Characterizing the fluorescence intermittency and photobleaching kinetics of dye molecules immobilized on a glass surface

The blinking behavior of single Atto565 molecules on a glass surface is studied under air or nitrogen atmospheres using confocal microscopy. The broad distributions for both on- and off-time durations obey power law kinetics that are rationalized using a charge tunneling model. In this case, a charge is transferred from the Atto565 molecule to localized states found on the glass surface. Subsequent charge recombination by back charge tunneling from trap to Atto565 cation (i.e., dark state) restores the fluorescence. The off-time distribution is independent of excitation intensity (I), whereas the on-time distribution exhibits a power law exponent that varies with I. Two pathways have been identified to lead to the formation of the radical dark state. The first involves direct charge tunneling from the excited singlet S1 state to charge traps in the surrounding matrix, and the second requires charge ejection from the triplet T1 state after intersystem crossing from S1. Monte Carlo simulation studies complement the two-pathway model. Photobleaching curves of both single and ensemble molecules do not exhibit monoexponential decays suggesting complex bleaching dynamics arising from triplet and radical states.     

Optical and dynamic properties of water-soluble highly luminescent CdTe quantum dots

CdTe quantum dots (QDs) were synthesized in aqueous solution using thioglycolic acid (HS-CH2COOH, TGA) as a stabilizer. The phenomenon of "on" and "off" luminescence intermittency (blinking) of CdTe QDs in PVA and trehalose was investigated by single-molecule optical microscopy, and we identified that the intermittencies of single QDs were correlated with the interaction of water molecules absorbed on the QD surface. The "off" times, the interval between adjacent "on" states, remained essentially unaffected with an increase in excitation intensity. Every QD showed a similar power law behavior for the "off" time distribution regardless of the excitation intensity and aqueous environment of the QDs. In the case of "on" time distribution, power law behavior with an exponential cutoff tail is observed at longer time scales. The time traces indicated that the "on" time was inversely proportional to the excitation intensity; the duration of "on" time became shorter with increasing excitation intensity. An increase in the duration of "on" time was observed in trehalose with respect to that in PVA. We obtained a clear decrease in the power law exponent when PVA was replaced with trehalose. These observations indicate that the luminescence blinking statistics of water-soluble single CdTe QDs is significantly dependent on the aqueous environment, which is interpreted in terms of passivation of the surface trap states of QDs.     

Power-law blinking in the fluorescence of single organic molecules.

The blinking behavior of perylene diïmide molecules is investigated at the single‐molecule level. We observe long‐time scale blinking of individual multi‐chromophoric complexes embedded in a poly(methylmethacrylate) matrix, as well as for the monomeric dye absorbed on a glass substrate at ambient conditions. In both these different systems, the blinking of single molecules is found to obey analogous power‐law statistics for both the on and off periods. The observed range for single‐molecular power‐law blinking extends over the full experimental time window, covering four orders of magnitude in time and six orders of magnitude in probability density. From molecule to molecule, we observe a large spread in off‐time power‐law exponents. The distributions of off‐exponents in both systems are markedly different whereas both on‐exponent distributions appear similar. Our results are consistent with models that ascribe the power‐law behavior to charge separation and (environment‐dependent) recombination by electron tunneling to a dynamic distribution of charge acceptors. As a consequence of power‐law statistics, single molecule properties like the total number of emitted photons display non‐ergodicity.     

Fluorescence blinking statistics from CdSe core and core/shell nanorods

We report fluorescence blinking statistics measured from single CdSe nanorods (NRs) of seven different sizes with aspect ratios ranging from 3 to 11. This study also included core/shell CdSe/ZnSe NRs and core NRs with two different surface ligands producing different degrees of surface passivation. We compare the findings for NRs to our measurements of blinking statistics from spherical CdSe core and CdSe/ZnS core/shell nanocrystals (NCs). We find that, for both NRs and spherical NCs, the off-time probability distributions are well described by a power law, while the on-time probability distributions are best described by a truncated power law, P(tau(on)) approximately tau(on)(-alpha)e((-tau)(on)/(tau)(c)). The measured crossover time, tau(c), is indistinguishable within experimental uncertainty for core and core/shell NRs, as well as for core NRs with different ligands, for the same core size, indicating that surface passivation does not affect the blinking statistics significantly. We find that, at fixed excitation intensity, 1/tau(c) increases approximately linearly with increasing NR aspect ratio; for a given sample, 1/tau(c) increases very gradually with increasing excitation intensity. Examining 1/tau(c)versus the single-particle photon absorption rate for all samples indicates that the change in NR absorption cross section with sample size can account for some but not all of the differences in crossover time. This suggests that the degree of quantum confinement may be partially responsible for the aspect ratio dependence of the crossover time.     

Two mechanisms for fluorescence intermittency of single violamine R molecules

The environment and temperature-dependent photoluminescence (PL) intermittency or "blinking" demonstrated by single violamine R (VR) molecules is investigated in two environments: poly(vinyl alcohol) (PVOH) and single crystals of potassium acid phthalate (KAP). In addition, temperatures ranging from 23 °C to 85 °C are studied, spanning the glass-transition temperature of PVOH (T(g) = 72 °C). The PL intermittency exhibited by VR is analyzed using probability histograms of emissive and non-emissive periods. In both PVOH and KAP, these histograms are best fit by a power law, consistent with the kinetics for dark state production and decay being dispersed as observed in previous studies. However, these systems have different temperature dependences, signifying two different blinking mechanisms for VR. In PVOH, the on- and off-event probability histograms do not vary with temperature, consistent with electron transfer via tunneling between VR and the polymer. In KAP the same histograms are temperature dependent, and show that blinking slows down at higher temperatures. This result is inconsistent with an electron-transfer process being responsible for blinking. Instead, a non-adiabatic proton-transfer between VR and KAP is presented as a model consistent with this temperature dependence. In summary, the results presented here demonstrate that for a given luminophore, the photochemical processes responsible for PL intermittency can change with environment.     

Photoblinking and photobleaching of rylene diimide dyes

We investigate photoblinking and photobleaching of perylene diimide (PDI) and its higher homologue terrylene diimide (TDI). Single molecule fluorescence trajectories of the dye molecules embedded in PMMA under ambient conditions exhibit "on"-"off" blinking in the time range from ms to s. Due to the limited statistics of individual trajectories we construct ensemble distributions of "on" and "off" times which follow power laws with similar power law coefficients (m(on) ≈ 1.18, m(off) ≈ 1.31). The blinking is attributed to reversible formation of radical cations which are presumably created by electron transfer from higher excited triplet states T(n) of the molecules to acceptor levels in the PMMA host. This view is corroborated by the properties of TDI, which blinks at an excitation wavelength of 520 nm but does not at lower energy excitation (647 nm). In line with this observation, T(1)-T(n) absorption data of TDI (and PDI) indicate that above a certain illumination wavelength population of higher excited triplet states T(n) does not occur, preventing blinking. It is furthermore argued that the long-lived dark ("off") states, i.e. the radical cations, are precursors for the photobleaching process of the dye molecules. Consequently, the photobleaching quantum yield Y(bl) for TDI is very small at an excitation wavelength of 647 nm (Y(bl) = 2 × 10(-10)) but increases by two orders of magnitude at 520 nm (Y(bl) = 2 × 10(-8)), which lies in the range observed for PDI investigated with an excitation wavelength of 488 nm. Additional studies of a PDI-TDI donor-acceptor dyad give further insights into the blinking and bleaching processes. Important findings include the observation of power law blinking of TDI and PDI (after bleaching of TDI) with similar coefficients as found for the isolated chromophores. Furthermore, in the dyad the photostability of TDI decreases due to efficient population of the states T(n) by singlet-triplet annihilation, while that of PDI (after bleaching of TDI) is the same as for the isolated dye. These findings support the conclusions drawn for the isolated chromophores, in particular the involvement of the triplet manifold in the blinking (and bleaching) behavior.     

Mechanisms of fluorescence blinking in semiconductor nanocrystal quantum dots

The light-induced spectral diffusion and fluorescence intermittency (blinking) of semiconductor nanocrystal quantum dots are investigated theoretically using a diffusion-controlled electron-transfer (DCET) model, where a light-induced one-dimensional diffusion process in energy space is considered. Unlike the conventional electron-transfer reactions with simple exponential kinetics, the model naturally leads to a power-law statistics for the intermittency. We formulate a possible explanation for the spectral broadening and its proportionality to the light energy density, the -32 power law for the blinking statistics of the fluorescence intermittency, the breakdown of the power-law behavior with a bending tail for the "light" periods, a lack of bending tail for the "dark" periods (but would eventually appear at later times), and the dependence of the bending tail on light intensity and temperature. This DCET model predicts a critical time t(c) (a function of the electronic coupling strength and other quantities), such that for times shorter than t(c) the exponent for the power law is -12 instead of -32. Quantitative analyses are made of the experimental data on spectral diffusion and on the asymmetric blinking statistics for the "on" and "off" events. Causes for deviation of the exponent from the ideal value of -32 are also discussed. Several fundamental properties are determined from the present experimental data, the diffusion correlation time, the Stokes shift, and a combination of other molecular-based quantities. Specific experiments are suggested to test the model further, extract other molecular properties, and elucidate more details of the light-induced charge-transfer dynamics in quantum dots.     

Aging correlation functions for blinking nanocrystals, and other on-off stochastic processes

Following recent experiments on power law blinking behavior of single nanocrystals, we calculate two-time intensity correlation functions I(t)I(t+t') for these systems. We use a simple two state (on and off) stochastic model to describe the dynamics. We classify possible behaviors of the correlation function and show that aging, e.g., dependence of the correlation function on age of process t, is obtained for classes of the on time and off time distributions relevant to experimental situation. Analytical asymptotic scaling behaviors of the intensity correlation in the double time t and t' domain are obtained. In the scaling limit I(t)I(t+t('))-->h(x), where four classes of behaviors are found: (i) finite averaged on and off times x=t' (standard behavior); (ii) on and off times with identical power law behaviors x=t/t' (case relevant for capped nanocrystals); (iii) exponential on times and power law off times x=tt' (case relevant for uncapped nanocrystals); (iv) for defected off time distribution we also find x=t+t'. Origin of aging behavior is explained based on simple diffusion model. We argue that the diffusion controlled reaction A+B <==>AB, when followed on a single particle level exhibits aging behavior.     

Single molecule fluorescence fluctuations of the cyanine dyes linked covalently to DNA

The intersystem crossing and isomerization dynamics of free-Cy3, Cy3-ssDNA, free-Cy5 and Cy5-ssDNA are obtained through simple analysis of rapid on/off blinking from single molecule fluorescence intensity time-traces and the fluorescence correlation spectroscopy (FCS). The on- and off-times observed in fluorescence time traces of single cyanine dyes are due to the formation of the triplet state and isomerization, where both the interaction with DNA and long central polymethine chain of cyanine dyes increase the barriers of isomerization, leading to long off-time. The results indicate that the single molecule fluorescence fluctuation together with the resulting second autocorrelation analysis are powerful methods for determining the triplet state and isomerization dynamics, which could be the simple techniques and complementary to other spectroscopic techniques, such as fluorescence decay measurement and laser flash photolysis to study the photophysical processes of complex molecules. Supported by the National Natural Science Foundation of China (Grant Nos. 20773139, 20833008 & 20825314), and State Key Project for Fundamental Research (Grant Nos. 2006CB806000 & 2007CB815200)     

Photoinduced hole trapping in single semiconductor quantum dots at specific sites at silicon oxide interfaces

Blinking dynamics of CdSe/ZnS semiconductor quantum dots (QD) are characterized by (truncated) power law distributions exhibiting a wide dynamic range in probability densities and time scales both for off- and on-times. QDs were immobilized on silicon oxide surfaces with varying grades of hydroxylation and silanol group densities, respectively. While the off-time distributions remain unaffected by changing the surface properties of the silicon oxide, a deviation from the power law dependence is observed in the case of on-times. This deviation can be described by a superimposed single exponential function and depends critically on the local silanol group density. Furthermore, QDs in close proximity to silanol groups exhibit both high average photoluminescence intensities and large on-time fractions. The effect is attributed to an interaction between the QDs and the silanol groups which creates new or deepens already existing hole trap states within the ZnS shell. This interpretation is consistent with the trapping model introduced by Verberk et al. (R. Verberk, A. M. van Oijen and M. Orrit, Phys. Rev. B, 2002, 66, 233202).     

Influence of the Dielectric Environment on the Photoluminescence Intermittency of CdSe Quantum Dots

We show experimentally that the photoluminescence intermittency (blinking) of single CdSe quantum dots (QDs) is influenced by the dielectric properties of the embedding environment (matrix), the type of ligands and the capping shell. For the on‐times, we observe (and tentatively explain) a strong deviation from the commonly reported inverse power law behaviour, which can be taken into account by an exponential cut‐off at long times. We assign this component to the photoejection of the electron, while the power law behaviour is a combination of hole‐ and electron‐trapping processes. The cut‐off times and their distributions depend strongly on the polarity of the environment. Also, the off‐times show, though on a much longer timescale, deviations from the inverse power laws. We suggest a model including surface states and self‐trapped states, which quantitatively explains the experimental observations.     

Single molecule studies of calix[4]arene-linked perylene bisimide dimers: relationship between blinking, lifetime and/or spectral fluctuations

We recorded fluorescence time traces, and simultaneously either the fluorescence lifetime or the emission spectra from single perylene bisimide (PBI) dimers embedded in a polystyrene matrix. In these traces three distinct intensity levels can be distinguished, which reflect the photo-induced radicalisation of one of the perylene subunits. Differences in the energy transfer rate between the neutral PBI and the reversibly formed radical anion give rise to variations in the chronological order of the appearance of the intensity levels, which allowed us to categorise the time traces into three distinct groups: Type 1 blinking corresponds to a high energy transfer rate, type 2 blinking to fluctuations between large and small transfer rates (dynamic quenching), and type 3 blinking results from small energy transfer rates together with Coulomb blockade. The information that we obtain from the distributions of the fluorescence lifetimes at the various signal levels allows us to relate these differences to properties of the local polymer environment of the dimers.     

Quenching and blinking of fluorescence of a single dye molecule bound to gold nanoparticles

A fluorescein derivative (SAMSA) bound to gold nanoparticles of different diameters is investigated by time-resolved fluorescence at the single molecule level in a wide dynamic range, from nanosecond to second time scale. The significant decrease of both SAMSA excited state lifetime and fluorescence quantum yield observed upon binding to gold nanoparticles can be essentially traced back to an increase of the nonradiative deactivation rate, probably due to energy transfer, that depends on the nanoparticle size. A slow single molecule fluorescence blinking, in the ms time scale, has a marked dependence on the excitation intensity both under single and under two photon excitation. The blinking dynamics is limited by a low probability nonlinear excitation to a high energy state from which a transition to a dark state occurs. The results point out a strong coupling between the vibro-electronic configuration of the dye and the plasmonic features of the metal nanoparticles that provide dye radiationless deactivation channels on a wide dynamic range.     

近红外聚乙烯醇荧光高分子材料的制备及性能

以一种方酸菁染料、水溶性石墨烯和聚乙烯醇为原料,设计合成了在近红外区具有强荧光(660~665 nm)特性及良好的光热稳定性的方酸菁/聚乙烯醇二元和方酸菁/石墨烯/聚乙烯醇三元高分子材料;与在水中相比,方酸菁染料在聚乙烯醇中的最大紫外-可见吸收和荧光波长红移,荧光强度和光稳定性大幅提高;石墨烯的存在增强了材料的光稳定性。     

Systematic study of the fluorescence decays of amino‐coumarin dyes in polymer matrices

We measured the fluorescence decays of seven different amino‐coumarin dyes in polymer films of poly(methyl methacrylate) (PMMA), poly(styrene) (PS), and ethylene‐butene rubber (EBR); as well as in the small molecule analogs ethyl acetate and toluene. Many of the dye‐solvent and dye‐polymer combinations exhibited single exponential decays with lifetimes ranging from 2.3 to 3.9 ns. Small deviations from single exponential behavior occurred for most of the dyes in EBR. Significant deviations from single exponential behavior occurred for 7‐(diethylamino)‐2‐oxo‐2H‐1‐benzopyran‐3‐carboxylic acid (coumarin‐3) in ethyl acetate and in all polymer matrices and 2,3,6,7‐tetrahydro‐11‐oxo‐1H,5H,11H‐[1]benzopyrano[6,7,8‐ij]quinolizin‐10‐carboxylic acid (coumarin‐343) in all of the polymer matrices. Time‐resolved fluorescence spectra indicated the presence of two different excited states for coumarin‐3 and coumarin‐343 in PMMA; these spectra were qualitatively different from the time‐resolved spectra of coumarin‐3 in ethyl acetate. We rationalize these results in terms of the chemical functionalities of the various dyes. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2333–2343, 2007     

Reduced blinking and long-lasting fluorescence of single fluorophores coupling to silver nanoparticles

The fluorescence signal of single organic fluorophores is characterized by random blinking and irreversible photobleaching. Photoinduced blinking of Cy5 has posed various limitations of this popular near-infrared (NIR) probe in biological applications. Here we show that fluorophore-metal nanoparticle (NP) complexes greatly suppress Cy5 blinking and noticeably reduce photobleaching events. The blinking behavior of single Cy5 molecules was investigated and compared in the absence and the presence of silver nanostructures. A power-law distribution of off time population was observed for single Cy5 molecules. Average off times were compared to evaluate the plasmonic effect of silver nanoparticles on the triplet decay rates. We furthermore demonstrate enhanced photostability in the presence of silver NPs. The results show that plasmonic-controlled fluorescence can lead to a novel physical mechanism to enhance fluorescence intensity, reduce blinking, and increase photostability.     

Preparation and fluorescent properties of poly(vinyl alcohol) bearing coumarin

A novel water‐soluble fluorescent material was prepared via ring‐opening reaction between 4‐methyl‐7‐(2,3‐expoxypropoxy) coumarin (MEC) and poly(vinyl alcohol) (PVA). The fluorescent behaviors of this material (PVA–MEC) in solution, solid and film were studied in detail. The results showed that the fluorescence of PVA–MEC arose from isolated dye molecules and had a good film forming ability. In addition, the effects of acid/base environments on PVA–MEC were studied and the results showed that it was less affected by environment than 7‐HMC. Moreover, relative fluorescence intensity of PVA–MEC had an excellent linear response in the temperature range of 0–60°C. These observations suggest that PVA–MEC is an excellent fluorescent macromolecular material with a convenient method of preparation and had a good water‐soluble ability. Copyright © 2007 John Wiley & Sons, Ltd.     

稀土配合物发光中的能量传递研究

就稀土发光的能量传递开展了3个方面的初步研究。首先,研究了稀土镧和铕掺杂后共发光的现象和机制;其次是研究了稀土铽配合物作为能量传递桥可以将能量从导电高分子聚乙烯咔唑（PVK）有效地传递给铕配合物;第三,发现通过物理掺杂的方法可以改变稀土铕配合物的激发光谱的形状,使其向着长波长方向移动,这样可以使铕配合物的激发光谱和PVK的发射光谱发生更大的重叠,从而增强它们之间的能量传递效率。     

Studies on the structural transitions and self-organization behavior of polyacrylic acids with complementary polymers in aqueous solution by laser flash photolysis method using the triplet state of covalently bound phenosafranine

The conformational transition of polyacrylic acids and the formation of interpolymer complexes with synthetic polymers in aqueous solution are investigated using the triplet state of the cationic dye phenosafranine covalently attached to the polymer chain. Laser excitation of the phenosafranine dye covalently bound to polymethacrylic acid at 532 nm shows that the absorption spectrum of the triplet state shifts to red region by 40 nm as compared to that of the free dye in aqueous solution and the triplet state lifetime is enhanced by 20-fold. Laser flash excitation shows that the environment of the triplet state of the dye bound to the polyelectrolyte at pH ?5.5 in aqueous solution is more rigid and less polar resulting in a highly compact globular nature of the polymer. The decay of the triplet state of the dye bound to the polymer is attributed to the quenching of the excited state by the carboxylate groups of polyacrylic acids and to the decay process of the triplet in the tightly coiled polymer environment in the pH range 2.0–5.0. The spectra of the triplet dye molecules bound to the polymer at different degree of ionization of the polyelectrolyte suggest that the structural transition from compact globular structure to stretched rod like structure is cooperative involving a series of structural transitions. The observation of diprotonated triplet state of the PMAA bound dye at higher pH (i.e. pH ∼7.0) reveals the existence of an intermediate structure akin to a micellar segment in PMAA prior to the formation of elongated linear chain. The self-organization of PMAA adduct formation with complementary macromolecules, PVP, PEO and PVA primarily due to hydrogen bonding makes the environment of the dye in the adduct more compact and rigid; in particular poly(vinylpyrrolidone), PVP, has the tendency to form more compact interpolymer complex at pH 4.5 than poly(vinyl alcohol), PVA, and poly(ethylene oxide), PEO as revealed from the laser flash photolysis studies of the polymer bound dye using triplet state of the phenosafranine as the marker.     

Sensitized phosphorescence of benzil-doped ladder-type methyl-poly(para-phenylene)

The delayed luminescence and phosphorescence of ladder-type methyl-poly(para-phenylene) (MeLPPP) doped with benzil at a concentration of 20% by weight has been measured. The introduction of benzil leads to a dramatic reduction of the polymer singlet emission. At the same time, a new band with maximum at 611 nm appears, corresponding to the phosphorescence of MeLPPP. The phosphorescence decay on the short time scale is close to an exponential law with a time decay of 15 ms. This indicates that benzil can efficiently sensitize the phosphorescence of the polymer. In addition, a broad and featureless emission is observed in the delayed luminescence spectra of benzil-doped MeLPPP, which is attributed to an exciplex formed between the polymer host and the dopant. We further observe that the delayed fluorescence is enhanced by the addition of benzil. It is concluded that the delayed fluorescence of benzil-doped MeLPPP is mainly due to the annihilation of triplet excitons on the polymer. Finally, efficient triplet-triplet energy transfer from the benzil-doped polymer to the red-emitting phosphorescent dye Pt(II)octaethylporphyrin is established.