Fluorinated Porphycenes: Synthesis, Spectroscopy, Photophysics, And Tautomerism

Six porphycenes have been synthesized, bearing one, two, or three fluorine atoms attached directly to the 18-pi-electron system at themesopositions. These novel compounds have been characterized by structural, electrochemical, and spectral techniques, combined with quantum chemical calculations. In three fluoroporphycenes, the unsymmetric substitution pattern leads to the presence of two nonequivalenttranstautomeric forms. They have been identified using electronic absorption, emission, and magnetic circular dichroism spectroscopies. Their relative energies have been estimated for the ground and lowest excited electronic states. Tautomerization potential is quasi-symmetric in S-0, but becomes strongly nonsymmetric in S-1. Femtosecond transient absorption studies allowed determination of tautomerization rates, larger and similar for both directions of the double hydrogen transfer in S-0, lower and disparate in S-1. Fluoroporphycenes emerge as good candidates for detailed studies of mechanisms of double hydrogen transfer, as well as processes responsible for rapid radiationless excited state depopulation.