Highly fluorescent and electroactive molecular squares containing perylene bisimide ligands

Article abstract of DOI:10.1039/a909892e

Large functional molecular squares have been assembled with ditopic perylene bisimide bridging ligands and Pt(II) and Pd(II) phosphine corner units and their optical, electrochemical and spectroelectrochemical properties have been studied.

Full text of DOI:10.1039/a909892e

Highly fluorescent and electroactive molecular squares containing perylene
bisimide ligands
Frank Würthner* and Armin Sautter
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) 16th December 1999, Accepted 7th February 2000,
Published on the Web, 29th February 2000
Large functional molecular squares have been assembled
with ditopic perylene bisimide bridging ligands and Pt(II
)
and Pd(II) phosphine corner units and their optical, electro-
chemical and spectroelectrochemical properties have been
studied.
Interest in functional supramolecular assemblies1 has been
triggered by examples from nature like the light harvesting
system (LHS) in purple bacteria.2 The supramolecular function-
ality of this natural assembly is realized by chlorophyll
molecules organized in cyclic arrangements which control the
electronic coupling of the dye molecules and allow accom-
modation of the reaction center of photosynthesis within the
largest ring of LHS.2 A straightforward access to cyclic
structures in high yields was introduced by Fujita and Ogura3
and Stang and Olenyuk4 via the concept of molecular squares
using ditopic bridging ligands and cis-coordinating square
planar Pt(II) or Pd(II) corners. Some very interesting examples
have been reported based on pyridyl-substituted porphyrin
ligands,5 where functionality is introduced by the ligand but
porphyrin fluorescence is quenched upon metal complexation.
Here, we report on a new ligand based on perylene tetra-
carboxylic acid bisimides6,7 which exhibits fluorescence quan-
tum yields FF near 100%6 as well as redox activity8 and which
conserves these properties in the metal-assembled state: a giant
photo- and redox-active molecular square (metal–metal diago-
nal ca. 3.4 nm).
Scheme 1 Reagents and conditions: i, dpppM(OTf)2, CH2Cl2, 24 h,
20 °C.
2b has not, as yet, been characterized by the ESI–MS
technique.
The optical and electrochemical properties of the perylene
ligand 1 remain almost unchanged upon formation of the
complexes 2. We ascribe this to the fact that in perylene
tetracarboxylic acid bisimides, the imide substituents have only
a small electronic coupling to the HOMO and LUMO of the
perylene chromophore.8,11 Therefore, the chromophoric per-
ylene unit is largely decoupled of the binding site and only small
bathochromic shifts of Dl = 5–7 nm in the absorption and
emission spectra are observed upon complexation. The absorp-
tion maximum of 2a is located at lmax = 591 nm, the emission
occurs at 625 nm. An even more important consequence is
negligible quenching of the perylene fluorescence in these
complexes with FF (2a, CHCl3) = 0.88.9
The perylene ligand 1 was synthesized in 66% yield by
condensation of the corresponding perylene bisanhydride7 with
4-aminopyridine in quinoline with catalytic amounts of zinc(II
)
acetate. The ligand exhibits an intense absorption band at lmax
= 585 nm and red fluorescence at lmax = 618 nm with FF
0.94 for 1 in chloroform.9
=
Equimolar mixing of ditopic perylene ligand 1 with
dpppM(OTf)2 [M = Pt(II) or Pd(II), dppp = 1,3-bis(diphenyl-
phosphino)propane; OTf = CF3SO32] in CH2Cl2 leads ex-
clusively to the formation of molecular squares 2a,b in 90 and
95% isolated yield, respectively (Scheme 1). The squares were
The ligand 1 and platinum complex 2a exhibit reversible
electrochemistry which was investigated by cyclic voltammetry
in CH2Cl2. 1 displays two, reversible reductions at potentials of
21.08 and 21.23 V (vs. Fc/Fc+), but oxidation is irreversible
causing adsorption of 1 on the platinum electrode. The
molecular square 2a shows two reversible waves in the
reductive cycle at similar potentials (21.01, 21.14 V vs. Fc/
Fc+) but the oxidative cycle, in contrast to 1 shows a reversible
wave at +0.93 V, probably as a consequence of the pyridyl
nitrogen lone pairs which are now blocked by platinum-
complexation (Fig. 1). The electrochemical studies of the
palladium complex 2b revealed a more complicated behavior
displaying an irreversible reduction at E < 21.15 V which we
1
characterized by elemental analysis, H and 31P{1H} NMR
spectroscopy and electrospray mass spectrometry (ESI–MS)
(2a).† In the 31P{1H} NMR spectra of 2a,b only one singlet is
observed owing to the high symmetry of the squares, addition-
ally there are Pt-satellites resulting from 195Pt–31P spin–spin
coupling in the spectrum of 2a. A typical change in chemical
shifts of the phosphorus nuclei of ca. 10 ppm results upon
complexation with 1.10 The 1H NMR spectra show only a single
set of signals for the ligand 1 and the dppp moiety with
significant changes in the chemical shifts of the a- and b-
pyridyl protons (ca. 0.3 ppm) owing to metal complexation.10 In
the ESI–MS of 2a (acetone solution) we observed signals at m/z
2575.0 which we assign to the M23OTf species with +3 charge
state (separation of peaks by 1/3 mass unit) which corresponds
to a molecular mass of 7725 for the square minus three triflate
anions in excellent agreement with the calculated mass of
7724.89 (C409H360N16O47F15P8S5Pt4). Furthermore, we ob-
served signals at m/z 1894, 1485 and 1213 which we assign to
(M24OTf)4+, (M25OTf)5+ and (M26OTf)6+ species (THF–
ethyl acetate solution). The less stable corresponding Pd-square
attribute to electron transfer processes involving the Pd(II
)
corners.
A closer insight into the electrochemical processes was
gained by spectroelectrochemical investigations of 2a and 1. By
increasing the potential in a stepwise manner to the first
reduction wave of 2a (21.01 V) the absorption bands of the
perylene bisimide chromophores at 591 nm decrease while three
bands in the UV–VIS–NIR appear with a maximum absorption
at 791 nm [Fig. 2(a)]. Based on identical spectroscopic changes
observed for the free perylene bisimide ligand 1, we assign these
optical transitions to perylene bisimide radical anionic species.
After a further increase of the potential to the second reduction
DOI: 10.1039/a909892e
Chem. Commun., 2000, 445–446
This journal is © The Royal Society of Chemistry 2000
445

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optical and electrical properties of the ligands are conserved in
the metal-assembled state 2a which shows a fluorescence
quantum yield of almost unity and multiple, fully reversible
redox couples. The giant inner volume of nanosquares 2 seems
ideal to accommodate large functional guest molecules for
studies on redox and photocatalytic processes.
We thank Dr Peter Weber (Scripps, La Jolla) for ESI mass
spectrometry and helpful discussions. We gratefully acknowl-
edge the Fonds der Chemischen Industrie, the BMBF, the DFG
and the Ulmer Universitätsgesellschaft for financial support, the
BASF AG and Degussa-Hüls AG for the donation of chem-
icals.
Fig. 1 Cyclic voltammogram of perylene–platinum square 2a in CH2Cl2;
scan rate 100 mV s–1. Working electrode: Pt disc, auxiliary electrode: Pt
wire, reference electrode: Ag/AgCl; concentration 2a = 2.5 3 10–4 M;
supporting electrolyte NBu4PF6 (0.1 M).
Notes and references
† Selected data for 2a: mp > 330 °C. 1H NMR (CDCl3, 500 MHz), d 9.13
(br s, 16H), 8.12 (s, 16H), 7.69 (br s, 32H), 7.40 (m, 32H), 7.32 (m, 16H),
7.20 (d, 32H, J 8.4 Hz), 7.09 (d, 16H, J 5.9 Hz), 6.77 (d, 32H, J 8.4 Hz), 3.29
(br s, 16H), 2.20 br s, 8H), 1.24 (s, 144H). 31P{1H} NMR (CDCl3, 85%
H3PO4, 202 MHz), d 215.11 (s). UV–VIS(CH2Cl2), l (e) 591 (217000),
550 (135000), 459 nm (72000 dm3 mol21 cm21). lem, max(CH2Cl2) 625 nm.
MS (ESI, acetone) Calc. (M23OTf)3+ m/z 2575.7. Found 2575.0. Found: C,
59.29; H, 4.59; N, 2.54; S, 3.03. C412H360N16O56F24P8Pt4S8·8H2O requires
C, 59.50; H, 4.56; N, 2.69; S, 3.08%.
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Fig. 2 Spectroelectrograms of perylene–platinum square 2a in CH2Cl2 with
NBu4PF6 (0.1 M) as supporting electrolyte. Working electrode: Pt disc;
auxiliary electrode: Pt wire; reference electrode: Ag/AgCl; concentration 2a
= 1.4 3 1024 M. Stepwise increase of the applied potential to radical
anionic perylene species (a) and to dianionic perylene species (b).
wave of 2a (21.14 V) the perylene radical anion bands
disappear completely whereas a broad and intense band
appears, with a maximum absorption at 679 nm [Fig. 2(b)],
corresponding to dianionic perylene bisimides. Oxidation to the
radical cationic state results in less pronounced spectral changes
in the visible but new broad bands appear in the NIR at 809, 908
and 1225 nm. The distinct isosbestic points in the absorption
spectra for all redox couples confirm the reversibility of all
oxidation and reduction processes. Therefore, we conclude that
(i) the redox processes take place exclusively at the perylene
ligands with the Pt(II) corner units being inert in the applied
potential range and acting solely as structural building blocks,
(ii) the perylene ligands assembled in the square do not interact
with one another even in a charged state, and (iii) each perylene
ligand in the square is reduced and oxidized at the same time at
the given potentials.12
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phenoxy-N,NA-bis(2,6-diisopropylphenyl)perylene-3,4,9,10-tetracar-
boxylic acid bisimide (FR = 0.96, CHCl3)6 by the optically dilute
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12 This conclusion is drawn from spectroelectrochemistry which shows
complete disappearance of the bands of the neutral and the radical-
anionic perylene species at the respective redox potentials.
In summary, we report on a new functional ditopic perylene
ligand 1 which has been used to construct nanosized molecular
squares 2 with Pt(II) and Pd(II) phosphine corner units. The
Communication a909892e
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Chem. Commun., 2000, 445–446