Symmetry-Breaking Charge Transfer in Boron Dipyridylmethene (DIPYR) Dimers

We
recently reported the photophysical properties of boron dipyridylmethene
(DIPYR) dyes, a class of intensely fluorescent pyridine-based chromophores,
which are structural analogues of both acenes and BODIPYs. In this
work, we endeavored to explore the properties of DIPYR dimers. The
synthesis and characterization of two novel homoleptic meso-linked dimers of boron dipyridylmethene dyes, bis-DIPYR and bis-α-DIPYR, are herein reported.
Their structural, electrochemical, and photophysical properties have
been probed using both steady-state and time-resolved techniques including
femtosecond and nanosecond transient absorption spectroscopies. Of
particular focus are the excited-state photophysical dynamics of the
dimers, which are studied in several solvents of varying polarity,
from methylcyclohexane to acetonitrile. It was found that both dimers
undergo symmetry-breaking charge transfer within 3 ps of photoexcitation,
forming a radical anion and radical cation, which were observed using
transient absorption and confirmed by spectroelectrochemical characterization.
Further, it was found that the emitting species is the symmetry-broken
state, which is stable for several nanoseconds before radiative recombination
to the ground state occurs. The efficiency and rapidity of symmetry
breaking, even in nonpolar media, is highly promising for application
of these materials to optoelectronic technologies requiring charge
transfer from an excitonic state.