Unraveling the significant role of inter-HBs blocking the dark state in the “OFF/ON” sensing mechanism of fluorescent probe detecting picric acid

Abstract

Photoinduced electron transfer (PET) and intermolecular hydrogen bonds (Inter-HBs) have been proposed as the sensing mechanism to detect picric acid (PA). However, the mechanism of turn-off/on fluorescence sensors is very complicated and needs theoretical investigations in detail. In this work, we attempted to theoretically address the photophysical properties of the boron dipyrromethene (BODIPY) and the acridine derivatives for the detection of PA. We chose two probes 5-(1,3,5,5,7,9-hexamethyl-9a,10-dihydro-5H-5 l4,6l4-dipyrrolo[1,2-c:2′,1′-f][1,3,2]diazaborinin-10-yl)benzene-1,3-diamine (BDP) and 9-(4-aminophenyl)-3,3,6,6-tetramethyl-10-phenyl-3,4,6,7,9,10-hexahydroacridine-1,8(2H,5H)-dione (ACR) to investigate the sensing mechanism. The results show that dispersion correction plays a crucial role in describing weak intermolecular interactions. The energy interleaving between the dark charge transfer (CT) and bright localized excited (LE) states is the driving force of the sensor's fluorescence quenching. After reacting with PA, the generated Inter-HBs of products BDP-PA and ACR-PA can significantly increase the energy of the dark CT state or directly changes the CT state to the LE state, thereby blocking the nonradiative decay of PET and opening the radiative pathway. This work reveals the important role played by the Inter-HBs in the fluorescence process and could provide a guide for the further design of novel fluorescent probes based on BODIPY and the acridine derivatives.