Vibrational photochemistry of porphine imbedded in a n-hexane-d₁₄ shpol'skii matrix

The near-infrared-induced tautomerization of free-base porphine incorporated in a n-hexane-d₁₄ matrix is reported. Porphine occupies two sites in a n-hexane-d₁₄ matrix as opposed to one site in a n-hexane matrix. Two spectroscopically distinct tautomers, differing by a 90° rotation of the NH bond axes within the molecular plane, exist for each site. Nernst glower irradiation on the NH stretch fundamental does not induce tautomerization, but irradiation in the regions 4000 < v < 5100 cm^-1 and 5630 < v < 7700 cm^-1 does induce tautomerization. Narrow-band (8 cm^-1 fwhm) laser irradiation studies in the NH symmetric plus antisymmetric stretch combination band region (6300 < v < 6600 cm^-1) show that the near-infrared-induced tautomerization occurs at select wavelengths. Tautomer conversion in absence of site conversion is strong evidence that the observed photochemistry is due to absorption of light by porphine and not by hexane. The estimated quantum yield is roughly 5 × 10^-4 for 6530-cm^-1 irradiation; this is 1-3 orders of magnitude larger than the quantum yield expected from RRKM theory.