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

Structures of the ground and tripler excited state of zinc meso-tetraphenyl porphyrin (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 obser ved trends, with TDDFT values in near quantitative agreement with experimen t. For ZnTPPBr8, we found that geometry distortion contributes about one-th ird of the redshifts observed for the Q and B bands in the ground-state spe ctrum. The results for ZnTPPX8 and the nonphenylated ZnPX8 provide insightf ul 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 up on beta-chlorination and beta-bromination.