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

Abstract

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.

Energy-dependent quenching adjusts the excitation diffusion length to regulate photosynthetic light harvesting

“…A range of photophysical mechanisms has been suggested to underlie qE (11): ( i ) energy transfer from excited Chl to carotenoid (Car) S1 states with lifetimes of ∼10 ps (31), ( ii ) energy transfer to mixed Chl–Car states that undergo charge transfer and subsequent recombination to the neutral ground states (32), and ( iii ) some form of conformation change-induced “concentration quenching,” which may involve symmetry-breaking charge transfer between two Chl molecules (33, 34). The first two mechanisms have rates consistent with weak quenching.…”

“…It is likely that both processes would give τnormalqnormalE values ≫5 ps. Enhanced internal conversion in closely spaced dimers, either of Chl a or of Chl a and Zea via symmetry-breaking charge transfer, may not produce clear spectroscopic observables as has been the case for concentration quenching in solution (33, 34).…”

Tuning the Triplet Excited State of Bis(dipyrrin) Zinc(II) Complexes: Symmetry Breaking Charge Transfer Architecture with Exceptionally Long Lived Triplet State for Upconversion

“…[20] All these conventional strategies however suffer from some drawbacks, such as the high cost and toxicityo fm etals, long synthetic routes and shortening of triplet excited state lifetimed ue to the heavy atom effect. [21,22] Recently,the symmetry breaking chargetransfer (SBCT)a pproachgained significant attention as an ew strategy to develop heavy atom free triplet PSs, [23] as it facilitates charge separation with negligible energetic loss and slow back charge recombination. [24] SBCT plays ak ey role in charge separation within photosynthetic reaction centers in living systems.…”

Modeling Infrared Spectral Dynamics upon Symmetry Breaking of a Photo-Excited Quadrupolar Dye

“…1 These systems include molecules containing two or more identical chromophores where the excited subunit can act either as electron donor (D) or acceptor (A). [2][3][4][5][6][7][8][9][10][11][12][13] Such symmetry-breaking (SB) charge separation process could be advantageously exploited for applications in photovoltaics and artificial photosynthesis. 9,14,15 Another class of compounds comprises multibranched molecules with a D(-π-A) n or A(-π-D) n (n = 2, 3) motif, that are attracting considerable interest for their promising two-photon absorption properties.…”