Biexcitons in molecules
Matthew Sfeir, ASRC and The Graduate Center, CUNY
The recent development of efficient singlet fission, the conversion of a singlet exciton to a triplet pair biexciton, as a intramolecular process (iSF) in isolated organic molecules has led to a wealth of fundamental information on multiexcitons in molecular systems. A highly tunable and flexible chemistry approach allows us to modify the relative orientation, proximity, bonding, and energetics of SF chromophores in molecular dimers, heterodimers, oligomers, and polymers. This approach has facilitated an understanding of how the triplet pair biexciton formation, conversion, and decay dynamics depend on the exact nature of the chromophores and their electronic interactions. The key factors that impact the multiexciton dynamics are the net multiplicity of the triplet pair, denoted as m(TT) with m = 1 (singlet), 3 (triplet), or 5 (quintet), and its evolution in time. I will discuss these molecular biexcitons in different limiting cases and illustrate how the nature of the electronic and spin coupling determines the primary recombination channel. I will propose and discuss a set for characteristics for “harvestable” biexcitons to address a pervasive misconception that a triplet pair must be “split” (spatially separated so that the binding energy is zero) to have utility for optoelectronics and photochemistry.