Elaborating the mechanism of a highly selective fluorescent ‘turn-on’ probe to detect the group IIIA ions: a detailed time-dependent density functional theory study

Aluminum (Al), gallium (Ga) and indium (In) are three essential elements in group IIIA of the periodic tables, which exhibit similar chemical behaviors in aqueous solutions. A novel fluorescent probe 4-hydrazino-7-nitrobenzofurazan-2-hydroxy-1-naphthaldehyde (NBD-hnap) by a simple conjugation of NBD and hnap molecules is found to detect the IIIA group ions based the ‘turn-on’ fluorescent. Here, the sensing mechanisms of probe NBD-hnap to detect IIIA group ions are explored by density functional theory (DFT) and time-dependent DFT (TDDFT) theoretical methods. In aqueous solution, the NBD-hnap has two absorption peaks, and the fluorescence intensity is almost zero during the emission process. After adding IIIA group ions, the absorption spectrum was blue shifted and showed strongly red fluorescence signal. In our calculation, the presence of the C=N bond may induce a twisted intramolecular charge transfer (TICT) state. Thus, the fluorescence quenching of NBD-hnap is caused by TICT process. When NBD-hnap is coordination with metal ions, the TICT state is eliminated.