Solvent tuning excited state structural dynamics in a novel bianthryl

Symmetry breaking charge separation (SBCS) is central to photochemical energy conversion. The widely studied 9,9-bianthryl (9,9’BA) is the prototype, but the role of bianthryl structure is hardly investigated. Here we investigate excited state structural dynamics in a bianthryl of reduced symmetry, 1,9-bianthryl (1,9’BA), through ultrafast electronic and vibrational spectroscopy. Resonance selective Raman in polar solvents reveals a Franck-Condon state mode that disappears concomitant with the rise of ring breathing modes of radical species. Solvent dependent dynamics show that CS is driven by solvent orientational motion, as in 9,9’BA. In nonpolar solvents the excited state undergoes multistep structural relaxation, including sub-picosecond Franck-Condon state decay and bi-exponential diffusion-controlled structural evolution to a distorted slightly polar state. These data suggest two possible routes to SBCS; the established solvent driven pathway in rapidly relaxing polar solvents and, in slowly relaxing media, initial intramolecular reorganisation to a polar structure which drives solvent orientational relaxation.