Global Analysis of Microwave-Induced Delayed Phosphorescence of Photoexcited Triplet States

Abstract

The microwave-induced delayed phosphorescence (MIDP) responses of several molecules in their excited triplet state are analyzed globally by least-squares minimization of the residuals, χ². The analysis considers not only the initial amplitudes of the MIDP responses which has been customary in the past, but also all data points (up to about 20,000) contained in a data set consisting of all three zero-field transitions and a range of delay times. It is shown that global fitting of the datacannotbe carried out successfully without including both sublevel decay to the ground singlet state and spin–lattice relaxation (SLR). Thus a complete data set containing at least two of the zero-field transitions 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 ionization of the hydroxyl proton, and 7-azatryptophan (7-Atrp). It is found that theT_zsublevel (zis the out-of-plane axis) decays principally by SLR even at about 1.2 K;k_z= 0 within experimental error (≈±0.01 s⁻¹) except for 5-Htrp anion, where it is measurable and is the smallest of thekvalues. For each triplet state, except for 5-Htrp anion, SLR is least efficient between theT_xandT_ysublevels. This relaxation process requires an angular momentum component alongz. Since thezcomponent of spin–orbit coupling is the weakest one in³(π, π*) states, the van Vleck relaxation process is suggested for Trp, 5-Htrp, and 7-Atrp. For the 5-Htrp anion, the most efficient SLR occurs betweenT_xandT_y, suggesting a different mechanism, possibly direct phonon modulation of the zero-field splittings.