Molecular Design Strategies for Near-Infrared Ratiometric Fluorescent Probes Based on the Unique Spectral Properties of Aminocyanines

In spite of the wide availability of various near-infrared (NIR) fluorophores as labeling reagents, there are few functional NIR fluorescent probes for which change in the absorption and/or fluorescence spectra upon specific reaction with biomolecules is seen. The widely used photoinduced electron-transfer mechanism is unsuitable for NIR fluorophores, such as tricarbocyanines, because their long excitation wavelength results in a small singlet ex-citation energy. We have reported the unique spectral properties of amine-substituted tricarbocyanines, which were utilized to develop two design strategies. One approach was based on control of the absorption wavelength by using the difference in electron-donating ability before and after a specific reaction with a biomolecule, and the other approach was based on control of the fluorescence intensity by modulating the Forster resonance energy-transfer efficiency through a change in the overlap integral that arises from the change in absorption under acidic conditions. These strategies were validated by obtaining tricarbocyanine-based ratiometric NIR fluorescent probes for esterase and for pH level.