Bromine-substituted p-nitrostilbene derivatives: synthesis, crystal structural studies, photoluminescence and the heavy atom effect on the singlet oxygen generation by two-photon absorption

A variety of linear and side-chained p-nitrostilbene derivatives with various numbers of bromine atoms were prepared to survey the crystal structural properties, the photoluminescence, and heavy atom effect on the singlet oxygen generation by two-photo absorption (TPA). Single crystals of 4-nitro-4′-(4″-bromo-phenyl-methyl-oxy)-diphenylethylene (C2), 4-nitro-4′-(3″,5″-dibromo-phenyl-methyl-oxy)-diphenylethylene (C3), 4-nitro-4′-(2′,3″,4′,5″,6″-pentabromo-phenyl-methyl-oxy)-diphenylethylene (C4) were obtained, and the structural characteristics were analyzed by X-ray diffraction. One- and two-photon optical properties of the photosensitizers are shown dependence on the numbers of substituted bromine atoms. While TPA cross-sections of the photosensitizers are diminished more considerably, singlet oxygen quantum yields of the photosensitizers are enhanced at some extents by the substituted bromine atoms. Side-chained photosensitizers display correspondingly higher singlet oxygen quantum yields and larger TPA cross-sections than the molecules with single bromine-substituted aromatic segment. Molecular modeling was also performed to reveal the fundamental reasons of the experimental observation. Photooxidation reaction of singlet oxygen with some substrates was employed to confirm singlet oxygen generation under one- and two-photon irradiation.