Color Tuning of Mechanochromic Luminescent β-Diketones via Boron Coordination and Donor-Acceptor Effects

Abstract

Owing to their facile synthesis and desirable optical properties, β-diketones (bdks) are attractive scaffolds for the production of new mechanochromic luminescent (ML) materials. However, ML-active bdk dyes that can access full color luminescent states (i.e., red, green, and blue) have yet to be realized. To synthesize such a set of bdk emitters, two strategies were used to red-shift the emission: substitution with varying donor and acceptor groups; and coordination of BF2 to the β-diketonate moiety. Additionally, 3,4,5-trimethoxy-substituted phenyl rings were used to disrupt the molecular packing to promote ML and enable aggregation-induced emission (AIE). Here, a series of trimethoxy-substituted bdks were prepared via Claisen condensation, each para-substituted with either F, I, or CN groups. Their boronated complexes were synthesized via reaction with boron trifluoride diethyl etherate. Optimized geometries generated from density functional theory calculations indicated that the dye para-substituted methoxy groups were directed out of the molecular plane, potentially leading to loose solid-state packing. The AIE studies using tetrahydrofuran/H2O solutions indicated AIE activity in both bdk dyes and boron complexes. High-contrast ML was also observed for each dye investigated. Furthermore, this set of dyes exhibited emissions crossing the entire visible spectrum, thereby demonstrating a strategy for color tuning and promoting ML and AIE in bdk systems.