부산대학교 도서관

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 π electron density of nearest neighbor molecules. The frequencies of four vibronic bands belonging to the 1Laand 1Lbsystems 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 1Lborigin 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 1Lborigin and the blue edge of the fluorescence spectrum suggests that the emitting state is 1La, that is known to be more stabilized than 1Lbby 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, formig H-bonds, are not in a favorable position to react, while reaction between equivalent molecules, for which a superposition of the π 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. [ABSTRACT FROM AUTHOR]