Synthesis and Fluorescence Mechanism of the Aminoimidazolone Analogues of the Green Fluorescent Protein: Towards Advanced Dyes with Enhanced Stokes Shift, Quantum Yield and Two-Photon Absorption

Novel small-molecular analogues the green fluorescence protein (GFP) chromophore are synthesised to expand and improve this fluorophore family and to deepen the understanding of their fluorescence mechanism. The introduction of an aminophenyl substituent and the repositioning of the hydroxyl group to enable strong intramolecular hydrogen bonding, not only enhances fluorescence emission, but also results in an increased Stokes shift and a considerable red shift. Experimental and computational results describe a dual fluorescence involving both excited-state intramolecular proton transfer and internal charge transfer (ESIPT?ICT) mechanism. The further improvement of the photophysical properties via the systematic variation of dialkylamino substituents at a single position of the chromophore led to a two-orders of magnitude enhancement in the quantum yields. In addition, the novel compounds also have significant two-photon absorption, which widens the possibilities for applications in the field of bioimaging.