Developing piezochromic luminescent materials via regioselective cyanation of naphthalimide–cyanostilbene derivatives

Abstract

Piezochromic materials (PCMs) are highly valuable in advanced photonics and intelligent technologies. However, predicting piezochromic responses, a priori, in the design stage remains a formidable challenge. Herein, a novel series of PCMs, NICN-R (R = 1C, 2C, 3C and 4C), are designed and developed by incorporating naphthalimide (NI) and cyanostilbene (CN) with various alkoxyl chains (–R). Within a broad pressure range of ≈10 GPa, the initially synthesized NICNα-R molecules exhibit remarkable changes in the visible colors of photoluminescence emission. The pressure coefficients of emission shifts, ranging from 13.1 nm GPa⁻¹ to 16.3 nm GPa⁻¹, are considerably large in PCMs. To enhance the piezochromic effects, NICNβ-R molecules are further synthesized through regioselective cyanation. The pressure coefficients are obviously increased to 17.8–20.4 nm GPa⁻¹, attributed to the restrained molecular twisting and promoted intramolecular charge transfer. This study unveils the pivotal influence of the substitution position/length in molecular contraction and planarization under high pressure, which ultimately determines the piezochromic responses. It not only elucidates the mechanisms behind piezallochromy, but also proposes innovative design concepts for developing sensitive PCMs across broad pressure ranges.