Temperature and deuterium effects on the phosphorescence lifetimes of dimethylbenzaldehydes in durene single crystals

Abstract

The phosphorescence decays of 2,4-, 2,5- and 3,4-dimethylbenzaldehyde-1 h 1 and −1 d 1 guests in perhydrogenated and perdeuterated durene single crystals have been studied between 4.2 and 120 K. The decays are exponential in the low (4.2 to ≈ 60 K) and high ( T > 90 K) temperature ranges and non-exponential in the intermediate range. The decrease in guest phosphorescence lifetimes σ observed between 15 and 60 K is essentially due to the thermal population of the nearby higher lying nπ* triplet state which decays with a much faster rate than the lowest ππ* triplet state. The significant increase in σ as temperature is reduced from 15 to 4.2 K can be accounted for by changes in the Boltzmann population of the spin sublevels of the lowest 3ππ* state. While no host crystal deuterium effect is detectable on σ, the observed guest intramolecular deuterium effect is predominantly due to the reduction of radiationless decay rates from the lowest 3ππ* states. One possible interpretation of the non-exponential phosphorescence decays in the intermediate temperature range is their attribution to the trans and cis conformers of the dimethylbenzaldehydes obtained by the rotation of the carbonyl group around the CC axis and present in thermally determined population in the durene host.