Oligothiophenes nano-organized on a cyclotetrasiloxane scaffold as a model of a silica-bound monolayer: Evidence for intramolecular excimer formation

Article abstract of DOI:10.1002/chem.200901307

Excimer formation in a new class of terthiophene-based fluorophores covalently bonded to a cyclotetrasiloxane scaffold has been demonstrated and the photophysical process ruling it has been investigated in detail and modeled theoretically. In contrast to

Full text of DOI:10.1002/chem.200901307

FULL PAPER
DOI: 10.1002/chem.200901307
Oligothiophenes Nano-organized on a Cyclotetrasiloxane Scaffold as a Model
of a Silica-Bound Monolayer: Evidence for Intramolecular Excimer
Formation
Wojciech Mrꢀz,[b] Jean Philippe Bombenger,[b] Chiara Botta,[b] Alessio Orbelli Biroli,[c]
Maddalena Pizzotti,[c] Filippo De Angelis,[d] Leonardo Belpassi,[d]
Riccardo Tubino,[a] and Francesco Meinardi*[a]
Abstract: Excimer formation in a new
class of terthiophene-based fluoro-
phores covalently bonded to a cyclote-
trasiloxane scaffold has been demon-
strated and the photophysical process
ruling it has been investigated in detail
and modeled theoretically. In contrast
to the conventional systems in which
long-living fluorophores such as pyrene
are linked in the same molecule, an ex-
cimer is formed only when two terthio-
phene-based branches nano-organized
on the same cyclotetrasiloxane scaffold
are close enough together when excita-
tion takes place. In such a case, exci-
mer formation is extremely efficient,
and the new bound excited states are
quite stable.
Keywords: excimers
·
lumines-
cence · nanostructures · siloxanes ·
thiophenes
Introduction
monolayers and multilayers. An experimental approach to
investigate these effects is the study of the photophysical
properties of simple models of fragments of a chemically en-
gineered silica surface covered by few molecular fluoro-
phores.[5–11] Functionalized cyclotetrasiloxane rings [4-
RC6H4Si(O)OSiMe3]4 (R=Cl, Br, CH=CH2, CH2Cl) can be
used as scaffolds for the synthesis of models of this kind.
These cyclic compounds, with a ring small enough to be suf-
ficiently rigid, can be considered as simple 2D models of a
fragment of a silica surface chemically engineered with aro-
matic moieties in an all-cis arrangement with respect to the
cycle.[12,13]
In the field of organic or organometallic materials with
emissive properties, much work has been directed toward
the translation of the emissive molecular properties into the
emissive properties of nanostructured materials.[1–4] It is par-
ticularly important to understand the emissive properties
caused by the packing of molecular fluorophores structurally
organized on a chemically engineered surface such as silica,
which is widely used as a substrate for the preparation of
Various organic fluorophores can be covalently linked to
these functionalized cyclotetrasiloxanes with the aim of pro-
ducing simple models of a monolayer of covalently bonded
fluorophores, whose photophysical properties can thus be
compared with those of monomeric fluorophores. These
cyclic systems can also be viewed as supramolecular fluoro-
phores with new emissive properties.
Photophysical investigation of such ensembles of molecu-
lar fluorophores provides relevant information about their
intermolecular interactions, which can modify their excited
state properties, giving rise in many cases to the formation
of excimers with peculiar optical properties. Excimer forma-
tion among planar molecules such as pyrene and naphtha-
lene is well known; many examples of inter- and intramolec-
ular excimers have been reported for this class of mole-
cules.[14] In particular, it has been shown that monitoring of
intramolecular excimer formation in molecules containing
[a] Prof. R. Tubino, Prof. F. Meinardi
CNR/INFM and Dipartimento di Scienza dei Materiali
Universitꢀ di Milano-Bicocca
Via Cozzi 53, 20125 Milano (Italy)
Fax : (+39)02-6448-5400
E-mail: meinardi@mater.unimib.it
[b] W. Mrꢁz, Dr. J. P. Bombenger, Dr. C. Botta
Istituto per lo Studio delle Macromolecole, CNR
Via Bassini 15, 20133 Milano (Italy)
[c] Dr. A. Orbelli Biroli, Prof. M. Pizzotti
Dipartimento di Chimica Inorganica
Metallorganica e Analitica “Lamberto Malatesta”
Unitꢀ di Ricerca dell’INSTM, Universitꢀ degli Studi di Milano
Via G. Venezian 21, 20133 Milano (Italy)
[d] Dr. F. De Angelis, L. Belpassi
Istituto di Scienze e Tecnologie Molecolari del CNR (ISTM-CNR)
c/o Diparimento di Chimica, Universitꢀ degli Studi di Perugia
Via Elce di Sotto 8, 06123 Perugia (Italy)
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pyrene subunits gives information on the spatial proximity
of the subunits during the photoexcitation process.[15–21] The
study of ensembles of other classes of molecules, such as the
oligothiophenes,[22] with potential applications in plastic
electronics is useful for an understanding of the influence of
the molecular interactions on their basic properties (charge
mobility, radiative and nonradiative recombination). More-
over, it raises the possibility of expanding the range of mole-
cules with which devices such as optical sensors can be
realized.
In this paper we report the synthesis and the investigation
of the photophysical processes occurring upon optical exci-
tation of a cyclotetrasiloxane functionalized with four ter-
thiophene-based fluorophores. We have compared its prop-
erties with those of the structurally related monomeric fluo-
rophores in order to investigate the effect of this kind of
nano-organization. Our findings about the interaction be-
tween the photoactive units shed light on the intriguing exci-
mer formation processes occurring in this class of materials.
semble of four terthiophene fluorophores covalently bonded
to the silica-based scaffold (all-cis-1,3,5,7-tetra{4-[5-
(2,2’:5’,2’’-terthiophenyl)]phenyl}-1,3,5,7-tetra(trimethylsil-
oxy)cyclotetrasiloxane, 4SiPT3), we also prepared the mon-
omeric fluorophore with an SiACHTNUTRGNEUNG(OSiMe3)3 group ({4-[5-
(2,2’:5’,2’’-terthiophenyl)]phenyl}tris(trimethylsiloxy)silane,
SiPT3), and the basic fluorophore (5-phenyl-2,2’:5’,2’’-ter-
thiophene, PT3).
To establish the best reaction conditions for the synthesis
of the cyclic compound 4SiPT3, SiPT3 was synthesized first
by palladium-catalyzed Suzuki coupling[23,24] of (4-bromo-
phenyl)tris(trimethylsiloxy)silane
and
2,2’:5’,2’’-terthio-
phene-5-boronic acid in THF as solvent with aqueous
K2CO3 as base. The reaction mixture was purified carefully
by column chromatography to eliminate the terthiophene
(T3) generated as a byproduct during the reaction, affording
the desired product in 65% yield. 4SiPT3 could not be ob-
tained under analogous conditions starting from all-cis-
1,3,5,7-tetra(4-bromophenyl)-1,3,5,7-(trimethylsiloxy)ciclote-
trasiloxane and 2,2’:5’,2’’-terthiophene-5-boronic acid, in
which case it was mainly T3 that was recovered from the re-
action mixture.
Results and Discussion
When we tried this reaction with an excess of a more
stable derivative of the boronic acid of T3, 2,2’:5’,2’’-terthio-
phene-5-boronic acid neopentyl ester, we obtained the de-
sired product in 20% yield
The molecules investigated are reported in Scheme 1. To an-
alyze the origin of the photophysical properties of the en-
after purification by column
chromatography with gradient
elution. Replacement of the
neopentyl ester with the com-
mercially available 2,2’:5’,2’’-ter-
thiophene-5-boronic acid pina-
colic ester gave the desired
product in a lower yield (about
10%).
Finally PT3 was obtained
starting from bromobenzene
and
2,2’:5’,2’’-terthiophene-5-
boronic acid pinacolic ester in
24% yield after flash column
chromatography.
All
com-
pounds were characterized by
1H NMR spectroscopy and in
the case of SiPT3 and 4SiPT3
by 29Si NMR and mass spectros-
copy, performed by using an
electron-spray ionization source
(ESI-MS). In particular, the
1H NMR spectrum in CDCl3 of
4SiPT3 is that expected for an
average C4v symmetry. The sig-
nals of the aromatic rings are
shifted slightly upfield com-
pared with those of reference
SiPT3, which suggests some
weak shielding due to long-
range ring-current effects be-
Scheme 1. Synthetic route to 4SiPT3, SiPT3, and PT3.
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Nano-organized Oligothiophenes
FULL PAPER
tween the aromatic rings. This effect, as already observed
for the basic cyclotetrasiloxane scaffold,[13] is more signifi-
cant for the more closely packed aromatic rings (C6H4Si)
linked to the cyclotetrasiloxane ring (Ddꢀ0.30 ppm) than
new broad band appears in the long-wavelength PL tail,
shifting the overall spectrum toward the red.
To rule out the possibility that this new red-shifted broad
band is simply due to the presence of aggregates of 4SiPT3,
the absorption of which is hindered by the broad band due
to the isolated molecules, additional optical measurements
have been performed for 4SiPT3 in a very good and in a
bad solvent: EPA (diethyl ether/pentane/ethanol, 5:5:2 by
volume), and ethanol, respectively. In ethanol both aggre-
gated and nonaggregated molecules are present, as evi-
denced by the appearance in the absorption spectrum of a
blue-shifted narrow peak at 340 nm, which can be assigned
to aggregate species and which partially overlaps the broad
absorption band of the isolated molecules at 380 nm (Fig-
ure 2A). At the same time the PL at short wavelengths is
1
for the thiophene rings of T3 (Ddꢀ0.10 ppm). H NMR sig-
nals of the CH3 groups of the OSiMe3 moieties of 4SiPT3
are shifted slightly downfield (Dd=0.08 ppm) when com-
pared with SiPT3. The 29Si NMR spectrum of 4SiPT3 shows,
in agreement with an average C4v symmetry, only one signal
around À79 ppm corresponding to the silicon atoms of the
cyclotetrasiloxane ring, shifted only slightly upfield when
compared with the signal at about À78 ppm of the reference
monomer SiPT3, and one signal around 11 ppm, assigned to
the silicon atoms of the OSiMe3 groups, shifted slightly
downfield when compared with the corresponding signals at
about 9 ppm of the reference monomer SiPT3. ESI-MS
shows the presence of the molecular peak [M+H]+ or
[M+Na]+ with the isotopic distribution expected for the
compounds 4SiPT3 and SiPT3, respectively.
Figure 1 shows the absorption and photoluminescence
(PL) spectra collected at room temperature (RT) of the
three molecules in dilute dichloromethane solution (10À5 m).
The normalized absorption spectra of all three molecules
are essentially identical, with a single broad band centered
around 380 nm. This behaviour is not preserved in the PL
spectra. Only SiPT3 and PT3 exhibit similar emissions char-
acterized by well-resolved peaks at 440 nm and 470 nm, and
two shoulders in the long-wavelength tail at about 505 nm
and 545 nm. The constant energy spacing of 0.180 meV be-
tween these peaks corresponds to two vibration modes of
the thiophene ring mainly involving the C=C double bonds
which, modulating the conjugation length most effectively,
are those with the strongest electron–phonon coupling.[25,26]
Since these two spectra overlap each other almost perfectly,
they provide direct evidence that the SiACTHNUGRTNEUNG(OSiMe3)3 group
does not influence the electronic levels of the fluorophores.
In contrast, the emission spectrum arising from the four
branches of 4SiPT3 is significantly different. The two peaks
around 440 nm and 470 nm are only barely detectable and a
Figure 2. Absorption (d), PL (c), and PLE spectra of 4SiPT3 in
A) ethanol, and B) EPA. a: PLE spectra of the short-wavelength
(440 nm) emission components; b: PLE spectra of the long-wave-
length (570 nm) emission components.
strongly reduced, while the component at longer wave-
lengths is strongly enhanced and red-shifted. Such blue-shift-
ed absorption accompanied by a red-shifted emission is the
typical fingerprint of oligothiophenes in the solid state, in
which an excitonic absorption band is present.[27] PL excita-
tion-profile (PLE) measurements recorded at different emis-
sion wavelengths further confirm the presence of two inde-
pendent phases. The PLE of the short-wavelength emission
component coincides with the broad absorption band at
380 nm of isolated 4SiPT3, whereas that of the long-wave-
length one is dominated by a peak at 340 nm coincident
with the new absorption band of 4SiPT3 in ethanol.
In EPA the 4SiPT3 absorption/emission pattern, as ex-
pected when aggregates are not present, is exactly the same
as in dichloromethane, with a single absorption peak at
Figure 1. Absorption and PL emission spectra of 4SiPT3, SiPT3, and PT3
in dichloromethane solution.
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F. Meinardi et al.
380 nm and a composite PL band (Figure 2B). In this case
the PLE is independent of the emission wavelength, always
reproducing the absorption peak.
These findings demonstrate clearly that, when aggregates
are present, they are characterized not only by a red-shifted
emission but also by an easily detectable absorption. More-
over, their PL can be selectively excited. In contrast, the
typical red-shifted emission of 4SiPT3 in dichloromethane
and in EPA (that is, in good solvents) is related to the for-
mation of new excited states, that is an excimer, which
arises from a change in the spatial conformation of the ter-
thiophene fluorophores nano-organized on the tetrasiloxane
scaffold after the photon-absorption process. Indeed, there
is additional evidence for the assignment of the red-shifted
emission of 4SiPT3 to the excimer formation upon intra-
fluorophore interaction among the PT3 branches: i) we
detect it regardless of the concentration; ii) the relative in-
tensities of the PL due to 4SiPT3 branches behaving as iso-
lated fluorophores (sharp peaks at 440 nm 470 nm) and that
due to interacting ones (broad and red-shifted band) are not
dependent on the concentration; iii) in the PL of SiPT3 so-
lution no excimer-like emission is detectable at any concen-
tration in dichloromethane solution.
In sterically constrained pyrene dimers, the excimer for-
mation is strongly related to the diffusive encounters be-
tween pyrene subunits which occur on the long timescale
(hundreds of nanoseconds) typical of the partially-forbidden
singlet transitions of these molecules.[15] To understand if a
similar process is also involved in the 4SiPT3 excimer for-
mation we performed time-resolved PL measurements at
RT as well as at low temperature (LT) on frozen solutions,
when the PT3 branches of 4SiPT3 were immobilized. At RT
the PL of SiPT3 in EPA solution showed a single exponen-
tial decay with a decay time of about 315 ps at all the emis-
sion wavelengths (Figure 3A), the same decay time as we
measured for PT3. The single exponential decay demon-
strates that the fluorophores do not interact with each other
and experience the same environment. The decay time is
two orders of magnitude faster than that reported for
pyrene.[15] This is due both to the large oscillator strength of
the transition involved in oligothiophenes, and to the pres-
ence of nonradiative decay channels that further shorten the
natural decay process. In the case of 4SiPT3 (Figure 3A) the
decay profile is more complex and strongly wavelength-de-
pendent. We will discuss the first few picoseconds below;
after that, the PL decay is still nearly single exponential but
the lifetime becomes slower and slower when the emission
wavelength at which the measurements are collected is in-
creased. In particular, at 415 nm, where the PL is dominated
by the single PT3-branch emission, its lifetime is very close
to 315 ps, the same as the value detected for SiPT3. At
520 nm, and for longer wavelengths, at which the red-shifted
emission ascribed to the excimer occurs, the PL lifetime ex-
ceeds 700 ps. In Figure 3B the PL spectra collected at differ-
ent delays after the excitation pulse are reported. As expect-
ed on the basis of the previous discussion, the PL in the first
100 ps closely resembles that of an isolated PT3 branch. At
Figure 3. A) PL lifetimes at RT of SiPT3 and 4SiPT3, at different emis-
sion wavelengths, in EPA solution; B) time evolution of the RT PL spec-
tra of 4SiPT3 in EPA solution at different delay times after the excitation
pulse.
intermediate delays (500–700 ps) the spectral features both
of the single PT3 and of the excimer-like emission are de-
tectable. Finally, by integrating the slow fraction of the PL
emission (900–1900 ps), only a very broad and structureless
band is observed, corresponding to the PL spectrum of the
pure excimer.
At LT the time-resolved PL measurements are much sim-
pler (Figure 4A). The PL decay of both SiPT3 and 4SiPT3
are single exponentials with a lifetime independent of the
wavelength. Their measured decay times are longer than at
RT (770 ps and 650 ps for SiPT3 and 4SiPT3, respectively),
consistently with the reduction of the nonradiative decay
channels introduced by thermally activated vibrational
modes. As a consequence, the PL spectra do not show any
evolution at different delays (Figure 4B). The SiPT3 PL
peaks are sharper than at RT but the overall shape of the
spectrum does not change. In contrast, the PL spectrum of
4SiPT3 is completely different. The broad red-shifted PL
due to the excimer presence disappears, and the overall
spectrum becomes almost identical to that of SiPT3, demon-
strating that formation of any excimer is prevented when
the branches of 4SiPT3 are immobilized. It appears that, as
in the case of pyrene-based dimers, a conformational read-
justment of the PT3 branches of 4SiPT3 is required for exci-
mer formation. Notably, neither the emission wavelengths
nor the decay lifetimes are exactly the same for 4SiPT3 and
SiPT3. This is probably simply because of the slightly differ-
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The overall PL decay scheme describes the processes
leading to the formation of the excimer. The key points are:
a) the decay time of the PL at LT corresponding to nonin-
teracting PT3 branches of the same 4SiPT3 molecule is ex-
actly that of SiPT3; b) the excimer PL at RT of 4SiPT3 is
prompt. The excimer formation can, in principle, be an alter-
native nonradiative decay pathway of the excited-state pop-
ulation of the four isolated PT3 branches of 4SiPT3; this
would imply that a faster PL decay time should be expected.
Since this is not the case, as demonstrated above, we must
deduce that for a fraction of 4SiPT3 molecules excimer for-
mation does not occur. This can be explained by considering
that, if the excimer formation requires structural readjust-
ment of the 4SiPT3 conformation with large relative motion
of the PT3 branches, the process takes place in the typical
time of the rotations of molecules in solution that is, on a
nanosecond timescale (or even more slowly, as in pyrene-
based molecules). However, because of the subnanosecond
decay time of the excited PT3 branches, only very few con-
formational changes of 4SiPT3 in the excited state are al-
lowed, thus in this case preventing excimer formation. How-
ever, point b) proves the presence of a fraction of 4SiPT3
which, upon excitation, immediately gives rise to formation
of the excimer. These are 4SiPT3 molecules in which at
least two PT3 branches are already very close to each other
when the excitation occurs. It follows that, for molecules
with fast-decaying excited states, as in the case of the thio-
phene-based 4SiPT3, the possibility of producing an excimer
is strictly related to the conformation of the 4SiPT3 struc-
ture at the excitation time.
Figure 4. A) PL lifetimes at LT of SiPT3 and 4SiPT3 in EPA solution;
B) time evolution of the LT spectra of 4SiPT3 and SiPT3 in EPA solution
at different delay times after the excitation pulse.
ent interaction of each fluorophore with the solvent mole-
cules.
The origin of the fast decay-time component detectable in
the first 100–200 ps of the time-resolved PL measurements
of 4SiPT3 recorded at RT (Figure 3A) deserves comment.
In model systems based on sterically constrained pyrene
dimers, it is possible to observe a strong non-exponential PL
decay at the wavelength corresponding to monomeric spe-
cies,[15] with a slow component due to the natural decay of
the monomeric species, and with a fast one ascribed to the
formation time of the excimer. The order of magnitude of
the fast decay component is in the range of several tens of
nanoseconds, corresponding to the time required for two
pyrene molecules to approach each other to form the exci-
mer. A similar model has also been used to describe the PL
dynamics of Si-linked oligothiophenes,[28] even if in this case
the emission lifetimes of the isolated molecules are in the
subnanosecond range.
Our time-resolved measurements on 4SiPT3 suggest that
the dynamics of the excimer formation should be different.
First, the excimer PL does not show any rise time, at least
within the time resolution of our setup (<15 ps). As clearly
shown in Figure 3A, the excimer emission reaches its maxi-
mum intensity immediately after the excitation pulse, and
then decreases monotonically with nearly single exponential
behavior. Therefore, the fast component in the PL decay
cannot be related to the excimer formation time, but is
probably connected with some excitation migration process
among the PT3 branches.
To give further insight into the proposed process, we per-
formed quantum-chemical DFT and time-dependent DFT
(TDDFT) calculations on 4SiPT3. The 20 lowest excited-
state energies were first calculated on the optimized struc-
ture of 4SiPT3 corresponding to the ground-state energy
minimum (min S0 in Figure 5). The calculated transitions
were broadened by a Gaussian convolution with full width
at half maximum (FWHM)=0.4 eV, thus simulating the
4SiPT3 absorption spectrum, with a calculated absorption
maximum at 2.996 eV (414 nm), only slightly red-shifted
(0.267 eV) compared with the experimental value (380 nm).
This agreement makes us confident of the accuracy of our
calculations. All the excited states are combinations of the
four PT3 lowest intrafluorophore transitions. The lowest ex-
cited state (S1) is calculated at 2.903 eV. We then performed
constrained ground-state geometry optimizations as a func-
tion of the distance between two opposite PT3 branches, al-
lowing all the residual geometrical parameters to relax fully
at their minimum energy values (Figure 5). For all struc-
tures, we performed time-dependent DFT calculations for
the lowest five excited states, thus tracing an approximate
excited-state profile along the interfluorophore distance as
selected coordinate (Figure 6). It has been found that, espe-
cially at relatively short distances, the approaching PT3
branches are in an “edge-to-face” conformation (Figure 5),
exactly as in the herringbone structure typical of the solid-
state oligothiophenes.[29]
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F. Meinardi et al.
sive interactions between the PT3 branches are significantly
large, and the ground-state energy rises to approximately
100 meV. The excited-state energy follows a different trend
as a function of the interfluorophore distance. Starting from
the Franck–Condon point (FC in Figure 6), a moderate
energy rise of S1 leads the system to overcome a small
energy barrier at 7.8 ꢃ (TS S1 in Figure 6), but a deep mini-
mum of S1 of more than 200 meV is found for a separation
of 5.8 ꢃ.
These data explain quite well the dynamics of the excimer
formation. At RT, because of the flatness of the ground-
state energy versus the distance between the PT3 branches,
all the possible conformations in the range 10–7 ꢃ are ther-
mally accessible with a comparable probability. Because the
energy of the lowest excited state is also largely distance-in-
dependent in the 10–7 ꢃ range, the absorption spectra of all
these conformations are the same and basically coincide
with that of SiPT3. However, large differences are expected
in the emitting properties as a function of the distance at ex-
citation time. Indeed, if the single PT3 branch is in the
min S0 conformation of 4SiPT3, characterized by a large
separation between the four branches (Figure 5), it will
decay as an isolated SiPT3 fluorophore, as the decay is too
fast to allow significant geometric readjustments. However,
if it is in a 4SiPT3 conformation with two PT3 branches rel-
atively close to each other (Figure 5), upon excitation at the
same energy of SiPT3 the excited system will fall immedi-
ately, with a negligible structural readjustment, to the bound
excited state (that is, the excimer) corresponding to an inter-
fluorophore distance of ꢀ6 ꢃ (Figures 5 and 6). The emis-
sion then corresponds to the prompt, unstructured, and red-
shifted PL of the excimer.
Figure 5. Optimized molecular structures of the 4SiPT3 system as a func-
tion of the interfluorophore distance. The upper structure corresponds to
the ground-state minimum (min S0), and the bottom structure to the ap-
proximate excited-state minimum (min S1). Distances calculated along
the dotted lines are indicated on the left.
The photophysical processes arising from excitation of
4SiPT3 are thus summarized in Scheme 2, with 4SiPT3 in
two conformations in equilibrium at RT: the conformation
M1, characterized by a large in-
terfluorophore distance be-
tween all four PT3 branches
and unable to form excimers
upon excitation, and the confor-
mation M2 with at least two flu-
orophores in close proximity,
which does form an excimer
Scheme 2. Proposed
kinetic
upon excitation. The parameter
a gives the absorption rate of
both the conformations. After
photoexcitation, M1* decays
with a rate constant kM1 that is
the same as that of monomeric
scheme for the photophysical
processes occurring upon exci-
tation of 4SiPT3.
Figure 6. Calculated ground- (S0) and excited-state (S1) energy profiles
(eV) for 4SiPT3 as a function of the interfluorophore distance.
SiPT3. M2* does not decay directly, due to the fast excimer
formation rate kMD. It is the intrafluorophore excimer D*
that then decays with rate constant kD. The back reaction
from the excimer D* to the monomer M2* is forbidden be-
cause of the large calculated energy barrier.
The calculated ground state energy is only very weakly
dependent on the interfluorophore distance of PT3 branches
(Figure 6). The absolute minimum has been found for a
large separation (around 8.9 ꢃ) but its energy does not in-
crease significantly on decreasing the distance to about 7 ꢃ,
at which it is only 25 meV higher. Just below 6 ꢃ the repul-
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Nano-organized Oligothiophenes
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transferred to a separator funnel, and extracted from CH2Cl2. The organ-
ic phase was neutralized by aqueous HCl (1n), then washed with brine
and dried over anhydrous MgSO4. The solvent was removed in vacuo.
The residue obtained was purified by column chromatography (silica gel,
n-hexane/CHCl3 9:1, Rf =0.32) to afford the pure product (69.5 mg, 65%
yield) as a yellow solid. 1H NMR (400.1 MHz, CDCl3): d=7.60 (m, 4H),
7.30 (d, 1H), 7.25 (dd, 1H), 7.21 (dd, 1H), 7.18 (d, 1H), 7.13 (m, 2H),
7.06 (m, 1H), 0.17 ppm (s, 27H); 29Si NMR (79.5 MHz, CDCl3): d=9.04
(s), À78.16 ppm (s). MS-ESI+: m/z: 619.8 [M+H]+; elemental analysis
calcd (%) for C27H38O3S3Si4: C 52.38, H 6.19; found C 52.36, H 6.26.
Conclusions
A complete description of the photophysical processes
giving rise to excimer formation in thiophene-based mole-
cules covalently bound to a cyclotetrasiloxane scaffold is
given. It has been demonstrated that, unlike similar systems
in which long-lived chromophores are linked in the same
molecule, excimer formation is possible only if the geometry
of the absorbing molecules is very close to that of the exci-
mer itself. In such a case excimer formation is extremely
fast, and the new excited species appear to be quite stable.
The simple model of a layer of rodlike fluorophores on
chemically engineered silica demonstrates that an excimer
state can be formed even in the short excited-state lifetime
of thiophene-based molecules. Contrary to previously re-
ported results on linear polyconjugated molecules,[30] theo-
retical modelling shows that, in the constrained geometry in-
vestigated, the two 4SiPT3 fluorophores forming the exci-
mer possess, in the excited state, a herringbone arrangement
similar to that assumed in unsubstituted oligothiophene
crystals.
Synthesis of 4SiPT3: In a dry Schlenk tube all-cis-1,3,5,7-tetra(4-bromo-
phenyl)-1,3,5,7-tetra(trimethylsiloxy)ciclotetrasiloxane
(80.2 mg,
0.069 mmol) and 2,2’:5’,2’’-terthiophene-5-boronic acid neopentyl ester
(193.0 mg, 0.536 mmol) were dissolved in THF (5 mL) and the solution
was de-aerated at À788C by three freeze–pump–thaw cycles. The solution
was allowed to reach room temperature, then a solution of [PdACHTUNGTRENNUNG(PPh3)4]
((27.1 mg, 8.5% mol) in THF (2 mL) and K2CO3 (1m aqueous solution,
3 mL), previously de-aerated, were added under an N2 atmosphere. The
reaction mixture was heated under vigorous stirring at reflux for 24 h.
The final mixture was allowed to cool to room temperature and the or-
ganic phase was removed in vacuo. The aqueous residue was extracted
from CH2Cl2 and the new organic phase was washed with HCl (10%
aqueous solution) and dried over anhydrous MgSO4. The solvent was re-
moved in vacuo and the crude product was purified by column chroma-
tography (silica gel, n-hexane, gradient to n-hexane/CH2Cl2 1:1, Rf =0.75)
to afford the pure product (25.0 mg, 20% yield) as a yellow solid.
1H NMR (400.1 MHz, CDCl3): d=7.34 (m, 16H), 7.21 (dd, 4H), 7.18 (d,
4H), 7.13 (dd, 4H), 7.09 (d, 4H), 7.01 (m, 12H), 0.25 ppm (s, 36H);
29Si NMR (79.5 MHz, CDCl3): d=11.02 (s), À79.37 ppm (s); MS-ESI+:
m/z: 1847.05 [M+Na]+; elemental analysis calcd (%) for C84H80O8S12Si8:
C 55.22, H 4.41; found: C 54.93, H 4.23.
Experimental Section
Materials: All reagents and anhydrous solvents were obtained from
Sigma Aldrich and used without additional purification except for THF,
which was freshly distilled from Na/benzophenone. (4-Bromophenyl)-
tris(trimethylsiloxy)silane,[31] all-cis-1,3,5,7-tetra(4-bromophenyl)-1,3,5,7-
tetra(trimethylsiloxy)cyclotetrasiloxane,[12] 2,2’:5’,2’’-terthiophene-5-bor-
onic acid,[32] and 2,2’:5’,2’’-terthiophene-5-boronic acid neopentyl ester[32]
were synthesized by using the procedure reported in the literature. 1H
and 29Si NMR spectra were recorded by using a Bruker Avance DRX-
300 or Bruker Avance DRX-400 spectrometer in CDCl3 (Cambridge Iso-
tope Laboratories, Inc.) as solvent. Mass spectra were obtained by using
a Thermo-Finnigan apparatus with an ion trap analyzer (positive mode)
and an electrospray ionization source (ESI) using an LCQ Advantage in-
strument or by using a Bruker-Daltonics ICR-FTMS Apex II instrument
with an ESI source.
Optical measurements: The absorption spectra in dichloromethane, etha-
nol, toluene, and EPA (diethyl ether/pentane/ethanol, 5:5:2 by vol.) were
recorded with a Perkin–Elmer Lambda 900 spectrometer. During contin-
uous-wave photoluminescence measurements, a xenon lamp was used as
the excitation source, with a Jobin Yvon Gemini 180 monochromator for
the wavelength selection. The emitted light passed through a Spex 270M
monochromator and was measured by a Spex Jobin Yvon CCD (charge-
coupled device) cooled with liquid nitrogen. Relative quantum yields
have been measured by comparison with a quinine sulfate standard. In
all PL measurements the solution concentration was below 2ꢄ105 m.
Time resolved spectra were collected by exciting the solution with the
second harmonic of a Ti:sapphire laser (pulse width<150 fs, repetition
rate 76 MHz). Signal were detected by a Hamamatsu streak camera cou-
pled with a Cromex spectrograph. The overall time resolution was better
than 15 ps.
Synthesis of PT3: In
a dry Schlenk tube bromobenzene (52.1 mg,
0.332 mmol) and 2,2’:5’,2’’-terthiophene-5-boronic acid pinacol ester
(140.1 mg, 0.374 mmol) were dissolved in toluene (15 mL) and the solu-
tion obtained was de-aerated at À788C by three freeze–pump–thaw
cycles. The solution was allowed to reach room temperature, then [Pd-
Theoretical methods: All the calculations have been performed with the
Gaussian 03 program package.[33] The B3LYP exchange-correlation func-
tional,[34] together with a 6-31G* basis set,[35] has been used for both
ground- and excited-state calculations.
ACHTUNGTRENNUNG(PPh3)4] (40.0 mg, 10% mol) and K2CO3 (1m aqueous solution, 1.2 mL),
previously de-aerated, were added under an N2 atmosphere. The reaction
mixture was heated under vigorous stirring at reflux for 24 h. The sol-
vents were removed in vacuo and the residue obtained was purified by
flash column chromatography (silica gel, n-hexane, Rf =0.24) to afford
the pure product (25.5 mg, 24% yield) as a yellow solid. 1H NMR
(300.1 MHz, CDCl3): d= 7.62 (d, 2H), 7.41 (t, 2H), 7.32 (d, 1H), 7.25
(m, 2H), 7.20 (d, 1H), 7.17 (d, 1H), 7.12 (m, 2H), 7.05 ppm (m, 1H); ele-
mental analysis calcd (%) for C18H12S3: C 66.63, H 3.73; found C 66.56,
H 4.00.
Acknowledgements
This work was supported by the Fondazione Cariplo (2005, research title:
Nuovi materiali con nanoorganizzazione di cromofori in sistemi Host-
Guest o su scaffold inorganico per dispositivi fotoluminescenti o optoe-
lettronici), and by the European Commission through the Human Poten-
tial Program RTN “Nanomatch” (MRTN-CT-2006-035884). F.D.A., M.P.,
and A.O.B. thank the CNR-INSTM PROMO 2006 and MUR (FIRB
2003, research title: molecular compounds and hybrid nanostructured
materials with resonant and non resonant optical properties for photonic
devices) for financial support. We thank Prof. Renato Ugo for useful dis-
cussions.
Synthesis of SiPT3: In a dry Schlenk tube (4-bromophenyl)tris(trimethyl-
siloxy)silane (78.7 mg, 0.174 mmol) and 2,2’:5’,2’’-terthiophene-5-boronic
acid (101.9 mg, 0.349 mmol) were dissolved in THF (3 mL) and the solu-
tion was de-aerated at À788C by three freeze–pump–thaw cycles. The so-
lution was allowed to reach room temperature, then a solution of [Pd-
ACHTUNGTRENNUNG(PPh3)4] (19.9 mg, 10% mol) in THF (3 mL) and K2CO3 (1m aqueous so-
lution, 1.2 mL), previously de-aerated, were added under an N2 atmos-
phere. The reaction mixture was heated under vigorous stirring at reflux
for 24 h. The final mixture was allowed to cool to room temperature,
Chem. Eur. J. 2009, 15, 12791 – 12798
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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Received: May 15, 2009
Published online: October 20, 2009
12798
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ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 12791 – 12798