development of a zinc porphyrin sensitizer reaping a η over
12%.4 However, thiswas achieved bythe cosensitization of
the zinc porphyrin sensitizer and a triphenylamine sen-
sitizer as well as the judicious usage of a Co(II/III) tris-
(bipyridyl)-based redox electrolyte. In parallel, there
has been keen interest in developing donorÀπ-bridgeÀ
acceptor (DÀπÀA)-type metal-free organic sensitizers.
Among the various organic donors, e.g., triphenylamine,5
coumarin,6 and indoline,7 that have been explored in
DSCs, triphenylamine has yielded η of ∼10%.5c Squaraine
dyes have also been designed to enhance the light-harvesting
property in the near-IR region.8 Succeeding developments
of such metal-free organic sensitizers need to address sig-
nificant issues such as further η boost, cost reduction, and
synthesis simplicity.
hasdemonstrateda performanceof themetalÀfreeorganic
DTF-C sensitizers comparable to that of Ru-based dye
N719 typically used in DSCs as a point of reference.
Tetrathiafulvene (TTF) is a well-known electron-donating
group, and the preparation of its derivatives has primarily
been motivated for applications as optoelectronic materials.9
Figure 1. Molecular structures of DTF-C1, DTF-C2, and
DTF-C3.
€
Recently, Gratzel et al. investigated the use of exTTF (with
extended π-conjugation) sensitizers in DSCs and obtained a
moderate η.10 By comparison, dithiafulvene (DTF)11 can be
regarded as a smaller version of the fulvene family charac-
terized by a terminal electron-donating group, which could
permit more concise synthesis, compact dye adsorption, and
effective charge separation.
To our knowledge, however, the potential of the dithia-
fulvenyl group as a donor for DSCs has not been explored.
Here, we report the facile synthesis and DSC performance
of a series of organic DÀπÀA-type sensitizers based on the
dithiafulvene derivative donor and a cyanoacrylic acid
acceptor with different bridge lengths. This initial work
Illustrated in Figure 1 are chemical structures of the
three DÀπÀA-type sensitizers we have synthesized, which
are designated as DTF-C1, DTF-C2, and DTF-C3 in the
order of increasing π-bridge length. To prevent aggre-
gation of these sensitizers as well as to retard charge
recombination,7b two n-hexyl groups were symmetrically
attached on the dithiafulvenyl donor unit. The synthesis
of the sensitizers involved two major steps (Scheme S1,
Supporting Information): (1) HornerÀWittig condensa-
tion of 4,5-bis(hexylthio)-1,3-dithiole-2-thione (HDT) as
the donor moiety and aromatic dialdehydes A1ÀA3 pro-
duced the key intermediate π-extended DTF-bearing alde-
hydes B1ÀB3; (2) Knoevenagel reaction of the resulting
aldehydes and cyanoacetic acid in the presence of piper-
idine afforded the target sensitizer compounds DTF-C. As
the starting reagents for the condensation in Scheme S1
(Supporting Information), HDT was prepared by the
reaction of [Et4N]2[Zn(DMIT)2] with 1-bromohexane fol-
lowing a protocol from the literature (Scheme S2, Support-
ing Information),12 and the aromatic dialdehydes were
obtained commercially or by Suzuki couplings (Scheme
S3, Supporting Information).
Figure 2 shows absorption spectra of the DTF-C sensi-
tizers, and the corresponding spectroscopic parameters
extracted are summarized in Table S1 (Supporting Infor-
mation). All three of the sensitizers have a relatively strong
absorption in the 400À500 nm region attributable to the
πÀπ* charge transfer transition in π-extended DTF
chromophores. The absorption maximum (λmax) was
observed at 428 nm (molar extinction coefficient ε =
1.87 Â 104 MÀ1 cmÀ1) for DTF-C1 and 406 nm (ε = 2.75 Â
104 MÀ1 cmÀ1) for DTF-C2. While the trend of extinction
coefficient is plausible, such a blue-shift as a result of the
lengthening the π-bridge is counter to our expectation.
From Table S1 (Supporting Information), we find that
the Stokes shift of DTF-C2 is much larger than that of
DTF-C1 (7908 cmÀ1 vs 6270 cmÀ1). This indicates a larger
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