Crystal Structures and Fluorescence Spectroscopic Properties of a Series of α,ω‐Di(4‐pyridyl)polyenes: Effect of Aggregation‐Induced Emission

Crystal structures and fluorescence spectroscopic properties were investigated for a series of all‐( E ) α,ω‐di(4‐pyridyl)polyenes ( 1 – 5 ) with different number of double bonds ( n ). Molecules 1 and 2 ( n =1, 2) in crystals are arranged to form partially π‐overlapped structures, whereas those of 3 – 5 ( n =3–5) are stacked in a herringbone fashion. All these molecules, the shorter polyenes in particular, are almost nonfluorescent in solution. In the solid state, 1 and 2 are highly emissive as pure organic solids [fluorescence quantum yields ( φ f )=0.3–0.5], while 3 and 4 are only weakly fluorescent ( φ f <0.05). The strongly n ‐dependent fluorescence properties can be attributed to the largely different molecular arrangements in the crystals. Although 5 is nonfluorescent in the solid state, we observe a very clear structure‐property relationship in 1 – 4 . Compounds 1 and 2 become much more emissive in the solid state than in solution as a result of the aggregation‐induced emission (AIE) effect.