Photophysics of untethered ZnTPP-fullerene complexes in solution

The spectroscopy and dynamic behavior of the self-assembled, Soret-excited zinc tetraphenylporphyrin (ZnTPP) plus fullerene (C(60)) model system in solution has been examined using steady state fluorescence quenching, nanosecond time-correlated single photon counting, picosecond fluorescence upconversion, and picosecond transient absorption methods. Evidence of ground state complexation is presented. Steady-state quenching of the S(2) and S(1) fluorescence of ZnTPP by C(60) reveals that the quenching processes only occur in the excited complexes, are ultrafast, and proceed at different rates in the two states. Only uncomplexed ZnTPP is observed by fluorescence lifetime methods; the locally excited complexes are either dark or, more likely, rapidly relax to products that do not radiate strongly. Both short-range (Dexter) energy transfer and electron transfer relaxation mechanisms are evaluated. Picosecond transient absorption data obtained from the subtle differences between the spectra of Soret-excited ZnTPP with and without a large excess of added C(60) reveal the formation, on a subpicosecond time scale, of relatively long-lived charge-separated species. Soret excitation of ZnTPP···C(60) does not produce a quantitative yield of species in the lower S(1) excited state.     

A close look at fluorescence quenching of organic dyes by tryptophan.

Understanding fluorescence quenching processes of organic dyes by biomolecular compounds is of fundamental importance for in-vitro and in-vivo fluorescence studies. It has been reported that the excited singlet state of some oxazine and rhodamine derivatives is efficiently and almost exclusively quenched by the amino acid tryptophan (Trp) and the DNA base guanine via photoinduced electron transfer (PET). We present a detailed analysis of the quenching interactions between the oxazine dye MR121 and Trp in aqueous buffer. Steady-state and time-resolved fluorescence spectroscopy, together with fluorescence correlation spectroscopy (FCS), reveal three contributing quenching mechanisms: 1) diffusion-limited dynamic quenching with a bimolecular quenching rate constant k(d) of 4.0 x 10(9) s(-1) M(-1), 2) static quenching with a bimolecular association constant K(s) of 61 M(-1), and 3) a sphere-of-action contribution to static quenching described by an exponential factor with a quenching constant lambda of 22 M(-1). The latter two are characterized as nonfluorescent complexes, formed with approximately 30 % efficiency upon encounter, that are stable for tens of nanoseconds. The measured binding energy of 20-30 kJ mol(-1) is consistent with previous estimates from molecular dynamics simulations that proposed stacked complexes due to hydrophobic forces. We further evaluate the influence of glycerol and denaturant (guanidine hydrochloride) on the formation and stability of quenched complexes. Comparative measurements performed with two other dyes, ATTO 655 and Rhodamine 6G show similar results and thus demonstrate the general applicability of utilizing PET between organic dyes and Trp for the study of conformational dynamics of biopolymers on sub-nanometer length and nanosecond time-scales.     

方菁染料敏化二氧化钛超微粒的光化学行为

通过吸收光谱、荧光猝灭、单光子计数等手段研究了2-(4-乙胺-2-羟基苯基)-4-(4-乙胺-苯基)-方菁染料(SQ)在TiO2超微粒体系中的光化学行为。结果表明, SQ强烈吸附在TiO2胶粒表面, 表观吸附常数为2.275×10^3mol^-^1 . dm^3。SQ的荧光能被TiO2有效地猝灭,猝灭的效率达97%。根据物质的氧化还原电位、光谱特性及荧光寿命的变化提出光诱导电子界面转移的荧光猝灭机理, 电子转移的速率常数为1.97×10^8s^-^1。     

The Effects of Heteroatoms Si and S on Tuning the Optical Properties of Rhodamine‐ and Fluorescein‐Based Fluorescence Probes: A Theoretical Analysis

The effects of the incorporated heteroatoms Si and S on tuning the optical properties of rhodamine‐ and fluorescein‐based fluorescence probes is investigated using DFT and time‐dependent DFT with four different functionals. As previously proposed, the large redshift (90 nm) produced by a Si atom in both the absorption and emission spectra can be attributed to the σ*–π* conjugation between the σ* orbital of the Si atom and the π* orbital of the adjacent carbon atoms. However, the presence of a Si atom does not alter the fluorescence quenching mechanism of the nonfluorescent forms of the investigated compounds. For the first time, these theoretical results indicate that the n orbital of the S atom plays an important role in determining the optical properties of the nonfluorescent form of rhodamine‐based fluorescence probes. It alters the fluorescence quenching mechanism by lowering the energy of the dark nπ* state, which is due to breakage of the C10?S52 bond upon photoexcitation.     

Electrochemical fluorescence switching properties of conjugated polymers composed of triphenylamine,fluorene, and cyclic urea moieties

The study of the electrochemical fluorescence switching properties of the conjugated copolymers containing fluorene, triphenylamine, and 1,3‐diphenylimidazolidin‐2‐one moieties is reported. The polymers show high fluorescence quantum yields, excellent thermal stability, and good solubility in polar organic solvents. While the polymer emits blue light under UV irradiation, the fluorescence intensity is quenched upon electrochemical oxidation. The fluorescent behavior can be reversibly switched between nonfluorescent (oxidized) state and strong fluorescence (neutral) state with a high contrast ratio (I_f/I_f0) of 16.3. The role of the electrochemical oxidation of the triphenylamine moieties is to generate the corresponding radical cations that lead to fluorescence quenching in the solid matrix. © 2012 Wiley Periodicals, Inc. J. Polym. Sci. Part A: Polym Chem, 2012     

Large scale synthesis of red emissive carbon dots powder by solid state reaction for fingerprint identification

Red emissive carbon dots(CDs) powder was synthesized on a large scale from phloroglucinol and boric acid by a novel solid state reaction with yield up to 75%. This method is safe and convenient, for it needs neither high pressure reactors nor complicated post-treatment procedures. The as-prepared carbon dots powder exhibited strong red fluorescence with excitation-independent behavior. XPS measurement and PL spectra suggest that such red fluorescence arise from boron-doped structures in CDs, which increases along with the boron concentration on CDs surface but decreases when the concentration quenching effect takes place. To overcome the aggregation induced fluorescence quenching of the solid CDs powder,the conventional methods are dispersing CDs into a large amount of inert substrates. But our present work provides a new strategy to realize strong red fluorescence of CDs in solid state. As a result, such carbon dots powder works well for latent fingerprint identification on various material surfaces.     

Exploring Reversible Quenching of Fluorescence from a Pyrazolo[3,4‐b]quinoline Derivative by Protonation

Pyrazolo[3,4‐b]quinoline derivatives are reported to be highly efficient organic fluorescent materials suitable for applications in light‐emitting devices. Although their fluorescence remains stable in organic solvents or in aqueous solution even in the presence of H₂O, halide salts (LiCl), alkali (NaOH) and weak acid (acetic acid), it suffers an efficient quenching process in the presence of protic acid (HCl) in aqueous or ethanolic solution. This quenching process is accompanied by a change in the UV spectrum, but it is reversible and can be fully recovered. Both steady‐state and transient fluorescence spectra of 1‐phenyl‐3,4‐dimethyl‐1H‐pyrazolo‐[3,4‐b]quinoline (PAQ5) during quenching are measured and analyzed. It is found that a combined dynamic and static quenching mechanism is responsible for the quenching processes. The ground‐state proton‐transfer complex [PAQ5 ??? H⁺] is responsible for static quenching. It changes linearly with proton concentration [H⁺] with a bimolecular association constant K_S=1.95 M ^?1 controlled by the equilibrium dissociation of HCl in ethanol. A dynamic quenching constant K_D=22.4 M ^?1 is obtained by fitting to the Stern–Volmer equation, with a bimolecular dynamic quenching rate constant k_d=1.03×10⁹ s^?1 M ^?1 under ambient conditions. A change in electron distribution is simulated and explains the experiment results.     

Influence of axial coordination on the photophysical property of excited state of zinc porphyrin

The phenomenon that metalloporphyrins are coordinated to axial ligands is very common especially in green-plant and photosynthetic bacteria, where such coordination impacts a signifi-cant effect on both the primary charge separation in the reaction center and energy transfer in an-tenna systems[1]. In addition, it plays a key role in photodynamic therapy and medicine synthesis in a sense that some metalloporphyrins have been assembled by coordination bonds to anti-cancer drugs, enhancing the se…     

发光共价有机骨架LCOF-NDT1荧光识别甲醛的理论研究

基于密度泛函理论(DFT)和含时密度泛函理论(TD-DFT),探究了发光共价有机骨架LCOF-NDT1与甲醛之间的分叉氢键作用.发现LCOF-NDT1与甲醛氢键作用后发光机理发生改变.氢键复合物的电子激发能减小,激发态下的氢键键长变短,氢键质子供体的~1H-NMR移向高场,氢键质子供体和受体的振动频率发生红移,均表明电子激发态下的氢键增强.氢键复合物的荧光速率系数减小,内转换速率系数增大,阐明电子激发态下氢键的增强有利于非辐射跃迁,不利于辐射跃迁,从而导致LCOF-NDT1荧光减弱或猝灭.计算结果表明LCOF-NDT1在荧光识别甲醛方面有潜在的应用前景.     

醇对水中DDAHPS分子聚集行为的影响

分别用稳态荧光猝灭技术和时间分辨荧光方法研究醇对两性表面活性剂 (十二烷基羟基磺化甜菜碱 ,DDAHPS)在水中聚集行为的影响 .结果表明 :在 4.0 0× 1 0 - 2 mol·L- 1DDAHPS水溶液中 ,随着正丁醇浓度的逐渐增大 ,表面活性剂的聚集数 (N)逐渐降低 .恒定醇的浓度为 2 .1 4× 1 0 - 2mol·L- 1时 ,醇碳链越长 ,N值越大 .与无醇体系相比 ,正丙醇和正丁醇使N值变小 ,正戊醇、正己醇和正庚醇使N值变大 .本文同时还测定了醇对微环境的极性 ,芘的荧光寿命及胶束内芘的激基缔合物形成效率的影响 .     

Detection of Explosive Vapors: The Roles of Exciton and Molecular Diffusion in Real‐Time Sensing

Time‐resolved quartz crystal microbalance with in situ fluorescence measurements are used to monitor the sorption of the nitroaromatic (explosive) vapor, 2,4‐dinitrotoluene (DNT) into a porous pentiptycene‐containing poly(phenyleneethynylene) sensing film. Correlation of the nitroaromatic mass uptake with fluorescence quenching shows that the analyte diffusion follows the Case‐II transport model, a film‐swelling‐limited process, in which a sharp diffusional front propagates at a constant velocity through the film. At a low vapor pressure of DNT of ≈16 ppb, the analyte concentration in the front is sufficiently high to give an average fluorophore–analyte separation of ≈1.5 nm. Hence, a long exciton diffusion length is not required for real‐time sensing in the solid state. Rather the diffusion behavior of the analyte and the strength of the binding interaction between the analyte and the polymer play first‐order roles in the fluorescence quenching process.     

Fluorescence quenching of CdS quantum dots by 4-azetidinyl-7-nitrobenz-2-oxa-1,3-diazole: a mechanistic study

Fluorescence quenching of CdS quantum dots (QDs) by 4‐azetidinyl‐7‐nitrobenz‐2‐oxa‐1,3‐diazole (NBD), where the two quenching partners satisfy the spectral overlap criterion necessary for Förster resonance energy transfer (FRET), is studied by steady‐state and time‐resolved fluorescence techniques. The fluorescence quenching of the QDs is accompanied by an enhancement of the acceptor fluorescence and a reduction of the average fluorescence lifetime of the donor. Even though these observations are suggestive of a dynamic energy transfer process, it is shown that the quenching actually proceeds through a static interaction between the quenching partners and is probably mediated by charge‐transfer interactions. The bimolecular quenching rate constant estimated from the Stern–Volmer plot of the fluorescence intensities, is found to be exceptionally high and unrealistic for the dynamic quenching process. Hence, a kinetic model is employed for the estimation of actual quencher/QD ratio dependent exciton quenching rate constants of the fluorescence quenching of CdS by NBD. The present results point to the need for a deeper analysis of the experimental quenching data to avoid erroneous conclusions.     

Dynamic Assemblies and Photophysical Changes of Zinc Porphyrin Dimer Regulated by N-Containing Ligands

Zinc porphyrin dimer （1） has been designed and synthesized as a novel host of N-containing ligands. The assembly behavior and photophysical changes of its host-guest complexes were evaluated by IH NMR, fluorescence, and UV-visible titrations, and the processes reveal that the host-guest assembly first creates a stable sandwich complex, then an axial coordination equilibrium appears between the sandwich complex and free ligand. The changes of absorption spectra of the assembly processes rely on the stabilities of the complexes, and fluorescence quenching depends on the axial coordination equilibrium, which indicates that the axial ligation/de-ligation dynamics is indeed a pathway from the excited state to the ground state for metalloporphyrin complexes.     

Uniform silica nanoparticles encapsulating two-photon absorbing fluorescent dye

We have prepared uniform silica nanoparticles (NPs) doped with a two-photon absorbing zwitterionic hemicyanine dye by reverse microemulsion method. Obvious solvatochromism on the absorption spectra of dye-doped NPs indicates that solvents can partly penetrate into the silica matrix and then affect the ground and excited state of dye molecules. For dye-doped NP suspensions, both one-photon and two-photon excited fluorescence are much stronger and recorded at shorter wavelength compared to those of free dye solutions with comparative overall dye concentration. This behavior is possibly attributed to the restricted twisted intramolecular charge transfer (TICT), which reduces fluorescence quenching when dye molecules are trapped in the silica matrix. Images from two-photon laser scanning fluorescence microscopy demonstrate that the dye-doped silica NPs can be actively uptaken by Hela cells with low cytotoxicity.     

Coenzyme Q functionalized CdTe/ZnS quantum dots for reactive oxygen species (ROS) imaging

Quantum dots (QDs) have been widely used for fluorescent imaging in cells. In particular, surface functionalized QDs are of interest, since they possess the ability to recognize and detect the analytes in the surrounding nanoscale environment based on electron and hole transfer between the analytes and the QDs. Here we demonstrate that fluorescence enhancement/quenching in QDs can be switched by electrochemically modulating electron transfer between attached molecules and QDs. For this purpose, a number of redox-active coenzyme Q (CoQ) disulfide derivatives [CoQC(n)S](2) were synthesized with different alkyl chain lengths (n=1, 5, and 10). The system supremely sensitive to NADH (nicotinamide adenine dinucleotide) and superoxide radical (O(2)(.)(-)), and represents a biomimetic electron-transfer system, modeling part of the mitochondrial respiratory chain. The results of our in situ fluorescence spectroelectrochemical study demonstrate that the reduced state of [CoQC(n)S](2) significantly enhanced the fluorescence intensity of CdTe/ZnS QDs, while the oxidized state of the CoQ conjugates quench the fluorescence to varying degrees. Fluorescence imaging of cells loaded with the conjugate QD-[CoQC(n)S](2) displayed strikingly differences in the fluorescence depending on the redox state of the capping layer, thus introducing a handle for evaluating the status of the cellular redox potential status. Moreover, an MTT assay (MTT=3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) proved that the cytotoxicity of QDs was significantly reduced after immobilization by CoQ derivatives. Those unique features make CoQ derivatived QDs as a promising probe to image redox coenzyme function in vitro and in vivo.     

荧光猝灭方法研究腺苷酸衍生物与嘧啶碱的弱相互作用──Stern-Volmer方程在竞争吸收体系中的修正式

已知在液氮温度下(77 K)核酸碱基之间能生成基态复合物^[1],这些复合物的荧光发射有比较大的红移.在室温条件下,核酸碱基的荧光发射量子产率很低,以至几乎无法检测.     

MOLECULAR CONFORMATION OF IONIC SURFACTANTS IN AQUEOUS SOLUTION

The molecular conformation of ionic surfactant in aqueous solution is investigated withfluorescent probes which are intrinsic insurfactant molecules or externally introduced. Quench-lng or pyrene monomer fluorescence by alkyltriphenylphosphonium or N-alkylpyridiniumobeys Stern-Voimer equation, being diffusi6n-controlled dynamic quenching, but the behaviorof quenching with different lengths of alkyl chain is "abnormal", i.e. the longer the chain,the greater the quenching rate constant. The pyrene excimer fluorescence is observed in theaqueous solution of cetyltrimethylammonium bromide (CTMAB), and the inhibition (for cationicquenchers) and promotion (for anionic quenchers) effects of CTMAB on the quenching ofpyrene fluorescence are also observed. The self-coiling conformation of ionic surfactantmolecules in aqueous solution is proposed to be responsible for these observations and theconformation to be dynamic.     

Photoluminescence of 2-aminofluorene: a relook

The absorption and fluorescence spectra of 2-aminofluorene (2-AF) have been studied as a function of solvent polarity and acid concentration. Using the multiparametric approach of Taft et al., it is clear that 2-AF is a better proton acceptor in the S₀ state and a proton donor in the S₁ state. Excitation of 2-AF to three electronically excited states has shown that fluorescence is always observed from the lowest excited singlet state, but fluorescence quantum yield increases with the increase of λ_exc. The decrease in fluorescence quantum yield with increase in solvent polarity and hydrogen bonding is due to solvent-induced quenching. A correspondence is observed between the decrease and increase in fluorescence intensities of the neutral and monocation, respectively, in the pH range from 6 to 3. Proton-induced fluorescence quenching of neutral 2-AF is noticed in the pH range 3 to 1. pK_a and values were determined for different prototropic equilibria and are discussed.     

Hybridization of a Flexible Cyclooctatetraene Core and Rigid Aceneimide Wings for Multiluminescent Flapping π Systems

The hybridization of flexible and rigid π‐conjugated frameworks is a potent concept for producing new functional materials. In this article, a series of multifluorescent flapping π systems that combine a flexible cyclooctatetraene (COT) core and rigid aceneimide wings with various π‐conjugation lengths has been designed and synthesized, and their structure/properties relationships have been investigated. Whereas these molecules have a V‐shaped bent conformation in the ground state, the bent structure changes to a planar conformation in the lowest excited singlet (S₁) state irrespective of the lengths of the aceneimide wings. However, the fluorescence behavior in solution is distinct between the naphthaleneimide system and the anthraceneimide system. The former has a nonemissive S₁ state owing to the significant contribution of the antiaromatic character of the planar COT frontier molecular orbitals, thereby resulting in complete fluorescence quenching in solution. In contrast, the latter anthraceneimide system shows an intense emission, which is ascribed to the planar but distorted S₁ state that shows the allowed transition between the π‐molecular orbitals delocalized over the COT core and the acene wings. The other characteristic of these π systems is the significantly redshifted fluorescence in the crystalline state relative to their monomer fluorescence. The relationship between the packing structures and the fluorescence properties was investigated by preparing a series of hybrid π systems with different sizes of substituents on the imide moieties, which revealed the effect of the twofold π‐stacked structure of the V‐shaped molecules on the large bathochromic shift in emission.     

THE QUENCHING OF EXCITED STATES OF HYPOCRELLIN A BY RARE EARTH IONS

The quenching processes of prompt and delayed fluorescence of hypocrellin A by rareearth ions were investigated. It is demonstrated that the prompt fluorescence of HA can bequenched efficiently by RE in methanol via a static quenching process including a groundstate complex formation. The thermally induced delayed fluorescence of HA can also bequenched effectively by complex formed, which quenching is of dynamic quenching behavior.The fluorescence quenching of HA in various micellar systems which protect the quenchingby the compartmentalization of micelles is discussed as well.