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Fluorinated Diphenylpolyenes:  Crystal Structures and Emission Properties

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posted on 27.12.2007, 00:00 by Yoriko Sonoda, Midori Goto, Seiji Tsuzuki, Nobuyuki Tamaoki
(E,E,E)-1,6-Diaryl(Ar)-1,3,5-hexatrienes (2, Ar = 4-fluorophenyl; 3, Ar = 2,4-difluorophenyl; 4, Ar = 2,4,6-trifluorophenyl; 5, Ar = perfluorophenyl) and (E,E,E)-1-perfluorophenyl-6-phenyl-1,3,5-hexatriene (6) were prepared. The absorption and fluorescence spectra in methylcyclohexane solution showed only a small dependence on the fluorine ring substituent, and were similar to those of the unsubstituted parent compound (1, Ar = phenyl). The solid-state absorption and fluorescence spectra shifted to red relative to those in solution and strongly depended on the substituent. The emission from crystals 15 originated mainly from monomeric species with the maximum wavelength (λf(max)) of 440−465 nm, which overlapped the emission from molecular aggregates (14) or excimeric species (5) in the red region. Crystal 6 exhibited red-shifted (λf(max) = 530 nm) and structureless emission due to excimers. The cocrystal of 1 and 5 (1/5) showed red-shifted (λf(max) = 558 nm) and distinctly structured emission, not from exciplexes but from the excited states of molecular aggregates in which molecules 1 and 5 strongly interact already in the ground state. These assignments were confirmed by the results of fluorescence lifetime and quantum yield measurements in the solid state. Single-crystal X-ray structure analyses showed that the molecules were basically planar in each crystal, whereas the crystal packing was strongly substituent-dependent. Weak π−π interactions in the herringbone (1 and 2) and in the π-stacked but largely offset structures (3 and 4) account for their predominantly monomeric origin of emission. The observation of excimer fluorescence from 5 was rather unexpected, since the molecules in this crystal were arranged in an offset stacking fashion due to perfluorophenyl−perfluorophenyl (C6F5···C6F5) interaction. The structures of 6 and 1/5 considerably resembled each other, in which molecules were π-stacked with more face-to-face geometries than those in 5, as a result of strongly attractive perfluorophenyl−phenyl (C6F5···C6H5) interaction. Nevertheless, the fluorescence origin was clearly different for 6 and 1/5. This can be ascribed to the difference in the strength of orbital−orbital interaction between molecular π-planes in the ground and excited states in crystals.