Fluorinated Diphenylpolyenes: Crystal Structures and Emission Properties

(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 1−5 originated mainly from monomeric
species with the maximum wavelength (λf(max)) of 440−465 nm, which overlapped the emission from molecular
aggregates (1−4) 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.