Laser photochemistry and hole-burning of chlorin in crystalline n-alkanes at low temperatures

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Abstract
Photochemical hole-burning has been used to study selective and reversible phototautomerism in chlorin (dihydroporphin) incorporated in matrices of n-hexane, n-octane, and n-decane at 4.2 K. The energy difference between the S1↔S0 transitions of the two forms, distinguished by the position of the two inner protons, is ∼1500 cm−1; the stable tautomer absorbing near 6350 Å. The back reaction from the less stable form to the stable one is 103 times faster than the forward reaction, and under wide band excitation in the Soret band a dynamic equilibrium exists between the two forms. Very narrow holes (∼40 MHz) can be burnt in the inhomogeneous lines of both tautomers. The width of these holes is consistent with a limit imposed by the fluorescence lifetime of 8 ns.