Disparity in piezofluorochromism for twisted mono-carbazole-based AIEgens resulting from interchanging electron-rich substituents: effect of coplanarity on twisted π-conjugates

Abstract

Carbazole-based aggregation-induced emission-active fluorogens (AIEgens) with piezo-fluorochromic (PFC) features have enhanced the scope of a wide variety of applications in the field of optical recording and security displays. However, ambiguity resulting from the various research outcomes motivates researchers to find a smart strategy that can generate a significant number of PFC-active materials. This article reports the design of mono-carbazole (CBZ)-linked unsymmetrically decorated anthracenyl π-conjugates TMBπCBZ (Φ_F = 61%) and CBZπTMB (Φ_F = 43%) [π = anthracene-linked vinyl] as AIEgens and strong solid-state emitters. The electron-rich trimethoxybenzene (TMB) unit is introduced within this CBZ-linked anthracenyl π-conjugated system and the strategy of interchanging TMB and CBZ affords two small organic positional isomers in an inexpensive and easily accessible route. Interestingly, after grinding in a mortar and pestle or quick pressing (in an infrared pellet maker: 20 MPa), TMBπCBZ displays reversible spectacular PFC features with a 62 nm redshift, whereas CBZπTMB remains unchanged. Thus, the interchange between the TMB and CBZ units in the same scaffold results in notable PFC-behavior and establishes a novel structure–property relationship. A single crystal study disclosed the difference in the molecular twisting and packing in the solid-state for these isomers. A powerful intramolecular interaction (2.15 Å) between –OMe and olefinic-CH (π) in CBZπTMB imposes greater coplanarity to the system and achieves a relatively stable conformation, whereas TMBπCBZ attains a more twisted molecular conformation, and a large number of weak intermolecular interactions (mainly C⋯H, no π⋯π) with cavities within the crystal motif induce the PFC-activity. Thus, a novel concept is reported, the coplanarity effect in these twisted systems appears to be unfavorable for the exhibition of PFC-features in these AIEgens. This fact is further supported by the results of the powder X-ray diffractometry, differential scanning calorimetry, and lifetime measurement studies. In addition, the intermolecular interactions were quantified using Hirshfeld-surface analyses, which substantiated the observed facts. Finally, TMB-An-π-CBZ is recognized as a promising platform for rewritable optical-recording and security-based applications.