Investigation of polar and stereoelectronic effects on pure excited-state hydrogen atom abstractions from phenols and alkylbenzenes

The fluorescence quenching of singlet-excited 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) by 22 phenols and 12 alkylbenzenes has been investigated. Quenching rate constants in acetonitrile are in the range of 10(8)-10(9) M(-1)s(-1) for phenols and 10(5)-10(6) M(-1)s(-1) for alkylbenzenes. In contrast to the quenching of triplet-excited benzophenone, no exciplexes are involved, so that a pure hydrogen atom transfer is proposed as quenching mechanism. This is supported by (1) pronounced deuterium isotope effects (kH/kD ca 4-6), which were observed for phenols and alkylbenzenes, and (2) a strongly endergonic thermodynamics for charge transfer processes (electron transfer, exciplex formation). In the case of phenols, linear free energy relationships applied, which led to a reaction constant of rho = -0.40, suggesting a lower electrophilicity of singlet-excited DBO than that of triplet-excited ketones and alkoxyl radicals. The reactivity of singlet-excited DBO exposes statistical, steric, polar and stereoelectronic effects on the hydrogen atom abstraction process in the absence of complications because of competitive exciplex formation.     

Investigations on the fluorescence quenching of 2,3-diazabicyclo[2.2.2]oct-2-ene by certain flavonoids

The fluorescence quenching of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) by seven flavonoids namely flavone, flavanone, quercetin, rutin, genistein, diadzein and chrysin has been investigated in acetonitrile and dichloromethane solvents. The bimolecular quenching rate constants lie in the range of 0.09-5.75 x 10(9)M(-1)s(-1) and are explained in terms of structure of the flavonoids studied. The reactivity of flavonoids are in the order: quercetin>rutin>genistein>diadzein>chrysin>flavone>flavanone. The quenching rate constants (k(q)) increase with increase in the number of -OH groups. The endergonic thermodynamic values of DeltaG(et) reveal that electron transfer quenching mechanism can be ruled out. Bond dissociation enthalpy calculations reveal that the position of -OH is important. Further in vitro-antioxidant activities of flavonoids were evaluated with rat liver catalase by gel electrophoresis. The deuterium isotope effect thus observed in this work provides evidence for hydrogen abstraction involved in the quenching process of singlet excited DBO by flavonoids. The data suggest the involvement of direct hydrogen atom transfer (radical scavenging) in the fluorescence quenching of DBO. Bond dissociation enthalpy calculation performed at B3LYP/6-31G(p')//B3LYP/3-21G level are in excellent agreement with the above observations and further reveal that the number OH groups and position of them decide the quenching ability of the flavonoids.     

A fluorescence-based method for direct measurement of submicrosecond intramolecular contact formation in biopolymers: an exploratory study with polypeptides.

A fluorescent amino acid derivative (Fmoc-DBO) has been synthesized, which contains 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) as a small, hydrophilic fluorophore with an extremely long fluorescence lifetime (325 ns in H2O and 505 ns in D2O under air). Polypeptides containing both the DBO residue and an efficient fluorescence quencher allow the measurement of rate constants for intramolecular end-to-end contact formation. Bimolecular quenching experiments indicated that Trp, Cys, Met, and Tyr are efficient quenchers of DBO (k(q) = 20, 5.1, 4.5, and 3.6 x 10(8) M(-1) x s(-1) in D2O), while the other amino acids are inefficient. The quenching by Trp, which was selected as an intrinsic quencher, is presumed to involve exciplex-induced deactivation. Flexible, structureless polypeptides, Trp-(Gly-Ser)n-DBO-NH2, were prepared by standard solid-phase synthesis, and the rates of contact formation were measured through the intramolecular fluorescence quenching of DBO by Trp with time-correlated single-photon counting, laser flash photolysis, and steady-state fluorometry. Rate constants of 4.1, 6.8, 4.9, 3.1, 2.0, and 1.1 x 10(7) s(-1) for n = 0, 1, 2, 4, 6, and 10 were obtained. Noteworthy was the relatively slow quenching for the shortest peptide (n = 0). The kinetic data are in agreement with recent transient absorption studies of triplet probes for related peptides, but the rate constants are significantly larger. In contrast to the flexible structureless Gly-Ser polypeptides, the polyproline Trp-Pro4-DBO-NH2 showed insignificant fluorescence quenching, suggesting that a high polypeptide flexibility and the possibility of probe-quencher contact is essential to induce quenching. Advantages of the new fluorescence-based method for measuring contact formation rates in biopolymers include high accuracy, fast time range (100 ps-1 micros), and the possibility to perform measurements in water under air.     

“Inverted” Solvent Effect on Charge Transfer in the Excited State

Faster in cyclohexane than in acetonitrile is the fluorescence quenching of the azoalkane 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) by amines and sulfides. Although this photoreaction is induced by charge transfer (CT; see picture) and exciplexes are formed, the increase in the dipole moment of the exciplex is not large enough to offset the solvent stabilization of the excited reactants, and an “inverted” solvent effect results.     

Reaction of singlet-excited 2,3-diazabicyclo[2.2.2]oct-2-ene and tert-butoxyl radicals with aryl-substituted benzofuranones

5,7-Di-tert-butyl-3-aryl-3H-benzofuran-2-ones are lactones with potential antioxidant activity, owing to their abstractable benzylic C-H hydrogens. The fluorescence quenching of the azoalkane 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO), an established probe for the hydrogen-donor propensity of chain-breaking antioxidants, was investigated for 16 aryl-substituted benzofuranone derivatives [m,m-(CF3)2, p-CN, m-CN, p-CF3, p-COOCH3, m-CF3, p-Cl, p-F, H, m-CH3, p-CH3, m,p-(CH3)2, p-OCH3, o-CH3, o-CF3, o,m-(CH3)2]. Analysis of the rate data in terms of a linear free energy relationship yielded a reaction constant of rho = +0.35. This implies that n,pi*-excited DBO acts as nucleophilic species. In contrast, hydrogen abstraction of tert-butoxyl radicals from the benzofuranones was accelerated by electron-donating substituents (rho = -0.23), in conformity with the electrophilic character of oxygen-centered alkoxyl radicals. Possible implications for the optimization of the hydrogen-donor propensity of antioxidants through structural variation are discussed.     

2,3-二氮杂二环-[2,2,2]-辛-2-烯与卤素分子间电荷转移相互作用的量子化学研究

对2,3-二氮杂二环-[2,2,2]-辛-2-烯 (DBO) 与Cl2、Br2和I2 3种卤素分子形成的s型电荷转移复合物进行了不同方法的理论计算。结果表明, MP2方法结合LanL2DZ*基组可以很好地描述DBO与卤素分子间的相互作用。其中, DBO与I2的结合能为33.6 kJ/mol, 与DBO…Br2的结合能接近, 明显大于DBO…Cl2相应的值。几何结构变化以及布居分析均与这一结果吻合。针对DBO的结构, 对DBO与I2间 形成p型电荷转移复合物以及DBO…(I2)2的可能性进行了理论分析和预测。     

Primary and secondary structure dependence of peptide flexibility assessed by fluorescence-based measurement of end-to-end collision rates

The intrachain fluorescence quenching of the fluorophore 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) is measured in short peptide fragments, namely the two strands and the turn of the N-terminal beta-hairpin of ubiquitin. The investigated peptides adopt a random-coil conformation in aqueous solution according to CD and NMR experiments. The combination of quenchers with different quenching efficiencies, namely tryptophan and tyrosine, allows the extrapolation of the rate constants for end-to-end collision rates as well as the dissociation of the end-to-end encounter complex. The measured activation energies for fluorescence quenching demonstrate that the end-to-end collision process in peptides is partially controlled by internal friction within the backbone, while measurements in solvents of different viscosities (H2O, D2O, and 7.0 M guanidinium chloride) suggest that solvent friction is an additional important factor in determining the collision rate. The extrapolated end-to-end collision rates, which are only slightly larger than the experimental rates for the DBO/Trp probe/quencher system, provide a measure of the conformational flexibility of the peptide backbone. The chain flexibility is found to be strongly dependent on the type of secondary structure that the peptides represent. The collision rates for peptides derived from the beta-strand motifs (ca. 1 x 10(7) s(-1)) are ca. 4 times slower than that derived from the beta-turn. The results provide further support for the hypothesis that chain flexibility is an important factor in the preorganization of protein fragments during protein folding. Mutations to the beta-turn peptide show that subtle sequence changes strongly affect the flexibility of peptides as well. The protonation and charge status of the peptides, however, are shown to have no significant effect on the flexibility of the investigated peptides. The meaning and definition of end-to-end collision rates in the context of protein folding are critically discussed.     

Theoretical study on the photolysis mechanism of 2,3-diazabicyclo[2.2.2]oct-2-ene

A CASPT2/CASSCF study has been carried out to investigate the mechanism of the photolysis of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) under direct and triplet-sensitized irradiation. By exploring the detailed potential energy surfaces including intermediates, transition states, conical intersections, and singlet/triplet crossing points, for the first excited singlet (S(1)) and the low-lying triplet states (T(1), T(2), and T(3)), we provide satisfactory explanations of many experimental findings associated with the photophysical and photochemical processes of DBO. A key finding of this work is the existence of a significantly twisted S(1) minimum, which can satisfactorily explain the envelope of the broad emission band of DBO. It is demonstrated that the S(1) (n-pi*) intermediate can decay to the T(1) (n-pi*) state by undergoing intersystem crossing (rather inefficient) to the T(2) (pi-pi*) state followed by internal conversion to the T(1) state. The high fluorescence yield and the extraordinarily long lifetime of the singlet excited DBO are due to the presence of relatively high barriers, both for intersystem crossing and for C-N cleavage. The short lifetime of the triplet DBO is caused by fast radiationless decay to the ground state.     

Synthesis of Dibenzo[hi,st]ovalene and Its Amplified Spontaneous Emission in a Polystyrene Matrix

A large number of graphene molecules, or large polycyclic aromatic hydrocarbons (PAHs), have been synthesized and display various optoelectronic properties. Nevertheless, their potential for application in photonics has remained largely unexplored. Herein, we describe the synthesis of a highly luminescent and stable graphene molecule, namely a substituted dibenzo[hi ,st ]ovalene (DBO 1 ), with zigzag edges and elucidate its promising optical‐gain properties by means of ultrafast transient absorption spectroscopy. Upon incorporation of DBO into an inert polystyrene matrix, amplified stimulated emission can be observed with a relatively low power threshold (ca. 60 μJ cm⁻²), thus highlighting its high potential for lasing applications.     

The synthesis of poly[6,8‐dioxabicyclo[3.2.1]octane‐b‐(ethylene glycol)‐b‐6,8‐dioxabicyclo[3.2.1]octane] by controlled cationic ring‐opening polymerization

The triblock copolymer poly[6,8‐dioxabicyclo[3.2.1]octane‐b‐(ethylene glycol)‐b‐6,8‐dioxabicyclo[3.2.1]octane] was prepared by the controlled cationic ring‐opening polymerization of 6,8‐dioxabicyclo[3.2.1]octane (6,8‐DBO) from a macroinitiator. The macroinitiator, poly(ethylene glycol) (PEG) di(1‐chloroethyl ether), was prepared via the addition of HCl to PEG divinyl ether and was characterized with ¹³C NMR, ¹H NMR, and gel permeation chromatography (GPC). Upon preparation, a small fraction of the chain ends underwent a cyclization reaction to form inactive chain ends. When the macroinitiator was used in polymerizations of 6,8‐DBO with ZnI₂ as an activator, linear kinetic plots were observed, a linear increase in the copolymer molecular weight with conversion was seen, and the molecular weight distributions of the copolymer samples remained constant at about 1.40. Confirmation of the triblock structure of the final product was obtained with ¹H NMR spectra, ¹³C DEPT spectra, and GPC chromatograms. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4081–4087, 2000     

Kinetics of end-to-end collision in short single-stranded nucleic acids

A novel fluorescence-based method, which entails contact quenching of the long-lived fluorescent state of 2,3-diazabicyclo[2.2.2]-oct-2-ene (DBO), was employed to measure the kinetics of end-to-end collision in short single-stranded oligodeoxyribonucleotides of the type 5'-DBO-(X)n-dG with X = dA, dC, dT, or dU and n = 2 or 4. The fluorophore was covalently attached to the 5' end and dG was introduced as an efficient intrinsic quencher at the 3' terminus. The end-to-end collision rates, which can be directly related to the efficiency of intramolecular fluorescence quenching, ranged from 0.1 to 9.0 x 10(6) s(-1). They were strongly dependent on the strand length, the base sequence, as well as the temperature. Oligonucleotides containing dA in the backbone displayed much slower collision rates and significantly higher positive activation energies than strands composed of pyrimidine bases, suggesting a higher intrinsic rigidity of oligoadenylate. Comparison of the measured collision rates in short single-stranded oligodeoxyribonucleotides with the previously reported kinetics of hairpin formation indicates that the intramolecular collision is significantly faster than the nucleation step of hairpin closing. This is consistent with the configurational diffusion model suggested by Ansari et al. (Ansari, A.; Kuznetsov, S. V.; Shen, Y. Proc.Natl. Acad. Sci. USA 2001, 98, 7771-7776), in which the formation of misfolded loops is thought to slow hairpin formation.     

Structural and environmental requirements for quenching of singlet oxygen by cyanine dyes

Singlet-oxygen quenching constants were measured for 19 cyanine dyes in acetonitrile. The most efficient quenchers were 1-butyl-2-[2-[3-[(1-butyl-6-chlorobenz-[cd]indol-2(1H)- ylidene)ethylidene]-2-chloro-1-cyclohexen-1-yl]ethenyl]-6-chlorobenz[cd] indolium and 6-chloro-2-[2-[3-(6-chloro-1-ethylbenz[cd]indol-2(1H)-ylidene) ethylidene]-2-phenyl-1-cyclopenten-1-yl]ethenyl]-1-ethyl-benz[cd]indolium, having quenching constants with diffusion-controlled values of 2.0 +/- 0.1 x 10(10) and 1.5 +/- 0.1 x 10(10) M-1 s-1, respectively. There was a trend toward increased quenching constants for cyanine dyes with the absorption band maxima at longer wavelengths. However, the quenching constants correlated better with the oxidation potentials of the cyanine dyes, suggesting that quenching proceeds by charge transfer rather than energy transfer. The quenching constants for 1,1',3,3,3',3'-hexamethylindotricarbocyanine perchlorate and 1,1'-diethyl-4,4'-carbocyanine iodide were measured in several solvents as well as in aqueous solutions of detergent micelles. In different solvents, the quenching constants varied by as much as a factor of 50. The quenching constants were largest in solvents with the highest values on the pi* scale of Kamlet, Abboud, Abraham and Taft. This was consistent with quenching occurring by charge transfer. Within cells, cyanine dyes concentrate in membrane-bound organelles. The quenching constants were substantial within detergent micelles. To the extent that micelles are models for biological membranes, cyanine dyes may be effective biological singlet-oxygen quenchers.     

Study of Fluorescence Quenching on Novel Coumarin Derivatives by Aniline in Different Solvents

The steady-state fluorescence quenching of novel coumarin derivatives; 4-(2, 6-dibromo-4-methyl-phenoxymethyl)-benzo[h]chromen-2-one [DMB] and 6-methoxy-4-p-tolyoxymethyl-chromen-2-one [TMC] has been studied in toluene, benzene, dioxane, acetonitrile and tetrahydrofuran [THF] using aniline as a quencher at room temperature with a view to understanding the role of diffusion in the quenching mechanism. The probability of quenching per encounter (p) is calculated in all the solvents. Further, an activation energy for quenching (E _a) was estimated using the values of p and the literature values of activation energy for diffusion (E _d). The magnitudes of these parameters indicate that the fluorescence quenching of these molecules by aniline is not solely due to the material diffusion but there is also a contribution of an activation energy.     

Azobridges from azines, XVII. Intra- and intermolecular [3+2] cycloaddition between nonstabilized azomethineimines and alkenes

Azo compounds 1 and 3 containing a CC-double bond in a parallel but distant position are quatemized by Me₃OBF₄ to 1,MeBF₄ and 3,MeBF₄, whereas MeI produces the cage compounds 2,HX and 4,HX. These [3+2] cycloadducts also are quantitatively formed from 1,MeBF₄ and 3,MeBF₄ with catalytic amounts of azo compounds. Intermolecular [3+2] cycloadditions occur with a mixture of DBH (5) or DBO (8), MeI and a variety of alkenes (→ HI) salts of (6, 7, 9 — 12). The intermediate azomethineimines, if stabilized by a fluorenylidene group, can be isolated (20, 22,24), but not, however, in the presence of a close parallel CC-bond (25 → 26).     

Dynamic and static quenching of fluorescence by 1-4 nm diameter gold monolayer-protected clusters

How the efficiency of molecular quenching by Au nanoparticles depends on nanoparticle size is reported for (a) dynamic (collisional) quenching of four different fluorophores by three Au nanoparticles having similar protective layers but differing core diameters (1.1, 1.6, and 2.0 nm) and (b) static quenching in the electrostatic association between [Ru(bpy)3]2+ and five tiopronin-protected Au nanoparticles having core diameters from 1.3 to 3.9 nm. The quenching constants systematically increase with core size. In (a), the dynamic constants scale with the molar absorbance coefficients of the nanoparticles, showing the essentially of the absorbance/emission spectral overlap, and the associated nanoparticle core density of electronic states, in energy-transfer quenching. In (b), the fluorescence of the Au nanoparticle itself was enhanced by energy transfer from the [Ru(bpy)3]2+ fluorophore.     

Measurement of electronic quenching rates in nitric oxide using two-photon spectroscopy

Two-photon excitation has been successfully applied to the direct measurement of electronic quenching rates. The value obtained for the quenching rate of NO[A] by NO[X] ($\text{Q}$ = 1.5±0.15 torr^-1) is in agreement with previous determinations.     

光谱法研究硫鸟嘌呤与七元瓜环及牛血清白蛋白的超分子相互作用

利用紫外-可见吸收光谱法和荧光光谱法研究了抗癌药物硫鸟嘌呤(6-TG)与七元瓜环(Q[7])及牛血清白蛋白(BSA)的相互作用. 结果表明, 6-TG与Q[7]及BSA可形成三元复合物, 且6-TG与Q[7]及BSA均可形成1:1的超分子配合物, 6-TG能引起BSA的荧光猝灭, 猝灭机制为静态猝灭. 此外, 还用同步荧光法和三维荧光法考察了6-TG对BSA构象的影响, 结果表明6-TG的加入使BSA的构象发生了变化, 而同步荧光光谱结果表明结合位点更接近于色氨酸.     

Photoinduced electron transfer of [ Ru (bpy)_2 (4, 4'-dcbpy)]~(2 ) with electron donors

Emission quenching of [Ru(bpy)2(4, 4'-dcbpy)] (PF6)2 (1) by benzenamine,4-[2-[5-[4-[4-dimethylamino]phenyl]-4,5-di-hydro-1-phenyl-1H-pyrazol-3-yl]-ethenyl]-N,N-dimetyl (2) or 1, 5-diphenyl-3-(2-phenothiazine)-2-pyrazoline (3) was observed. Measurements of the emission decay of 1 before and after addition of 2 or 3 by single photon counting technique con-finned the observations. The emission quenching of 1 by 2 or 3 was submitted to Stern-Volmer equation. It was calculated that the quenching rate constants (kq) are 5.5 × 109(mol/L)-1s-1 for 2 and 4.0 × 109(mol/L)-1s-1 for 3, respectively. These results indicated a character of dynamic quenching process. The singlet-state of 2 or 3 was also quenched by 1. The quenching behaviors did not conform to the Stern- Volmer equation and involved both static and dynamic quenching processes. The apparent quenching rate constant (kapp) was calculated to be 3 × 109 (mol/L)-1 for the interaction of excited 2 with 1, and 1.2 × 109 (mol/L)-1 for that of excited 3 wit     

Protonation and coordination properties towards Zn(II), Cd(II) and Hg(II) of a phenanthroline-containing macrocycle with an ethylamino pendant arm

Protonation and Zn(II), Cd(II) and Hg(II) coordination with the ligand 5-aminoethyl-2,5,8-triaza-[9]-10,23-phenanthrolinophane (L2), which contains an aminoethyl pendant attached to a phenanthroline-containing macrocycle, have been investigated by means of potentiometric, 1H NMR and spectrofluorimetric titrations in aqueous solutions. The coordination properties of L2 are compared with those of the ligand 2,5,8-triaza-[9]-10,23-phenanthrolinophane (L1). Ligand protonation occurs on the aliphatic amine groups and does not involve directly the heteroaromatic nitrogens. The fluorescence emission properties of L2 are controlled by the protonation state of the benzylic nitrogens: when not protonated, their lone pairs are available for an electron transfer process to the excited phenanthroline, quenching the emission. As a consequence, the ligand is emissive only in the highly charged [H3L2]3+ and [H4L2]4+ species, where the benzylic nitrogens are protonated. Considering metal complexation, both [ML1]2+ and [ML2]2+ complexes (M = Zn(II) and Cd(II)) are not emissive, since the benzylic nitrogens are weakly involved in metal coordination, and, once again, they are available for quenching the fluorescence emission. Protonation of the L2 complexes to give [MHL2]3+ species, instead, leads to a recovery of the fluorescence emission. Complex protonation, in fact, occurs on the ethylamino group and gives a marked change of the coordination sphere of the metals, with a stronger involvement in metal coordination of the benzylic nitrogens; consequently, their lone pairs are not available for the process of emission quenching.     

Study on the interaction between an ionic liquid and L-tryptophan by fluorescence spectroscopic technique

In this work, the interaction of an ionic liquid (IL) 1-octyl-3-methylimidazolium chloride ([Omim]Cl) and L-tryptophan was investigated with fluorescence spectroscopic method. The quenching mechanism and binding characteristics of [Omim]Cl with L-tryptophan at different temperatures were studied from fluorescence measurement. The thermodynamic parameters such as free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS), were calculated by thermodynamic equations. The experimental results indicated that [Omim]Cl and L-tryptophan had weak interaction and the mechanism of quenching belonged to static quenching. Hydrogen bonds and van der Waals interactions played important roles in the binding of [Omim]Cl with L-tryptophan.