The near-infrared photochemistry of porphine imbedded in an N-hexane matrix

The near-infrared induced tautomerization of free-base porphine incorporated in a n-hexane-d₁₄ matrix is reported. Nernst glower irradiation on the NH stretch fundamental does not induce tautomerization, but irradiation in the regions 4000 < ν < 5100 cm^-1 and 5630 < ν < 7700 cm^-1 does induce tautomerization. Narrowband (8 cm^-1 fwhm) laser irradiation studies in the NH symmetric plus antisymmetric stretch combination band region (6300 < ν < 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 porphine absorption and not hexane absorption. The estimated quantum yield of 3×10^-5 for 6530 cm^-1 irradiation is at least one order of magnitude larger than the quantum yield expected from RRKM theory.