3 years ago

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

Intramolecular Electron Transfer in Frozen Solvents: Charge Transfer and Local Triplet States Population Dynamics Revealed by Dual Phosphorescence
Ewelina Karolak, Alina Majka, Jerzy Karpiuk, Jacek Nowacki
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 that 1[D•+–A•–] decays to charge-separated (3[D•+–A•–]) and donor-centered ([3D*–A]) triplets, which display dual phosphorescence. 3[D•+–A•–] and [3D*–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 in 3[D•+–A•–] resulting in excited open ring biradical.

Publisher URL: http://dx.doi.org/10.1021/acs.jpclett.7b02020

DOI: 10.1021/acs.jpclett.7b02020

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