Utilizing the heterocyclic effect towards high contrast ratios of mechanoresponsive luminescence based on aromatic aldehydes

Abstract

A high contrast ratio is required by mechanoresponsive luminescent (MRL) materials to enable their adoption in mechanical sensors. However, the effective design principle remains restricted by the lack of understanding of the necessary features of such luminophores. In this contribution, a series of simple aromatic aldehydes, namely, APCz, APAd, APPo, and APPt, was designed and synthesized by combining different heterocyclic cores with 4-benzoyl substituted at the symmetrical positions. The heterocyclic cores N-butyl-carbazole (APCz), N-butyl-9,9-dimethyl acridine (APAd), N-butyl-phenoxazine (APPo), and N-butyl-phenothiazine (APPt) endowed the moderately twisted compounds with the ability of emitting luminescence in the solid state and in adopting metastable states in their pristine-crystal state. These features enabled phase and multicolor changes under external-force stimuli. Single-crystal analyses revealed that decreasing the type of noncovalent interactions by changing the heterocyclic cores from five- to six-membered rings with additional heteroatoms led to the formation of compounds sensitive to external force and to a dramatic increase in MRL contrast ratio from Δλ = 10 nm to Δλ = 53 nm. Overall, this work provided new insights into the design of high-contrast stimuli-responsive luminescent materials.