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

Substituent effects on the energies of electronic transitions (ETs) between the triplet excited and ground states of gem-diphenyltrimethylenemethane biradicals (32aradical dotradical dot) 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 32radical dotradical dot* gave reasonable correlations with the substituent constant, σradical dot. 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 (3radical dot*). The results suggest that TL of 32radical dotradical dot* and PL of 3radical dot* derive from a common diarylmethyl radical fluorophore. This interpretation is also supported by the DFT and TDDFT calculated electronic structures and ET energies of 32radical dotradical dot and 3radical dot. Thermodynamic and kinetic analyses of the charge recombination (CR) process between 2radical dot+ and 1radical dot−, which generates 32radical dotradical dot*, revealed that substituents not only alter the TL energies but also the TL intensities of 32radical dotradical dot*. The observations made in this effort demonstrate that 32radical dotradical dot* as well as 32radical dotradical dot and 2radical dot+ have greatly twisted molecular geometries and highly localized electronic structures.