Crystalline Indole at High Pressure: Chemical Stability, Electronic, and Vibrational Properties

Vibrational and electronic spectra of crystalline indole were measured up to 25.5 GPa at room temperature in a diamond anvil cell. In particular, Fourier transform infrared (FTIR) spectra in the mid-infrared region and two-photon excitation profiles and fluorescence spectra in the region of the HOMO-LUMO transitions were obtained. The analysis of the FTIR spectra revealed a large red-shift of the N-H stretching mode with increasing pressure, indicating the strengthening of the H-bond between the NH group and the pi electron density of nearest neighbor molecules. The frequencies of four vibronic bands belonging to the L-1(a) and L-1(b) systems were obtained as a function of pressure. Comparison with literature data shows that the crystal acts as a highly polar environment with regard to the position of the L-1(b) origin and of the fluorescence maximum, which are largely red-shifted with respect to the gas phase or to solutions in apolar solvents. A large, and increasing with pressure, frequency difference between the L-1(b) origin and the blue edge of the fluorescence spectrum suggests that the emitting state is L-1(a) that is known to be more stabilized than L-1(b) by dipolar relaxation. Crystalline indole was found to be very stable with respect to pressure-induced reactivity. Only traces of a reaction product, containing saturated C-H bonds, are detected after a full compression-decompression cycle. In addition, differently from many unsaturated compounds at high pressure, irradiation with light matching a two-photon absorption for a HOMO-LUMO transition has no enhancing effect on reactivity. The chemical stability of indole at high pressure is ascribed to the crystal structure, where nearest neighbor molecules, forming H-bonds, are not in a favorable position to react, while reaction between equivalent molecules, for which a superposition of the pi electron clouds would be possible, is hindered by H-bonded molecules. Consistently, no excimer emission was observed except at the cell opening at the end of the compression-decompression run. Extremely limited chemical reactivity and excimer formation likely occur at crystal defects, evidencing the strict connection between the two phenomena.