Development of green/red-absorbing chromophores based on a coumarin scaffold that are useful as caging groups

Abstract

We report the design, synthesis and spectroscopic characterization of a series of pushpull chromophores based on a novel coumarin scaffold in which the carbonyl of the lactone function of the original coumarin dyes has been replaced by the cyano(4- nitrophenyl)methylene moiety. The skeleton of the compounds was synthesized by condensation of a thiocoumarin precursor with the corresponding arylacetonitrile derivatives, and their photophysical properties were fine-tuned through the incorporation of electron-withdrawing groups (EWG) like nitro and cyano at the phenyl ring, leading to absorption in the green to red region. Although fluorescence emission was weakened or even cancelled upon introduction of two or three strong EWGs, it is noticeable the emission of the mononitro-containing coumarin derivatives in the red region upon excitation with green light, as well as their significant large Stokes' shifts. The new coumarin derivatives can be useful as photocleavable protecting groups, as demonstrated through the synthesis and characterization of a series of coumarin-based photocages of benzoic acid. Preliminary photolysis studies with green light have demonstrated that the structure of the coumarin chromophore influences on the rate of the uncaging process, opening the way to exploiting these new coumarin scaffolds as caging groups removable with visible light.