Optical and conformational studies on benzobisthiazole derivatives.

2,6-Didodecyl-4,8-diphenyl-benzo[1,2-d;4,5-d']bisthiazole (3) and 2,6-didodecyl-4,8-dipyrrole-2-yl-benzo[1,2-d;4,5-d']bisthiazole (5) were synthesized, and their optical properties were investigated in solution and in the solid state. Compounds 3 and 5 were excited with the 325 nm He-Cd laser line to produce blue and green luminescence, respectively. The luminescence of 5 (Phi = 14%) was more efficient than that of 3 (Phi = 5%). Structural and optical properties were further determined with DFT and ZINDO calculations. The planar structure of 5 results in pi --> pi* electronic transitions from the pyrrole moiety to the benzobisthiazole frame, while the twisted geometry of 3 results in luminescence strongly associated with the pi --> pi* transitions within the benzothiazole frame. The effect of solvent on the luminescence properties of 5 is summarized as competition between intra- and intermolecular NH...N hydrogen bonds.

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