Intersystem crossing to higher triplet states in isolated molecules

Nonradiative electronic relaxation under collision-free conditions

“…Fischer and Lim146 have observed that the fluorescence quantum yield of anthracene vapor (0.02 Torr and 1-nrh bandpass) exhibits a step-like drop at about 255 nm, which corresponds to the onset of the 1B3u+ -1A1g absorption band. Their interpretation of the origin of the step-like drop in the quantum yield is different from that of Laor et al 145 Fischer and Lim assume that the important deactivating process is intersystem crossing from 1B2u+(St) to 3B1g+(T2) (3B1g+ lies 500 cm-1 below 1B2u+). Initial excitation of the 1B3u+ state is followed by 1B3u+ 1B1g ^1B2u+ internal conversion which produces a distribution of vibrational levels that enhances the 1B2u+ w 3B1g+ intersystem crossing.…”

“…Laor et al 145 were the first to study anthracene under isolated molecule conditions (0.2 Torr and 30-Á excitation bandwidth). The authors measured fluorescence lifetimes as a function of excess energy up to an excess energy of ~19 000 cm-1.…”

Recent Advances in Self-Exciting Photodynamic Therapy

“…Electrons excited by the incoming radiation will jump to the higher energy orbital in the PS, after which two things may happen. One is the electron immediately relaxes back to the ground state (time scale ∼10 −10 s) (Laor et al, 1973;DeRosa, 2002;Zhao et al, 2013b); this is the unproductive pathway for PDT. The other pathway is to undergo intersystem crossing, a spin forbidden electronic orbital transition (∼10 −8 s), and then react with, most typically, an oxygen molecule generating singlet oxygen (Menzel and Thiel, 1998;DeRosa, 2002;Zhao et al, 2013b).…”