Anion induced modulation of self-assembly and optical properties in urea end-capped oligo(p-phenylenevinylene)s

Article abstract of DOI:10.1039/b413909g

The non-emissive supramolecular assembly of urea end-capped oligo(p-phenylenevinylene) flourophores turned strongly emissive in the presence of tetraburylammonium flouride which has implications in the anion controlled design of supramolecular architectur

Full text of DOI:10.1039/b413909g

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Anion induced modulation of self-assembly and optical properties in
urea end-capped oligo(p-phenylenevinylene)s{
Reji Varghese, Subi J. George and Ayyappanpillai Ajayaghosh*
Received (in Cambridge, UK) 10th September 2004, Accepted 22nd October 2004
First published as an Advance Article on the web 8th December 2004
DOI: 10.1039/b413909g
chloroform (1 6 1025 M) showed a strong emission maximum
The non-emissive supramolecular assembly of urea end-capped
at 454 nm with a shoulder at 487 nm (Wf 5 0.35) (Fig. 1(b)).
However in cyclohexane, the fluorescence of BU-OPV1 is
significantly quenched (Wf 5 0.02) indicating strong intermolecular
interactions between OPVs which is a thermoreversible process as
revealed by temperature dependent emission studies (Fig. 1(b),
inset). The BU-OPV1 aggregates remain stable up to a dilution of
1027 M and up to 60 uC as indicated in the absorption and
emission changes. Based on the above observations it is clear that
BU-OPV1 forms a stable one-dimensional assembly by the
cooperative interaction of H-bonding between the urea groups
and p-stacking between the aromatic moieties.4
oligo(p-phenylenevinylene) flourophores turned strongly emis-
sive in the presence of tetrabutylammonium flouride which has
implications in the anion controlled design of supramolecular
architectures with tunable emission properties.
Design of functional supramolecular architectures is one of the
main goals of interdisciplinary research in chemistry, biology and
material science. Toward this end, control on the supramolecular
assemblies of oligo(phenylenevinylene)s (OPVs) is at the centre
stage of research.1,2 Recently, we have reported the gelation of
OPVs and the resultant modulation of optical properties for the
purpose of selective light harvesting.3 Here we report on the
construction of supramolecular stacks of bisurea end-capped
OPVs and their anion induced destruction leading to a remarkable
modulation of the emission behavior thereby opening the avenue
for the anion controlled modulation of supramolecular architec-
tures with tunable optical properties.
The bisurea tethered OPVs, BU-OPV1 and BU-OPV2, have
been synthesized by a multistep synthetic strategy adopting
standard procedures and characterized by spectral analyses
(see ESI{). The UV/Vis spectra of BU-OPV1 in chloroform (1 6
1025 M, e 5 6.22 6 104 M21 cm21) showed characteristics of
molecularly dissolved OPVs indicating that they are not aggre-
gated in chloroform (Fig. 1(a)). However, in non-polar solvents
such as cyclohexane, the UV/Vis spectrum of BU-OPV1 (1 6
1025 M) is broad with an absorption maximum at 410 nm
(e 5 9.59 6 103 M21 cm21) with a shoulder band at 460 nm
(e 5 5.4 6 103 M21 cm21), characteristic of self-assembled OPV
chromophores. The fluorescence spectrum of BU-OPV1 in
Fig. 1 (a) Absorption and (b) emission spectra of BU-OPV1 (1 61025 M)
…
in cyclohexane ( ) (Wf 5 0.02) and in chloroform (—) (Wf 5 0.35).
{ Electronic supplementary information (ESI) available: Spectrocopic
data, Benesi–Hildebrand plots, synthesis and characterization of BUOPVs
and experimental procedures. See http://www.rsc.org/suppdata/cc/b4/
b413909g/
The emission spectrum in cyclohexane is zoomed 56 to make it
visible. Inset: Changes in the emission (lex 5 380 nm) spectra in
cyclohexane with increasing temperature.
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Fig. 4 Time resolved fluorescence decay profiles of BU-OPV1 in
chloroform ($), cylohexane–chloroform (16:1) (m) and after the addition
of 30 equivalents of TBAF (&).
to the concentration of BU-OPV1 (1.1 6 1025 M) resulted in a
14-fold fluorescence enhancement, with saturation being observed
after the addition of ca. 32 equivalents.
The effect of F2 on BU-OPV1 was visible by the naked eye due
to the strong fluorescence upon illumination with a UV lamp
(Fig. 2(a), inset). However, for chloride, bromide and iodide ions,
the enhancement in fluorescence is relatively weak, particularly for
iodide ions. This is clear from plots of the emission changes at
468 nm as a function of the salt concentration, which show higher
selectivity for F2 over other halide ions, especially at low
concentrations of halide anions (10–20 mM) (Fig. 2(b)). The
selectivity of the BU-OPV1 self-assembly to anions was found to
be in the order F2 . Cl2 . Br2 . I2 which is rationalized on the
basis of anion basicity. Fluoride ions being more basic than the
other halide ions, will form strong hydrogen bonded complexes
with the BU-OPV1 self-assembly. BU-OPV2 is found to exhibit a
similar selectivity for fluoride ions.
Fig. 2 (a) Changes in the emission spectra of BU-OPV1 (1.1 6 1025 M)
in cyclohexane–chloroform (16:1) upon the addition of TBAF (2.2 6
1026–3.96 6 1024 M). Inset: photograph under UV light showing strong
emission of BU-OPV1 in the presence of TBAF. Emission of BU-OPV1
(left) and emission of the BU-OPV1?TBAF complex (right). (b)
Fluorescence intensity of BU-OPV1 (1.1 6 1025 M) at 468 nm upon
titration with 0–2 equivalents of quaternary ammonium halides (TBAX);
TBAF (&), TBAC (.), TBAB ($), TBAI (m).
The observed quenching of the emission in cyclohexane and the
anion induced recovery as shown in Fig. 2 is rationalized on the
basis of the one-dimensional stacking and destacking of the OPVs
as shown in Fig. 3. At the initial stages of addition, the
fluorescence titration curve could be fitted well to a 1:1 binding
isotherm, indicating the formation of a 1:1 complex (see ESI{).
The persistence of the vibronic shoulder of the self-assembled
species at 460 nm and the resemblance of the excitation spectrum
of BU-OPV1 with the absorption spectrum after the addition of 2
equivalents of TBAF suggest strong chromophore interactions in
the resulting 1:1 complexes. The Benesi–Hildebrand plot of the
Since halide anions are known to compete with urea H-bonds,5,6
we were keen to know whether anions can induce the breaking of
the H-bond and p-stack assisted self-assembly of BU-OPV1 and
BU-OPV2 thereby reinstating the strong emission of the OPV
units. Fig. 2(a) shows the fluorescence enhancement observed
when the BU-OPV1 self-assembly is titrated with increasing
amounts of tetrabutylammonium fluoride (TBAF). A gradual
increase in the TBAF concentration up to 28 equivalents relative
Fig. 3 Interaction of F2 with the BU-OPV1 self-assembly which justifies the enhancement in fluorescence.
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change in the emission intensity against the reciprocal of the TBAF
concentration gave a linear fit, characteristic of a 1:1 complexation
from which the association constants is estimated as 2.8 6
104 M21. However, an increase of TBAF concentration beyond 2
equivalents results in a dramatic increase in the fluorescence and
the Benesi–Hildebrand plots beyond 2 equivalents could be fitted
to the binding isotherm of a 1:2 stoichiometry, which gave a
binding constant of 5.1 6 107 M22. At this situation, a gradual
increase in the absorbance at 410 nm with the disappearance of the
shoulder band corresponding to the self-assembly at 460 nm was
observed in the UV/Vis spectra indicating a complete breaking of
the OPV stacks (see ESI{). Thus, with the increase of TBAF
concentration, the 1:1 stoichiometry of the sandwich complex
of India for research fellowships. This is contribution number
RRLT-PPD-192 from RRL, Trivandrum.
Reji Varghese, Subi J. George and Ayyappanpillai Ajayaghosh*
Photosciences and Photonics Division, Regional Research Laboratory,
CSIR, Trivandrum, 695019, India
Notes and references
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gradually changes to
a 1:2 stoichiometry with no inter-
chromophore interactions (Fig. 3(c)), thereby exhibiting a high
quantum yield of emission (Wf 5 0.32).
Time-resolved single photon counting experiments (Fig. 4) of
BU-OPV1 in chloroform showed a single exponential decay with a
lifetime of 1.32 ns (x2 5 1.03) corresponding to the non-assembled
BU-OPV1. In cyclohexane, a multiexponential fit (x2 5 1.11)
corresponding to lifetimes of 0.19 ns (36.89%), 0.67 ns (41.64%)
and a relatively long-lived species (1.96 ns, 21.47%) could be
obtained, indicating the presence of self-assembled species. Upon
addition of 30 equivalents of TBAF to BU-OPV1 the behavior
was best reproduced by a biexponential fit (x2 5 1.1) in which the
major fraction corresponds to a lifetime of 0.66 ns (82.43%) with a
minor contribution of a longer-lived species corresponding to
1.79 ns (17.59%). These observations clearly show that F2 ions
enter into a competitive H-bond interaction with the urea groups,
thereby disrupting the supramolecular stacks of the BU-OPV1 and
thus restoring the strong emission.
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In conclusion, we have demonstrated a novel supramolecular
approach to ‘‘turn off’’ and ‘‘turn on’’ the emission of OPV
self-assemblies, taking advantage of the competitive H-bonding
interaction of halide ions with urea groups. We are currently
exploring the possibilities of using this idea in controlling
the morphology and optical properties of OPV based
nanoarchitectures.
ˆ
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This work is supported by the Department of Science and
Technology (DST), Government of India, New Delhi (SF/C6/99-
2000 and SR/S5/OC-31/2003). R. V. and S. J. G. thank the
Council of Scientific and Industrial Research (CSIR), Government
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