Singlet-triplet Annihilation Research Articles

Singlet-triplet annihilation in single LHCII complexes.

In light harvesting complex II (LHCII) of higher plants and green algae, carotenoids (Cars) have an important function to quench chlorophyll (Chl) triplet states and therefore avoid the production of harmful singlet oxygen. The resulting Car triplet states lead to a non-linear self-quenching mechanism called singlet-triplet (S-T) annihilation that strongly depends on the excitation density. In this work we investigated the fluorescence decay kinetics of single immobilized LHCIIs at room temperature and found a two-exponential decay with a slow (3.5 ns) and a fast (35 ps) component. The relative amplitude fraction of the fast component increases with increasing excitation intensity, and the resulting decrease in the fluorescence quantum yield suggests annihilation effects. Modulation of the excitation pattern by means of an acousto-optic modulator (AOM) furthermore allowed us to resolve the time-dependent accumulation and decay rate (∼7 μs) of the quenching species. Inspired by singlet-singlet (S-S) annihilation studies, we developed a stochastic model and then successfully applied it to describe and explain all the experimentally observed steady-state and time-dependent kinetics. That allowed us to distinctively identify the quenching mechanism as S-T annihilation. Quantitative fitting resulted in a conclusive set of parameters validating our interpretation of the experimental results. The obtained stochastic model can be generalized to describe S-T annihilation in small molecular aggregates where the equilibration time of excitations is much faster than the annihilation-free singlet excited state lifetime.

Effect of Conjugated Backbone Protection on Intrinsic and Light-Induced Fluorescence Quenching in Polythiophenes

Polythiophenes (PTs), particularly regioregular poly(3-hexylthiophene-2,5-diyl) (rr-P3HT), are important materials in photovoltaics. The photophysical properties of PTs are still poorly understood, because of their aggregation tendency and formation of interchain species which can be avoided by insulating the conjugated backbone via self-threading. We investigated two polymers, rr-P3HT and its insulated analog, imbedded in PMMA at low concentrations. The exciton decay dynamics and fluorescence quantum yield were analyzed as a function of excitation power densities over the range from 1 × 10–4 to 100 W/cm2. For both polymers, substantial (up to 5 times) photoinduced fluorescence quenching was observed owing to singlet–triplet annihilation and quenching by other long-living charged photoproducts. We found that chain insulation eliminates static (or ultrafast) fluorescence quenching, but has no effect on slow dynamic quenching at time scales longer than 10 ps. We propose that static quenching is solely due t...

Temperature dependence of the exciton dynamics inDCM2:Alq3

We study the temperature dependence of the triplet and singlet exciton dynamics in the archetype small molecule fluorescent guest-host system tris(8-hydroxyquinolinato) aluminum $({\mathrm{Alq}}_{3})$ doped with 4-(dicyanomethylene)-2-methyl-6-julolidyl-9-enyl-4H-pyran (DCM2). We develop a comprehensive model of the exciton dynamics, and use it to fit the transient photoluminescence under different pulsed optical pumping in the temperature range of $80<T<295\phantom{\rule{4pt}{0ex}}\mathrm{K}$. The triplet decay has a significantly different temperature dependence than that of singlets. From 295 to 80 K, the triplet-triplet annihilation (TTA) rate decreases by two orders of magnitude, whereas the singlet-triplet annihilation rate decreases by $<50%$, primarily a result of the different energy transfer mechanisms of singlets and triplets. The temperature dependence of the TTA rate reveals two regimes separated by a transition at 180 K from Marcus to Miller-Abrahams transfer. This work deepens our understanding of exciton dynamics and energy transfer in small molecule organic materials.

Influence of Phospholipid Composition on Self-Assembly and Energy-Transfer Efficiency in Networks of Light-Harvesting 2 Complexes

In the photosynthetic membrane of purple bacteria networks of light-harvesting 2 (LH2) complexes capture the sunlight and transfer the excitation energy. In order to investigate the mutual relationship between the supramolecular organization of the pigment-protein complexes and their biological function, the LH2 complexes were reconstituted into three types of phospholipid membranes, consisting of L-α-phosphatidylglycerol (PG), L-α-phosphatidylcholine (PC), and L-α-phosphatidylethanolamine (PE)/PG/cardiolipin (CL). Atomic force microscopy (AFM) revealed that the type of phospholipids had a crucial influence on the clustering tendency of the LH2 complexes increased from PG over PC to PE/PG/CL, where the LH2 complexes formed large, densely packed clusters. Time-resolved spectroscopy uncovered a strong quenching of the LH2 fluorescence that is ascribed to singlet-singlet and singlet-triplet annihilation by an efficient energy transfer between the LH2 complexes in the artificial membrane systems. Quantitative analysis reveals that the intercomplex energy transfer efficiency varies strongly as a function of the morphology of the nanostructure, namely in the order PE/PG/CL > PC > PG, which is in line with the clustering tendency of LH2 observed by AFM. These results suggest a strong influence of the phospholipids on the self-assembly of LH2 complexes into networks and concomitantly on the intercomplex energy transfer efficiency.

Analysis of exciton annihilation in high-efficiency sky-blue organic light-emitting diodes with thermally activated delayed fluorescence

We study external quantum efficiency (ηEQE) roll-off in organic light-emitting diodes (OLEDs) using thermally-activated delayed fluorescence (TADF) of 4,5-di (9H-carbazol-9-yl) phthalonitrile (2CzPN). Using 2CzPN intramolecular rate constants from optical analyses, we construct an exciton quenching model incorporating intersystem crossing and reverse intersystem crossing. The model indicates that singlet–triplet annihilation and triplet–triplet annihilation dominate ηEQE roll-off because of the relatively long 2CzPN triplet lifetime of 273μs. This work yields a method to relax the exciton quenching process in TADF based OLEDs.

Ab Initio Studies of Triplet-State Properties for Organic Semiconductor Molecules

Triplet–triplet annihilation (TTA) leads to a reduced efficiency of organic light-emitting diodes (OLEDs) at high current densities. Spacial confinement of the triplet excitons, which is mainly dependent on triplet energy differences, can reduce the TTA rate. Therefore, a deliberate choice of the organic semiconductor materials with particular attention to their triplet energies can help to considerably increase the device efficiency. Organic solid-state lasers are, on the other hand, efficiently quenched by singlet–triplet annihilation (STA), which is closely related to the triplet–triplet absorption of the organic semiconductors. To establish a useful set of parameters related to the processes in organic semiconducting devices, we provide theoretical estimates for the triplet energy of 31 organic semiconductor molecules using state-of-the art ab initio quantum chemical methods. For a subset of 22 molecules, the triplet–triplet absorption spectra were calculated as well. We also discuss related features ...

Triplets Contribute to Both an Increase and Loss in Fluorescent Yield in Organic Light Emitting Diodes

Nonradiative triplets in fluorescent organic light emitting diodes (OLEDs) can lead to increased efficiency through triplet-triplet annihilation, or to decreased efficiency due to singlet-triplet annihilation. We study the tradeoff between the two processes from the electroluminescence transients of an OLED comprising a tetraphenyldibenzoperiflanthene (DBP) doped rubrene emissive layer, whose emission spectrum peaks at a wavelength of 610 nm. The electroluminescent transients in the current density range, 4 mA/cm(2)<J<57 A/cm(2), are modeled based on singlet and triplet density dynamics. Our analysis shows that triplets positively contribute to the OLED efficiency at J<2.2 A/cm(2), while decreasing the efficiency at higher J. The high OLED peak external quantum efficiency of 6.7% and rapid efficiency roll-off with J are quantitatively explained by the tradeoff between triplet-triplet and singlet-triplet annihilation. The model suggests optimal materials properties needed for achieving high efficiency at high brightness in fluorescent OLEDs.

Mechanisms of Suppression and Enhancement of Photocurrent/Conversion Efficiency in Dye-Sensitized Solar-Cells Using Carotenoid and Chlorophyll Derivatives as Sensitizers

The mechanisms of suppression and enhancement of photocurrent/conversion efficiency (performance) in dye-sensitized solar cells, using carotenoid and chlorophyll derivatives as sensitizers, were compared systematically. The key factor to enhance the performance was found to be how to minimize interaction among the excited-state dye-sensitizer(s). In a set of retinoic-acid (RA) and carotenoic-acid (CA) sensitizers, having n conjugated double bonds, CA7 gave rise to the highest performance, which was reduced toward RA5 and CA13. The former was ascribed to the generation of triplet and the resultant singlet-triplet annihilation reaction, while the latter, to the intrinsic electron injection efficiency. In a set of shorter polyene sensitizers having different polarizabilities, the one with the highest polarizability (the highest trend of aggregate formation) exhibited the higher performance toward the lower dye concentration and the lower light intensity, contrary to our expectation. This is ascribed to a decrease in the singlet-triplet annihilation reaction. The performance of cosensitization, by a pair of pheophorbide sensitizers without and with the central metal, Mg or Zn, was enhanced by the light absorption (complementary rather than competitive), the transition-dipole moments (orthogonal rather than parallel) and by the pathways of electron injection (energetically independent rather than interactive).

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.

Probing the Electronic State of a Single Coronene Molecule by the Emission from Proximate Fluorophores

We measured electronic transitions of the 2D graphene-type molecule hexa-peri-hexabenzocoronene (HBC) at the single-molecule level. The large intersystem crossing rate and long triplet state lifetime in the range of seconds are prohibitive for direct single-molecule observation. By covalently coupling fluorescent acceptor molecules (perylenecarboximide, PMI) to HBC, efficient singlet energy transfer gives rise to strong PMI fluorescence. Confocal single-molecule fluorescence microscopy with two excitation colours matching the HBC and PMI transition frequencies, respectively, was conducted. Single HBC-6PMI molecules were readily observed via the PMI emission. It was found that after selective excitation of the HBC the PMI emission is interrupted by dark intervals whose length of several seconds is in agreement with the triplet state lifetime of HBC. Accordingly, the presence/absence of PMI emission permits to read out the spin state of a single HBC molecule. Moreover, due to spectral overlap, the HBC triplet state acts as an energy acceptor for PMI in the excited singlet state, thus leading to efficient singlet-triplet annihilation (STA) during its lifetime. Hence, intersystem crossing into the HBC triplet state serves as a collective fluorescence switch for individual multichromophores.

Absorption, Luminescence, and Sizing of Organic Dye Nanoparticles and of Patterns Formed Upon Dewetting

Organic nanoparticles made of a push-pull triarylamine dye with an average diameter of 60 nm, were prepared by reprecipitation. We study their photophysical properties by a combination of photothermal and fluorescence microscopy. Photothermal contrast provides a quantitative measure of the number of absorbers. The size of nanoparticles estimated from the absorption measurements was compared with sizes measured by AFM. Fluorescence and absorption microscopy provide quantum yield on the single-particle level as a function of excitation intensity. The quantum yield strongly decreases at high intensities because of singlet-singlet or singlet-triplet annihilation. We also report the formation of molecular thin layers and of labyrinth-shaped structures on glass substrates, presumably induced by dewetting.

The Electronically Excited States of LH2 Complexes from Rhodopseudomonas acidophila Strain 10050 Studied by Time-Resolved Spectroscopy and Dynamic Monte Carlo Simulations. I. Isolated, Non-Interacting LH2 Complexes

We have employed time-resolved spectroscopy on the picosecond time scale in combination with dynamic Monte Carlo simulations to investigate the photophysical properties of light-harvesting 2 (LH2) complexes from the purple photosynthetic bacterium Rhodopseudomonas acidophila. The variations of the fluorescence transients were studied as a function of the excitation fluence, the repetition rate of the excitation and the sample preparation conditions. Here we present the results obtained on detergent solubilized LH2 complexes, i.e., avoiding intercomplex interactions, and show that a simple four-state model is sufficient to grasp the experimental observations quantitatively without the need for any free parameters. This approach allows us to obtain a quantitative measure for the singlet-triplet annihilation rate in isolated, noninteracting LH2 complexes.

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.

Temporal response of optically pumped organic semiconductor lasers and its implication for reaching threshold under electrical excitation

We study the step response of optically pumped organic semiconductor lasers and find that lasing is sustained for $l150\text{ }\text{ns}$ following pump turn on due to annihilation quenching of guest singlet with host triplet excitons. We derive a general criterion for reaching threshold that critically depends on the pump rise time, the singlet-triplet quenching rate, and the spin statistics of exciton formation. These results imply that singlet-triplet annihilation alone fundamentally restricts lasing by electrical excitation to conditions that are not practically feasible in organic semiconductors.

Exciton annihilation in a polyfluorene: Low threshold for singlet-singlet annihilation and the absence of singlet-triplet annihilation

Ultrafast photoinduced absorption measurements have been used to directly investigate singlet-singlet annihilation in polyfluorene. The pump fluence threshold for annihilation to dominate the decay was measured to be $\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{J}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ corresponding to an excitation density of $1.5\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. The annihilation rate was found to be faster than that expected from a simple dipole-dipole interaction. This is ascribed to the additional influence of diffusion which, because of the dispersive nature of the exciton migration, has strong time dependence as the singlet excitons thermalize in the density of states as well as the expected intrinsic time dependence from a diffusion controlled process. Also, a comparable background level of triplets was created in the film to study the effect of singlet-triplet annihilation, which surprisingly, given the low threshold for singlet-singlet annihilation, was found to be negligible.

A dye-sensitized solar cell using pheophytin–carotenoid adduct: Enhancement of photocurrent by electron and singlet-energy transfer and by suppression of singlet–triplet annihilation due to the presence of the carotenoid moiety

Titania-based solar cells were fabricated by the use of a pheophytin derivative and a pheophytin–carotenoid adduct. The addition of the carotenoid moiety increased the photocurrent ( J sc = 3.7 → 6.0 mA cm −2), reduced the photovoltage ( V oc = 0.53 → 0.46 V), and enhanced the solar energy-to-electricity conversion efficiency ( η = 1.4 → 1.8%). The increase in photocurrent was explained in terms of electron and singlet-energy transfer from the carotenoid to the chlorin moiety as well as the prevention of the chlorin aggregate by the bulky carotenoid moiety and the resultant suppression of the singlet–triplet annihilation reaction.

The influence of annihilation processes on the threshold current density of organic laser diodes

We examine the impact of various annihilation processes on the laser threshold current density of a multilayer organic laser diode by numerical simulation. Our self-consistent numerical model treats the dynamics of electrons, holes, and singlet as well as triplet excitons in the framework of a drift-diffusion model. The resulting particle distributions enter into an optical model. In our approach, a three layer waveguide structure is taken into account and the resulting laser rate equations are solved. Various annihilation processes are included as reactions between the different particle species in the device employing typical annihilation rates and material properties of organic semiconductors. By systematically varying the device dimensions and the annihilation rate coefficients, the dominating quenching processes are identified. The threshold current density is found to depend sensitively on the thickness of the emission layer. The influence of annihilation processes on the threshold current density is quantified as a function of the emission layer thickness and various annihilation rate coefficients. Using typical annihilation rate coefficients singlet-polaron annihilation is found to be the dominating quenching process. Maximum annihilation rate coefficients are calculated allowing a threshold current density below 1kA∕cm2. Singlet-triplet annihilation is recognized as another main loss process for singlet excitons. In our model the singlet exciton density is increased by triplet-triplet annihilation whereas it is diminished by singlet-triplet annihilation. The ratio of the rate coefficients for singlet-triplet and triplet-triplet annihilations is identified to be critical for the total number of singlet excitons being quenched by triplet excitons.

Photocurrents of solar cells sensitized by aggregate-forming polyenes: Enhancement due to suppression of singlet–triplet annihilation by lowering of dye concentration or light intensity

Titania-based Grätzel-type solar cells were fabricated by the use of polyene dyes with various transition dipole moments. In the dye having the largest transition dipole among the samples, an aggregate was readily formed through dispersive interaction, and the photocurrent was increased when the dye concentration or the light intensity was lowered. This observation was ascribed to the suppression of the singlet–triplet annihilation reaction. In the dye having the smallest transition dipole, there was no sign of aggregate formation, and the photocurrent was decreased when the dye concentration or the light intensity was lowered.

Dye-sensitized solar cells using retinoic acid and carotenoic acids: Dependence of performance on the conjugation length and the dye concentration

Titanium oxide-based dye-sensitized solar cells (DSSC) were fabricated by the use of retinoic acid and carotenoic acids having the number of conjugated double bonds, n = 5–13. The incident photon-to-current conversion efficiency, the photocurrent density and the solar energy-to-electricity conversion efficiency exhibited the highest values at n = 7, and then decreased toward both sides. The effects of dilution of CA7 with deoxycholic acid were also examined. The above parameters per unit CA7 concentration progressively increased toward the lowest concentration, which is ascribed to the isolated excitation free from singlet–triplet annihilation in the dye molecules on the TiO 2 layer.

Multichromophoric Dendrimers as Single-Photon Sources: A Single-Molecule Study

Single-molecule fluorescence spectroscopy was performed on a number of multi-peryleneimide substituted polyphenylene dendrimers for the purpose of investigating the parameters which influence the efficiency of the dendrimers as single-photon sources at room temperature. Analysis of fluorescence intensity, lifetime, and interphoton arrival time distribution revealed that all measured first-generation dendrimers behave as single-photon emitters when more than one chromophore is excited by a single excitation pulse regardless of the number of constituent chromophores. This is a result of efficient singlet-singlet annihilation, which becomes less efficient in higher-generation dendrimers when the interchromophoric distance increases. The efficiency of the investigated dendrimers as single-photon sources depends on several other parameters, such as the nature of the surrounding polymer matrix, the number of chromophores, and the extent of interchromophoric interactions. These parameters mainly affect the frequency of singlet-triplet annihilation, which in turn dominates the quality of these multichromophoric dendrimers as single-photon sources. The results reported here are important not only for the design of single-photon sources based on single organic molecules but also for a fundamental understanding of natural and artificial multichromophoric systems.