Ground and triplet excited structures and spectroscopic properties of halogenated zinc meso-tetraphenylporphyrin

Structures of the ground and tripler excited state of zinc meso-tetraphenylporphyrin (ZnTPP) and its beta-octahalogenated derivatives (ZnTPPX8 with X = F, Cl, and Br) have been predicted using density functional theory (DFT) with Becke's three-parameter functional. The excitation energies: computed by singles CI and time-dependent DFT (TDDFT), are consistent with the observed trends, with TDDFT values in near quantitative agreement with experiment. For ZnTPPBr8, we found that geometry distortion contributes about one-third of the redshifts observed for the Q and B bands in the ground-state spectrum. The results for ZnTPPX8 and the nonphenylated ZnPX8 provide insightful accounts of effects of phenyl, fluoro, chloro, and bromo substituents on the excitation energies of these systems. The computed singlet-triplet (S-0-T-1) splitting of ZnTPP is in excellent agreement with experiment. The S-0-T-1 splitting is predicted to be significantly (0.4-0.5 eV) redshifted upon beta-chlorination and beta-bromination.