Ortho green fluorescence protein synthetic chromophore; excited-state intramolecular proton transfer via a seven-membered-ring hydrogen-bonding system.

Department of Chemistry, National Taiwan University, Taipei 106, Taiwan, R.O.C.
Journal of the American Chemical Society (Impact Factor: 10.68). 05/2007; 129(15):4534-5. DOI: 10.1021/ja070880i
Source: PubMed
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    ABSTRACT: Synthesis of the 1-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)naphthalen-2-ol [PPIN] is reported, spectral and fluorescent properties of the title compound are investigated. The feasibility of excited state intramolecular proton transfer (ESIPT) has been argued from the changeover of relative stability of the enol and the keto forms on photoexcitation from the S0-PES to the S1-PES. Critical evaluation on the modulations of geometrical parameters other than the proton transfer reaction coordinate has also been undertaken. The intramolecular hydrogen bonding (IMHB) interaction in PPIN has been explored by calculation of the hyperconjugative charge transfer interaction from the lone electron pair of ring nitrogen atom to the σ(∗) orbital of O-H bond, under the provision of Natural Bond Orbital (NBO) analysis. However, DFT calculations together with the experimental results reveal that the excited species with the intramolecular N⋯HO hydrogen bond type undergoes rapid radiationless deactivation. This leads to a conclusion that the low-intensity dual-band fluorescence of the title compound in solution originates from the traces of the conformation with the -OH bond to azomethine nitrogen atom (ESIPT).
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    ABSTRACT: The intramolecular proton transfer reactions in 2-(2'-hydroxyphenyl)benzoxazole (HBO) and its naphthalene-fused analogs, (HNB1-3) in both S0 and S1 states at the PBE1PBE/6-311++G(2d,2p) level of theory in the gas phase and water have been investigated to find the effects of extension of aromaticity on the intramolecular proton transfer and photophysical properties. The results show that the ground state intramolecular proton transfer (GSIPT) in the studied species is impossible. Excited states potential energy surface calculations support the existence of ESIPT process. Structural parameters, relative energy of isomers, H-bonding energy, adsorption and emission bands, vertical excitation and emission energies, oscillator strength, fluorescence rate constant, dipole moment, atomic charges and electron density at critical points were calculated. Orbital analysis shows that vertical S0→S1 transition in the studied molecules corresponds essentially to the excitation from HOMO (π) to LUMO (π(*)). The potential of HNB2 molecule as an emissive and electron transport material in designing improved organic white light emitting diodes is predicted in this work. Our calculations are also supported by the experimental observations.
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    ABSTRACT: 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.
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