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

Jerzy Karpiuk; Alina Majka; Ewelina Karolak; Jacek Nowacki

doi:10.1021/acs.jpclett.7b02020

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

J Phys Chem Lett. 2017 Oct 5;8(19):4659-4667. doi: 10.1021/acs.jpclett.7b02020. Epub 2017 Sep 14.

Authors

Jerzy Karpiuk¹, Alina Majka², Ewelina Karolak², Jacek Nowacki¹

Affiliations

¹ Institute of Physics, Polish Academy of Sciences , Al. Lotników 32/46, 02-668 Warsaw, Poland.
² Institute of Physical Chemistry, Polish Academy of Sciences , Kasprzaka 44/52, 01-224 Warsaw, Poland.

PMID: 28892399
DOI: 10.1021/acs.jpclett.7b02020

Abstract

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.