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.