Flanking-donor-controlled diversity in mechanical-force-induced reversible fluorochromism and enhanced emission for carboxylic acid and ester linked solid-state emitters

Abstract

The invention of small organic molecules that conserve their exciting optical properties in the solid state is in high demand. However, new strategic molecular designs are still valuable due to their irregularity. Herein, we present the variance in the reversible mechanical-force-induced fluorescence switching (MFC) and mechanical-force-induced enhanced emission (MIEE) for suitably substituted mono-carboxylic acid or ester-linked anthracenyl π-conjugates. When comparing the MFC/MIEE effects of carboxyl or ester-linked emitters, the carboxyl group primarily induces MFC behavior through H-bonding modulation, while the ester analogs display MIEE, which is governed by variation in the molecular packing style and structural orientation. A similar pattern is observed in the present systems when thiophene is used as a small flanking substituent. Interestingly, the replacement of thiophene with dimethoxybenzene results in notable MFC and MIEE features for both the carboxylic ester and acid analogs. These donor-substituent-dependent features of the carboxylic acid/ester analogs are fascinating and are identified here for the first time. Detailed single crystal X-ray diffraction (SC-XRD) studies for all the molecules reveal the intrinsic variations in the molecular conformation and packing due to the various intermolecular interactions. The dimethoxybenzene moiety results in more flexible intermolecular interactions and offers diversity. Furthermore, the observed outcomes are supported by powder XRD, excited state lifetime, and differential scanning calorimetry (DSC) studies. The Hirshfeld surface analysis from a single crystal is carried out to partly demonstrate the collective role of non-covalent interactions and available void space in these outcomes. We have demonstrated the use of the carboxylic acid and ester analogs as MFC and MIEE platforms for reusable security writing.