GLOBAL ANALYSIS OF MICROWAVE-INDUCED DELAYED PHOSPHORESCENCE OF PHOTOEXCITED TRIPLET-STATES

The microwave-induced delayed phosphorescence (MIDP) responses of seve ral molecules in their excited triplet state are analyzed globally by least-squares minimization of the residuals, chi(2). The analysis cons iders not only the initial amplitudes of the MIDP responses which has been customary in the past, but also all data points (up to about 20,0 00) contained in a data set consisting of all three zero-held transiti ons and a range of delay times. It is shown that global fitting of the data cannot be carried out successfully without including both sublev el decay to the ground singlet state and spin-lattice relaxation (SLR) . Thus a complete data set containing at least two of the zero-field t ransitions when analyzed globally yields both sublevel decay constants and the SLR rate constants. Results are reported for tryptophan (Trp) , 5-hydroxytryptophan (5-Htrp), the anion of 5-Htrp obtained by ioniza tion of the hydroxyl proton, and 7-azatryptophan (7-Atrp). It is found that the T-z sublevel (z is the out-of-plane axis) decays principally by SLR even at about 1.2 K; k(z) = 0 within experimental error (appro ximate to+/-0.01 s(-1)) except for 5-Htrp anion, where it is measurabl e and is the smallest of the k values. For each triplet state, except for 5-Htrp anion, SLR is least efficient between the T-x and T-y suble vels. This relaxation process requires an angular momentum component a long z. Since the z component of spin-orbit coupling is the weakest on e in (3)(pi, pi) states, the van Vleck relaxation process is suggeste d for Trp, 5-Htrp, and 7-Atrp. For the 5-Htrp anion, the most efficien t SLR occurs between T-x and T-y, suggesting a different mechanism, po ssibly direct phonon modulation of the zero-field splittings. (C) 1996 Academic Press, Inc.