Twisted molecular geometry and localized electronic structure of the triplet excited gem-diphenyltrimethylenemethane biradical: substituent effects on thermoluminescence and related theoretical calculations

Abstract

Substituent effects on the energies of electronic transitions (ETs) between the triplet excited and ground states of gem-diphenyltrimethylenemethane biradicals ( 3 2a ) were explored by using thermoluminescence (TL) spectroscopy and density functional theory (DFT) including time-dependent (TD) DFT. Linear free energy (Hammett) analyses of TL energies of a variety of para-substituted aryl derivatives of 3 2 * gave reasonable correlations with the substituent constant, σ . The slope of Hammett plots of the data are nearly identical to one obtained from a similar analysis of the photoluminescence (PL) energies of the structurally-related 1,1-diarylethyl radicals ( 3 *). The results suggest that TL of 3 2 * and PL of 3 * derive from a common diarylmethyl radical fluorophore. This interpretation is also supported by the DFT and TDDFT calculated electronic structures and ET energies of 3 2 and 3 . Thermodynamic and kinetic analyses of the charge recombination (CR) process between 2 + and 1 −, which generates 3 2 *, revealed that substituents not only alter the TL energies but also the TL intensities of 3 2 *. The observations made in this effort demonstrate that 3 2 * as well as 3 2 and 2 + have greatly twisted molecular geometries and highly localized electronic structures.