Excited state structural dynamics and Herzberg–Teller coupling of tetraphenylporphine explored via resonance Raman spectroscopy and density functional theory calculation

Abstract

Resonance Raman spectra of free-base tetraphenylporphine (TPP) were obtained with 397.9, 416, and 435.7 nm excitation wavelengths and density functional calculations were done to elucidate the electronic transitions and the resonance Raman spectra (RRs) of TPP. The RRs indicate that the Franck–Condon region photodynamics for S 0 → S 4 electronic state is predominantly along the C m–ph stretch while that for S 0 → S 3 electronic state is predominantly along the porphin ring C β C β stretch. Non-totally symmetric vibrational modes were regularly presented in resonance Raman spectra: the shorter the excitation wavelengths were, the stronger intensity the modes had, which can be interpreted in terms of electric dipole transition moments caused by Franck–Condon and Herzberg–Teller coupling. Four non-total symmetry vibrational mode υ 52, υ 64, υ 97 and υ 130 in A 2 irreducible representative of TPP were observed in 397.9, 416 and 435.7 nm resonance Raman spectrum. With the shorter wavelength laser excitations at 416 or 397.9 nm, the A 2 vibrational modes show more enhanced Raman intensity by comparison with those in the TPP spectrum excited at 435.7 nm.