Amphiphilic conjugated thiophenes for self-assembling antenna systems in water

Article abstract of DOI:10.1039/b823268g

Newly developed conjugated terthiophene surfactants are able to aggregate in water and to act as a host for hydrophobic chromophores, creating a multiple donor-acceptor energy transfer (ET) system by self-assembly.

Full text of DOI:10.1039/b823268g

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Amphiphilic conjugated thiophenes for self-assembling antenna
systems in waterwz
Patrick van Rijn,a Tom J. Savenije,b Marc C. A. Stuartc and Jan H. van Esch*a
Received (in Cambridge, UK) 24th December 2008, Accepted 11th February 2009
First published as an Advance Article on the web 6th March 2009
DOI: 10.1039/b823268g
Newly developed conjugated terthiophene surfactants are able to
aggregate in water and to act as a host for hydrophobic
chromophores, creating a multiple donor–acceptor energy trans-
fer (ET) system by self-assembly.
which in water self-assemble into micelles, and can be turned
into an ET-system by incorporation of suitable hydrophobic
acceptor molecules into the hydrophobic micellar interior.
A variety of hydrophobic acceptor molecules can be hosted
in the hydrophobic micellar core, leading to stable water-
soluble self-assembled ET-systems solely composed of donor
and acceptor molecules (Fig. 1). In the case of Nile Red as an
acceptor, the system displays an antenna effect and overall a
very efficient ET within the assemblies.
The transfer of excitation energy in the light harvesting
complexes of the photosynthetic system is one of the most
important physical processes for life.1 In the light harvesting
complexes multiple donor molecules like chlorophyll absorb
light and transfer the excitation energy to a central acceptor
molecule where it is further processed to form a charge
separated state.2 Over the past decade a variety of interesting
model systems have been designed and studied to mimic the
natural light harvesting antenna system and gain insight into
the underlying physical principles.3–7 For instance, dendrimers
consisting of several donors covalently attached to a central
acceptor core have been found to display the antennae effect3
which is one of the key features that make natural light
harvesting systems so successful. Even though this covalent
strategy has produced a number of interesting systems4,5
changing the donors and/or acceptor in such covalent systems
remains cumbersome.
Amphiphilic polythiophenes reported before are based
on an alternating sequence of hydrophilic and hydrophobic
substituted thiophenes, leading to lamellar structures at the
air/water interface and in Langmuir–Blodgett multilayers.9
It was expected that shorter oligo-thiophenes form more
dynamic and small water-soluble aggregates. We synthesized
a conjugated (1) and cross-conjugated (2) amphiphilic terthio-
phene by step-wise Stille couplings of two thiophenes with
hydrophilic tetraethylene glycol tails to the central thiophene
bearing a hydrophobic hexadecane chain. By slight variation
of the structures, we anticipate to vary the type of aggregate as
well as the spectroscopic properties. Amphiphilic thiophenes 1
and 2 have been obtained by a multistep synthetic procedure
in good yields, and have been fully characterized (see ESIz).
Terthiophenes 1 and 2 are soluble in water up to concentra-
tions of at least 50 mM to give transparent yellow solutions.
Turbidity measurements revealed that the surfactants have
cloud points of 23 ꢀ 1 1C and 38 ꢀ 2 1C for compounds 1 and
2, respectively, which are typical for non-ionic oligoethylene
The self-assembly of donors and acceptors is an attractive
alternative to covalent approaches because of its versatility
and flexibility, and moreover, also the natural light harvesting
systems are formed completely by self-assembly. Several ex-
amples of self-assembled multiple donor–acceptor systems
have been reported,6,7 however, these systems are either still
based on covalently connected donors or display an antenna
effect only in some cases.8 Therefore, the challenge remains to
create light harvesting antenna systems that are completely
formed through self-assembly of multiple donors and an
acceptor in water.
In our pursuit to develop water-soluble conjugated oligo-
mers, we discovered a fully self-assembling antenna-ET-
system. Here we report on amphiphilic conjugated thiophenes
a Self Assembling Systems, Faculty of Applied Sciences, Delft
University of Technology, Julianalaan 136, 2628 BL Delft,
The Netherlands. E-mail: j.h.vanesch@tudelft.nl;
Tel: þ31 15 2782682
b Opto-electronic materials, Faculty of Applied Sciences, Delft
University of Technology, Julianalaan 136, 2628 BL Delft,
The Netherlands
c Electron microscopy, Groningen Biomolecular Sciences and
Biotechnology Institute, University of Groningen, Nijenborgh, 4,
9747 AG Groningen, The Netherlands
w This communication is dedicated to Prof. Dr Roeland J. M. Nolte, a
master of supramolecular chemistry.
z Electronic supplementary information (ESI) available: Experimental
details, absorption and emission spectra, NOE 1H-NMR, LC-MS. See
DOI: 10.1039/b823268g
Fig. 1 Schematic representation of the ET-system that is formed
from the donor (D) thiophene amphiphiles (1 and 2) and the hydro-
phobic acceptor (A).
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based surfactants.10 Surface tension measurements confirmed
their amphiphilic character, and the critical micelle concentra-
tions (CMC) for isomers 1 and 2 were reached at 1.0 ꢀ 0.1 mM
for both. The surface tension plots show for both 1 and 2 a
pretransition at 0.1 mM, however, no changes in absorbance
or fluorescence spectra occur around this concentration
(vide infra). Most likely, this pretransition is due to the
formation of premicellar aggregates of 1 and 2, pointing to a
less cooperative association process. Dynamic light scattering
confirmed the formation of micellar aggregates above 1.0 mM
(20 1C) with diameters of 6 ꢀ 2 nm for 1 and 21 ꢀ 5 nm for 2.
Cryo transmission electron microscopy experiments indicated
the formation of spherical micellar and cylindrical micellar
morphologies formed by 1 and 2, respectively.
entrapment of the acceptor in the hydrophobic core. Suitable
hydrophobic acceptor molecules are for instance tetraphenyl-
porphyrin (TPP) and Nile Red, because their absorption
spectra (lmax(TPP)
¼
421 nm, and
lmax(Nile Red)
¼
550 nm) nicely overlap with the emission spectra of 1 and 2
(Fig. 2; ESI, SI9z). Nile Red is a well-known fluorescence
probe for hydrophobic micro-environments.14 Addition of
Nile Red to an aqueous solution of 1 or 2 leads to a
pronounced blue shift and an increase in the intensity of the
Nile Red emission. The emission maxima of Nile Red above
the CMC of 1 and 2 are comparable to the emission found
when Nile Red is solubilized in tert-butanol, indicating the
formation of hydrophobic domains in micelles of 1 and 2 in
which Nile Red resides. The dependency of the Nile Red
emission on the concentrations of 1 and 2 again showed two
transitions (ESI, SI4, SI7z). One transition occurs at a high
concentration of 1.0 mM for 1 and 2 which is in excellent
agreement with the CMC. The other transition is observed at a
markedly lower concentration of approximately 0.01 mM for
both 1 and 2. Most likely, this transition is due to the
formation of premicellar aggregates which can be initiated
by hydrophobic probes like Nile Red. Also, the otherwise
water-insoluble TPP can be solubilized in water by 1 or 2
above the CMC up to a molar ratio of 1 : 4 (TPP : 1 or 2).
The terthiophene isomers described here have spectroscopic
properties similar to most other terthiophenes.11 The thio-
phene amphiphiles have absorption maxima at 350 and 330
nm, and emission maxima at 455 nm and 470 nm for 1 and 2,
respectively. For these and similar compounds it is known that
aggregation can affect the electronic properties of chromo-
phores due to electronic coupling between transition dipole
moments.12 The absorption and emission maxima of thio-
phene amphiphiles 1 and 2 do not shift and the intensities
increase linearly with the concentration up to 1.0 mM.
However, above 1.0 mM the absorption spectra of isomer 1
do not follow the Lambert–Beer law, whereas the emission
intensities of both 1 and 2 deviate from linearity with
increasing concentration, indicating that self-assembly occurs.
The above data agree with the formation of micellar aggre-
gates of 1 and 2 in which there is a small electronic coupling
between the terthiophene groups at concentrations above
1.0 mM, whereas the pretransition at 0.1 mM should be
ascribed to the formation of small pre-micellar aggregates with
no significant electronic interactions between the chromo-
phores. The optical properties of the micelles are summarized
in Table 1. Observed lifetimes, and quantum yields are close to
previously published data on terthiophene analogues.13
Micelles of 1 and 2 are in fact aqueous assemblies of
multiple chromophores, and therefore they are of potential
interest for the construction of self-assembled ET- and
antenna systems. A straightforward approach towards such
systems would be co-assembly with an acceptor molecule by
Table
in water
1
Photophysical properties of thiophenes and mixtures
Donor (D)
tD/psb
kF/109 sꢂ1
kNR/109 sꢂ1
c
jF
Isomer 1
Isomer 2
95
125
0.10
0.07
1.1
0.56
9.5
7.4
D/A mix (20 : 1)
w(IDA/ID)
kET/1010 sꢂ1a
jET
1 þ Nile Red
1 þ TPP
0.25
0.34
0.39
0.13
3.2
2.0
1.3
8.9
0.75
0.66
0.61
0.92
Fig. 2 Normalized abs. (solid) and em. (dashed) of isomer 1 (black)
(lmax ¼ lexc: 350 nm, lem: 455 nm) in water and TPP (grey) (lmax: 421 nm,
2 þ Nile Red
l
em: 655 nm) in THF, depicting the overlap integral (A); normalized
2 þ TPP
abs. (solid) and em. (dashed) of isomer 1 (black) (lmax ¼ lexc: 350 nm,
a
kET was determined using 1/(wtD) ꢂ 1/(tD) where w is the ratio in
lem: 455 nm) in water and Nile Red (grey) (lmax: 550 nm, lem: 632 nm)
emission of D/A (1 mM : 50 mM) and D (lex: 350 nm and lem: 455 nm
in EtOH, depicting the overlap integral (B); normalized em. of micellar
solutions of isomer 1 (solid black) (1.0 mM), (lem: 455 nm) without
and with 50 mM acceptor (Nile Red (dashed black), lem: 618 nm; TPP
(dashed grey), lem: 655 nm) (C).
b
for isomer 1; lex: 330 nm and lem: 480 nm for isomer 2). Lifetimes tD
were measured (lex: 407 nm) (see ESI, SI11). The quantum yield
(jF) was determined by using 9,10-diphenylanthracene as a reference.
c
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Apparently, TPP can efficiently be accommodated by isomers
1 and 2 in their hydrophobic micro-environments, which
directly place donor and acceptor in close proximity.
The authors thank Aurelie Brizard and Wesley Browne for
helpful discussions.
It was found that when 1 or 2 were combined with the
hydrophobic chromophores as acceptors an ET-system in
water could be formed, based solely on self-assembly of
donors and acceptor due to hydrophobic interactions. To
1.0 mM solutions of amphiphilic thiophenes 1 or 2, different
quantities of TPP or Nile Red were added to give acceptor
concentrations of 0 to 250 mM. It was observed that the
emissions of 1 and 2 are increasingly quenched by the addition
of increasing amounts of TPP as indicated by w (see Table 1),
until a plateau is reached at a molar ratio of 1 or 2 to TPP of
about 20 : 1 (ESI, SI6, SI10z). The quantum efficiencies are
66% and 92% for combinations 1/TTP and 2/TPP, respec-
tively. Simultaneously an increase in the emission intensity of
TPP is observed. These results indicate that there is efficient
ET from the thiophenes to the porphyrin. The large overlap of
the excitation spectrum with the absorption spectra of the
thiophenes, and the similarity of the TPP emission maxima by
direct excitation of TPP or via the thiophenes, confirm this
conclusion and exclude the formation of excimers and ground
state interaction of 1 or 2 with TPP.
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In conclusion, we successfully developed conjugated terthio-
phene surfactants that aggregate into micellar type assemblies
at low concentrations. These micelles can act as a host for
hydrophobic chromophores, in which efficient ET can take
place between the donor aggregate and the acceptor. This
approach gives access to new antenna systems which are
completely formed by self-assembly of small molecular
components and can easily be modified by varying the hydro-
phobic acceptor.
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This work was supported by Nanoned/STW and the
Netherlands Organization for Scientific Research (NWO).
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Chem. Commun., 2009, 2163–2165 | 2165