Redox-switchable polyester dendrimers incorporating both π-donor (tetrathiafulvalene) and π-acceptor (anthraquinone) groups

Article abstract of DOI:10.1039/a807062h

The convergent synthesis of (TTF)(x)(AQ)(y) (TTF = tetrathiafulvalene; AQ = anthraquinone) polyester dendrimers is reported; the molecules undergo clean amphoteric redox behaviour with reversible switching between cationic and anionic states being achieve

Full text of DOI:10.1039/a807062h

Redox-switchable polyester dendrimers incorporating both p-donor
(tetrathiafulvalene) and p-acceptor (anthraquinone) groups
Martin R. Bryce,* Pilar de Miguel and Wayne Devonport
Department of Chemistry, University of Durham, Durham, UK DH1 3LE. E-mail: m.r.bryce@durham.ac.uk
Received (in Liverpool, UK) 9th September 1998, Accepted 16th October 1998
The convergent synthesis of (TTF)x(AQ)y (TTF = tetra-
thiafulvalene; AQ = anthraquinone) polyester dendrimers is
reported; the molecules undergo clean amphoteric redox
behaviour with reversible switching between cationic and
anionic states being achieved under electrochemical control;
the higher generation system (TTF)8(AQ)4 displays an
intradendrimer charge-transfer interaction in solution.
Herein we report the prototype systems 8 and 9 containing
both TTF and AQ units, built around a benzene 1,3,5-triester
core. The synthesis of the (TTF)4 dendron 3 is shown in Scheme
1.† The reaction of phenol derivative 19a,d (2.1 equiv.) with
benzene-1,3,5-tricarbonyl chloride (1.0 equiv.) in the presence
of DMAP as base (CH2Cl2, 20 °C) afforded the carboxylic acid
derivative 2 (56% yield) after hydrolysis of the unreacted acid
chloride group during workup. Reaction of 2 with oxalyl
chloride in toluene afforded acid chloride derivative 3 (82%
yield). The synthesis of the (AQ)2 reagent 7 is shown in Scheme
2. 2-(Hydroxymethyl)anthraquinone 4 reacted with 5-(tert-
butyldimethylsiloxy)isophthalic acid 513 using the DMAP-
catalysed DCC method14 to form compound 6 in 55% yield.
Attempted desilylation of 6 using TBAF in THF resulted in
cleavage of the ester linkages;15 however, heating a solution of
Dendritic molecules1 which possess functional groups at the
exterior surface and/or embedded within their structure2 are of
current interest. In this context, dendrimers which carry
multiple cationic or anionic sites are attracting attention, due to
their potential applications as macromolecular polyelectrolytes,
catalysts and charge transfer materials. The charged sites may
be introduced into the structure in two conceptually different
ways: (i) during the synthesis, e.g. by using quaternary
ammonium linkages,3 by incorporating transition metals,4 or by
attaching polyanionic surfaces;5 or (ii) by electrochemical
redox processes or chemical doping of a neutral dendrimer
which contains redox-active groups.6 For these systems, the
redox centres may behave independently in multi-electron
processes (n identical non-interacting electroactive centres
giving rise to a single n-electron wave) or they may interact
intra- or inter-molecularly, in which case overlapping or
closely-spaced redox waves are observed at different potentials.
Representative electron donor groups are ferrocene,7 metal-
(bipyridyl)8 and tetrathiafulvalene (TTF),9 while interior an-
thraquinone (AQ)10 and peripheral naphthalene diimide11
groups have been used as electron acceptor units. Dendritic
systems containing both strong p-donor and p-acceptor groups
covalently bonded into their framework12 are especially
attractive targets as they should possess amphoteric redox
properties under electrochemical control,8b and may engage in
inter- and/or intra-molecular charge-transfer interactions.
6 in a mixture of THF–aq. HCl (1 ) gave the alcohol derivative
7 (81% yield) which was only sparingly soluble in most organic
M
O
OH
O
4
HO2C
CO2H
i
OSiMe2But
5
O
O
O
O
O
O
O
O
OR
6 R = SiMe2But
7 R = H
ii
OH
S
S
S
S
iii
O
O
O
O
O
O
O
O
S
1
S
O
O
O
S
i
O
S
O
O
R
O
O
O
O
O
O
S
S
S
S
O
O
O
O
O
S
S
S
S
O
O
O
O
S
S
S
S
O
O
S
S
S
S
O
O
O
O
O
O
S
O
S
S
S
S
S
S
S
S
S
S
2 R = CO2H
3 R = C(O)Cl
S
S
ii
S
S
8
S
Scheme 1 Reagents and conditions: i, benzene-1,3,5-tricarbonyl chloride,
DMAP, CH2Cl2, 20 °C, silica gel chromatography; ii, oxalyl chloride,
toluene 80 °C.
Scheme 2 Reagents and conditions: i, DCC, DMAP, CH2Cl2, 0–20 °C; ii,
HCl (1
M)–THF (7:1 v/v), 50 °C; iii, 3, DMAP, 1,4-dioxane, reflux.
Chem. Commun., 1998, 2565–2566
2565

AQ
transfer from TTF to anthraquinone units. Based on the different
redox potentials of these donor and acceptor moieties, the
degree of charge-transfer is expected to be small ( < 0.1)18 and
that it is observed only in 9 appears to be an interesting
consequence of the more densely packed structure of the higher
generation molecule.
In summary, an efficient route has been established to
polyester ‘co-block’ dendrimers containing both p-donor and
p-acceptor moieties at the periphery. A future direction will be
the incorporation into these structures of stronger p-acceptor
groups,18,19 in conjunction with TTF, enabling the study of
intramolecular charge-transfer interactions within a dendritic
microenvironment. Such materials could find applications in
the development of electrooptical switches.
O
O
O
AQ
O
O
AQ
O
O
O
O
O
O
TTF
O
O
AQ
O
O
O
O
TTF
TTF
O
O
O
O
O
O
O
O
O
O
O
O
O
O
TTF
O
O
O
O
This work was funded by EPSRC (W. D.) and Universidad
Complutense de Madrid (P. de. M.).
O
O
O
O
O
TTF
TTF
O
O
Notes and references
TTF
TTF
† All new compounds gave 1H NMR spectra, mass spectra (FAB or plasma
desorption) and analytical data which were entirely consistent with their
structures. Selected data for 8: dH(CDCl3) 9.16 (3 H, s), 8.42 (3 H, t, J 1.5),
8.39 (6 H, d, J 1.5), 8.22–7.85 (14 H, m), 7.06 (4 H, s), 6.72 (8 H, s), 5.54
(4 H, s) and 5.20 (8 H, s).
9
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Fig. 1 Cyclic voltammogram of 8 (MeCN, 20 °C, Bu4N+PF62 electrolyte,
Pt electrode, vs. Ag/AgCl, scan rate 50 mV s21).
solvents. Esterification of 7 to afford G1 dendrimer 8 (60%
yield, after column chromatography on silica gel) was achieved
by reaction with acid chloride derivative 3 in the presence of
DMAP in refluxing 1,4-dioxane. Analogous iterative proce-
dures, starting with the known octa-TTF building block,9d gave
the (TTF)8(AQ)4 G2 system 9. Compounds 8 and 9 are soluble
in a range of organic solvents (e.g. CH2Cl2, CHCl3, THF,
dichlorobenzene and CS2): they darken on storage in air, and
can be stored for several weeks under vacuum in the dark at
0 °C.
The solution electrochemistry of 8 and 9 has been studied by
cyclic voltammetry (CV) in MeCN solution (Fig. 1). Scanning
anodically, 8 exhibits two reversible four-electron oxidation
waves to form, sequentially, the radical cation and dication of
each of the TTF moieties;16 scanning cathodically, two
reversible two-electron waves are observed, corresponding to
the reduction of each anthraquinone unit to the radical anion and
the dianion.17 Thus, 8 demonstrates clean amphoteric redox
behaviour with reversible switching between the +8, +4, 0, 22
and 24 charged states being achieved. The CV of 9 is very
similar (the +16, +8, 0, 24 and 28 states are clearly observed)
although the second TTF wave was slightly narrower than the
first wave, which was probably due to adsorption phenomena,
as observed previously with some higher generation sys-
tems.9d
An important difference between the G1 and G2 systems 8
and 9 is manifested in their UV–VIS spectra. Compound 9
shows a very weak (e < 250) broad absorption band in the l
460–750 nm region in MeCN, which is not present in 8. The
solvent dependency of this band and experiments at various
concentrations of 9 suggest that this low energy band arises
from intramolecular (rather than intermolecular) p–p charge-
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Chem. Commun., 1998, 2565–2566