Effect of the Electron Donor/Acceptor Orientation on the Fluorescence Transduction Efficiency of the d-PET Effect of Carbazole-Based Fluorescent Boronic Acid Sensors

We have synthesized three new carbazole-based fluorescent boronic acid sensors to investigate the fluorescence transduction efficiency of the novel d-PET effect, in which the fluorophore acts as the electron donor and the protonated amine/boronic acid group as the electron acceptor of the photoinduced electron transfer process (PET). Aryl ethynyl groups are attached at the 3,6-position of carbazole (aryl = 4-dimethylaminophenyl for sensor 1 or phenyl for sensor 2). Sensor 3 is without 3,6-substitutions. The phenylboronic acid moiety is attached at the 9-position (N-atom) of the carbazole in these sensors. We found that 1 and 3 are d-PET sensors (fluorophore as the electron donor, supported by DFT/TDDFT calculations), which show diminished emission at acidic pH but intensified emission at neutral/basic pH, which is in stark contrast to the normal a-PET (fluorophore as the electron acceptor) sensors, e.g., 2, which shows intensified emission at acidic pH but diminished emission at neutral pH. The fluorescence modulation efficiency of the d-PET effect of the new sensors, i.e., the emission intensity enhancement upon switching from acidic pH to neutral pH, is up to 10-fold, which is greatly improved compared to our previous d-PET sensors (ca. 3-fold). The efficient d-PET effect of the new sensors is attributed to the proper orientation of the electron donor/acceptor; i.e., the dipole moment and the transition moment (the direction of PET) of the new sensors are oriented in the same direction, and the dipole moment values of the new sensors along the vector direction of the PET are larger than that of the reported d-PET sensors. Selective recognition of a-hydroxyl carboxylic acids, such as tartaric acid, was achieved with the d-PET sensors, and a novel fluorescence transduction profile of enhancement/diminishment for chemoselectivity was observed. Herein we propose that the orientation of the electron donor/acceptor may significantly affect the fluorescence modulation efficiency of the PET effect; this discovery will be important for the future design of PET sensors with improved fluorescence transduction efficiencies.