Influence of local microenvironment on the double hydrogen transfer in porphycene

We performed time-resolved transient absorption and fluorescence anisotropy measurements in orderto study tautomerization of porphycene in rigid polymer matrices at cryogenic temperatures. Studieswere carried out in poly(methyl methacrylate) (PMMA), poly(vinyl butyral) (PVB), and poly(vinyl alcohol)(PVA). The results prove that in all studied media hydrogen tunnelling plays a significant role in thedouble hydrogen transfer which becomes very sensitive to properties of the environment below approx.150 K. We also demonstrate that there exist two populations of porphycene molecules in rigid media:‘‘hydrogen-transferring’’ molecules, in which tautomerization occurs on time scales below 1 ns and‘‘frozen’’ molecules in which double hydrogen transfer is too slow to be monitored with nanosecondtechniques. The number of ‘‘frozen’’ molecules increases when the sample is cooled. We explain thiseffect by interactions of guest molecules with a rigid host matrix which disturbs symmetry of porphyceneand hinders tunnelling. Temperature dependence of the number of hydrogen-transferring moleculessuggests that the factor which restores the symmetry of the double-minimum potential well inporphycene are intermolecular vibrations localized in separated regions of the amorphous polymer