Ortho Green Fluorescence Protein Synthetic Chromophore; Excited-State Intramolecular Proton Transfer via a Seven-Membered-Ring Hydrogen-Bonding System

Abstract

Green fluorescence protein (GFP), which serves as an energy acceptor and emitter for bioluminescence in the sea pansy Renilla reniformis and the jellyfish Aequorea Victoria, has drawn much attention because of its applications in molecular biology and biochemistry. 1 GFP takes advantage of the presence of a chromophore that is anchored both covalently and via a hydrogen-bond network, 4-(4-hydroxybenzylidene)-1,2-dimethyl-1H-imidazol-5(4H)-one (p-HBDI, see Scheme 1), which undergoes excited-state proton transfer (ESPT) 2 via the proton relay of water molecules and a remote residue such as E222, 3 resulting in a very effective and intense anion fluorescence.Nevertheless, studies reveal a strong cutoff between the properties of wild type GFP (or certain GFP mutants) and the synthetic analogue chromophores of p-HBDI. 4 In view of photophysics, the fluorescence yield of the protein-free chromophore in fluid solvents is much weaker and strongly temperature dependent. The results suggest an efficient radiationless transition operating in p-HBDI, most probably induced by conformational relaxation along torsional deformation of the two exocyclic C-C bonds to a nonfluorescent twisted intermediate. 5 More recently, it has been proposed that the shallow potential energy surface of the intermediates may conically intersect with that of the ground state, inducing the dominant radiationless deactivation. 4c-d,6 Such a conformational relaxation is greatly suppressed in GFP by its proton relay, rigid environment.In view of chemistry, most of the research has been focused on the chemical modification of p-HBDI analogues at the C(1) position. 4c,7 Conversely, in this study, we are interested in the derivatization on the phenyl ring. As an ingenious approach, switching the hydroxyl group from the C(8) position to the C(6) position (see Scheme 1), forming 4-(2-hydroxybenzylidene)-1,2-dimethyl-1H-imidazol-5(4H)-one (o-HBDI), a structural isomer of p-HBDI may reveal several novel features with respect to p-HBDI. The geometry optimization (B3LYP/cc-pVDZ and aug-cc-pVDZ, see ESI) of o-HBDI unveils the existence of a seven-memberedring intramolecular hydrogen bond between -OH and the N(2) atom. This intramolecular hydrogen-bonding configuration should, in part, hinder the exocyclic torsional deformation such that the radiationless deactivation may be reduced. More importantly, theoretical approaches also predict that excited-state intramolecular proton transfer (ESIPT) from the OH proton to the N(2) atom is thermally favorable (vide infra), forming a zwitterionic tautomer species (see Table of Content, TOC).In light of these perspectives, we have thus expended great effort to make a facile synthesis of o-HBDI. Briefly, the o-methoxybenzaldehyde was used as a starting reactant (see Scheme 1). Because of the lack of the o-hydroxyl group and hence the intramolecular lactonation, 3 was obtained with a good yield (70%). Subsequent reaction of 3 with methylamine, followed by deprotection of the methyl group of o-MBDI by BBr 3 , afforded o-H...

Molecular-Size Fluorescence Emitters

“…Some of the compounds synthesized along this line showed relatively bright fluorescence (Wu and Burgess 2008;Baranov et al 2014). Of special interest was the design of fluorophores exhibiting the excited-state reactions, particularly ESIPT (Chen et al 2007). …”

Efficient enhancement of internal proton transfer of branched π-extended organic chromophore under one-photon and near-infrared two-photon irradiation

“…It is deduced that C3 can have more possibility to undergo internal proton transfer in the excited state than C1 [22]. The linear fluorescence spectra of the molecules were measured to get the key evidences of internal proton transfer in the excited state.…”

Synthesis, Crystal Structure, Spectroscopic Properties, and DFT Studies of 7,9-Dibromobenzo[h]quinolin-10-ol

“…The resulting proton-transfer tautomer (keto-form) exhibits considerable differences in molecular structure and electronic configuration from its corresponding ground state, i.e., a large Stokes shifted tautomer fluorescence. This unique optical property has found many diverse applications such as probes for solvation dynamics [7][8][9] and biological environments [10,11], fluorescence microscopy imaging [12], near-infrared fluorescent dyes [13], nonlinear optical materials [14], photochromic materials [15], chemosensors [16][17][18][19][20], and organic light-emitting diodes [21][22][23][24][25][26]. To expand the scope of 2-based chromophores available for designing systems for laser dyes and fluorescent materials, the present research reports the synthesis of a benzo[h]quinolin-10-ol derivative 7,9-dibromobenzo[h]quinolin-10-ol (1) as well as its X-ray structure, spectroscopic properties, and complementary time-dependent density functional theory (TD-DFT) calculations.…”