Kinetics Of Delayed Fluorescence Research Articles

A general framework for non-exponential delayed fluorescence and phosphorescence decay analysis, illustrated on Protoporphyrin IX

Delayed fluorescence (DF) is a long-lived luminescence process used in a variety of applications ranging from oxygen sensing in biological tissues to organic Light Emitting Diodes. In common cases, DF results from the de-excitation of the first excited triplet state via the first excited singlet state of the chromophore, which produces a mono-exponential light signal whose amplitude and lifetime give an insight into the probed environment. However, non-linear de-excitation reactions such as triplet-triplet annihilation, which can cause decays to lose their mono-exponential nature, are often neglected. In this work, we derive a global framework to properly interpret decays resulting from a combination of linear and non-linear de-excitation processes. We show why the standard method of using multi-exponential models when decays are not mono-exponential is not always relevant, nor accurate. First, we explain why the triplet de-excitation and light production processes should be analyzed individually: we introduce novel concepts to precisely describe these two processes, namely the deactivation pathway – the reaction which mainly contributes to the triplet state de-excitation – and the measurement pathway – the reaction which is responsible for light production. We derive explicit fitting functions which allow the experimenter to estimate the reaction rates and excited state concentrations in the system. To validate our formalism, we analyze the in vitro Transient Triplet Absorption and DF of Protoporphyrin IX, a well-known biological aromatic molecule used in photodynamic therapy, cancer photodetection and oxygen sensing, which produces DF through various mechanisms depending on concentration and excitation intensity. We also identify the precise assumptions necessary to conclude that triplet-triplet annihilation DF should follow a mono-exponential decay with a lifetime of half the triplet state lifetime. Finally, we describe why the commonly used definitions of triplet / DF lifetime are ill-defined in the case where second-order reactions contribute to the deactivation process, and why the fitting of precise mixed-orders DF kinetics should be preferred in this case. This work could allow the correct interpretation of various long-lived luminescence processes and facilitate their understanding.

Polystyrene and Poly(ethylene glycol)-b-Poly(ε-caprolactone) Nanoparticles with Porphyrins: Structure, Size, and Photooxidation Properties.

The transport of a photosensitizer to target biological structures followed by the release of singlet oxygen is a critical step in photodynamic therapy. We compared the (photo)physical properties of polystyrene nanoparticles (TPP@PS) of different sizes and self-assembled poly(ethylene glycol)-b-poly(ε-caprolactone) core/shell nanoparticles (TPP@PEG-PCL) with different lengths of copolymer blocks, both suitable for the transport of the tetraphenylporphyrin (TPP) photosensitizer. The singlet oxygen was formed inside both nanoparticles after irradiation with visible light. Its kinetics was controlled by the size of TPP@PS; its lifetime (τΔ) increased with increasing nanoparticle size (from 6.5 to 16 μs) because of hindered diffusion into the external aqueous environment, where it was quickly deactivated. Accordingly, the prolongation of the singlet oxygen-sensitized delayed fluorescence kinetics was found for TPP@PS of high size. The TPP@PEG-PCL self-assemblies allowed for enhanced oxygen diffusion, and the estimated low values of τΔ ≈ 3.7 μs were independent of the size of building blocks. The delayed fluorescence in oxygen-free conditions originating from triplet-triplet annihilation indicated a high mobility of TPP in the PCL core in comparison with fixed molecules in the PS matrix. Photooxidation of uric acid revealed the highest efficacy for TPP@PS of small sizes, whereas the largest TPP@PS exhibited the lowest activity, and the efficacy of TPP@PEG-PCL remained independent of the sizes of the building blocks.

Thermal Dynamics of Xanthene Dye in Polymer Matrix Excited by Double Pulse Laser Radiation

Double-pulse laser excitation of the eosin and silver nanoparticles embedded into polymer media is known to be a method of electronic-vibrational energy deactivation kinetic process information obtaining and polymer thermal dynamics investigation. We have studied the vibrational relaxation processes in dye molecules (eosin) and nanoparticles in polyvinyl alcohol after two time-shifted laser pulses with fast and delayed fluorescence kinetics study. In order to simulate thermal and photophysical processes caused by double photon excitation, we solved heat transfer and energy deactivation differential equations numerically. The simulation allowed us to obtain the value of heat conductivity coefficient of polymer matrix.

Singlet oxygen-sensitized delayed fluorescence of common water-soluble photosensitizers

Six common water-soluble singlet oxygen ((1)O2) photosensitizers - 5,10,15,20-tetrakis(1-methyl-4-pyridinio) porphine (TMPyP), meso-tetrakis(4-sulfonathophenyl)porphine (TPPS4), Al(III) phthalocyanine chloride tetrasulfonic acid (AlPcS4), eosin Y, rose bengal, and methylene blue - were investigated in terms of their ability to produce delayed fluorescence (DF) in solutions at room temperature. All the photosensitizers dissolved in air-saturated phosphate buffered saline (PBS, pH 7.4) exhibit easily detectable DF, which can be nearly completely quenched by 10 mM NaN3, a specific (1)O2 quencher. The DF kinetics has a biexponential rise-decay character in a microsecond time domain. Therefore, we propose that singlet oxygen-sensitized delayed fluorescence (SOSDF), where the triplet state of a photosensitizer reacts with (1)O2 giving rise to an excited singlet state of the photosensitizer, is the prevailing mechanism. It was confirmed by additional evidence, such as a monoexponential decay of triplet-triplet transient absorption kinetics, dependence of SOSDF kinetics on oxygen concentration, absence of SOSDF in a nitrogen-saturated sample, or the effect of isotopic exchange H2O-D2O. Eosin Y and AlPcS4 show the largest SOSDF quantum yield among the selected photosensitizers, whereas rose bengal possesses the highest ratio of SOSDF intensity to prompt fluorescence intensity. The rate constant for the reaction of triplet state with (1)O2 giving rise to the excited singlet state of photosensitizer was estimated to be ~/>1 × 10(9) M(-1) s(-1). SOSDF kinetics contains information about both triplet and (1)O2 lifetimes and concentrations, which makes it a very useful alternative tool for monitoring photosensitizing and (1)O2 quenching processes, allowing its detection in the visible spectral region, utilizing the photosensitizer itself as a (1)O2 probe. Under our experimental conditions, SOSDF was up to three orders of magnitude more intense than the infrared (1)O2 phosphorescence and by far the most important pathway of DF. SOSDF was also detected in a suspension of 3T3 mouse fibroblast cells, which underlines the importance of SOSDF and its relevance for biological systems.

Features of the delayed fluorescence kinetics of exogenous fluorophores in biological tissues

Kinetic regularities of the delayed fluorescence and phosphorescence of exogenous fluorophores (organic molecule dyes) in biological tissues were established. Differences in the delayed fluorescence kinetics of the healthy and pathogenic tissues stained with erythrosine in vitro are discussed.

Pathology development stage and its influence on the delayed fluorescence kinetics of molecular probes

This paper describes regularity of the delayed fluorescence kinetics changes of xanthene dyes in the cancer cells received from tumors of different diameter. It shows that the rate constant of bimolecular reactions (singlet–triplet T−1Δg(O2) and triplet–triplet annihilations) as well as thermoactive fluorescence decrease with increasing pathological process development stage. It could be a result of both more expressed cancer cell hypoxia and a change in cytoplasm viscosity.

The Kinetics of Exogenous Phosphors Delayed Fluorescence in Tissues

The kinetics of delayed fluorescence (DF) and phosphorescence (Ph) of xanthene dyes in tumourous and normal mammary tissues of the BYRB line mice was investigated. Spontaneous mammary cancer tumours are characteristic of the mice of this line with the possible exogenous MMTV- retrovirus, found in leukocytic fraction. The kinetics of DF and Ph molecules of the dyes was measured by means of laser flash-photolysis. The molecules in basic S0→S1 absorption band were excited by the second harmonic of pulsed laser to YAG:Nd3+ with wave length 532 nm. At the expiration of the process S1→T1 intersystem crossing (IC) and the formation of triplet T1 molecule state, the registration system of delayed luminescence started. The luminescence was registered by PMT and monochromator. The parameters of the exciting pulse: pulse duration 10 ns, density of energizing power not more than 5 MWt/cm2. The DF was registered at the wave length of 560 nm, and Ph at the wave length of 680 nm. The DF of the examined molecules can occur both as the result of termoactivated reverse to T1→S1 IC and as the result of the two T1 states annihilation with the sequent formation of S1 states. Moreover under the certain conditions in the presence of oxygen the cross – annihilation of the dye and oxygen stimulated molecules can occur. It was demonstrated that the most effective quencher of the triplet T1 states of phosphors in the tissues is the molecular oxygen 3!g"(O2). In the result of the interaction of the dye T1 molecules with the molecular oxygen singlet 1∆g (O2) oxygen is formed: T1+3!g"(O2)#$#S0+1%g(O2). Then as the result of singlet-triplet T1→1∆g(O2) annihilation of the rest triplet T1 states of the dyes with 1∆g(O2): T1+1!g(O2)"#"S1+3$g%(O2) the singlet S1 states of dyes are formed, thus contributing to the DF. As a result the registered kinetics of the DF of the dyes is made up of the three signals of various nature: termoactivated DF; T1-T1 annihilated DF; and luminescence due to the singlet-triplet T1→1∆ g (O2) cross–annihilation. The kinetic curve transforms and becomes no monotonously dependent on time. It was specified that in the tissues during short periods of time the most significant contribution to the total signal was made by singlet-triplet T1→1∆ g (O2) annihilation. However, other things equal, the delayed luminescence kinetics in normal and pathogenous tissues differ. The contribution of the singlet-triplet annihilation to the total signal of the DF in the normal tissues is significantly less than in tumour, which indicates the less effectiveness of the triplet states of the dyed molecules interaction with the singlet oxygen. Unlike DF the Ph intensity of dye molecules decreases monotonously with the period of time. Phosphorescence kinetics as well as DF differs within different tissues. The luminescence peculiarity depends on the phase of the tumour, the biotissue condition and other factors. However in all our experiments the common regularity is evident, namely the Ph lifetime in tumours is shorter than that in normal tissues. Reliable registered differences in the dye tissue delayed luminescence kinetics can be used when developing an alternative method of optical diagnosis of biotissues. We assume that the method based on the measurement of lifetime of the delayed luminescence phosphor is fairly promising. Any combinations possessing delayed luminescence and meeting the requirements set to such specimen may serve as exogenous phosphor.

Mechanism study on the origin of delayed fluorescence by an analytic modeling of the electronic reflux for photosynthetic electron transport chain

A mathematical–physical analysis model, which describes individually the electronic reflux of several significant components in the photosynthesis electron transport chain, was firstly developed. The process of electrons flowing back to the oxidized reaction center P 680 + was simulated by a series of photochemical reaction equations, resulting in getting the linked differential equations of delayed fluorescence (DF) intensity. MATLAB provided a computationally efficient method to solve these linked equations. Simulations based on this model showed that the decay kinetics of DF accord with double exponential. DF components decaying in the millisecond range (fast phase) are related to the charge recombination of P 680 + and Q A - . The components decaying in the seconds range are associated with the recombination of P 680 + with Q B 2 - . The developed model was tested in maize leaves treated with different electron blockers to induce changes in photosynthesis electron transport chain. The experimental results demonstrated that the developed model can accurately determine the regulatory effects of electron blockers on photosynthesis electron transport chain. Therefore, the model presented here could be potentially useful for studying the electron transfer in plant. It also provides an experimental workbench for testing hypotheses as to the underlying mechanism controlling the change for different phases of DF.

Triplet exciton state and related phenomena in the β-phase of poly(9,9-dioctyl)fluorene

Using both time-resolved emission and cw photoinduced absorption spectroscopy as a function of temperature, the aggregation phenomena ($\ensuremath{\beta}$-phase formation) observed in poly(9,9-dioctyl)fluorene is studied. All spectra of the $\ensuremath{\beta}$ phase, including absorption, prompt and delayed fluorescence, phosphorescence, and photoinduced triplet absorption feature very narrow linewidths, which are unique within the class of conjugated polymers. From the comparison of the latter data with amorphous polyfluorene, poly(9,9-diethylhexyl)fluorene, as well as with the fully planar ladder-type poly(paraphenylene), we conclude that the origin of the $\ensuremath{\beta}$ phase cannot simply be an extended intrachain conjugation, but interchain interactions are involved. Furthermore, the $\ensuremath{\beta}$ phase acts as an energetic trap for both singlet and triplet excitons initially created on amorphous chain segments. The delayed fluorescence kinetics of the $\ensuremath{\beta}$ phase were measured at different temperatures. From the analysis of these decays within the framework of dispersive triplet migration in a Gaussian density of states distribution, further evidence is provided that the delayed fluorescence originates from triplet-triplet annihilation. At room temperature, it is clear that triplet excitons migrate over large distances, exceeding that of singlet excitons. Also, the segregation time between dispersive triplet migration and classical thermally activated hopping, is in the case of $\ensuremath{\beta}$-phase containing samples, dependent on the separation of the $\ensuremath{\beta}$-phase domains.

Intra-chain triplet–triplet annihilation and delayed fluorescence in soluble conjugated polymers

Pulse radiolysis has been used to generate high triplet populations on isolated conjugated polymer chains in benzene solutions. This allows direct observation of delayed fluorescence (DF) arising from intramolecular triplet–triplet annihilation (TTA). In poly(2-methoxy,5-(2 ′-ethyl-hexyloxy)- p-phenylenevinylene) the triplet lifetime at low triplet concentration is 90 μs which decreases to 15 μs as the number of triplets saturates at ca. 30 triplets per chain. Analysis of the DF kinetics gives an upper limit of 1.5% for the number of triplets that contribute to TTA. We also observed DF in a range of other conjugated polymers.

The triplet state of the ladder-type methyl-poly( p-phenylene) as seen by pulse radiolysis-energy transfer

The lowest triplet state of a ladder poly( p-phenylene) (MeLPPP) has been produced by sensitised energy transfer following pulse radiolysis of benzene solution, and absorbs at 1.34 eV with a lifetime ⩾170 μs. An S 0–T 1 energy separation of 2.15 ± 0.07 eV was determined, in excellent agreement with recent phosphorescence measurements, confirming the validity of the technique for the determination of the triplet energies of conjugated polymers. At high triplet populations intrachain triplet–triplet annihilation (TTA), leading to delayed fluorescence (DF) occurs. The DF spectrum is in good agreement with the prompt fluorescence and the DF kinetics support a TTA mechanism.

Delayed Fluorescence Induced by Molecular Oxygen Quenching of Zinc Tetraphenylporphyrin Triplets at Gas/Solid Interfaces of Silica and Zeolite

The quenching of Zn(II) tetraphenylporphyrin (ZnTPP) triplets adsorbed onto porous silica or NaA zeolite by molecular oxygen in the gas phase was studied at different temperatures by the diffuse-reflectance laser flash photolysis technique. The quenching is enhanced with decrease in temperature (apparent activation energies are within the range −1.5 to −2.0 kcal/mol) and occurs with high rate constants (2.1 × 105 to 2.4 × 105 Torr-1 s-1). While the ZnTPP delayed fluorescence (DF) is undetectable in evacuated samples, it appears in the presence of O2. The yield of this oxygen-induced DF of ZnTPP corresponds to 10−20% of the prompt fluorescence in a wide range of O2 concentration (partial O2 pressure from 0.001 up to 10 Torr). DF kinetics exhibits a distinct rise part at relatively high O2 pressure. Experimental results are described in terms of a singlet oxygen feedback mechanism which includes the efficient triplet energy transfer from 3ZnTPP to 3O2 and formation of 1O2 followed by a much more efficient e...

The analysis of oxygen-induced change in the luminescence kinetics

A new technique of precision measurement of molecular oxygen concentration is proposed. It is based upon a phenomenological analytical treatment of the kinetics of the delayed fluorescence (DF) arising from the annihilation of triplet excited states of phosphor molecules in solid solution with mobile oxygen molecules previously activated to the singlet level (1 Delta g) by energy transfer. Eosin and erythrosin are embedded in polymer polyvinylbutyral matrices, and used to study the dependences of the suggested parameters of DF kinetics on air pressure (from 0.05 to 1 atm) and matrix temperature (from 15 to 18 degrees C). This method may be readily applied to an oxygen concentration measuring device.

Annihilation processes and interaction with oxygen in micellar solutions of aromatic hydrocarbons

Cationic SAC, cetyltrimethylammonium bromide (CTAB), and anionic SAC, sodium dodecylsulfate (SDS), were selected for this study. Aromatic hydrocarbons (AH), anthracene and fluoranthene, both sparingly soluble in water (<I0 -~ mole/liter) and well soluble in watermicelle solutions (WMS), were selected as luminescent probes. The TTA in WMS was monitored by measuring delayed fluorescence (DF) of the solutions with a pulsed laser apparatus [5]. The third harmonic of a YAG:Nd 3+ laser system (X = 355 nm, W = 3  10 -3 J) was used for the excitation. The detection system of the apparatus has been described elsewhere [6]. The AH were purified by zone refining, and the SAC (Merck) were used as received. Doubly distilled water and D20 (99.8% deuteration) were used as solvents. The solutions were thermostated at 20~ The solutions were degassed by freezepump--thaw cycles. Anthracene and fluoranthene were introduced into WMS by two methods. The first method was to add a 1% hexane solution of the luminophore to the WMS and subsequently evacuate hexane by stirring the solution vigorously. The second method was to add anthracene and fluoranthene crystals to the WMS and stir the solution with a magnetic stirrer for 72 h at t = 70~ The amount of AH in the WMS was monitored by fluorescence excited by an LGI-505 nitrogen laser. Both methods of introducing the luminophore into solution produced identical results. The first method was used as a method of choice. In all experiments the concentration of CTAB was 2.5  10 -3 mole/liter and that of SDS was 2 x 10 -2 mole/liter. The critical micelle concentrations (CMC) for CTAB and SDS were taken as 9.5  10 -4 [7] and 8.1 x 10 -3 mole/liter, respectivel~ [8]. The laser excitation of deoxygenated AH in WMS led to complex decay kinetics of DF. Figure la presents typical oscilloscope decays of the luminophore DF in WMS. A decay analysis of the luminophore DF for the AH concentration range under study (10-6-5  10 -6 mole/ liter) on a semilogarithmic scale revealed two exponential regions at the initial (AB) and final (CD) stages of the decay. The decay constants for AB and CD regions of the decay curve were calculated from the plots in Fig. ib and ic (KAB - 10 s and KCD - 10 3 sec-Z). We will now discuss possible reasons for the complex excited triplet decay behavior of AH in WMS. Both anthracene and fluoranthene exhibit a large singlet-triplet

Kinetics of incoherent exciton annihilation in nonideal one-dimensional structures

The annihilation of incoherent excitons in a quasi-one-dimensional (Q1D) crystal containing impurities (which form traps or reflecting barriers) is studied. The distribution function and the survival probability of an annihilating pair of quasi-particles diffusing in a chain with absorbing or reflecting ends are obtained. These results are used to calculate the kinetic curves of the delayed fluorescence (DF) caused by triplet-triplet annihilation in Q1D crystals containing impurities whose concentration is much higher than the concentration of triplet excitons. It is shown that at long times the kinetics of DF is described by the exponential dependence exp( -const t 1/3) which differs qualitatively from that predicted for pure Q1D crystals, where a power law of DF decay α t -3/2 is expected.

Determination of the probability of excited singlet state generation for molecules during triplet-triplet heteroannihilation

An increased intensity of delayed fluorescence (DF) of dyes in alcohols as a result of addition of aromatic hydrocarbons has been observed. The authors understanding of this process is incomplete and mainly based on information extracted from the rate constant of the excited singlet state formation as a result of triplet pair annihilation. It is therefore important to develop new methods for determining the probability of occurrence of the luminescence process, i.e., the fraction of annihilation acts which lead to the excited singlet state formation. In the present work this parameter is determined from analysis of some peculiarities in the DF kinetics for the studied system. Binary mixtures of anthracene with dyes in propyl alcohol were used as model systems.

Differential equations for delayed fluorescence kinetics in living plants

The slow decay of the delayed fluorescence (DF) of living plants seems to have a complicated kinetics. A system of differential equations is derived, which takes into account that DF results from a recombination of electrons at the oxidized reaction-center P680 of Photosystem II (PS II). We use a resistor-accumulator (capacitor) equivalent-circuit to describe the Z-scheme. The main characteristic of the model is that the electron-pools of the transport-chain of PS I and PS II are coupled in series by the plastoquinon-molecules.

Triplet Exciton–Charge Carrier Interaction in Anthracene

AbstractThe delayed fluorescence (DF) quenching of anthracene crystals caused by hole injection is investigated. The kinetics of DF and DF quenching due to triplet‐hole interaction are measured over a wide range of light intensities when the triplet exciton decay converts from unimolecular to bimolecular. The values of photoenhanced space‐charge‐limited current have provided the ratio of free to trapped hole concentrations in crystal. The increased role of triplet‐free hole interaction at high light intensity is shown. The rate constants of triplet‐trapped hole ((0.6 ± 0.2) × 10−11 cm3 s−1) and of triplet‐free hole ((2 ± 1) × × 10−10 cm3 s−1) interactions leading to triplet quenching are measured.