Insight into Isomeric Effect on the Photoluminescence and Mechanoluminescence of Cyanostilbene Derivatives

Molecular structures, packings, and intermolecular interactions
significantly affect the photophysical properties of organic luminogens.
In this work, the photoluminescence (PL) and mechanoluminescence (ML)
of two pairs of isomers, 1/2 and 3/4, were systematically explored. The fluorescence of
crystals 1c and 4c is much brighter than
that of their isomers 2c and 3c, respectively.
Only 1c is ML-active among all four molecules. Single-crystal
structural analysis revealed that isomerization of a substituent group
affected their molecular packing and intermolecular interactions.
Stronger intermolecular interaction and intact three-dimensional hydrogen-bonded
networks were formed only in crystal 1c, which were essential
for preventing slippage of molecular layers and generating ML; the
other molecules were either lacking π–π interactions
or C–H···π interactions. Theoretical calculation
suggested that the energy barrier between the Franck–Condon
(FC) structure and minimum energy crossing point (MECP) structure
of 2/3 was much lower than that of 1/4. Nonradiative decay channels of molecules 2 and 3 were thus more easily activated, which
led to their lower quantum yield.

分子结构、堆积方式与分子间相互作用，对有机发光体（organic luminogens）的光物理性质具有显著影响。本研究系统探究了两对同分异构体（1/2与3/4）的光致发光（photoluminescence, PL）与机械发光（mechanoluminescence, ML）性能。晶体1c与4c的荧光强度，分别显著高于其同分异构体2c与3c。四种分子中仅晶体1c具备机械发光活性。单晶结构分析表明，取代基的异构化会改变分子的堆积模式与分子间相互作用。仅晶体1c形成了更强的分子间相互作用与完整的三维氢键网络，这对阻止分子层滑移、产生机械发光至关重要；其余分子要么缺乏π-π堆积相互作用，要么缺失C-H···π相互作用。理论计算结果显示，2/3的弗兰克-康登（Franck–Condon, FC）结构与最低能量交叉点（minimum energy crossing point, MECP）结构之间的能垒，远低于1/4对应的能垒。因此分子2与3的非辐射衰变通道更易被激活，进而导致其量子产率更低。