A red-emission diketopyrrolopyrrole-based fluoride ion chemosensor with high contrast ratio working in a dual mode: Solvent-dependent ratiometric and “turn on” pathways

A new red-emission diketopyrrolopyrrole (DPP)-based fluoride ion chemosensor (DPP-4) with two strong electron-donating triphenylamine groups was designed and synthesized. The sensing properties of DPP-4 for F− were extensively investigated both in acetone and in acetonitrile solutions. Deprotonation of the N–H groups on DPP-4 provided rapid and selective detection of F− in which an apparent color change from red to purple with large red shift in ambient light can be observed by the naked-eye in both two solvents. However, very distinct sensing behaviors were obtained in the fluorescent changes exhibiting solvent-dependent dual mode approaches, i.e. ratiometric and “turn on” modes in acetone and acetonitrile, respectively. In acetone, ratiometric channel was activated upon addition of n-Bu4NF, and more than 50 nm red shift from 590 nm to 641 nm was obtained and accompanied with emission color from orange to purple red. In acetonitrile, a “turn on” response with red emission at 631 nm can be easily recognized, and more than 50-fold enhancement were obtained. TD-DFT calculations indicated that DPP-4 performed an intramolecular charge transfer (ICT) process between triphenylamine to DPP core. Interestingly, the reverse charge transfer process from deprotonated DPP core to triphenylamine moieties was obtained after addition of F−, which resulted in red-shift in absorption spectra of the compound. Furthermore, using fluoride and HSO4− anions as two chemical inputs and absorption or fluorescence as output, compound DPP-4 was demonstrated to be a parallel double-INH logic gate in acetonitrile solution.