Pyrene Photochemistry in Solid n-Alkane Matrices: Comparisons with Liquid-Phase Reactions

The photochemically induced attachment of pyrene to a series of n -alkanes (C n, where n = 8, 19, 20, 21, 24, and 28), cyclohexane (CyH), and 1-eicosene (C=20) has been investigated as a function of solvent phase, pyrene concentration, radiation wavelength (especially below and above 300 nm), and alkane chain length. Qualitatively, the efficiency and selectivity of attachment are much greater in the solid than in the liquid phases of the alkanes. Efficiency is decreased significantly when the concentration of pyrene in the solid phases is increased above ca. 10 -5 M. The solid-state reactions are very selective when n ≥ 21 and the irradiation wavelength is > 300 nm. Under these conditions, only one photoattachment product, a 1-alkylpyrene, can be detected in each case. Mechanistic models consistent with these observations are advanced. They explain the changes in selectivity and efficiency based on the location of the pyrene molecules in the solid phase matrices and on the energy available to the highly excited states of pyrene which initiate the attachment processes. The procedures outlined provide a one-step recipe for the syntheses of some rather difficult to obtain photophysical probes.