TRIPLET-STATE PROPERTIES OF 7-AZATRYPTOPHAN AND 5-HYDROXYTRYPTOPHAN -ANALYSIS OF DELAYED SLOW-PASSAGE ODMR RESPONSES

The triplet state properties of 7-azatryptophan (7AW), 5-hydroxytrypto phan (5HW), and the 5HW anion (pH 12.3) have been investigated by opti cally detected magnetic resonance, ODMR, in zero applied field. Also t he triplet state properties of tryptophan (Trp) have been reexamined. Analysis of ''slow-passage'' ODMR responses during phosphorescence dec ay (delayed ODMR) that contain the effects of fast passage is introduc ed using Marquardt-Levenberg nonlinear least-squares fitting. The para meters determined by the analysis of delayed ODMR are the band center frequency (nu(0)) and bandwidth (nu(1/2)) of the inhomogeneously broad ened ODMR band, the apparent decay constants of the triplet sublevels, and their relative radiative rate constants. The values of nu(0) and nu(1/2) are obtained with greater accuracy than is possible with analy sis that does not account for fast passage effects on the band shape. The kinetic parameters are influenced somewhat by residual spin-lattic e relaxation (SLR) effects, however. The zero-field magnetic resonance frequencies and band widths have been determined for Trp, 7AW, 5HW, a nd 5HW anion, along with the apparent kinetic parameters for Trp and 7 AW. Microwave-induced delayed phosphorescence transient responses of 5 HW and its anion are analyzed globally to give kinetic parameters free of residual effects. The ODMR band widths of 5HW and its anion greatl y exceed those of Trp, while those of 7AW are significantly narrower. The zero-field-splitting (ZFS) parameter E is reduced in 5HW and its a nion to about half its value in Trp, while the E-parameter of 7AW incr eases by about 10%. Sublevel decay constants relative to Trp are hardl y changed in 5HW, but increase significantly in the anion. Decay const ants of Trp increase moderately with 7-aza substitution, as expected f rom introduction of spin-orbit coupling at the N atom. The pattern of ODMR bandwidths of Trp and 7AW is consistent with a positive correlati on of solvent induced shifts in the ZFS D- and E-parameters.