Triplet Excited States of Free-Base Porphin and Its β-Octahalogenated Derivatives

Density functional theory (DFT) electronic structure calculations were carried out to predict the structures, energetics, and triplet−triplet (T−T) spectra for the low-lying triplet states of free-base porphin (PH 2 ) and its β-octahalogenated derivatives (β-PH 2 X 8; X = F, Cl, Br). The lowest triplet excited state of PH 2 and β-PH 2 X 8 was found to retain D 2 h symmetry with stretched C β −C β and C β −C m bond distances. For free-base porphin, the singlet−triplet (S 0 −T 1 ) gap obtained with the B3LYP functional was in excellent agreement with the experimental phosphorescence value. Excitation energies computed by time-dependent DFT also provided a fine account of the observed T−T spectrum. β-Halogenation had little effect on the singlet−triplet gaps of porphin. The S 0 −T 1 and S 0 −T 2 splittings for β-PH 2 X 8 were within 0.1 eV of the corresponding splittings in the unsubstituted porphin. All bands in the T−T spectra of β-PH 2 X 8 were predicted to be significantly (up to 0.65 eV) red-shifted in comparison to corresponding bands of the unsubstituted porphin.