Synthesis and solid-state fluorescence properties of structural isomers of novel benzofuro[2,3-c]oxazolocarbazole-type fluorescent dyes

Structural isomers of novel benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type (3a) and benzofuro[2,3-c]oxazolo[5,4-a]carbazole-type fluorophores (4a), which differ in the position of oxygen and nitrogen in the oxazole ring, and their N-alkylated (R = butyl, benzyl and 5-nonyl) carbazole derivatives (3b-d and 4b-d) have been synthesized, and their photo-physical properties in solution and in the solid state have been investigated. Considerable differences in the absorption and fluorescence spectra were observed between structural isomers in both states. In solution, the fluorophore 3a exhibits much stronger absorption and fluorescence intensities than the fluorophore 4a. However, the two isomeric fluorophores exhibit similar fluorescence intensities in the crystalline state. In solution, the dyes 3a-d and 4a-d exhibited almost the same absorption and fluorescence spectra in each compound series. On the other hand, their solid-state fluorescence excitation and emission spectra were quite different, and a drastic fluorescence enhancement was found to be caused by the N-alkylation of the fluorophores; the fluorophores 3d and 4d (R = 5-nonyl) with sterically hindered substituents exhibited strong solid-state fluorescence properties. Semi-empirical molecular orbital calculations (AM1 and INDO/S) and solid-state fluorescent spectral analyses have been carried out to elucidate the effects of the substituents and chromophore skeleton on the photophysical properties of the two isomers (3 and 4) both in solution and in the crystalline state. On the basis of the results of calculations and the spectral analyses, the relations between the observed photophysical properties and the chemical structures of the benzofuro[2,3-c]oxazolocarbazole-type fluorophores are discussed. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).