Two-photon absorption of tetraphenylporphin free base

We study two-photon absorption (TPA) spectra for toluene solution of 5,10,15,20-tetraphenyl-21H,23H-porphin (H2TPP) in B and Q bands regions and find the maximum TPA cross-section values of 25 and 1–6 GM in laser wavelength ranges, 730–790 and 1100–1400 nm, correspondingly. In the 730–790 nm range the spectrum is attributed to parity allowed two-photon transition into g parity state, positioned nearby B state. Much lower TPA cross-section of ∼1 GM is measured for the transition into pure electronic Q state and it is due to the contribution of only those low-symmetry H2TPP confomers, where mutual orientation of the porphyrin plane and the four phenyl rings lifts the center of inversion. The intermediate values of TPA cross-section of ∼6 GM are observed for the transition into vibronic Q states and are explained by TPA-allowed transition into vibronic states, which can occur even for totally centrosymmetrical molecules. Measurement of two-photon polarization ratio, Ω=σcirc/σlin, shows that for parity allowed g→g transition Ω =1.05±0.05 and for transitions into electronic and vibronic Q-states, Ω=0.62±0.06 and 0.79±0.1 (depending on wavelength), respectively. Quantum-chemical calculations of both u and g parity excited energy levels of H2TPP molecule are performed by CNDO/S method and the results are in good agreement with the experimental data.