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 aminesubstituted 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-do-nating 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 Förster resonance energytransfer efficiency through a change in the overlap integral that arises from the change in absorption under acidic conditions. These strategies were validated by obtaining tricarbocyaninebased ratiometric NIR fluorescent probes for esterase and for pH level.