Study on the triplet state of triphenylene by microwave induced delayed phosphorescence and T←S excitation spectroscopy

Abstract

The lowest triplet state of triphenylene has been studied with the aid of microwave induced delayed phosphorescence (MIDP) and T←S photo-excitation methods at very low temperatures. The phosphorescence spectrum consists mainly of vibronic bands due to the e′ modes, the band at 0–1624 cm^-1 being extremely intense. The relative radiative decay rates were found to be in the order of k_x ^r ≫ k_y ^r > k_z ^r for the 0,0 band and k_x ^r ≃ k_y ^r ≫ k_z ^r for the e′ vibrational bands. Thus the spin sublevels acquire radiative activity through mechanisms similar to those for benzene.

The T←S excitation spectrum was found to be quite different from the mirror image of the emission spectrum, thereby indicating a more complicated vibronic coupling scheme for the intensity borrowing in the absorption processes than in the emission processes. The structure of the excitation spectrum, however, was satisfactorily analysed by using a simplified model of vibronic coupling between the T ₁ and T ₂ states through one C-C-H bending and two C=C stretching vibrations.