Two-Photon Spectroscopy of the Q‑Bands of meso-Tetraphenyl-Porphyrin and -Chlorin Framework Derivatives

The two-photon absorption and excitation spectra in the Q-band region (900–1360 nm) were measured for a number of porphyrinoids, including simple porphyrins (porphin, meso-tetraphenyl-porphyrin), chlorins (meso-tetraphenyl-2,3-dimethoxychlorin), and porphyrin and chlorin-like analogues (meso-tetraphenyl-porpholactone and meso-tetraphenyl-porpholactol, respectively). These molecules were chosen to provide a series of compounds that differed structurally only in “single points”. Vibronic structure is observed in the two-photon spectra for all porphyrinoids investigated, but their relative intensities show distinct differences from the vibronic development observed in the corresponding one-photon spectra. A Franck–Condon analysis provides insight into the observed differences. The calculations also indicate that the two-photon absorptivities are associated primarily with Type I processes involving multiple intermediate states, and that accurate assignment requires a summation over at least 30 intermediate states. The Q-bands of the meso-tetraphenyl-2,3-dimethoxychlorin exhibit anomalously high two-photon absorptivities, which we have traced to facile conformational distortion of the chlorin chromophore. Calculations indicated that the relative Q-band absorptivities are sensitive to the phase and magnitude of the chlorin ring distortions.