Pyridazine–carbazole based fluorescent probes for volatile acid detection

Abstract

Stimuli-responsive materials based on donor–acceptor systems have great potential for sensing applications, including in the solid state. Herein, two push–pull-type molecules, 3,6-di(9H-carbazol-9-yl)pyridazine (CzPyr–H) and 3,6-bis(3,6-dimethoxy-9H-carbazol-9-yl)pyridazine (CzPyr–OMe), each of which contain two carbazole donors and a pyridazine acceptor, were designed and synthesized. The photophysical properties of CzPyr–H and CzPyr–OMe were systematically explored with the aim of switching the absorption and emission properties using trifluoroacetic acid (TFA) as an external analyte, thus realizing volatile acid sensing. The protonation of the central pyridazine unit led to a broad absorption band at longer wavelengths owing to intramolecular charge transfer (ICT), and the emission intensity gradually decreased with increasing TFA concentration. Density functional theory calculations of the HOMO–LUMO energy gaps and orbital distributions of CzPyr–H and CzPyr–OMe as well as their protonated forms, CzPyr–H(H+) and CzPyr–OMe(H+), confirmed that charge transfer occurred in the protonated species. In their aggregated solid states, both compounds showed interesting emission properties including aggregation-enhanced emission (AEE). Utilizing their TFA sensitivity and the AEE phenomenon, CzPyr–H and CzPyr–OMe were successfully applied as fluorescent probes for volatile acid detection and as security ink for information encryption on paper.