface and the emission is self-waveguided
toward the edges. This is in agreement with the above
theoretical predictions, given that the upper exciton dipole
moment is nearly perpendicular to the exposed face of the
crystal. It is instead very difficult recognizing a well-defined
vibronic progression in the emission spectrum and, in parti-
cular, assigning the weak shoulder at 2.41 eV. This shoulder
could be due to the 0-0 emission from the thermally popu-
lated k = 0 lower exciton or, alternatively, to the 0-0 emis-
sion from the lowest emissive state (k ¼ 0) weakly allowed by
disorder.53 The low separation between these two emitting
states (predicted 0.02 eV, as mentioned above) prevents a
definitive attribution. Temperature-dependent emission
spectra would allow clarifying this point, since very different
behaviors are expected in the two cases.19,20,53
incidence, the plane formed by the normal to the surface and
the polarization direction of the minimum absorption at
normal incidence (bc plane of incidence). No relevant differ-
ences have been found for either positive or negative angles
of incidence. An emerging intense structure is detected at
oblique incidence. Its maximum is at 3.94 eV, and several
replicas are detected on its low-energy tail. This structure is
attributed to the ac component coming from the L molecular
transition, which is also responsible for the main structure in
Figure 4. This further confirms the presence of a strong
transition out of the accessible face. However, the latter
configuration is slightly complicated by the choice of the
plane of incidence which is not a principal plane and can
determine a modification of the state of polarization of light
during its propagation inside the crystal.50
On the basis of the calculations and the measurements in
solution, other excitonic transitions are expected originating
from the higher molecular electronic transitions. Indeed, in
the b polarized spectrum taken at normal incidence, a
relatively weak structure is also detected between about 3.4
and 4.0 eV, showing poorly defined replicas. A similar band
cannot be excluded in the a-polarized spectrum below the ac
exciton of L origin. The data in the inset of Figure 3b and in
Table 2 suggest that these structures are attributable to the
excitonic components coming from the molecular transition
at 4.2 eV or to the superposition of the ones coming from the
transitions at about 4.2 and 4.5 eV, with nonvanishing
intensity both along b and along the a axis.
The observation of a strong and almost free excitonic state
coming from the molecular electronic transition at lowest
energy motivated us to study the emission properties. Since
TPP crystals are almost pure H aggregates, the lowest b
polarized excitonic level is expected to give rise to a very weak
0-0 b-polarized PL transition coming from the correspond-
ing band at the center of the Brillouin zone (k = 0). On the
contrary, the PL replicas are expected to originate from
several states with different momentums and to be both b
and ac polarized.19,20 Indeed, emission properties depend on
the contribution of all the states with different momentums
which, at room temperature, are thermally populated. Nu-
merical calculations performed by the transition charge
The emission properties of crystals grown by the melted
material have also been investigated. We underline the easy
processability of this method and the possibility to follow the
process by optical characterization in real time during the
growth. Figure 7 shows the images taken under the fluore-
scence microscope at different temperatures during the
growth of a sample of TPP. The starting polycrystalline
powder was placed in the hot stage apparatus and the
temperature was changed as reported in the inset (where
diamonds indicate the temperature corresponding to the

2348 Crystal Growth & Design, Vol. 10, No. 5, 2010
Tavazzi et al.
Figure 8. Images taken under the fluorescence microscope of the
TPP sample grown from the melted material (Figure 7).
to orange-green where the melting point is approached
(panels 2 and 3). After decreasing the temperature, crystal-
lization takes place and a few dark regions can be observed
on the surface of the sample, which is also characterized by
many cracks where photoluminescence is observed (panel 4).
After melting the material again, the emission from the
melted compound is clearly green (panel 5). By decreasing
the temperature, crystallization takes place and abruptly the
green regions clearly show orange borders (panel 6), which
are even more evident when further decreasing the tempera-
ture (panel 7). We underline that a discrepancy between the
nominal temperatures and the temperature of the sample is
expected due to the presence of a quartz substrate between
the material and the heating element. Figure 8 shows the
images taken at room temperature on the grown sample after
removing it from the hot stage. Many crystals are observed,
with a dark surface and orange emitting borders, similar to
the crystal grown by PVT reported in the inset of Figure 6.
Smaller crystalline domains are found in the case of the
melted sample. The macroscopic order and the presence of
cracks in this type of crystal is likely to be improved by
adopting a slower and proper profile of temperature during
the growth.
Conclusions
The molecular and crystal structure of a novel oligothio-
phene-based co-oligomer, namely, thiophene-phenyl-pyrrole,
is reported and discussed. The growth from the melted com-
pound is also discussed and suggested for the exploitation of
this material in its crystalline form in real working devices and
for crystal engineering applications. The structural and opti-
cal characterization/simulation allowed the attribution of the
polarized optical bands observed in the UV-visible spectra.
The results show that (i) the electronic molecular transition at
Figure 7. Panels 1-7: Images taken under the fluorescence micro-
scope during the growth of a sample of TPP by the melting method.
Inset: the corresponding temperature profile (diamonds indicate the
nominal temperatures corresponding to the images).
images). The starting polycrystalline powder shows a disor-
dered orange photoluminescence (panel 1). The color shifts

Article
Crystal Growth & Design, Vol. 10, No. 5, 2010 2349
lowest energy is polarized along the long molecular axis and is
quite strong compared to other conjugated molecules such as
oligothiophenes, (ii) the crystals show a H-type aggregation of
the corresponding transition moments with strong intermo-
lecular coupling, (iii) the main excitonic state is almost free
and polarized along the normal to the surface, (iv) the
resulting excitonic splitting is of about 1.2 eV, (v) the emission
is dominated by a transition polarized along the normal to the
exposed face and the orange emission is self-waveguided. Its
origin is attributed to transitions involving states of nonzero
momentum in the Brillouin zone.
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Acknowledgment. S.T., L.M., L.S., S.M., and P.S. thank
Fondazione Cariplo for financial support. A.Y. and L.M.
thank PolyNet Network of Excellence for financial support.
L.M. thanks EU Commission for Marie Curie Grant
(Proposal No. 236101 - Acronym: TETRA-FUN). A.K.D.
D. thanks the French Embassy in Dakar (Senegal) for a Ph.D.
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