between compound 5 and 3 is at the π-bridge, in which two
phenyls in 3 are replaced by two thienyls in 5. Compounds
2 and 4, which have been reported by others,15,16 were
synthesized by a TiCl4/Zn-induced reductive coupling and
standard Wittig reaction, respectively. The synthesis of
boron-containing compounds 1, 3, and 5 are shown in
Scheme 1. The Wittig reactions were used to prepare the
Scheme 1a
Figure 1. Molecular structure of compounds 1-5; Mes ) 2,4,6-
trimethyl-phenyl.
tendency that the σ values of A-π-A molecules are smaller
than those of their D-π-D analogues with identical π-bridges.5a
To systematically explore the TPA and TPEF potentials
of A-π-A-type quadrupoles, three new A-π-A-type com-
pounds with dimesitylboryl as acceptors, together with other
two related D-π-D-type compounds, have been synthesized
and investigated in this letter. Noticing that a trivalent
nitrogen atom, with a lone pair of electrons in its p-orbital,
has been widely adopted as the donor center,3-12 we chose
the three-coordinate boron atom, which has an empty
p-orbital, as the acceptor center. Indeed, trivalent boranes
have found application as optoelectronic materials in recent
years.13 However, the reports concerning the role of boron
in TPA materials are very few.14
a Reagents and condition: (a) (i) NBS/BPO/CCl4/reflux/3 h, (ii)
PPh3/toluene/reflux/3 h; (b) t-KOBu/THF/0 °C to rt/24 h; (c)
n-LiBu/FB(Mes)2/THF/-78 °C to rt/24 h.
precursors 1′, 3′, and 5′. Substituting the Br atoms of 1′ and
3′ or the R-H of the thienyls of 5′ with a dimesitylboryl group
by means of the lithium derivative at low temperature affords
object compounds 1, 3, and 5 in good yield. All of these
compounds are stable in air and in common solvents. The
detailed characterization data can be found in Supporting
Information.
Compounds 1 and 3 differ from 2 and 4 mainly in the
terminal electron-donating/-accepting groups. The difference
As summarized in Table 1, the spectra (both absorption
and emission) positions of the A-π-A type compounds are
considerably blue-shifted relative to those of their D-π-D
analogues. The quantum yields Φ of 1 and 3 are also slightly
higher than that of 2 and 4, respectively. Due to the electron-
rich nature of thienyl and its low stabilization energy relative
to benzene,17 both the absorption and emission spectra of 5
are greatly red-shifted compared with those of 3 (see Table
1 and Figure 2). On the other hand, the quantum yield of 5
is slightly smaller than that of others. Despite this, compound
5 is still the strongest emitter among 1-5 because of its
largest absorbance and higher product of ꢀ‚Φ.
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