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

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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.