Pressure-Induced Fluorescence of Pyridine

Abstract

Two-photon excitation profiles and fluorescence spectra have been measured as a function of pressure in a diamond anvil cell up to 15.5 GPa in crystal phases I and II and in the glassy form of pyridine. The fluorescence emission intensity increases by about 6 orders of magnitude in going from the liquid to the crystalline phases at 3 GPa and further increases with pressure. This is explained by an energy inversion of the lowest (1)B(1) (nπ*) and (1)B(2) (ππ*) excited states likely due to the involvement of the lone pair of the N atom in intermolecular CH···N bonds. These interactions characterize the crystal phases and are stabilized by pressure. The glassy form, accordingly, is characterized by a much weaker fluorescence. Excimer emission is also observed. Comparison of the emission of several samples with different compression and annealing histories, the lack of reversibility in the excimer emission with decompression, and the larger relative intensity of the excimer band in the glassy form suggest that excimer formation occurs at crystal defects. This results support the conclusions of a previous investigation proposing that pressure-induced reactivity of pyridine is limited to crystal defects and agrees with the present knowledge of the solid-state chemistry of aromatic crystals.