A fluorescence emission study of nitro- and nitromethyl-substituted 1,4-diarylbutadienes in solid state

摘要

1-(4-Nitrophenyl)-4-phenylbuta-1E,3E-diene 1, 1,4-di (4-nitrophenyl)buta-1E,3E-diene 2, 1-phenyl-2-methyl-4-(4-nitrophenyl) buta-1E,3E-diene 3, and 1-4-di (4-nitrophenyl)buta-1E,3E-diene 2, 1-phenyl-2-methyl-4-(4-nitrophenyl) buta-1E,3E-diene 3, and 1-4-di (4-nitrophenyl)-2-methylbuta-1E,3E-diene 4 have been synthesized and their fluorescence emissions in solid state in comparison with their ambient temperature fluorescence emissions in organic solvents of varying relative permittivity and i ethanol-methanol matrix at 77 K have been investigated. It has been found that while the UV-vis absorption of the dienes in solution phase largely remains unaffected by solvent polarity, the fluorescence maximum (#lambda#_f max) gets significantly altrered and a much red-shifted #lambda#_f max is observed in solvents of relatively higher relative permittivity (3.g. polar solvents like acetonitrile and methanol). In ethanol-methanol glass at 77 K, the fluorescence of the dienes gets significantly blue-shifted. In solid state also a red-shifted as compared to in solvents having relatively lower relative permittivity (e. g. the non-polar solvents like 1,4-diooxane and THF). The fluorescence spectra of nitro-dienes, particularly 2 and 4 in solid state are significantly red-shifted as compared to their fluorescence emission in relatively non-polar solvents. Further, for the mono-nitro substituted dienes 1 and 3, the solid state fluorescence emission is similar to that observed in relatively non-polar solvents. The results have been discussed in terms of the singlet excited states of the dienes having different polarity and geometry. It has been suggested that in solution phase in addition to the fluorescence emission from the initially prepared planar locally excited state, the fluorescence emission in these dienes can occur from intramolecular charge transfer excited state and in some cases even from a non-planar, dipolar conformationally relaxed intramolecular charge transfer excited state. The red-shifted emission of these dienes insolid state when compared tothat in relatively less polar solvents, and in another highly condensed media such as ethanol-methanol glass at 77 K indicates that the solid state fluorescence of the dienes does not originate from conformationally relaxed excited states. The solid state fluorescence emission of the dienes has been attributed to excimer and intramolecular charge transfer excited state with planar geometry. Further, the role of energy level re-ordering of the two lowest excited states of the dienes, influencing the fluorescence properties has also been discussed.