Proton tunneling, isotopic effects, and dealkylation reaction in tetra-<i>tert</i>-butyl porphycene: Supersonic jet spectroscopy revisited

Abstract

Jet-cooled 2,7,12,17-tetra-tert-butylporphycene exhibits a very complicated electronic spectrum, previously interpreted in terms of three different conformers and/or tautomers present in the supersonic expansion. Based on experiments employing two different techniques of introducing molecules into the jet—heating and laser desorption—we now correct this assignment. The observed species are attributed to tetra-, tri-, and di-tert-butylporphycenes. These conclusions are supported by mass spectrometry and density functional theory calculations; the latter, in combination with the experiment, allow elucidation of the most stable tautomeric forms and understanding of vibrational patterns observed in laser-induced fluorescence and dispersed fluorescence spectra. Tetra-substituted porphycene shows tunneling splitting due to coherent delocalization of the two hydrogens in the inner cavity. In the tri- and di-substituted derivatives, tunneling is suppressed because of symmetry lowering. In order to explain the spectral shifts observed upon consecutive H/D substitution of the inner protons, we propose a simple model that allows estimating the tunneling splitting magnitude from experimentally observed transition energies compared to those estimated assuming additivity of the isotope shifts upon successive H/D replacements. The procedure may be useful for analysis of complex spectra.