considered to be independent despite the existence of
specific spiro interactions in the excited states.1a,2 Mole-
cules possessing a spiro-configured central unit and tailor-
made optical and redox properties have thus found ap-
plications in electrochemiluminescence studies,3 organic
light-emitting devices,4 electrochromic devices,5 field-
effect transistors,1a lasers,6 and organic- or dye-sensitized
solar cells (DSSC).7 In this context, 2,207,70-tetrakis (N,N-
di-p-methoxyphenylamine)-9,90-spirobifluorene (spiro-
MeOTAD) and related compounds have received great
attention as hole transporters in solid-state DSSC since
1998.7a,8 The potential of the spiro arrangement in the
design of organic photosensitizers for DSSC, with parti-
cular regard to the suppression of aggregation-induced
self-quenching processes and to the introduction of multi-
ple anchoring groups on the same molecule, has been
demonstrated more recently.7e,9
perpendicularly aligned donor/acceptor chromophore
couples attached to the SCPDT core. The new spiro-
configured dye has been investigated as a component of
DSSC, showing that the replacement of the spirobifluo-
rene core with SCPDT has positive effects on the photo-
voltaic performance of the device. The new dye extends the
sensitization effect to the red zone of the solar spectrum,
without enlarging the size of the conjugated bridge.
Most spiro compounds for optoelectronics developed so
far, including those used in solar cells, are based on the
easily accessible 9,90-spirobifluorene core. In order to
further expand this field of research, the availability of
alternative spiro cores with different electronic char-
acteristics would be highly beneficial. As also suggested
by computational studies,10 the replacement of the 9,90-
spirobifluorene core with 4,40-spirobi[cyclopenta[2,1-
b;3,4-b0]dithiophene] (SCPDT, Figure 1) is particularly
attractive.11 However, the synthesis of spiro compounds
based on SCPDT still remains a challenge. Thus, the few
reported examples of spirocyclopenta-derivatives of 2,20-
bithiophene concern either heteroatom-bridged spiro-
bithiophenes,12 or mixed spiro compounds where only
one-half of the central core consists of a bithiophene
unit.7e,13 In an outstanding paper,11a Salbeck and co-
workers actually described the preparation of a tetraphe-
nyl-substituted derivative of SCPDT by Suzuki coupling
of phenylboronic acid and 2,20,6,60-tetrabromo-4,40-
spirobi[cyclopenta[2,1-b;3,4-b0]dithiophene] (4Br-SCPDT,
Figure 1), but the synthetic pathway leading to this valu-
able starting material was not disclosed there.
Figure 1. 4,40-Spirobi[cyclopenta[2,1-b;3,4-b0]dithiophene] derivatives.
Inspired by previous literature reports,13 we first tried to
generate SCPDT by acid-mediated intramolecular Friedelꢀ
Crafts cyclization of a tertiary carbinol obtained in turn
by the reaction of cyclopenta[2,1-b;3,4-b0]dithiophen-4-
one and 3-lithio-2,20-bithiophene formed in situ by a
selective lithium-bromo exchange reaction of 3-bromo-
2,2-bithiophene with n-BuLi (see Supporting Informa-
tion, SI). Among the various cyclization conditions
tested, only the use of H2SO4 in n-octane provided traces
of SCPDT, hardly separable from tars. Other Lewis and
Brønsted acids completely failed to give the desired
product, in line with earlier findings of Wynberg and
co-workers who did not succeed in preparing spiranes
using cyclopenta[2,1-b;3,4-b0]dithiophen-4-one as start-
ing material since the intermediate carbinols containing
two R-thienyl units decomposed prior to ring closure.14 In
order to overcome this limitation and to avoid the
occurrence of intermolecular side reactions,13a we de-
cided to protect the electron-rich R-positions of the
thiophene units by bromination prior to the cyclization
step, as outlined in Scheme 1.
We here report the preparation of SCPDT from simple
2,20-bithiophene derivatives and its further elaboration
to give the dipolar dye SCPDT1 (Figure 1) featuring two
(8) (a) Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weissortel, F.;
€
Salbeck, J.; Spreitzer, H.; Gratzel, M. Nature 1998, 395, 583. (b) H.; Hsu,
C.-Y.; Chen, Y.-C.; Lin, R. Y.-Y.; Ho, K.-C.; Lin, J. T. Phys. Chem.
Chem. Phys. 2012, 14, 14099. (c) Fantacci, S.; De Angelis, F.; Nazeeruddin,
€
M. K.; Gratzel, M. J. Phys. Chem. C 2011, 115, 23126.
(9) (a) Heredia, D.; Natera, J.; Gervaldo, M.; Otero, L.; Fungo, F.;
Lin, C.-Y.; Wong, K.-T. Org. Lett. 2010, 12, 12. (b) Macor, L.;
Gervaldo, M.; Fungo, F.; Otero, L.; Dittrich, T.; Lin, C.-Y.; Chi,
L.-C.; Fang, F.-C.; Lii, S.-W.; Wong, K.-T.; Tsai, C.-H.; Wu, C.-C.
RSC Adv. 2012, 2, 4869.
(10) Yang, S. Y.; Kan, Y. H.; Yang, G. C.; Su, Z. M.; Zhao, L. Chem.
Phys. Lett. 2006, 429, 180.
(11) (a) Londenberg, J.; Saragi, T. P. I.; Suske, I.; Salbeck, J. Adv.
Mater. 2007, 19, 4049. (b) Saragi, T. P. I.; Londenberg, J.; Salbeck, J.
J. Appl. Phys. 2007, 102, 046104.
(12) (a) Ohshita, J.; Lee, K.-H.; Hamamoto, D.; Kunugi, Y.; Ikadai,
J.; Kwak, Y.-W.; Kunai, A. Chem. Lett. 2004, 33, 892. (b) Lee, K.-H.;
Ohshita, J.; Tanaka, D.; Tominaga, Y.; Kunai, A. J. Organomet. Chem.
2012, 710, 53.
The synthetic sequence leading to SCPDT started with
the selective lithiation of 3,5,50-tribromo-2,20-dithiophene
1 (conveniently obtained in two steps from commer-
cially available thiophene precursors, see SI) followed by
quenching with chlorotrimethylsilane (TMSCl) to give
the TMS-capped bithiophene 2. Further treatment of
€
(13) (a) Mitschke, U.; Bauerle, P. J. Chem. Soc., Perkin Trans. 1 2001,
740. (b) Ong, T.-T.; Ng, S.-C.; Chan, H. S. O.; Vardhanan, R. V.;
Kumura, K.; Mazaki, Y.; Kobayashi, K. J. Mater. Chem. 2003, 13, 2185.
(14) Wynberg, H.; Heeres, G. J.; Jordens, P.; Sinnige, H. J. M. Recl.
Trav. Chim. Pays-Bas 1970, 89, 545.
B
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