Effect of Alkyl Substituents on Molecular Packing and Luminescence Properties of Crystalline Thioxanthone Derivatives

Thioxanthone (TX) and its derivatives as important triplet sensitizers and photoinitiators have attracted a great deal of interest in photochemical studies; however, their luminescence properties receive less attention, which limits the understanding of the correlation between their structures and properties. Here, a series of alkyl chains (methyl (Me), ethyl (Et), propyl (n-Pr), and isopropyl (i-Pr)) are introduced on the 2-position and 4-position of TX to mainly investigate the correlation between the packing structure and the luminescence property of the resulting crystals. The alkyl substituents are found to have a negligible effect on the photophysical properties of TX-2-alkyl and TX-4-alkyl compounds in their dispersed states; however, single-crystal X-ray diffraction analysis and photophysical measurement demonstrate that the TX-2-alkyl and TX-4-alkyl crystals present distinctive packing motifs, leading to different luminescence properties. The TX-2-Me, TX-4-Me, TX-4-n-Pr, and TX-4-i-Pr crystals show a dimeric π–π packing motif; the TX-2-n-Pr and TX-4-Et crystals exhibit a long-range π–π monomer packing motif and the TX-2-Et and TX-2-i-Pr crystals display a herringbone packing motif. It is found that a change in molecular packing motif from dimeric π–π packing and monomeric π–π packing to herringbone packing generally corresponds to a gradually blue-shifted fluorescence and room-temperature phosphorescence (RTP) emission wavelengths. This work will provide a comprehensive understanding of the effect of the alkyl substituents on the packing structures and luminescence properties of the TX luminophore.