Exchange coupling and triplet exciton migration in semiconducting (NN′-dibenzyl-4,4′-bipyridilium)2+(7,7,8,8-tetracyanoquinodimethane) 2–4 crystals

Abstract

Spin-triplet excitons have been observed by electron spin resonance spectroscopy in mono crystals of (NN′-dibenzyl-4,4′-bipyridilium)²⁺(6,6,8,8-tetracyanoquinodimethane)^2–₄. The dipole–dipole fine-structure of these triplets was resolved in the slow exciton exchange temperature region of the crystal. The dipole–dipole splittings, line widths and relative intensities of the exciton signals were monitored with respect to temperature, and used to derive the principal values of the dipole–dipole fine-structure, the singlet to triplet energy gap, and the activation energy of exciton exchange. The results are interpreted in terms of electron exchange-coupling between the TCNQ molecules and molecular anions in this crystal. It is proposed that this exchange-coupling changes from a narrow band type below 100 K to a “dimer-pair” between 100 and 200 K and to a “dimer” above 200 K.