A Highly Sensitive ESIPT-Based Ratiometric Fluorescence Sensor for Selective Detection of Al3+

An excited-state intramolecular proton transfer (ESIPT)-based highly sensitive ratiometric fluorescence sensor, 1H was developed for selective detection of aluminum (Al(3+)) in acetonitrile as well as in 90% aqueous system. Single-crystal X-ray diffraction analysis reveals almost planar and conjugated structure of 1H. Photophysical properties of the sensor as well as its selectivity toward Al(3+) are explored using UV-visible, steady-state, and time-resolved fluorescence spectroscopic studies. The bright cyan (λem = 445 nm) fluorescence of 1H in acetonitrile turns into deep blue (λem = 412 nm) with ∼2.3-fold enhancement in emission intensity, in the presence of parts per billion level Al(3+) (detection limit = 0.5 nM). Interestingly, the probe 1H exhibits increased selectivity toward Al(3+) in H2O/acetonitrile (9:1 v/v) solvent system with a change in fluorescence color from pale green to deep blue associated with ca. sixfold enhancement in emission intensity. Density functional theoretical (DFT) calculations provide the ground- and excited-state energy optimized structures and properties of the proposed aluminum complex [Al(1) (OH)]2(2+), which is in harmony with the solution-state experimental findings and also supports the occurrence of ESIPT process in 1H. The ESIPT mechanism was also ascertained by comparing the basic photophysical properties of 1H with a similar O-methylated analogue, 1'Me.