Filling the gap between molecular and aggregate states: how does molecular packing affect photophysical properties?

Current research shows that photophysical behaviors of fluorescent materials are used to be regulated by molecular design, but in fact it remains elusive to directly correlate molecular structures with the macroscopic properties at aggregate state. Herein, a series of natural alicycle-incorporated salicylideneanilines with aggregation-induced emission activity as well as their corresponding simple ones have been synthesized to fill the gap between molecular and aggregate level. Unexpectedly, the largely stronger emission of the simple salicylideneaniline with Br substituent over its alicycle-fused compound contrasts the previous molecular design strategy of rigidification effect via structural incorporation of alicycles, suggesting the influential effect of packing, which can be further verified by quenched emission of the simple Cl-substituted salicylideneaniline due to its compact packing. Moreover, comprehensive stimuli-responsive chromic investigations prove that the packing modes correlate well with the excited-state intramolecular proton transfer and cis to trans isomerization processes, enabling the Me-substituted simple salicylideneaniline with loosest packing photochromic, while the one (Cl group) with most compact packing mechanochromic. Hence, this work helps to bridge the gap between molecular structures and macroscopic properties via molecular packing.