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

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.