Study of higher excited singlet states of zinc(II)-tetraphenylporphin

The mechanism for the production of the S2 state fluorescence following the excitation to the S1 states of zinc (II)-tetraphenylporphin has been investigated by using two-photon absorption and optical–optical double resonance techniques. The main process populating the S2 state is found to be a stepwise two-photon absorption through the S1 state (the Q state) even by the excitation of relatively low power density (<5×1015 photons cm−2 pulse−1) of a nitrogen pumped dye laser. The absorption cross sections σ1n of the Sn ← S1 transition are determined from the kinetic analyses based on the two-photon absorption measurement. The Sn ← S1 absorption spectrum obtained is structureless and its σ1n values vary from 4.74×10−16 to 1.02×10−15 cm2 molecule−1 in the wavelength region of 490–560 nm. This observation suggests that the corresponding Sn ← S1 transition possesses a strongly allowed nature and there exists a higher excited singlet state of gerade symmetry in the 17 800–20 400 cm−1 region above the ground state. The result of MO calculation taking into account doubly excited configurations and higher singly excited configurations actually indicates the presence of a strongly two-photon allowed 1A2g state in this energy region.