Photochemical Hole Burning in the Soret Absorption Band of Zinc-tetrabenzoporphin Derivative by Two-photon Excitation

Line-narrowing spectroscopic techniques, such as site-selection spectroscopy (fluorescence line narrowing) and spectral hole burning, have, to our knowledge, so far failed to eliminate the effect of inhomogeneous broadening of optical transitions to the second and/or higher excited singlet electronic states (Soret absorption band) of porphin-type dyes in low temperature matrices, mainly due to: (1) Perturbations between different electronic states and resulting broadening of the homogeneous spectral bands [1], and (2) Increasing density of vibronic states with increasing transition energy and lack of correlation between individual states of different molecules. However, in a class of porphin derivatives - free-base and metallotetrabenzoporhines (TBP) - the former effects are substantially reduced. As a result, quasiline structure of S0-S2 transitions of ZnTBP in an Ar matrix [2], and later of ZnTBP in supersonic expansions [3] were observed. Further, fluorescence from S2 state of ZnTBP [2,4] and recently also of Zn-tetratolyl-TBP [5] and Zn-TPP [6] were reported.