Decorating BODIPY with electron-rich unit THDTAP: An ICT-based fluorometric sensor toward peroxide, acid, and electrochemical stimuli

Abstract

A new type of BODIPY dye, compound 2 which possesses donor-acceptor (D-A) type structure, is synthesized by decorating electron-rich THDTAP unit onto the meso-position of BODIPY. The THDTAP has two chemically active sites (S and N atoms) that are utilized to manipulate the optoelectronic properties. By stepwise oxidation of two S-atoms on THDTAP moiety with m-CPBA, the 2 was converted to 3, trans-4, syn-4, 5, and 6. This transformation results in large influence on optoelectronic properties. The theoretical calculations reveal that the LUMOs of 2–6 are all located on BODIPY core, whereas their HOMOs show the different distributions. The 2 and 3 show the intramolecular charge-transfer (ICT) transition as proved by TD-DFT calculations, red-shifts of absorption and emission bands, and solvatochromism. The ICT process is inhibited in trans-4, syn-4, 5, and 6 due to the high oxidation states of S-atoms on THDTAP moieties. As a result, these four compounds display characteristic fluorescence of BODIPY. The femtosecond transient absorption (fs-TA) spectroscopic study reveals that 2 shows photoinduced hole-transfer (PHT) process in dichloromethane (DCM) solution. Upon acidification and neutralization of N-atom on THDTAP moiety, the ICT/PHT process of 2 in DCM is reversibly manipulated to result in fluorescence ON/OFF of BODIPY core. Moreover, the fluorescence of BODIPY core on 2 can be switched on through electrochemical oxidation of N-atom on THDTAP moiety. The compound 2 is thus a promising fluorometric sensor toward peroxide, acid, and electrochemical stimuli.