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.

(E,E,E)-1,6-二芳基(Ar)-1,3,5-己三烯（2, Ar=4-氟苯基；3, Ar=2,4-二氟苯基；4, Ar=2,4,6-三氟苯基；5, Ar=全氟苯基）以及(E,E,E)-1-全氟苯基-6-苯基-1,3,5-己三烯（6）均已合成。
甲基环己烷溶液中的吸收光谱与荧光光谱仅随环上氟取代基发生微小变化，且与未取代母体化合物（1, Ar=苯基）的光谱特征相似。
固态吸收与荧光光谱则相较于溶液态发生红移，且显著受取代基影响。
晶体1~5的发射主要来自单体物种，荧光最大波长（λf(max)）为440~465 nm，其发射与红色区域内分子聚集体（1~4）或激基缔合物（excimer）的发射发生重叠。
晶体6则因激基缔合物产生红移（λf(max)=530 nm）且无精细结构的发射。
共晶体1/5的发射同样红移（λf(max)=558 nm）且具有清晰的精细结构，其发射并非来自激基复合物（exciplex），而是来自分子聚集体的激发态——该聚集体中分子1与5在基态已存在强相互作用。
上述归属通过固态下的荧光寿命与量子产率测试结果得到了验证。
单晶X射线结构分析表明，各晶体中的分子基本呈平面构型，但晶体堆积方式显著依赖于取代基。
晶体1和2采用人字型堆积（herringbone），晶体3和4则为π堆叠（π-stacked）但大幅错位的结构，这两种情况下的弱π-π相互作用是其发射主要源自单体的原因。
晶体5的激基缔合物荧光现象颇为意外，因为该晶体中的分子因全氟苯基-全氟苯基（C6F5···C6F5）相互作用而采用错位堆叠排布。
晶体6与共晶体1/5的结构高度相似，二者的分子均因强吸引力的全氟苯基-苯基（C6F5···C6H5）相互作用而采取π堆叠构型，且相较于晶体5具有更面对面的堆叠几何结构。
然而，晶体6与1/5的荧光起源却明显不同，这可归因于晶体中分子π平面在基态与激发态下的轨道-轨道相互作用强度存在差异。