Structural relaxation in the 1,2,6-trimethyl-3,5-diphenyl-4-pyridone triplet

Abstract

On 308 nm or 337.1 nm laser pulse excitation, 1,2,6-trimethyl-3,5-diphenyl-4-pyridone (2) forms triplets (λ_max ^T = 330 - 335 nm) characterized by short lifetimes (2 - 3 μs) in fluid solutions at room temperature. The triplet yields (0.07 - 0.5) decrease on going from non-polar solvents to polar/hydrogen-bonding solvents. The triplet exhibits a reluctant quenching behavior toward quenchers with triplet energies in the range 40 - 66 kcal mol^-1, although the spectroscopic triplet energy of 2 is estimated to be 74 kcal mol^-1 (from phosphorescence spectra in low temperature glasses). This inefficient energy transfer quenching is explained in terms of extensive relaxation in the triplet state of 2 in fluid solutions, most probably in the form of twisting about the double bonds of the heterocyclic ring (thereby relieving the steric strain). The triplets of 2 and its oxygen analogue (1) are shown to be efficient donors of electrons and energy in their interactions with paraquat and oxygen respectively.