Anisotropic Intersystem Crossing from the Upper Excited Triplet States of Anthracenes: Two-laser Time-Resolved EPR Study

A two-laser, two-color time-resolved EPR (TREPR) technique has been applied to the investigation of the electron spin polarization in the excited triplet states of anthracene and its halogen anthracene in rigid glassy matrixes. The one-laser excitation of anthracenes with 355 nm gave the triplet TREPR spectra of the lowest excited triplet (T 1 ) states with EEE/AAA polarization pattern. For 9-bromoanthracene and 9,10-dibromoanthracene, the reexcitation by a dye laser (442 nm) to the upper excited triplet ( T n: n > 3) states brought about the opposite phase polarization in the T 1 states. The phenomena were interpreted in terms of the sublevel selective reverse intersystem crossing (RISC) from the T n ( n = 2 or 3) state to the S 1 state. The time profiles of the polarization were analyzed taking into account the anisotropic RISC processes. On the other hand, for anthracene and 9,10-dichloroanthracene, little effect was observed on the electron spin polarization by two-step laser irradiation.