Development of colorimetric and fluorescent sensor based on a combination of ICT (intramolecular charge transfer) and FRET (Förster resonance energy transfer) for the detection of water

Abstract

Colorimetric and fluorescent sensors for water are crucial to environmental and quality control monitoring, industrial process, food inspection and so on. Although the optical sensing mechanism of sensors for water has been of considerable concern in analytical chemistry, photochemistry, and photophysics in recent years, a further fundamental study is necessary to provide a direction in molecular design toward creating highly sensitive fluorescent sensors for detecting, quantitating and visualizing a trace amount of water in solids, liquids, or gases. In this work, we have newly designed and synthesized an ICT (intramolecular charge transfer)/FRET (Förster resonance energy transfer)-type colorimetric and fluorescent sensor, pyridine-boron trifluoride complex KOY-1-BF₃, where the thienylpyridine-carbazole-based D-(π-A)₂ skeleton and boron-dipyrromethene (BODIPY) skeleton are the ICT-type donor fluorophore and the acceptor fluorophore in the FRET process, respectively. It was found that the addition of water to the KOY-1-BF₃ solution causes its dissociation into thienylpyridine-carbazole-based D-(π-A)₂ fluorophore KOY-1 and then the energy transfer from the thienylpyridine-carbazole-based D-(π-A)₂ skeleton to the BODIPY skeleton through the FRET process, and thus resulting in a large pseudo-Stokes shift of 7942 cm⁻¹ (150 nm) and an enhancement of fluorescence emission originating from the BODIPY skeleton as well as the blue-shift of the ICT-based photoabsorption band. Furthermore, in the high water content region, a decrease in the fluorescence intensity was observed due to the formation of hydrogen-bonded proton transfer complex KOY-1-H₂O with water molecules, which shows a feeble fluorescence emission property, leading to low FRET efficiency. Consequently, this work is the first report on the development and optical sensing mechanism of an ICT/FRET-type colorimetric and fluorescent sensor for water possessing a large pseudo-Stokes shift.