Synthesized Blue Fluorescent Protein Analogue with Tunable Colors from Excited-State Intramolecular Proton Transfer through an N–H···N Hydrogen Bond

A synthesized blue fluorescent protein
(BFP) chromophore analogue
2-BFP ((4Z)-4-[(1H-imidazol-2-yl)­methylene]-1-methyl-2-phenyl-1H-imidazol-5­(4H)-one)
displays dual fluorescent emission that arises from the same Z-isomer. The larger Stokes shift emission is a result of
excited-state intramolecular proton transfer (ESIPT) mediated by an
N–H···N type of hydrogen bond. Compared to other
green fluorescent protein (GFP) analogues with ESIPT such as o-HBDI, 2-BFP possesses greatly enhanced quantum yields
and much slower proton-transfer rates. In addition, fluorescence up-conversion
experiments revealed two rising components of lifetime for the tautomer
formation of 2-BFP. The results imply that the relaxation of the N*
state in 2-BFP triggers the proton transfer of the molecule. The weaker
photoacidity of N–H is proposed to be crucial for these photophysical
and photochemical properties. Finally, the ESIPT process in 2-BFP
is inhibited in protic solvents (MeOH) or by the formation of metal–chelate
complexes, providing insights for further developments and applications
of ESIPT molecules.