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

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Abstract
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 (15 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.

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