Formyl-Group Multiplicity Regulates Nonradiative Decay Through Intermolecular Locking in Tetraphenylethylene-Based Aggregation-Induced Emission Luminogens.

Aggregation-induced emission (AIE) is commonly attributed to the restriction of intramolecular motion upon aggregation. However, aggregation alone does not guarantee high solid-state photoluminescence efficiency, because residual molecular motions may still promote nonradiative decay. Here, four formyl-substituted tetraphenylethylene (TPE) derivatives, denoted TPE-nCHO, were comparatively investigated to clarify how substituent multiplicity and arrangement influence aggregate-state emission. Although all compounds exhibit typical AIE behavior, their solid-state photoluminescence quantum yields differ markedly: TPE-CHO shows only 20 ± 2%, whereas (Z)-TPE-2CHO, (E)-TPE-2CHO, and TPE-4CHO reach 89 ± 2%, 99 ± 1%, and 99 ± 1%, respectively. Fluorescence lifetime analysis shows that the low efficiency of TPE-CHO is associated with a substantially larger nonradiative decay contribution. Single-crystal structures and quantitative short-contact statistics indicate that the highly emissive (E)-TPE-2CHO and TPE-4CHO crystals possess more extensive and spatially distributed intermolecular contact networks than TPE-CHO. These structural trends are consistent with stronger intermolecular confinement and reduced nonradiative relaxation. Overall, efficient solid-state emission in this TPE series requires both a twisted geometry that avoids detrimental cofacial π-π stacking and a sufficiently developed intermolecular contact network; the actual degree of confinement is governed by both substituent multiplicity and packing topology.