The influence of crystal packing on the solid state fluorescence behavior of alkyloxy substituted phenyleneethynylenes

Article abstract of DOI:10.1039/b902937k

The study reports the solid state photophysical properties of a series of alkyloxy-substituted oligo(phenyleneethynylene)s, methoxy to hexyloxy, supported by a detailed analysis of molecular packing obtained from single crystal X-ray diffraction data. While the emission peaks are highly red shifted (by as much as 114 nm) in the solid state, all molecules exhibit similar absorption and emission in dilute solutions. The red shift is maximum in ethoxy and minimum in methoxy, while other crystalline films exhibit intermediate values. In the crystal structures, the spacing between the molecular pairs forming the J-aggregates is varied between 3.48 to 4.65 , with no systematic dependence on the chain length. The red shifted emission maximum is found to vary linearly with the spacing between the interacting molecules in the J-aggregate. Thus, the emission in the solid state is determined by the extent of dipolar coupling between the molecules, the alkyl chain length influencing the properties only indirectly. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b902937k

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
The influence of crystal packing on the solid state fluorescence behavior
of alkyloxy substituted phenyleneethynylenes†‡
a
Reji Thomas, Shinto Varghese and G. U. Kulkarni
b
a
*
Received 16th February 2009, Accepted 30th March 2009
First published as an Advance Article on the web 30th April 2009
DOI: 10.1039/b902937k
The study reports the solid state photophysical properties of a series of alkyloxy-substituted
oligo(phenyleneethynylene)s, methoxy to hexyloxy, supported by a detailed analysis of molecular
packing obtained from single crystal X-ray diffraction data. While the emission peaks are highly red
shifted (by as much as 114 nm) in the solid state, all molecules exhibit similar absorption and emission
in dilute solutions. The red shift is maximum in ethoxy and minimum in methoxy, while other
crystalline films exhibit intermediate values. In the crystal structures, the spacing between the molecular
˚
pairs forming the J-aggregates is varied between 3.48 to 4.65 A, with no systematic dependence on the
chain length. The red shifted emission maximum is found to vary linearly with the spacing between the
interacting molecules in the J-aggregate. Thus, the emission in the solid state is determined by the extent
of dipolar coupling between the molecules, the alkyl chain length influencing the properties only
indirectly.
properties of various single molecular conductors, particularly
Introduction
oligo(phenyleneethynylene)s. A major portion of such studies
focus on the effect of the functional groups, especially donor–
acceptor pairs, on the conducting properties of the phenyl-
eneethynylene backbone.11a,12 Among them, the molecule
carrying nitro and amino groups showed a negative differential
resistance behavior, a prototype switch action.13 In addition to
the chemical effects, structural attributes such as the planarity of
the phenyleneethynylene backbone also seem to have a large
impact on the electronic and the photophysical properties of the
p-phenylenethynylenes.14 Towards this end, a number of studies
have been carried out during the last decade on the molecular
structure and packing in the solid state and the associated pho-
tophysical properties.3c,15 The two-dimensional aggregation and
the associated emission of merocyanine dyes in LB films have
also been investigated.16 Accordingly, the weak interactions
responsible for molecular packing have come under focus in
many studies.15,17 Scanning probe microscopic techniques have
also been employed to study the two-dimensional arrangements
of the conjugated chains on solid surfaces.18 A recent study
on a phenyleneethynylene-phenylenevinylene copolymer has
shown the photophysical properties to vary with long chain
substitutions.19 A few years ago, Bunz reviewed the literature on
phenyleneethynylenes.20
Conjugated oligomers and polymers are of immense interest in
current research due to their interesting electronic and photo-
physical properties.1 Recent literature contains numerous reports
on the synthesis and properties of molecular systems having
conjugated backbones, such as p-phenylenevinylenes,2 p-phe-
nyleneethynylenes,3 p-phenylenes4 and thiophenes.5 The elec-
tronic properties exhibited by these systems have made them
important in single molecular electronics6 and display applica-
tions.7 In addition, these molecular systems have been tested for
solar cell applications8 as well as for nonlinear optical applica-
tions such as second or third harmonic generation.9 In contrast
to their inorganic counterparts, the light emitting properties of
these molecular systems are easily tunable based on structural
and functional group modifications.10
Among the various conjugated molecules mentioned above,
p-phenyleneethynylenes have taken centre stage as model
systems with the possibility of tuning properties by way of
substituting functional groups in the middle phenyl ring.11
The last decade witnessed a lot of efforts, experimental as well
as theoretical, towards designing and understanding the
aChemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for
Advanced Scientific Research, Jakkur (P. O.), Bangalore, 560064, India.
E-mail: kulkarni@jncasr.ac.in; Fax: +91-80-220827676
bPhotosciences and Photonics, Chemical Sciences and Technology Division,
National Institute for Interdisciplinary Sciences, Trivandrum, 695019,
India
We considered it interesting to investigate systematically the
influence of crystal packing of molecules on the solid state fluores-
cence behavior. In this paper, we report our results on dialkyloxy-
substituted oligo(phenyleneethynylene)s, where the alkyl chain
length has been varied from methyl to hexyl (Scheme 1). We have
obtained the crystal structures by single crystal X-ray diffraction
and measured fluorescence spectra from the corresponding solid
films. We have analysed the weak interactions prevalent in the
crystal structures and identified the electronically interacting pairs
of molecules. Our study has shown that the fluorescence emission
depends directly on the molecular spacing in the crystalline state.
† This paper is part of a Journal of Materials Chemistry issue in
celebration of the 75th birthday of C. N. R. Rao.
‡ Electronic
supplementary
information
(ESI)
available:
Crystallographic details; NMR characterisation data; table of weak
interactions; excitation spectra; XRD patterns; diagrams showing
possible intermolecular interactions. CCDC reference numbers
695299–695304. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b902937k
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cooled to room temperature. Fluorescence measurements of the
films were carried out using the front face emission scan mode on
the SPEX Fluorolog F112X spectrofluorimeter. Solid state
quantum efficiency was measured using a calibrated integrating
sphere in the SPEX Fluorolog spectrofluorimeter. A Xe arc lamp
was used to excite the thin film sample placed in the sphere, with
360 nm excitation wavelength. The solid state quantum yield was
determined by comparing the spectral intensities of the lamp and
sample emission using a reported procedure.25 In order to test the
calibration of the experimental system, the solid state quantum
yield of a thin film of Alq3 was determined (0.19), which is
consistent with previously reported values.25
Scheme 1 Molecules presented in this study. R ¼ C1: 1,4-bis(phenyle-
thynyl)-2,5-bis(methoxy)benzene; C2: 1,4-bis(phenylethynyl)-2,5-bis-
(ethoxy)benzene; C3: 1,4-bis(phenylethynyl)-2,5-bis(n-propoxy)benzene;
C4: 1,4-bis(phenylethynyl)-2,5-bis(n-butyloxy)benzene; C5: 1,4-bis(phe-
nylethynyl)-2,5-bis(n-pentyloxy)benzene; C6: 1,4-bis(phenylethynyl)-2,5-
bis(n-hexyloxy)benzene.
Crystallography measurements
Experimental
In order to carry out X-ray diffraction measurements, the
recrystallised samples were analyzed under a polarizing micro-
scope and single crystals were separated for X-ray structure
determination. The single crystal data were collected on
a Bruker-Nonius diffractometer with Kappa geometry attached
with an APEX - II-CCD area detector and a graphite mono-
chromator for Mo Ka radiation (50 kV, 40 mA) at room
temperature (298 K). The intensity data were processed using
SAINT software26 of the Bruker suite of programs. The struc-
tures were solved and refined using the SHELXTL package.27
The structure was solved by direct methods and refined by full-
matrix least-squares techniques. All hydrogen atoms were
located from a difference Fourier map. The intermolecular
interactions were analyzed using the PLATON package.28 The
crystalline nature of the film samples used for solid state fluo-
rescence was confirmed by performing diffraction on a Bruker
D8 system. The obtained patterns matched well with the powder
patterns simulated based on the single crystal data (see ESI‡
Fig. S3).
Alkyloxy-substituted oligo(phenyleneethynylene)s (C1, C2, C3,
C4, C5 and C6; see Scheme 1) were synthesized following
reported procedures.21,22 All the chemicals required for synthesis
were procured from a commercial source (Aldrich) and used as
obtained. The solvents were distilled and dried using general
procedures.23
Synthesis of 2,5-bis(phenylethynyl)-1,4-bis(alkyloxy)benzenes
(C1–C6)
To an oven dried 100 mL two necked round bottom flask, 2,5-
dibromo-1,4-dialkyloxybenzene (6 mmol) was dissolved in THF
(7 ml). To the solution bis(triphenylphospine) palladium(II)
dichloride (0.12 mmol) was added and stirred for 15 minutes. To
the reaction mixture triphenylphosphine (0.24mmol), diisopro-
pylethylamine (24 mmol) and copper(I)i odode (0.24) were added
and the atmosphere replaced with argon (3ꢀ). To this reaction
mixture phenylacetylene (14.2 mmol) was added and once again
the argon atmosphere maintained and the round bottom flask
was sealed. The reaction mixture was heated at 65 ꢁC for 24
hours and filtered over celite to remove the inorganic products.
The organic portion was extracted using dichloromethane and
purified using flash column chromatography (silica gel and using
hexanes as the eluent). The dialkyloxy-substituted oligo(pheny-
leneethynylene)s were produced in yields of 80 to 90% and were
characterized using NMR (1H and 13C) spectroscopy (see ESI‡)
in addition to single crystal X-ray diffraction.
Results and discussion
The solution phase absorption spectra of C1 to C6 recorded from
their dilute toluene solutions (15 mM) are shown in Fig. 1a. The
absorption spectra exhibit two intensity maxima centered around
307 and 367 nm. The presence of two features in the absorption
spectrum is understandable. A theoretical study29 on C6 has
shown that in solution, the molecule exists in the planar form and
the alkyloxy-substitution modifies the p-orbitals of the central
arene ring through resonance interaction with the oxygen lone
pairs resulting in electronic transitions from both HOMO and
HOMO ꢂ 1 to LUMO. The scenario is expected to be similar in
other molecules as well. Fig. 1b shows the fluorescence emission
spectra of the compounds C1 to C6. All the molecules exhibit
similar spectral features with the maximum intensity around 401
nm and a shoulder at ꢃ420 nm. As the emission behavior is
essentially similar, the chain length (alkyloxy) induced chemical
effects should have a minimal role, if any. The excitation spectra
(see ESI,‡ Fig. S1) showed features with maxima at 310 and 367
nm, similar to the absorption spectra (Fig. 1a), indicating that
the absorbing and the emitting species are similar in all the
molecules. Table 1 lists the details of the absorption and emission
characteristics of the molecules in solution. In all the cases, the
emission exhibited a monoexponential decay confirming the
Photophysical property measurements
The solution phase absorption spectra were recorded on a Shi-
madzu UV-3101PC UV-vis-NIR spectrophotometer while the
excitation and emission spectra were recorded on a SPEX Flu-
orolog F112X spectrofluorimeter. Fluorescence quantum yields,
with an estimated reproducibility of around 2%, were determined
by comparison with diphenylanthracene (Ff ¼ 0.9), which was
used as the standard. The fluorescence lifetimes were determined
using the time-correlated single photon counting (TCSPC)
technique.24 Crystals of the compounds C1 to C6 were obtained
by the slow evaporation of their saturated solutions in toluene
and the crystals were separated from the mother liquor. For solid
state photoluminescence measurements, the crystals were spread
in between two glass plates and heated to melting and slowly
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Fig. 1 (a) Absorption in dilute solutions (15mM in toluene) and (b)
fluorescence in dilute solutions (excitation wavelength, 360 nm) of alky-
loxy-substituted oligo(phenyleneethynylene)s.
Table 1 Absorption and emission properties of alkyloxy substituted
oligo(phenyleneethynylene)s (15mM) in toluene
Fig. 2 (a) Fluorescence emission (excitation 360 nm) spectra for the
crystalline films of C1 to C6. (b) Chromaticity diagram (CIE 1931)
showing the emission colours.
Molecule
3 ꢀ 10ꢂ4 Mꢂ1 cmꢂ1
t (ns)
c2
F(Fl.)
C1
C2
C3
C4
C5
C6
3.198
2.964
3.001
3.209
3.186
3.093
1.23
1.32
1.37
1.45
1.45
1.35
1.02
1.01
1.07
1.05
1.02
1.09
0.90
0.92
0.91
0.91
0.89
0.91
a number of intensity maxima are observed indicating the pres-
ence of various emitting levels in the crystalline state. The
emission colors from the crystalline samples are represented with
the help of a chromaticity diagram, CIE 1931 standard (see
Fig. 2b). The C1 system lies in the blue region while C2 is
distinctly in the green region and the rest of the molecules exhibit
intermediate colors in the crystalline state.
presence of a single emitting species. Singlet excited state life-
times of C1 to C6 lie within a narrow range of 1.23 to 1.45 ns with
no systematic dependence on the chain length. All the molecules
showed a high quantum yield of ꢃ0.9.
In order to gain an insight into the solid state fluorescence
behavior of the various molecules, it is important to look into the
details of the molecular packing in their single crystals. Single
crystals can be considered as self-assembled species par excel-
lence since the nature of the weak interactions between neigh-
boring molecules can be determined with a high degree of
precision.15 These intermolecular forces typically decide the bulk
alignment of the molecules and thereby the photophysical
properties of the materials. In the present study, we have carried
out fluorescence measurements on crystalline films. The XRD
patterns from the films were essentially similar compared to the
simulated powder patterns from the single crystal data (Fig. S3,
see ESI‡).
The solid state behavior of the molecules is quite different.
Fig. 2 shows the fluorescence spectra from the crystalline films of
C1 to C6. On comparing the behavior in solution (see Fig. 1b),
we find that the solid state emission peaks are red shifted. The
spectrum from C1 exhibits the maximum emission intensity at
ꢃ444 nm with distinct shoulders at 425 and 390 nm and a broad
tail at higher wavelengths. With an increase in the chain length by
one methylene unit, the C2 spectrum is highly red shifted to
around 515 nm, and it is relatively symmetric. The spectra of C3,
C4, C5, and C6 lie between those of C1 and C2, with C4, C5 and
C6 almost overlapping. Thus, a systematic trend in the variation
of the emission with alkyloxy chain length is not quite apparent
from this data. In the excitation spectra shown in Fig. S2 (ESI‡),
All the dialkyloxy-substituted oligo(phenyleneethynylene)s
(C1 to C6) crystallize in monoclinic space groups (C2/c, P21/c or
P21/n) with a half molecule in the asymmetric unit. The crystal
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depending on the alkyl chain length. The molecule C1 shows
a maximum deviation from planarity in contrast to the usual
planar geometry of phenyleneethynylenes.30 The end phenyl
rings are twisted by as much as ꢃ80ꢁ in relation to the middle
ring. The crystal structure shows a slipped herring bone packing
along the ac-diagonal plane as shown in Fig. 3a. Fig. 3b contains
three molecular layers in the lateral view. The major interactions
in crystal packing are C–H/p in nature, where the triple bond
serves as a p acceptor for four interactions originating from the
phenyl ring hydrogens. The stability provided by the C–H/p
interactions is primarily responsible for such an unfavorable
twist in the molecule. The deviations of the middle phenyl ring
from the planarity in the other molecules, C2 to C6, are within
the range of 4.5ꢁ to 16ꢁ as shown in Fig. 4a. These molecules
adopt a herringbone type of packing arrangement as shown in
Fig. 4b taking C2 as example. The alkyl chain in C3 exhibits
a gauche conformation, while in the other molecules, the chains
are in trans conformation. The C–H/p interactions in these
structures originate from both phenyl and alkyl chain hydrogens
with both the phenyl ring and the triple bond acting as p-
acceptors. In C4, we found an additional interaction of C–H/O
type. The details of hydrogen bonding are listed in Table S2
(ESI‡). The interchain interaction seems to build up as the chain
length increases and accordingly the distance between them, as
measured with respect to lines drawn bisecting the methylene
Fig. 3 (a) Herringbone packed layers of C1 along the ac-diagonal plane
of the crystal lattice. (b) A lateral view of the slipped stack of molecular
layers.
˚
chains, varies as 4.85, 4.31 and 3.88 A for C4, C5 and C6
respectively.
A detailed analysis of the molecular packing revealed that for
each molecule, there exist two neighbors. Thus, one finds two
types of molecular pairs as shown for C1 in Fig. 5. In Fig. 5a, the
partner molecules extend an angle of 78.5ꢁ between them, while
˚
in Fig. 5b the pair is parallel with a spacing of 4.65A. Based on
earlier theoretical and experimental studies, it is reasonable to
consider the first pair (Fig. 5a) to be weakly interacting or non-
interacting.31 In the second pair (Fig. 5b), the molecules are
arranged in a nearly ‘head to tail’ fashion, akin to a ‘J-aggre-
gate’.32 However the intermolecular distance is fairly large
˚
(4.65A) which would result in minimal electronic communication
between the two molecules. The molecular packing pattern of the
interacting molecules was similar in all the cases (Fig. S4 to S8,
see ESI‡). Depending on the C–H/p interactions prevalent, the
spacing between the interacting molecules is found to vary in the
˚
range 3.48–4.62 A. This class of molecules prefer to remain in all-
planar conformation while the solid state packing can bring
about significant distortions affecting the intermolecular spacing
Fig. 4 (a) Deviation of the phenyl ring from planarity with respect to the
alkyl chain length. (b) Herringbone packing of molecule C2.
structures of compound C6 is previously known in the literature
and the structure solution obtained in this study matched closely
with that reported.17b The crystal structures of C1, C2, C3, C4
and C5 are being reported by us for the first time. Crystallo-
graphic and refinement details of the compounds are provided in
the ESI‡ (Table S1). Obviously, the molecular geometry varies
Fig. 5 (a) Crossed and (b) slipped stacking pairs in molecule C1.
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Table 2 Fluorescence lifetimes and quantum yields of dialkyloxy-
substituted oligo(phenyleneethynylene)s in the solid state
Lifetime (ns)
Compound t1
F1% t2
F2% t1
F3% c2
F (solid)
C1
C2
C3
C4
C5
C6
2.83 61
3.10 55
2.24 84
4.59 25
4.68 42
0.24
2.63 97
0.27
2.55 95
0.72 14
0.35
4.13 12
1.01 0.51
1.01 0.53
1.06 0.82
1.07 0.73
1.01 0.74
1.01 0.78
3
4
0.22
2.22 66
0.20
3
—
—
3
3.35 31
5.29
2
3
(Fig. 7) are blue shifted with respect to pristine C6. The intensity
maximum in the sample containing 14.4 mg of C6 occurs at 462
nm which remains nearly the same in the case of the 7.2 mg
sample albeit with reduced intensity. Both spectra do contain
a shoulder feature around 494 nm corresponding to the parent
aggregate structures. The blue shifted emission is likely to arise
due to the microcrystalline nature of the sample where the
J-aggregates near the particle surface may suffer from a reduced
interaction in contrast to those in the bulk lattice. We observe
a rise in intensity around 400 nm due to increasing proportions of
the isolated molecular species from the surface. On further
diluting the sample (3.6 mg in KBr), the emission from both
surface (462 nm) and bulk species (494 nm) diminishes while
emission due to monomeric species (400 nm) increases further.
This is understandable as the concentration of the emitting
species is in the nanomolar range.15b,35 The fluorescence spectrum
of the isolated monomeric species matches closely with the
solution phase spectrum of C6.
Fig. 6 Variation of the emission maximum with the spacing between the
interacting molecules in crystals of C1 to C6 (excitation 360 nm).
and in turn the transition energies.30 An interesting correlation
was observed between the solid state emission maxima (from
Fig. 2a) and the spacing between the interacting molecules
forming the J-aggregates. As presented in Fig. 6, the emission
maximum shows a linear dependence on the molecular spacing,
exhibiting a blue shift with increasing spacing. This is not only
a clear indication of the presence of J-aggregates, but is also
clearly a measure of the strength of the dipolar coupling between
the interacting molecules.33,34 Such a direct correlation between
the crystal packing and the emission property is indeed note-
worthy. Corresponding to emission from the dilute solutions
˚
(401 nm), a spacing value of 5.55 A can be derived from the plot
in Fig. 6. This value should indicate a very weakly coupled
aggregate structure.
Table 2 shows the summary of the fluorescence lifetime
measurements. All the compounds show tri-exponential decay
except the molecule C4, which shows a bi-exponential decay with
the longest lifetime species as the major one. Molecule C2 shows
two lifetimes with comparable relative abundance, whereas for
molecules C1, C3, C5 and C6 the species with intermediate life-
time are the major components. All the crystals exhibited
quantum yields (up to 0.8) but these are low compared to the
values from the solution phase (compare Tables 1 and 2). Also,
there is a spread in the values (0.51–0.82) in the crystalline state
which is absent in solution. A systematic trend with respect to the
alkyl chain length is however not quite obvious. When examined
with respect to the weighted average of lifetime, a trend seems to
emerge: the higher the lifetime, the less the quantum yield,
implying that the decay of the excited state take place through
non-radiative channels.
We have also performed an experiment to physically influence
the aggregate structure by way of breaking up the crystals into
smaller and smaller particles. This was accomplished by gently
grinding known quantities (14.4 mg, 7.2 mg and 3.6 mg) of C6
crystals with 100 mg of dry KBr powder. The spectra obtained
Conclusions
The present study deals with the crystal structures and the fluo-
rescence properties of alkyloxy-substituted oligo(phenyleneethy-
nylene)s (methyl, C1, to hexyl, C6). The crystalline films of C1 and
C2 exhibit fluorescence maxima at ꢃ444 and 515 nm respectively,
while for C3, C4 and C5, the maxima are positioned in between,
with hardly any trend with respect to the chain length. This is in
clear contrast to the behavior in dilute solutions, where all the
molecules, C1 to C6, exhibit fluorescence maxima around 401 nm.
An analysis of the crystal packing of the molecules revealed the
Fig. 7 Fluorescence spectra (excitation 360 nm) of C6. Fluorescence
from the crystalline film, spectrum a (as in Fig. 2a). The spectra b, c and
d correspond respectively to 14.4, 7.2 and 3.6 mg of C6 diluted with 100
mg of KBr powder. The isosbestic point observed at 440 nm is marked
with an arrow. The solution phase spectrum (e) is shown for comparison
with arbitrary intensity.
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J. Mater. Chem., 2009, 19, 4401–4406 | 4405

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ꢁ
A, 2001, 105, 791; (c) Y. Karzazi, J. Cornil and J. L. Bredas,
presence of J-aggregate pairs, which accounted for the red shifted
emission in the solid state. Interestingly, the emission maximum
was found to vary linearly with the spacing between the inter-
acting pair of molecules in the J-aggregates: the lower the spacing,
the greater the red shift. In dilute solutions, the molecules
exhibited monoexponential decay of the excited state with high
quantum yields (ꢃ0.9). In the case of the crystalline films, the
quantum yields were still appreciable but varied (0.5 to 0.8).
Nanotechnology, 2003, 14, 165; (d) J. D. Le, Y. He, T. R. Hoye,
C. C. Mead and R. A. Kiehl, Appl. Phys. Lett., 2003, 83, 5518; (e)
S. Lakshmi and S. K. Pati, Phys. Rev. B, 2005, 72, 193410.
13 J. Chen, M. A. Reed, A. M. Rawlett and J. M. Tour, Science, 1999,
286, 1550.
14 (a) Z. J. Donhauser, B. A. Mantooth, K. F. Kelly, L. A. Bumm,
J. D. Monnell, J. J. Stapleton, D. W. Price Jr, A. M. Rawlett,
D. L. Allara, J. M. Tour and P. S. Weiss, Science, 2001, 292, 2303;
(b) S. I. Khondaker, Z. Yao, L. Cheng, J. C. Henderson, Y. Yao
and J. M. Tour, Appl. Phys. Lett., 2004, 85, 645; (c) N. Li, K. Jia,
S. Wang and A. Xia, J. Phys. Chem. A, 2007, 111, 9393; (d)
Y. Sonoda, M. Goto, S. Tsuzuki and N. Tamaoki, J. Phys. Chem.
A, 2006, 110, 13379.
Acknowledgements
The authors thank the Department of Science and Technology,
Government of India for research grants under DST Unit on
Nanoscience. They are grateful to Prof. C. N. R. Rao for his
constant encouragement. The authors also thank Dr Suresh Das,
Dr George Thomas and A. R. Ramesh for useful discussions.
15 (a) R. Davis, N. P. Rath and S. Das, Chem. Commun., 2004, 74; (b)
R. Davis, N. S. Saleesh Kumar, S. Abraham, C. H. Suresh,
N. P. Rath, N. Tamaoki and S. Das, J. Phys. Chem. C, 2008, 112,
2137; (c) N. S. S. Kumar, S. Varghese, N. P. Rath and S. Das,
J. Phys. Chem. C, 2008, 112, 8429.
16 M. Miruta, M. Villeneuve and H. Nakahara, Chem. Phys. Lett., 2005,
405, 416.
17 (a) H. Li and R. West, Macromolecules, 1998, 31, 2866; (b) H. Li,
D. R. Powell, R. K. Hayashi and R. West, Macromolecules, 1998,
31, 52.
References
´
€
18 (a) P. Samorı, V. Francke, K. Mullen and J. P. Rabe, Chem. Eur. J.,
1999, 5, 2312; (b) B. G. Sumpter, P. Kumar, A. Mehta, M. D. Barnes,
W. A. Shelton and R. J. Harrison, J. Phys. Chem. B, 2005, 109, 7671;
(c) K. Yoosaf, P. V. James, A. R. Ramesh, C. H. Suresh and
K. G. Thomas, J. Phys. Chem. C., 2007, 111, 14933.
19 E. Tekin, D. A. M. Egbe, J. M. Kranenburg, C. Ulbricht,
S. Rathgeber, E. Birckner, N. Rehmann, K. Meerholz and
U. S. Schubert, Chem. Mater., 2008, 20, 2727.
1 (a) T. A. Skotheim and J. R. Reynolds, Handbook of Conducting
Polymers, Third Edition, Conjugated Polymers: Processing and
€
Applications, C R C Publishers, USA, 1998; (b) W. Brutting,
Physics of Organic Conductors, Wiley-VCH: Berlin, Germany, 2005.
2 (a) S. Fratiloiu, F. C. Grozema and L. D. A. Siebbeles, J. Phys. Chem.
B, 2005, 109, 5644; (b) G. D. Scholes and G. Rumbles, Nature Mater.,
2006, 5, 683; (c) A. Ajayaghosh and S. J. George, Chem. Eur. J., 2005,
5, 593.
20 U. H. F. Bunz, Chem. Rev., 2000, 100, 1605.
21 H. Meier, D. Ickenroth, U. Stalmach, K. Koynov, A. Bahtiar and
C. Bubeck, Eur. J. Org. Chem., 2001, 23, 4431.
22 S. M. Dirk, D. W. Price, S. Chanteau, D. V. Kosynkin and
J. M. Tour, Tet. Lett., 2001, 57, 5109.
23 A. I. Vogel, A. R. Tatchell, B. S. Furnis, A. J. Hannaford and
P. W. G. Smith, Vogel’s Textbook of Practical Organic Chemistry
(5th Edition), ELBS-Longman Singapore publishers; Singapore 2005.
24 Manual, Time Correlated Single Photon Counting, HORIBA Jovin
Yvon Inc., 2006.
25 (a) M. Muccini, M. Murgia, C. Taliani, A. D. Esposti and
R. Zamboni, J. Opt. A: Pure Appl. Opt., 2000, 2, 577; (b) J. C. de
Mello, H. F. Wittmann and R. H. Friend, Adv. Mater., 1997, 9,
230; (c) L. –O. Palsson and A. P. Monkman, Adv. Mater., 2002, 14,
757.
ꢀ
3 (a) J. C. Collings, A. C. Parsons, L. Porres, A. Beeby, A. S. Batsanov,
J. A. K. Howard, D. P. Lydon, P. J. Low, I. J. S. Fairlamb and
T. B. Marder, Chem. Commun., 2005, 2666; (b) J. Kim, Pure Appl.
Chem., 2002, 74, 2031; (c) J. Kim and T. M. Swager, Nature, 2001,
411, 1030; (d) M. Levitus, K. Schmieder, Holly Ricks,
K. D. Shimizu, U. H. F. Bunz and M. A. Garcia-Garibay, J. Am.
Chem. Soc., 2001, 123, 4259; (e) P. K. Sudeep, P. V. James,
K. G. Thomas and P. V. Kamat, J. Phys. Chem., 2006, 110, 5642.
4 (a) G. Leising, S. Tasch, F. Meghdadi, L. Athouel, G. Froyer and
U. Scherf, Synth. Met., 1996, 81, 185; (b) S. Tasch, A. Niko,
G. Leising and U. Scherf, Appl. Phys. Lett., 1996, 68, 1090.
€
5 (a) G. Gigli, O. Inganas, M. Anni, M. De Vittorio, R. Cingolani,
G. Barbarella and L. Favaretto, Appl. Phys. Lett., 2001, 78, 1493;
(b) T. Otsubo, Y. Aso and K. Takimiya, Bull. Chem. Soc. Jap.,
2001, 74, 1789.
26 Bruker AXS, SAINT Software Reference Manual v. 6.23, Madison,
WI, 2002.
27 G. M. Sheldrick, SHELXL-97. Crystal Structure Refinement, Version
97-1, Institut Anorg. Chemie, University of Gottingen, Germany,
1997.
28 A. L. Spek, J. Appl. Cryst., 2003, 36, 7.
29 P. V. James, P. K. Sudeep, C. H. Suresh and K. G. Thomas, J. Phys.
Chem. A, 2006, 110, 4329.
30 A. Beeby, K. Findlay, P. J. Low and T. B. Marder, J. Am. Chem. Soc.,
2002, 124, 8280.
6 (a) G. Barbarella, M. Melucci and G. Sotgiu, Adv. Mater., 2005, 17,
1581; (b) L. De. Cole. Molecular Wires from Design to Properties,
Springer: Berlin, Germany, 2005.
7 (a) H. A. Al Attar, A. P. Monkman, M. Tavasli, S. Bettington and
M. R. Bryce, App. Phys. Lett., 2005, 86, 121101; (b) C. J. Tonzol,
A. P. Kulkarni, A. P. Gifford, W. Kaminsky and S. A. Jenekhe,
Adv. Funct. Mater., 2007, 17, 863.
8 (a) W. Ma, C. Yang, X. Gong, K. Lee and A. J. Heeger, Adv. Funct.
Mater., 2005, 15, 1617; (b) J. Y. Kim, K. Lee, N. E. Coates, D. Moses,
T. Q. Nguyen, M. Dante and A. J. Heeger, Science, 2007, 317, 222.
9 (a) Y. Kawabe, H. Ikeda, T. Sakai and K. Kawasaki, J. Mater. Chem.,
1992, 2, 1025; (b) A. Mathy, K. Ueberhofen, R. Schenk, H. Gregorius,
31 (a) Z. Xie, B. Yang, F. Li, G. Cheng, L. Liu, G. Yang, H. Xu, L. Ye,
M. Hanif, S. Liu, D. Ma and Y. Ma, J. Am. Chem. Soc., 2005, 127,
14152; (b) J. Cornil, D. A. dos Santos, X. Crispin, R. Silbey and
€
R. Garay, K. Mullen and C. Bubeck, Phys. Rev. B, 1996, 53, 4367.
ꢁ
J. L. Breedas, J. Am.Chem. Soc., 1998, 120, 1289; (c) J. Cornil,
D. Beljonne, J. P. Calbert and J. L. Breedas, Adv. Mater., 2001, 13,
1053.
10 (a) T. Mitsumori, I. M. Craig, I. B. Martini, B. J. Schwartz and
F. Wudl, Macromolecules, 2005, 38, 4698; (b) J. M. Hancock and
S. A. Jenekhe, Macromolecules, 2008, 41, 6864.
ꢁ
32 A. Mishra, R. K. Behera, P. K. Behera, B. K. Mishra and
G. B. Behera, Chem. Rev., 2000, 100, 1973.
33 E. G. McRae and M. Kasha, J. Chem. Phys., 1958, 28, 721.
34 A. Datta and S. K. Pati, Chem. Soc. Rev., 2006, 35, 1305.
35 J. Dong, K. M. Solntsev and L. M. Tolbert, J. Am. Chem. Soc., 2009,
131, 662.
11 (a) A. Salomon, D. Cahen, S. Lindsay, J. Tomfohr, V. B. Engelkes and
C. D. Frisbie, Adv. Mater., 2003, 15, 1881; (b) W. Haiss, C. Wang,
I. Grace, A. Batsanov, D. J. Schiffrin, S. J. Higgins, M. R. Bryce,
C. J. Lambert and R. J. Nichols, Nature Mater., 2006, 5, 995.
12 (a) S. Lakshmi and S. K. Pati, J. Chem. Phys., 2004, 121, 11998; (b)
J. M. Seminario, A. G. Zacarias and P. A Derosa, J. Phys. Chem.,
4406 | J. Mater. Chem., 2009, 19, 4401–4406
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