Triplet-Singlet Emission Spectra of Xylenes in Crystalline State at 4.2° and 77°K, and in EPA at 77°K

Abstract

The triplet-singlet emission spectra of xylenes in the crystalline state at 4.2° and 77°K and in a rigid glass at 77°K have been photographed and analyzed. The samples were obtained from the National Bureau of Standards. They were kept sealed under an oxygen-free atmosphere of helium gas throughout the experiments. Large well-formed crystals were grown from the vapor on a surface cooled by liquid nitrogen, and some were grown from the melt. At 4.2°K o-xylene produced two sharp spectra similar in structure and separated by 370 cm-1 from each other. These spectra are ascribed to two distinct crystalline phases. At 77°K o-xylene did not produce a significant phosphorescence. The spectrum of m-xylene at 4.2°K consisted of a strong diffuse spectrum and a weak sharp spectrum toward shorter wavelengths. The diffuse spectrum appeared to be associated with the amorphous phase. At 77°K a broad spectrum appeared which was found to be characteristic of an aldehyde. At 4.2°K p-xylene showed a sharp spectrum beginning at 25 254 cm-1. It differs in vibrational structure from the spectrum of this compound in a rigid glass. A vibrational analysis indicated that the carrier of this spectrum is p-methylbenzaldehyde. With crystals of the p-xylene grown from the melt the genuine spectrum due to p-xylene was observed between 28 246 and 25 416 cm-1, over-lapped by the spectrum of p-methylbenzaldehyde in the longer wavelength region. These results suggest a temperature-activated sensitization which had been first observed in toluene.