In this review, we will summarize our recent progress in the design and application of novel organic sensors with emission in the near-infrared region (600–900 nm). By coupling different functional groups with excited-state intramolecular proton transfer (ESIPT) segments, new probes are developed to achieve a large Stokes shift, high sensitivity, and selectivity and to tune the emission toward the near-infrared region. The developed probes exhibit attractive optical properties for bioimaging and environmental science applications. In addition, we further discuss the photophysical properties of ESIPT dyes and how their fluorescence could be affected by structural/environmental factors, which should be considered during the development of robust ESIPT-based fluorescence probes. Their potential applications as imaging reagents are illustrated for intracellular membranes, mitochondria, lysosomes, and some biomolecules.

Molecular probes based on the excited-state intramolecular proton-transfer (ESIPT) mechanism have emerged to be attractive candidates for various applications. Although the steady-state fluorescence mechanisms of these ESIPT-based probes have been reported extensively, less information is available about the fluorescence lifetime characteristics of newly developed NIR-emitting dyes. In this study, four NIR-emitting ESIPT dyes with different cyanine terminal groups were investigated to evaluate their fluorescence lifetime characteristics in a polar aprotic solvent such as CH2Cl2. By using the time-correlated single-photon counting (TCSPC) method, these ESIPT-based dyes revealed a two-component exponential decay (τ1 and τ2) in about 2–4 nanoseconds (ns). These two components could be related to the excited keto tautomers. With the aid of model compounds (5 and 6) and low-temperature fluorescence spectroscopy (at −189 ℃), this study identified the intramolecular charge transfer (ICT) as one of the major factors that influenced the τ values. The results of this study also revealed that both fluorescence lifetimes and fractional contributions of each component were significantly affected by the probe structures.

Accurate determination of Cu2+ in solution is crucial for preventing several disease conditions. Spectroscopy-based techniques for metal ion detection are promising methods due to their excellent sensitivity and rapid response time. In this work, we are reporting a newly synthesized 2-(2′-Hydroxyphenyl) benzoxazole-based compound, probe 2, by incorporating a vinyl pyridinium segment into the bis(HBO) 4 system. Probe 2 exhibited excellent specificity toward Cu2+ in solution. The ratiometric absorbance (λ440/λ370) and the quenching of fluorescence at λem ≈585 nm exhibited an excellent linear correlation. The formation of the 2-Cu complex can be utilized as a highly sensitive spectroscopic method for the detection of Cu2+ in solution with a detection limit of 0.15 µM. In addition, Cu2+-induced fluorescence quenching in probe 2 occurs mainly via a static quenching mechanism by forming a 2-Cu complex, and the stability constant for the 2-Cu complex was calculated based on spectroscopic measurements.