Photoluminescent supramolecular polymers: Metal-ion directed polymerization of terpyridine-functionalized perylene bisimide dyes

Article abstract of DOI:10.1039/b205478g

Perylene bisimide dyes bearing one and two 2,2′:6′,2″-terpyridine receptor groups were prepared and the structural and optical properties of their Zn²⁺ complexes were investigated by ¹H NMR and fluorescence experiments.

Full text of DOI:10.1039/b205478g

Photoluminescent supramolecular polymers: metal-ion directed
polymerization of terpyridine-functionalized perylene bisimide dyes
Rainer Dobrawa and Frank Würthner*
Abteilung Organische Chemie II, Universität Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany.
E-mail: frank.wuerthner@chemie.uni-ulm.de
Received (in Cambridge, UK) 7th June 2002, Accepted 15th July 2002
First published as an Advance Article on the web 25th July 2002
Perylene bisimide dyes bearing one and two 2,2A+6A,2B-
terpyridine receptor groups were prepared and the struc-
tural and optical properties of their Zn2+ complexes were
investigated by 1H NMR and fluorescence experiments.
quantified by integration of the NMR signals. The titration
curve in Fig. 2 shows the concentrations of the different species
during the titration experiment. Interestingly, the addition of an
excess amount of zinc triflate gives rise to a second process in
which complex 2 is dissociated again in favour of an open form
with the zinc ion only attached to one tpy-unit and its other
coordination sites are presumably saturated by solvent mole-
cules. Both processes, the formation of complex 2 and its
decomposition, are therefore slow on the NMR timescale but
fast on the laboratory timescale and fully reversible.
The same NMR titration studies were carried out with the
ditopic ligand 3. The spectra show analogous changes of the
chemical shifts upon complexation and the disappearance of
ligand signals exactly at a 1+1 stoichiometry. In addition, line
broadening is observed as is expected for NMR signals of
polymers (Fig. 1). From these data the formation of coordina-
Photoluminescent polymers have received significant interest in
the last decade especially in the fields of organic light emitting
diodes1 and plastic solar cells.2 Introduction of metal ions into
conjugated polymers has been investigated with regard to
higher electroluminescence quantum yields by harvesting
triplet state emission.3 However the purification and processing
of such polymers is a difficult task. Therefore new concepts
such as supramolecular polymerization4 of photoactive building
blocks might be of interest. Here we report on the metal-directed
polymerization of highly fluorescent perylene bisimide dyes
provided with terpyridine receptor units and a dimeric assembly
which proved to be useful to elucidate the impact of the metal
ion on the photophysical properties of the perylene dye.
The highly soluble bis-2,2A:6A,2B-terpyridine (tpy)-function-
alized perylene bisimide dye 3 was synthesized by condensation
of 4A-aminophenyl-tpy5 and tetra(p-tert-butylphenoxy)perylene
bisanhydride in quinoline with zinc acetate as catalyst in 50%
yield (Scheme 1). The monotopic compound 1 is available
under the same conditions in 40% yield using the corresponding
perylene imide anhydride.6 Both dyes exhibit strong absorption
in the red region (lmax = 583 nm, e = 55000 dm3 mol21 cm21
in CH2Cl2) and intense fluorescence (lmax = 620 nm, Ffl
0.92 in CH2Cl2).†
=
The complexation of the monotopic dye 1 with an octahed-
rally coordinating metal ion yields a dimeric complex which can
be seen as a model reaction for the formation of the coordination
polymer. Complexation can be achieved with zinc(II) tri-
fluoromethanesulfonate (triflate) in a mixture of chloroform and
acetonitrile and can be easily followed in an 1H NMR titration
experiment from the characteristic change of the chemical shift
values d of the tpy-signals upon complexation.7 The spectra
clearly show the formation of the dimeric complex 2 and the
disappearance of the uncomplexed ligand at a dye+Zn2+ ratio of
2+1. As illustrated in Fig. 1 no dynamical processes appear but
all species can be distinguished and their concentrations can be
Fig. 1 NMR titration of dye 1 with zinc triflate in CDCl3–CD3CN (80+20)
with the corresponding dye+Zn2+ ratios; bottom spectrum: NMR spectrum
of polymer 4 showing analogous chemical shifts d as the dimer.
Scheme 1 Complexation of terpyridine functionalized perylene bisimide dyes. Reagents: i) Zn(CF3SO3)2, CHCl3–CH3CN (80+20), Ar = p-tert-
butylphenyl.
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Fig. 2 Concentration of uncomplexed ligand 1 (-), dimer 2 (5) and the
open form (:) upon addition of Zn(CF3SO3)2 to a 5.5 mM solution of 1 in
CDCl3–CD3CN (80:20) calculated from integrated 1H NMR signals.
Fig. 3 Fluorescence titration experiment of 1 (1025 M in CHCl3–CH3OH
(60+40), lexc = 550 nm) with iron(II) perchlorate; inset: fluorescence
quantum yields upon addition of zinc(II) triflate (5) and iron(II) perchlorate
(-) calculated from the integrated fluorescence spectra.
tion polymer 4 can be concluded. Owing to the extremely rigid
structure of both the perylene bisimide and the metal–tpy-
complex unit a rigid rod-like structure is expected for polymer
4. After precipitation from CHCl3–CH3CN the polymer can
yield of only 0.04 in CHCl3. The complexation can be followed
by a fluorescence titration experiment which shows only a
marginal change in the case of zinc(II) but a significant decrease
for iron(II) (Fig. 3). This result shows that by application of
different metal ions the M(tpy)2-unit can be changed from a
purely structural to a functional unit which opens up interesting
possibilities with regard to the investigation of directional
electron or energy transfer processes within the rigid-rod
polymers. Incorporation of functional metals such as Ru2+ and
Os2+ might be especially appealing.8
In summary, two new perylene bisimide dyes bearing
terpyridine receptor units have been synthesized and assembled
to fluorescent and non-fluorescent dimeric and polymeric
structures by metal ion coordination. It is anticipated that
incorporation of other metal ions in these polymers will lead to
interesting multinuclear assemblies with new photophysical
properties.
1
only be redissolved in DMF. Nevertheless, H NMR spectra
recorded in deuterated DMF prove that the polymeric structure
remains intact also in polar solvents. As for the dimer, evidence
for the reversibility of the tpy-complexation by Zn2+ ions is
given for the polymer, since exceeding the 1+1 stoichiometry of
zinc ions leads to the formation of the open zinc complex and
results in a degradation of the polymeric strands into small
monomeric and oligomeric fragments.
Although the coordination of zinc(II) metal ions to the tpy-
units of 1 and 3 is strong there is little electronic interaction
between the Zn(tpy)22+ unit and the perylene bisimide chromo-
phore according to UV/Vis and fluorescence spectroscopy.
Thus the UV/Vis spectra show almost unchanged absorption
properties of the chromophore upon dimerization (2) and
polymerization (4) at wavelengths l > 350 nm. At shorter
wavelengths, i.e. 250–350 nm, a small increase in the
absorption attributed to tpy-complexation can be observed.
Table 1 shows the fluorescence quantum yields determined in
chloroform and DMF. The intense fluorescence of the ligand is
preserved in the dimer and only decreased by 20% for the
polymer (which could only be investigated in DMF for
solubility reasons).
Notes and references
† The synthesis of these compounds will be published elsewhere. All
1
compounds have been characterized by H NMR, mass spectrometry and
elemental analysis.
1 R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks,
C. Taliani, D. D. C. Bradley, D. A. Dos Santos, J. L. Brédas, M. Lögdlund
and W. R. Salaneck, Nature, 1999, 397, 121.
2 C. J. Brabec, N. S. Sariciftci and J. C. Hummelen, Adv. Funct. Mater.,
2001, 11, 15.
Additional functional possibilities for such photoluminescent
supramolecular polymers are exemplified by an experiment in
which the zinc(II) metal salt is replaced by iron(II) perchlorate
hexahydrate. Here NMR titrations indicate that the complex
formation is not reversible at r.t. so that no decomposition of the
respective Fe(tpy)22+ complexes takes place in the presence of
excess iron(II). Even more importantly, the perylene bisimide
fluorescence is drastically quenched to a fluorescence quantum
3 J. S. Wilson, A. S. Dhoot, A. J. A. B. Seeley, M. S. Khan, A. Köhler and
R. H. Friend, Nature, 2001, 413, 828.
4 For reviews on metal containing supramolecular polymers see: L.
Brunsveld, B. J. B. Folmer, E. W. Meijer and R. P. Sijbesma, Chem. Rev.,
2001, 101, 4071; M. Rehahn, Acta Polym., 1998, 49, 201.
5 G. D. Storrier, S. B. Colbran and D. C. Craig, J. Chem. Soc., Dalton
Trans., 1997, 3011.
6 F. Würthner, A. Sautter, D. Schmid and P. J. A. Weber, Chem. Eur. J.,
2001, 7, 894.
Table 1 Fluorescence quantum yields of ligand 1, dimer 2 and polymer 4
7 R. P. Thummel and Y. Jahng, Inorg. Chem., 1986, 25, 2527; E. C.
Constable and A. M. W. Cargill Thompson, J. Chem. Soc., Dalton Trans.,
1994, 1409.
8 A. Harriman and R. Ziessel, Chem. Commun., 1996, 1707; J.-P. Sauvage,
J.-P. Collin, J.-C. Chambron, S. Guillerez, C. Coudret, V. Balzani, F.
Barigelletti, L. De Cola and L. Flamigni, Chem. Rev., 1994, 94, 993.
Ffl
CHCl3
DMF
Ligand 1
Dimer 2
Polymer 4
0.92
0.90
—
0.75
0.74
0.61
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