Laser-induced photochemical isomerization of free base porphyrin in ann-octane crystal at 4·2 K

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Abstract
It is known that the porphyrin free base (H2P) can be incorporated in an n-octane Shpol'skii matrix. In slowly grown crystals the solute molecules occur in two orientations, which are co-planar and have their N-H … H-N axes at right angles; in the S 1↔S 0 ‘quasi-line’ absorption and fluorescence spectra they can be distinguished by a relative shift of 65 cm-1 of their spectral features. By irradiation with light the two species are transformed into one another via a shift of the two inner protons. Here a study is presented of the photoisomerism following selective excitation with a dye laser into the 0-0 band of the S 1←S 0 transition of either of the two forms, for H2P and D2P (two inner hydrogens replaced by deuterium). At 4·2 K the reaction proceeds in both directions; for H2P the reaction in one direction appears to be 10 per cent faster than in the other, for D2P the absolute rates are nearly ten times lower than for H2P and not measurably different in the two directions. The reactions are not significantly enhanced when the experiment is carried out at 77 K, or by excitation into the Soret band, i.e. via absorption of photons that are 8000 cm-1 more energetic. A possible reaction mechanism is discussed: the intersystem crossing from S 1 into the triplet manifold may lead to a displacement of the inner hydrogens because of vibronic coupling between the two lower triplet states. Finally the isotope effect on the T 0→S 0 decay rate has been determined: for H2P =92 s-1, for D2P =46 s-1.