Using 7-hydroxy-1-indanone as a prototype (I), which exhibits excited-state intramolecular proton transfer (ESIPT), chemical modification has been performed at C(2)-C(3) positions by fusing benzene (molecule II) and naphthalene rings, (molecule III). I undergoes an ultrafast rate of ESIPT, resulting in a unique tautomer emission (λ(max) ∼530 nm), whereas excited-state equilibrium is established for both II and III, as supported by the dual emission and the associated relaxation dynamics. The forward ESIPT (normal to proton-transfer tautomer species) rates for II and III are deduced to be (30 ps)(-1) and (22 ps)(-1), respectively, while the backward ESIPT rates are (11 ps)(-1) and (48 ps)(-1). The ESIPT equilibrium constants are thus calculated to be 0.37 and 2.2 for II and III, respectively, giving a corresponding free energy change of 0.59 and -0.47 kcal/mol between normal and tautomer species. For III, normal and tautomer emissions in solid are maximized at 435 and 580 nm, respectively, achieving a white light generation with Commission Internationale de l'Eclairage (CIE) (0.30, 0.27). An organic light-emitting diode based on III is also successfully fabricated with maximum brightness of 665 cd m(-2) at 20 V (885 mA cm(-2)) and the CIE coordinates of (0.26, 0.35). The results provide the proof of concept that the white light generation can be achieved in a single ESIPT system.
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...
Fluorescence spectroscopy and femtosecond relaxation dynamics of 2-{[2-(2-hydroxyphenyl)benzo[d]oxazol-6-yl]methylene}malononitrile (diCN-HBO) and 2-{[2-(2-hydroxyphenyl)benzo[d]thiazol-6-yl]methylene}malononitrile (diCN-HBT) are studied to probe the excited-state proton transfer (ESPT) coupled charge transfer (ESCT) reaction. Unlike most of the ESPT/ESCT systems previously designed, in which ESCT takes place prior to ESPT, both diCN-HBO and diCN-HBT undergo ESPT, concomitantly accompanied with the charge transfer process, such that the ESPT reaction dynamics are directly coupled with solvent polarization effects. The long-range solvent polarization interactions result in a solvent-induced barrier that affects the overall proton transfer reaction rate. In cyclohexane, the rate constant of ESPT of diCN-HBO is measured to be 1.1 ps (9.1 x 10(11) s(-1)), which is apparently slower than that of 150 fs for the parent molecule 2-(2'-hydroxyphenyl)benzoxazole (HBO). Upon increasing solvent polarity to, for example, CH 3CN, the rate of ESPT is increased to 300 fs (3.3 x 10(12) s(-1)). The results are rationalized by the stabilization of proton transfer tautomer, which possesses a large degree of charge transfer character via an increase of the solvent polarity, such that the corresponding solvent-induced barrier is reduced. We thus demonstrate a prototypical system in which the photon-induced nuclear motion (proton transfer) is directly coupled with solvent polarization and the corresponding mechanism is reminiscent of that applied in an electron transfer process.
Acenes are a class of aromatic hydrocarbons composed of linearly fused benzene rings. Noteworthy features of these molecules include their extended flat structure and the narrow gap between the HOMO and LUMO energy levels. However, the preparation of larger acenes, those that are larger than pentacene, has been challenging. These molecules are relatively unstable and have low solubility in typical solvents. Recently researchers have developed a new synthesis route for higher acenes using stable and soluble "precursors," which generate these structures on demand by either heating or irradiation of light. Using this method, nonsubstituted hexacene, heptacene, octacene, and nonacene were successfully prepared. In this Account, we summarize the preparation of nonsubstituted acenes from corresponding precursors, describe their physical properties, and discuss potential applications including potential usage in organic semiconductor devices. We first introduced the concept of using a precursor in the work with pentacene. Overall, we divide this methodology into two categories: masking pentacene itself with a dienophile to form a cycloadduct and the construction of higher acenes through conventional synthetic procedures. For the first category, a diverse array of dienophiles could be chosen, depending on the processing needs, especially for use in field-effect transistors (FETs). For the second category, researchers synthesized the pentacene precursor molecules using a multistep procedure. Upon proper activation, these molecules expel small fragments to generate pentacene readily. This strategy enabled the production of pentacene andunprepared higher acenes ranging from hexacene to nonacene. This new method provides a way to unravel the fascinating chemistry of higher acenes.
o-Hydroxy analogues, 1a-g, of the green fluorescent protein chromophore have been synthesized. Their structures and electronic properties were investigated by X-ray single-crystal analyses, electrochemistry, and luminescence properties. In solid and nonpolar solvents 1a-g exist mainly as Z conformers that possess a seven-membered-ring hydrogen bond and undergo excited-state intramolecular proton transfer (ESIPT) reactions, resulting in a proton-transfer tautomer emission. Fluorescence upconversion dynamics have revealed a coherent type of ESIPT, followed by a fast vibrational/solvent relaxation (<1 ps) to a twisted (regarding exo-C(5)-C(4)-C(3) bonds) conformation, from which a fast population decay of a few to several tens of picoseconds was resolved in cyclohexane. Accordingly, the proton-transfer tautomer emission intensity is moderate (0.08 in 1e) to weak (∼10(-4) in 1a) in cyclohexane. The stronger intramolecular hydrogen bonding in 1g suppresses the rotation of the aryl-alkene bond, resulting in a high yield of tautomer emission (Φ(f) ≈ 0.2). In the solid state, due to the inhibition of exo-C(5)-C(4)-C(3) rotation, intense tautomer emission with a quantum yield of 0.1-0.9 was obtained for 1a-g. Depending on the electronic donor or acceptor strength of the substituent in either the HOMO or LUMO site, a broad tuning range of the emission from 560 (1g) to 670 nm (1a) has been achieved.
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