Regulation of aggregation-induced emission and reversible remarkable mechanofluorochromism by substitution position of the carbazole unit in D-A type carbazole-functionalized dicyanoethylene π-systems

Abstract

In this study, two twisted D-A type dibenzothiophene- and carbazole-functionalized dicyanoethylene derivatives were formulated and synthesized. They were labeled as 2-substituted carbazole (BT-CE-C2) and 3-substituted carbazole (BT-CE-C3), respectively. Furthermore, an investigation into the correlation between the molecular structure of these derivates and their photophysical properties was conducted. The characteristic D-A structures and the highly distorted spatial conformations of the two fluorophores led to the emergence of a unique intramolecular charge transfer (ICT) effect and remarkable aggregation-induced emission (AIE) behavior. Notably, the position of substitution of the carbazole unit regulated the reversible high-contrast mechanofluorochromic (MFC) properties demonstrated by BT-CE-C2 and BT-CE-C3. The as-prepared solid powders of BT-CE-C2 and BT-CE-C3 produce blue-green and orange-yellow emissions at 508 and 567 nm, respectively. However, their ground powders emitted orange (at 589 nm) and orange-red (at 619 nm) lights. Notably, BT-CE-C2 exhibited enhanced fluorescence upon external stimulation, increasing its solid-state luminescence efficiency from 0.035 to 0.189. In contrast, the BT-CE-C3 exhibited a decrease in fluorescence, causing its solid-state luminescence efficiency to change from 0.426 to 0.348. The fluorescence lifetimes, powder X-ray diffraction (PXRD), and spectral analysis collectively identified the phase transition between the crystalline and amorphous states as the underlying cause of the MFC behavior observed in BT-CE-C2 and BT-CE-C3. Additionally, the red shift in photoluminescence (PL) spectra under external stimulus can be attributed to planarized-induced charge transfer (PICT), exciton coupling, increased π-π interactions, and enhanced orbital overlap between adjacent molecules. This combination of factors led to the observed MFC properties of these compounds.