Pressure effect on the nonradiative process of thioflavin-T

Time-resolved emission techniques were employed to study the nonradiative process of thioflavin-T (ThT) in 1-propanol, 1-butanol, and 1-pentanol as a function of the hydrostatic pressure. Elevated hydrostatic pressure increases the alcohol viscosity, which in turn strongly influences the nonradiative rate of ThT. A diamond-anvil cell was used to increase the pressure up to 2.4 GPa. We found that the nonradiative rate constant, k(nr), decreases with pressure. We further found a remarkable linear correlation between a decrease in k(nr) (increase in the nonradiative lifetime, τ(nr)) and an increase in the solvent viscosity. The viscosity was varied by a factor of 1000 and k(nr) was measured at high pressures, at which the nonradiative rate constant of the molecules decreased from (7 ps)(-1) to (13 ns)(-1), (13 ps)(-1) to (17 ns)(-1) and (17 ps)(-1) to (15 ns)(-1) for 1-propanol, 1-butanol, and 1-pentanol, respectively. The viscosity-dependence of k(nr) is explained by the excited-state rotation rate of the two-ring systems, with respect to each other.     

Effect of Delocalization and Rigidity in the Acceptor Ligand on MLCT Excited-State Decay

In its most simple form, the energy gap law for excited-state nonradiative decay predicts a linear dependence of ln k(nr) on the ground- to excited-state energy gap, where k(nr) is the rate constant for nonradiative decay. At this level of approximation, the energy gap law has been successfully applied to nonradiative decay in a wide array of MLCT excited states of polypyridyl complexes of Re(I), Ru(II), and Os(II). This relationship also predicts a dependence of k(nr) on the structural characteristics of the acceptor ligand. We report here a brief survey of the literature which suggests that such effects exist and have their origin in the extent of delocalization of the excited electron in the ligand pi framework and on acceptor ligand rigidity.     

Temperature dependence of the fluorescence properties of curcumin

Steady-state and time-resolved techniques were employed to study the nonradiative process of curcumin dissolved in ethanol and 1-propanol in a wide range of temperatures. We found that the nonradiative rate constants at temperatures between 175-250 K qualitatively follow the same trend as the dielectric relaxation times of both neat solvents. We attribute the nonradiative process to solvent-controlled proton transfer. We also found a kinetic isotope effect on the nonradiative process rate constant of ~2. We propose a model in which the excited-state proton transfer breaks the planar hexagonal structure of the keto-enol center of the molecule. This, in turn, enhances the nonradiative process driven by the twist angle between the two phenol moieties.     

Modeling the nonradiative decay rate of electronically excited thioflavin T

A computational model of nonradiative decay is developed and applied to explain the time-dependent emission spectrum of thioflavin T (ThT). The computational model is based on a previous model developed by Glasbeek and co-workers (van der Meer, M. J.; Zhang, H.; Glasbeek, M. J. Chem. Phys. 2000, 112, 2878) for auramine O, a molecule that, like ThT, exhibits a high nonradiative rate. The nonradiative rates of both auramine O and ThT are inversely proportional to the solvent viscosity. The Glasbeek model assumes that the excited state consists of an adiabatic potential surface constructed by adiabatic coupling of emissive and dark states. For ThT, the twist angle between the benzothiazole and the aniline is responsible for the extensive mixing of the two excited states. At a twist angle of 90°, the S(1) state assumes a charge-transfer-state character with very small oscillator strength, which causes the emission intensity to be very small as well. In the ground state, the twist angle of ThT is rather small. The photoexcitation leads first to a strongly emissive state (small twist angle). As time progresses, the twist angle increases and the oscillator strength decreases. The fit of the experimental results by the model calculations is good for times longer than 3 ps. When a two-coordinate model is invoked or a solvation spectral-shift component is added, the fit to the experimental results is good at all times.     

Deactivation pathways for metal-to-ligand charge-transfer excited states of ruthenium polypyridyl complexes with triphenylphosphine as a ligand

Emission, excitation spectra, quantum yields, and emission lifetimes are reported for the mixed-ligand, bis(2.2'-bipyridine)ruthenium(II) complexes, cis-[Ru(bpy)(2)(PPh(3))X](n+) with X = Cl(-), Br(-), CN(-), and NO(2)(-) (n = 1) and pyridine (py), 4-aminopyridine (NH(2)py), 4,4'- bipyridine (4,4'-bpy), NH(3), and MeCN (n = 2) in EtOH-MeOH, 4:1 (v:v), at 77 K. Radiative, k(r), and nonradiative, k(nr), decay rate constants were determined for the series of complexes, and a linear dependence of ln k(nr) on E(00), with E(00) being the 0-0 energy gap determined by emission spectral fitting, was obtained with a slope of -(0.6 ± 0.1) × 10(-3). On the basis of emission quantum yields and apparent k(r) values, possible metal-to-ligand charge-transfer (MLCT) deactivation by direct population of excited (1)dd states from initially excited (1)MLCT states is discussed.     

Enhanced deep-red luminescence of tris(hexafluoroacetylacetonato)samarium(III) complex with phenanthroline in solution by control of ligand coordination

The effect of solvent molecule on the emission properties of Sm(hfa)3(phen)2 (hfa = hexafluoroacetylacetonato, phen = phenanthoroline) was investigated using acetone, acetonitrile, and pyridine. 5G(5/2) --> 5H(9/2) transition intensities in pyridine were found to be larger than those in corresponding acetone and acetonitrile. The radiative rate constant in pyridine (4.8 x 10(2) s(-1)) was 2 times larger than those in acetonitrile (2.6 x 10(2) s(-1)) and acetone (2.3 x 10(2) s(-1)), although the nonradiative transition via vibrational relaxation (k(nr) = 1.7 x 10(4) s(-1)) in pyridine was the same as those in acetone and acetonitrile (k(nr) = 1.8 x 10(4) s(-1)), resulting in the enhanced emission quantum yield of Sm(III) complex in pyridine (2.7%). The coordination structures of Sm(hfa)3(phen)2 in acetonitrile, acetone, and pyridine were estimated by X-ray single-crystal analyses. These results indicate that enhancement of the emission properties in pyridine is due to faster radiative rate related to formation of asymmetrical nine-coordinated structure, Sm(hfa)3(phen)(py) (py = pyridine).     

Short polyglutamine peptide forms a high-affinity binding site for thioflavin-T at the N-terminus

Thioflavin-T is one of the most important amyloid specific dyes and has been used for more than 50 years; however, the molecular mechanism of staining is still not understood. Chemically synthesized short polyglutamine peptides (Q(n), n = 5-10) were subjected to the thioflavin-T (ThT) staining assay. It was found that the minimum Q(n) peptide that stained positive to ThT was Q(6). Two types of ThT-binding sites, a high-affinity site (k(d1) = 0.1-0.17 μM) and a low-affinity site (k(d2) = 5.7-7.4 μM), were observed in short polyQs (n = 6-9). (13)C{(2)H}REDOR NMR experiments were carried out to extract the local structure of ThT binding sites in Q(8) peptide aggregates by observing the intermolecular dipolar coupling between [3-Me-d(3)]ThT and natural abundance Q(8) or residue-specific [1,2-(13)C(2)] labeled Q(8)s. (13)C{(2)H}REDOR difference spectra of the [3-Me-d(3)]ThT/natural abundance Q(8) (1/9) complex indicated that all of the five carbons of the glutamine residue participated in the formation of ThT-binding sites. (13)C{(2)H}DQF-REDOR experiments of [3-Me-d(3)]ThT/residue-specific [1,2-(13)C(2)] labeled Q(8) (1/50) complexes demonstrated that the N-terminal glutamine residue had direct contact with the ThT molecule at the high-affinity ThT-binding sites.     

Excited state properties of mixed phosphine 2-(2'-pyridyl)quinoline complexes of ruthenium(II)

Mixed ligand complexes of the type Ru(pq)(2)(PP)(2+) (pq = 2,2'-pyridylquinoline and PP = one bidentate or two monodentate phosphine ligands) have been prepared from the appropriate phosphine and Ru(pq)(2)Cl(2). The room temperature absorption spectra and low temperature (77 K) emission spectra, emission lifetimes, and quantum yields have been measured for the series of complexes and compared with those of Ru(pq)(3)(2+) and analogous Ru(bpy)(2)(PP)(2+) complexes (bpy = 2,2'-bipyridine) where possible. Emission spectra have been fit using a single mode Franck-Condon analysis. The visible absorption bands and emission bands are assigned to MLCT transitions that are blue shifted relative to Ru(pq)(3)(2+), while the emission lifetimes and quantum yields are increased. The trends in the nonradiative rate constants, k(nr), are described in terms of the energy gap, E(0), and the Huang-Rhys factor, S(M), which were obtained from the spectral fittings, and are correlated with the phosphine ligand structures.     

Spectroscopic and excited-state properties of tri-9-anthrylborane I: Solvent polarity effects

Spectroscopic and excited-state properties of tri-9-anthrylborane (TAB), showing unique absorption and fluorescence characteristics originating from p(boron)-pi(anthryl group) orbital interactions, were studied in 12 solvents. Although the absorption maximum energy (nu(a)) of TAB which appeared at around 21 x 10(3) cm(-1) (band I) was almost independent of the solvent polarity parameter, f(X) (f(X) = (D(s) - 1)/(2D(s) + 1) - (n(2) - 1)/(2n(2) + 1) where D(s) and n represent the static dielectric constant and the refractive index of a solvent, respectively), the fluorescence maximum energy (nu(f)) showed a linear correlation with f(X). The f(X) dependence of the value of nu(a) - nu(f) demonstrated that the change in the dipole moment of TAB upon light excitation was approximately 8.0 D, indicating that absorption band I was ascribed to an intramolecular charge-transfer transition in nature. The excited electron of TAB was thus concluded to localize primarily on the p orbital of the boron atom. Furthermore, it was shown that the fluorescence lifetime and quantum yield of TAB varied from 11.8 to 1.1 ns and from 0.41 to 0.02, respectively, with an increase in f(X). The present results indicated that the nonradiative decay rate constant (k(nr)) of TAB was influenced significantly by f(X). Excited-state decay of TAB was understood by intramolecular back-electron (charge) transfer from the p orbital of the boron atom to the pi orbital of the anthryl group, which was discussed in terms of the energy gap dependence of k(nr). Specific solvent interactions of TAB revealed by the present spectroscopic and photophysical studies are also discussed.     

Study of thioflavin-T immobilized in porous silicon and the effect of different organic vapors on the fluorescence lifetime

Steady-state and time-resolved emission techniques have been employed to study the fluorescence properties of thioflavin-T (ThT) adsorbed on oxidized porous silicon (PSi) surfaces, with an average pore size of ～10 nm. We found that the average fluorescence decay time of ThT, when it is adsorbed on the PSi surface, is rather long, τ(av) = 1.3 ns. We attribute this relatively long emission lifetime to the effect of the immobilization of ThT on the PSi surface, which inhibit the rotation of the aniline with respect to the benzothiazole moieties of ThT. We also measured the fluorescence properties of ThT in PSi samples in equilibrium with vapors of several liquids, such as methanol, acetonitrile, and water. We found that the fluorescence intensity drops by a factor of 10, and the average decay time, measured by a time-correlated single-photon counting technique, decreases by a factor of 3. We explain these results in terms of liquid condensation of the vapors in the PSi pores, which leads to partial dissolution of the ThT molecules in the liquid pools.     

Physicochemical studies of green phosphorescent light-emitting materials from cyclometalated heteroleptic iridium(III) complexes

A series of novel imidazole ligands were synthesized and characterized. Phosphorescence studies of series of heteroleptic cyclometalated iridium(III) complexes reveal that these complexes possess dominantly (3)MLCT and (3)π-π* excited states and the solvent shifts of these complexes are interpreted by Richardt-Dimroth and Marcus solvent functions. The results consistent with prior assignments on the absorption band to a metal-to-ligand charge transfer excited state associated with chelating ligand. Emission kinetic studies exploited that the radiative transition (k(r)), increases with increasing λ(em) and linear correlation exists between ln(k(nr)) and energy gap. Electronic transition theory is applied to study the effect of E(g) and ΔQ(e) on non-radiative transition (k(nr)). With a larger ΔQ(e), favouring vibrational overlap and leading to a larger value for k(nr).     

2-芳基丙酸类手性药物色谱拆分的热力学研究

以0.5%和1.0%(体积分数)正丙醇-50 mmol/L磷酸盐缓冲液(pH 6.41)为流动相,温度74~313 K,在Chiral-AGP柱手性固定相上,考察了萘普生和布洛芬对映体在手性柱上的保留和分离行为。在实验范围内,温度升高对分离不利,随着温度的升高,对映体的保留时间、分离度和选择性因子都减少;萘普生对映体的分离度均比布洛芬大;流动相含1.0％正丙醇时,萘普生对映体和布洛芬对映体达到完全分离应控制的最高温度分别为298和288 K。用ln k对1/T作图得到的Van’t Hoff曲线都具有良好的     

Torsional Dynamics of Thioflavin T in Room‐Temperature Ionic Liquids: An Effect of Heterogeneity of the Medium

The ultrafast excited‐state dynamics of a fibril binding dye, thioflavin T (ThT), has been studied in two room‐temperature ionic liquids (RTILs): 1‐Butyl‐1‐methylpyrrolidinium bis(trifluoromethylsulfonyl)imide and methyltrioctylammonium bis(trifluoromethylsulfonyl)imide. Previously, in most studies, it was observed that the excited‐state dynamics of the ThT dye were dependent on the viscosity of the medium. In our study, by using RTILs, we have demonstrated that the excited‐state dynamics of ThT are not only viscosity dependent, but also dependent on the heterogeneous nature of the medium. The effect of structural heterogeneity present in neat RTILs on the excited‐state dynamics of ThT was observed. For both RTILs, excitation wavelength dependency on the emission properties of ThT was observed.     

1H NMR研究溶质浓度对三碳多元醇水溶液氢键作用以及玻璃化的影响

为考察溶质浓度对三碳多元醇水溶液氢键作用以及不同物质玻璃态形成能力的影响,采用¹H NMR外标法研究不同浓度的1-丙醇（NPA）,2-丙醇（IPA）,1,2-丙二醇（PG）,1,3-丙二醇（PD）和丙三醇（glycerol）的水溶液在室温常压下质子化学位移.结果表明:具有CH₃CH（OH）一基团的PG烷基质子的化学位移变化趋势与其他几种醇相比有较大差异.醇羟基质子与水分子的氧形成较强O-H…O氢键.相同摩尔分数下,羟基数的增加导致水质子和羟基质子的化学位移降低,而且羟基位置不同也会导致水质子和羟基质子化学位移差异.这几种三碳多元醇碱性强弱的顺序和降低冰的均相成核温度能力的顺序一致,即glycerol>PG>PD>IPA>NPA,¹H NMR技术表明glycerol和PG更适合用作低温保护剂.     

DEUTERIUM-ISOTOPE EFFECT ON THE FLUORESCENCE YIELDS AND LIFETIMES OF INDOLE DERIVATIVES–INCLUDING TRYPTOPHAN AND TRYPTAMINE

Abstract— The fluorescence yields and lifetimes of indole, five of its alkyl detivatives, tryptophan, and tryptamine have been determined in degassed, heavy and light water at room temperature. All of the compounds have radiative lifetimes nearly identical to the parent compound indole, and a comparison of these results with recently reported data on tryptophyl derivatives disclosed a striking uniformity in radiative lifetimes between indole and many amino acids and peptides which contain the indole group as the fluorescence unit. The fluorescence rate k _f in H₂O, was found to be 4.5 × 10⁷ sec^-1. The nonradiative decay rates were found to vary between 5.1 and 46 × 10⁷ sec^-1 and from a study of the deuterium-solvent isotope effect and the deuterium-substituent effect a mechanism for nonradiative deactivation is proposed which includes an isotopically dependent proton transfer and a pathway involving energy loss via the ring carbon hydrogen vibrations. Tryptophan at pH 7 was found to have a unique nonradiative decay scheme not evidenced at a pH 1 or pH 10.     

Spectroscopic structure-property relationships of a series of polyaromatic platinum acetylides

To develop a structure-spectroscopic property relationship in platinum acetylides having poly(aromatic hydrocarbon) ligands, we synthesized a series of chromophores with systematic variation in the number of fused aromatic rings (nFAR) and ligand topology (polyacene (L), polyphenanthrene (Z), or compact(C)). We measured ground-state absorption, fluorescence, and phosphorescence spectra. We also performed nanosecond and femtosecond transient absorption experiments. To extend the range of compounds in the structure-property relationship, we did DFT calculations on an expanded series of chromophores. Both the DFT results and experiments show that the S(1) and T(1) state energies are a function of both nFAR and the ligand topology. In the L chromophores, the S(1) and T(1) state energies decrease linearly with nFAR. In contrast, the S(1) and T(1) state energies of the Z chromophores oscillate around a fixed value with increasing nFAR. The C chromophores have behavior intermediate between the L and Z chromophores. A parallel series of calculations on the ligands shows the same behavior. The S(1)-S(n) energy obtained from ultrafast time-resolved spectra has a linear variation in nFAR. The rate constant for nonradiative decay, k(nr), was calculated from the S(1) state lifetime and decreases with an increasing number of π electrons in the aromatic ring. The result is consistent with the spin-orbit coupling caused by the central platinum heavy atom decreasing with larger nFAR. The present work shows that the framework developed for the analysis of poly(aromatic hydrocarbon) properties is useful for the understanding of the corresponding platinum acetylide complexes.     

Evidence for the existence of an effective interfacial tension between miscible fluids. 2. Dodecyl acrylate-poly(dodecyl acrylate) in a spinning drop tensiometer

We studied drops of dodecyl acrylate in poly(dodecyl acrylate) (molecular weight of 25,000) in a spinning drop tensiometer to determine whether an effective interfacial tension (EIT) existed between these two miscible fluids. We found convincing evidence. We estimated the mechanical relaxation time from an immiscible analogue (1-propanol and poly(dodecyl acrylate)) and showed that the dodecyl acrylate drops maintained quasi-steady diameters long after this relaxation period. Drops continuously grew in length and became more diffuse, but the width of the transition zone did not grow with t(1/2) as expected from Fick's law although this system had been shown to follow Fick's law in a static configuration (Antrim, D.; Bunton, P.; Lewis, L. L.; Zoltowski, B. D.; Pojman, J. A. J. Phys. Chem. B 2005, 109, 11842-11849). The EIT was determined from Vonnegut's equation, EIT = (Deltarho)omega(2)r(3)/4; both the inner and outer diameters were measured, yielding values of 0.002 and 0.02 mN m(-1), respectively. The EIT was found to be independent of the rotation rate above 6000 rpm and independent of the initial drop volume. The EIT was found to decrease with temperature and increase with the difference in concentration between the monomer drop and polymer-monomer fluid. The square gradient parameter, k, was determined from EIT = k(Deltac(2)/delta), where Deltac is the difference in mole fraction and delta is the width of the transition zone. The square gradient parameter was on the order of 10(-9) N. The square gradient parameter was found to decrease with temperature, to be independent of concentration, and to increase with the molecular weight of the polymer.     

Retention factor and retention index of homologous series compounds in microemulsion electrokinetic chromatography employing suppressed electroosmosis

The retention factor (k) and retention index (I) of homologous series compounds such as alkylbenzenes (BZ), alkylaryl ketones, alkylbenzoates, and alkylparabens in microemulsion electrokinetic chromatography (MEEKC) with suppressed electroosmosis were investigated in a wide range of SDS concentrations ([SDS]), temperatures, and concentrations of organic cosolvents (phi). Using BZ as standards, the retention indices of other homologous series compounds were determined and they were found to be independent of [SDS] and temperature, while are dependent on the types and concentrations of organic cosolvents. The retention factor linearly increases with increasing [SDS], while linearly decreases with increasing temperature. The value of log k linearly decreases with increasing phi for methanol, ethanol, or ACN, while decreases by a second-degree polynomial with increasing phi for 2-propanol. Excellent agreement was found between the observed and predicted values of log k of analytes in MEEKC at given [SDS] and phi, where the predicted values were obtained from modified equations of the linear relationship of log k as functions of [SDS], the number of carbons, and phi. Therefore, both k and I can be used for peak identification of homologous series compounds.     

Photophysical properties of coumarin-7 dye: role of twisted intramolecular charge transfer state in high polarity protic solvents

Photophysical studies on coumarin-7 (C7) dye in different protic solvents reveal interesting changes in the properties of the dye on increasing the solvent polarity (Deltaf; Lippert-Mataga solvent polarity parameter) beyond a critical value. Up to Deltaf approximately 0.31, the photophysical properties of the dye follow good linear correlations with Deltaf. For Deltaf > approximately 0.31, however, the photophysical properties, especially the fluorescence quantum yields (Phi(f)), fluorescence lifetimes (tau(f)) and nonradiative rate constants (k(nr)), undergo large deviations from the above linearity, suggesting an unusual enhancement in the nonradiative decay rate for the excited dye in these high polarity protic solvents. The effect of temperature on the tau(f) values of the dye has also been investigated to reveal the mechanistic details of the deexcitation mechanism for the excited dye. Studies have also been carried out in deuterated solvents to understand the role of solute-solvent hydrogen bonding interactions on the photophysical properties of the dye. Observed results suggest that the fluorescence of the dye originates from the planar intramolecular charge transfer (ICT) state in all the solvents studied and the deviations in the properties in high polarity solvents (Deltaf > approximately 0.31) arise due to the participation of a new deexcitation channel associated with the formation of a nonfluorescent twisted intramolecular charge transfer (TICT) state of the dye. Comparing present results with those of a homologous dye coumarin 30 (C30; Photochem. Photobiol., 2004, 80, 104), it is indicated that unlike in C30, the TICT state of the C7 dye does not experience any extra stability in protic solvents compared to that in aprotic solvents. This has been attributed to the presence of intramolecular hydrogen bonding between the NH group (in the 3-benzimidazole substituent) of the C7 dye and its carbonyl group, which renders an extra stability to the planar ICT state, making the TICT state formation relatively difficult. Qualitative potential energy diagrams have been proposed to rationalize the differences observed in the results with C7 and C30 dyes in high polarity protic solvents.     

Theoretical Insights into the Substituent Effects on the Fluorescence of Boron Complexes of 2-(Benzothiazol-2-yl)phenols: a Quantum Mechanics Study

The performance of organic fluorescent materials can be improved by chemical substitutions with auxochrome groups such as amino to increase solubility, alter emitting color, or modify film quality. The complex 6,6-difluoro-6-bora-5-oxa-11-thia-6 a-aza-benzo[a]fluorine(BOBTP) and its derivatives, which possess excellent luminescent property at room temperature, were theoretically simulated by density functional theory. The geometries of the ground state and the first excited state of BOBTPs complexes were investigated and their bond parameters were obtained. Further, these bond parameters are compared with each other, and the computational wavelengths of maximum absorption and emission of studied complexes match up with the experimental values. It was found that amino substituent bonding to appropriate positions of BOBTP can reduce the reorganization energy significantly, which is ascribed to electron-donating effect of the amino group. The reorganization energy also plays an important role in the fluorescence quantum yield of all the BOBTPs. In particular, the radiative decay of complexes 3 and 4 is dominant due to the smaller reorganization energies, so their fluorescence quantum yield is almost 1, on the contrary the non-radiative decay and intersystem crossing rate of both the 1 and 2 can not be ignored for the larger reorganization energies, and the corresponding fluorescence quantum yields were calculated when the radiative decay rate(kr) and nonradiative decay rate(k(nr)) were taken into account.