Confined Electrochemiluminescence Generation From Individual Molecular Crystals Assembled via the Coffee-Ring Effect

Molecular crystals (MCs) can confine and propagate self-generated electrochemiluminescence (ECL), yet the relationship between crystal morphology and ECL performance remains largely unexplored. The main challenge lies in achieving precise morphological control. Herein, we exploit the coffee-ring effect in evaporating droplets as a spatiotemporal confinement strategy to direct the self-assembly of 9,10-bis(phenylethynyl)anthracene (BPEA) MCs. Real-time optical microscopy reveals that the coffee-ring effect generates ring-shaped deposits on the electrode surface, where crystals exhibit three distinct distribution patterns from the edge to the center, namely orderly packing at the periphery, well-separated dispersion and clustered aggregation in the inner portion. The well-separated crystals in the intermediate zone enable in situ ECL imaging of four distinct morphologies, including rod-like, etched, needle-like, and branched, within a single assembly. Notably, rod-like crystals exhibit strong waveguiding behavior with enhanced ECL emission at their terminals, whereas branched crystals show no obvious waveguiding effect. This work provides new insights into the correlation between crystal morphology and ECL performance at the single entity level.