Intramolecular Electron Transfer in Frozen Solvents: Charge Transfer and Local Triplet States Population Dynamics Revealed by Dual Phosphorescence.

In frozen solvents at 77 K, ultrafast (≤250 fs) photoinduced intramolecular electron transfer (ET) in bichromophoric donor-acceptor ([D-A]) diarylmethane lactones produces a covalently linked radical ion pair,1 [D•+ -A•- ]. Steady state and time-resolved luminescence measurements reveal that1 [D•+ -A•- ] decays to charge-separated (3 [D•+ -A•- ]) and donor-centered ([3 D*-A]) triplets, which display dual phosphorescence.3 [D•+ -A•- ] and [3 D*-A] are formed in parallel via two intersystem crossing mechanisms: spin orbit charge transfer (SOCT) and hyperfine coupling (HFC), with solvent dependent branching ratio. The solvent drives the D-A alignment during the freezing process to adapt to increasing solvent polarity, producing inhomogeneous ground-state population distribution with solvent-dependent D-A exchange interaction, which plays a key role in partitioning into SOCT and HFC mechanisms. In polar glasses, a third phosphorescence band appears due to dissociative back ET in3 [D•+ -A•- ] resulting in excited open ring biradical.