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.
The relaxivity of the complex was modulated by Cu2+, that is, in the absence of Cu2+ the complex exhibited a relatively low relaxivity value of 6.40 mM−1 s−1, while the addition of Cu2+ triggered the relaxivity to 11.28 mM−1 s−1, an enhancement of approximately 76%.
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