Development of fluorescent sensors based on a combination of PET (photo-induced electron transfer) and FRET (Förster resonance energy transfer) for detection of water†
Fluorescent sensors DJ-1 and DJ-2 with a large Stokes shift (SS) based on a combination of PET (photo-induced electron transfer) and FRET (Förster resonance energy transfer) have been developed for the detection of water in organic solvents. DJ-1 is composed of anthracene-(aminomethyl)phenylboronic acid ester as the PET-type donor fluorophore and a BODIPY skeleton as the acceptor fluorophore in the FRET process. In contrast, DJ-2 is composed of an anthracene skeleton as the donor fluorophore and a BODIPY-(aminomethyl)phenylboronic acid ester skeleton as the PET-type acceptor fluorophore in the FRET process. In fact, the addition of water to organic solvents containing DJ-1 or DJ-2 caused both PET suppression and energy transfer from the donor fluorophore to the acceptor fluorophore through the FRET process, thus resulting in an enhancement of the fluorescence band originating from the BODIPY skeleton. In addition, the pseudo-SS values of DJ-1 and DJ-2 between the photoabsorption maximum of the anthracene fluorophore and the fluorescence maximum of the BODIPY fluorophore are 7563 cm−1 (141 nm) and 8017 cm−1 (153 nm), respectively, which are significantly higher than those of a typical PET-based fluorescent sensor. It was found that the FRET efficiency for DJ-1 is quantitative, but that for DJ-2 was estimated to be ca. 50% based on time-resolved fluorescence lifetime measurements. Moreover, the detection limit of DJ-1 for water is superior to that of DJ-2. Based on the fluorescence sensing mechanism of DJ-1 and DJ-2 for water, we propose that a combination of a PET-type donor fluorophore and an acceptor fluorophore in the FRET process is one of the most promising molecular designs to create an efficient fluorescent sensor for the detection of water in organic solvents.