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-alpha-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.