Angewandte
Chemie
DOI: 10.1002/anie.200802034
Electrochemiluminescence
Tuning of Electrogenerated Silole Chemiluminescence**
Christina Booker, Xin Wang, Samar Haroun, Jigang Zhou, Michael Jennings,
Brian L. Pagenkopf,* andZhifeng Ding*
Increasing interest in p-conjugated compounds containing
silole rings (1-silacyclopentadiene)[1] is to a large extent due to
recent exploitation for applications such as electrochemilu-
minescent sensors and light-emitting diodes.[2–10] Their unique
photophysical and electronic properties[11] arise from the
particularly low lying LUMO owing to s*–p* conjugation
between the s* orbital of two exocyclic s bonds on the silicon
atom and the p* orbital of the butadiene moiety.[12,13] We
recently reported the synthesis of donor–acceptor siloles[14]
and oligomeric ethynyl siloles,[15] and the electrogenerated
chemiluminescence (ECL)[16] of several silole-based chromo-
phores.[17] We also reported a series of 2,5-bis(arylethynyl)
siloles in which a curious improvement of photoluminescence
(PL) quantum efficiency from 9 to 63% was achieved by
increasing the steric bulk on the silicon atom and 2,5-
substituents.[18] It seemed plausible that the enhanced lumi-
nescence resulting from the highly improved quantum
efficiency was due to increasing the energy barriers for
nonemissive decay processes. However, these electronic
improvements did not translate to ECL, and the efficiencies
of some ethylene- and ethynyl-substituted siloles[17] were only
in the range of 0.001 to 0.1 relative to 9,10-diphenylanthra-
cene (DPA),[19] owing to the instability of their radical cations
needed for ECL generation.[20] Herein we report that
successful tuning of the electrochemical potentials and of
silole–thiophene hybrid chromophores results in higher
stability of the radical cations and ultimately in improved
ECL efficiency.
luminescence and ECL. Therefore the target chromophores,
3b–d and 4b–d, were prepared as shown in Scheme 1.
Intramolecular cyclization of bis(phenylethynyl) dialkyl
Scheme 1. Synthesis of thienyl-containing silole chromophores.
silanes 1 with an excess of lithium naphthalenide followed
by reaction with ZnCl2 afforded 2,5-dizinc intermediates 2,
which were used in situ to prepare silole chromophores 3 and
4 by Negishi cross-coupling reactions catalyzed by [PdCl2-
(PPh3)2].[15,22] Good yields were obtained by carrying out the
entire reaction sequence, consisting of cycloreduction, trans-
metalation, and cross-coupling, from silane 1 to silole 3 or 4 in
one pot.
Considering that 1,1-dimethyl-2,5-bis(2-thienyl)-3,4-
diphenylsilole (3a) and 1,1-dimethyl-2,5-bis[(2,2’-bithio-
phen)-5-yl]-3,4-diphenylsilole (4a) are efficient electron-
transporting materials,[21] we expected that replacement of
the methyl substituents with larger isopropyl, tert-butyl and n-
hexyl groups would augment the energy barriers for non-
emissive decay processes, stabilize the radical cations gen-
erated electrochemically, and thus result in enhanced photo-
Figure 1 shows cyclic voltammograms (CVs) of 0.78 mm
4a (a) and 1.34 mm 4c (b) in dichloromethane solution
containing 0.1m tetrabutylammonium perchlorate as support-
ing electrolyte. Electrochemical data for the other six siloles
are summarized in Table 1. As the potential is scanned from
zero to the positive region, 4a loses an electron to become a
radical cation at a half-wave potential of 0.818 V, and loses a
second electron to likely become a radical dication at a half-
wave potential of 0.972 V (Figure 1a). The half-wave poten-
tials for the two consecutive oxidations of 4c are 0.697 and
0.840 V, respectively (Figure 1b). The calculated (DFT/
B3LYP/6-31G*) electron density in the HOMOs of 4c and
its radical cation are spread out over the conjugated
thiophene units. In the X-ray crystal structure of 4c
(Figure 2), the two bithiophenyl groups are almost in the
same plane as the silole core. Coplanarity of silole and
bithiophenyl moieties may lead to a second oxidation wave
instead of one two-electron oxidation reaction, which agrees
well with the results of quantum chemical calculation (see the
Supporting Information). Because the conjugated system is so
extensive in 4c, the molecule can compensate for the missing
[*] C. Booker, X. Wang, S. Haroun, Dr. J. Zhou, Dr. M. Jennings,
Prof. Dr. B. L. Pagenkopf, Prof. Dr. Z. Ding
Department of Chemistry, The University of Western Ontario
1151 Richmond St., London ON N6A5B7 (Canada)
Fax: (+1)519-661-3022
E-mail: zfding@uwo.ca
[**] This work was supported by NSERC, PREA, CFI, OIT, and UWO. We
thank FMC Lithium, a division of FMC Corporation, for a generous
donation of tBu2Si(OTf)2, and we thank Lindsay Kelland for ECL
experiments.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2008, 47, 7731 –7735
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7731