Rival photochemical hole-burning mechanisms in porphyrine doped polymers

The phototransformations of H2-tetraphenylporphine (TPP) in polymer glasses were investigated at 77 K. The use of a relatively high experimental temperature enabled us to burn holes with a large area. This provided us with the opportunity to unambiguously identify the photoproduct and measure its spectrum with high precision. Depending on the irradiation conditions two photoprocesses can be initiated: (a) proton tautomerization (PT), which results in an intensity redistribution in the absorption band of the educt; and (b) another photoprocess (Ph-2), which leads to a partial obliteration of the educt absorption bands and an appearance of short-wavelength shifted photoproduct absorption bands. Both photoreactions are photo- and thermo-reversible. A very interesting feature of the photoprocesses investigated is the dependence of the photoreaction output on the sample irradiation history. When a freshly cooled sample is irradiated by a red narrow-band laser, the main photoreaction is Ph-2. If the sample is preliminary irradiated by a broad-band source, PT dominates.