Triplet-triplet Annihilation Rate Research Articles

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 ...

Influence of magnetic field on delayed fluorescence of coumarin dye in Langmuir–Blodgett films

The triplet–triplet annihilation (TTA) was studied in mixed Langmuir–Blodgett films of 7-decyloxy-3 (4′-ethoxyphenyl)-coumarin molecules and palmitic acid. The thermal activation of the TTA process is observed due to the inhomogeneous broadening of triplet energy levels. Modulation of the TTA rate constant in the external magnetic field for the multilayer films shows the dependence that is characteristic of crystals. Only negative magnetic effect is observed for the monolayers. Time dependence of the magnetic effect in conditions of high rate constant of triplet excitons migration is connected with the dominating contribution of ordered clusters or randomly oriented molecule clusters into the TTA process.

Organic light-emitting diodes under high currents explored by transient electroluminescence on the nanosecond scale

We investigate organic light-emitting diodes (OLEDs) comprising the singlet emitter system 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) doped into aluminium tris(8-hydro-xyquinoline) (Alq${}_{3}$) at high excitation densities. With the OLED active area reduced to $100\ifmmode\times\else\texttimes\fi{}100\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}$m${}^{2}$, current densities up to 800 A/cm${}^{2}$ are achieved in pulsed operation. These devices exhibit an intense electroluminescence (EL) turn-on peak on the nanosecond time scale. With the help of streak camera measurements, we prove that the steady state EL of the fluorescent OLEDs is reduced due to singlet-triplet quenching. We demonstrate that short electrical pulses with a rise time of 10 ns make the separation of singlet emission and singlet-triplet quenching in time domain possible. By modeling the singlet and triplet population dynamics in the emission layer, we find that the triplet-triplet annihilation-rate coefficient in doped fluorescent materials is triplet-density dependent at high excitation density. The increased triplet lifetime usually observed in host:guest systems due to triplet trapping on guest molecules vanishes at high current densities. An increase in current density leads to an increased triplet-triplet annihilation rate, while the triplet density in the emission layer stays constant.

Supermolecular-Chromophore-Sensitized Near-Infrared-to-Visible Photon Upconversion

Selective near-IR (NIR) excitation (780 nm) of the conjugated supermolecule ruthenium(II) [15-(4'-ethynyl-(2,2';6',2''-terpyridinyl))-bis[(5,5',-10,20-di(2',6'-bis(3,3-dimethylbutoxy)phenyl)porphinato)zinc(II)]ethyne][4'-pyrrolidin-1-yl-2,2';6',2''-terpyridine] bis(hexafluorophosphate) (Pyr(1)RuPZn(2)) in solutions containing N,N-bis(ethylpropyl)perylene-3,4,9,10-tetracarboxylicdiimide (PDI) or tetracene gives rise to a substantial anti-Stokes energy gain (PDI, 0.70 eV; tetracene, 0.86 eV). Experimental data clearly demonstrate that this upconverted fluorescence signal is produced via Pyr(1)RuPZn(2)-sensitized triplet-triplet annihilation (TTA) photochemistry. The TTA process was confirmed by the quadratic dependence of the integrated (1)PDI* emission centered at 541 nm derived from 780 nm laser excitation. The T(1)→T(n) excited state absorption decay of Pyr(1)RuPZn(2), monitored at 900 nm as a function of PDI concentration, revealed Stern-Volmer and bimolecular quenching constants of 10 048 M(-1) and 5.9 × 10(8) M(-1) s(-1), respectively, for the PDI triplet sensitization process. The T(1)→T(n) PDI extinction coefficient at 560 nm (ε(T) = 6.6 × 10(4) M(-1) cm(-1)) was determined through the triplet energy transfer method utilizing anthracene as the donor chromophore. (3)PDI* transient triplet absorption dynamics observed as a function of 485 nm incident nanosecond pump laser fluence demonstrate a bimolecular (3)PDI*-(3)PDI* TTA rate constant (k(TT) = 1.0 ± 0.2 × 10(9) M(-1) s(-1)). The maximum quantum yield of the supermolecule-sensitized PDI upconverted emission (Φ(UC) = 0.0075 ± 0.0002) was determined relative to [Os(phen)(3)][PF(6)](2) at an incident laser power of 22 mW at 780 nm. This study successfully demonstrates NIR-to-visible photon upconversion and achieves a new record anti-Stokes shift of 0.86 eV for sensitized TTA, using the supermolecular Pyr(1)RuPZn(2)sensitizer. The stability of the Pyr(1)RuPZn(2)/PDI chromophore combination is readily apparent as continuous irradiation at 780 nm produces 541 nm centered fluorescence with no significant decrease in intensity measured over time domains exceeding several hours. The molecular components of these NIR-to-vis upconverting compositions illustrate that substantial anti-Stokes energy gains via a TTA process can be effortlessly realized.

Study on the Transient Electroluminescence of Phosphorescent OLEDs

We have studied the turn-off dynamics of phosphorescent organic light-emitting diodes (OLEDs) by investigating the temperature, pulse width, and voltage dependences of the radiative triplet lifetime with the dispersive parameter γ, calculated from the second luminescence of the transient electro-luminescence. This behavior is caused by the redistribution and recombination of electrons and holes accumulated near the interfaces. The lifetime increases with increasing carrier density and with decreasing temperature. This is due to the decrease in the hopping and the triplet-triplet annihilation rates. The experimental data satisfy the relationship between the dispersive parameter γ and temperature T, γ − 1 ∝ exp(−A/T ), with a constant A.

Low Power Visible-to-UV Upconversion

Low power visible-to-UV photon upconversion is demonstrated for the first time, achieved from two simple organic chromophores dissolved in benzene. Selective 442 nm excitation of the triplet sensitizer 2,3-butanedione (biacetyl) in the presence of the laser dye 2,5-diphenyloxazole (PPO) results in the observation of singlet fluorescence from the latter in the UV centered at 360 nm, anti-Stokes shifted by a record 0.64 eV with respect to the excitation. All of the experimental data are consistent with the upconverted singlet PPO fluorescence being produced as a result of biacetyl-sensitized triplet-triplet annihilation (TTA) of triplet excited PPO chromophores. Nanosecond laser flash photolysis performed under pseudo-first-order conditions revealed the bimolecular rate constant of triplet-triplet energy transfer between the biacetyl sensitizer and PPO acceptor, k(q) = 9.0 x 10(8) M(-1)s(-1). The TTA process was confirmed by the quadratic dependence of the upconverted integrated PPO emission intensity measured with respect to incident 442 nm light power density. The maximum quantum yield of the upconverted emission (0.0058 +/- 0.0002) was determined relative to 1,8-diphenyl-1,3,5,7-octatetraene, both measured with 0.389 W/cm(2) incident power density. The PPO triplet-triplet annihilation rate constant (k(TT)) was determined from transient absorption decays monitored at the peak of its characteristic triplet-to-triplet excited-state absorption (500 nm) as a function of incident pulsed laser fluence; this process attains the diffusion limit in benzene at room temperature, k(TT) = 1.1 +/- 0.1 x 10(10) M(-1) s(-1).

Kinetics of the decelerated fluorescence of organic molecules in n-paraffins at 77 K and its mathematical model

Results of investigations into the kinetics of decelerated fluorescence decay for 1,2-benzpyrene in dodecane and coronene in n-octane at 77 K are presented in the paper. It is demonstrated that the reason for the nonexponential kinetics is a statistical spread of the triplet-triplet annihilation rate constant. A mathematical model is suggested which describes well the decelerated fluorescence decay in these systems.

Stochastic Model of Photodynamics in Multichromophoric Conjugated Polymers

A stochastic model of triplet exciton dynamics in multichromophoric conjugated polymers is presented and analyzed in detail, with a focus on the single molecule spectroscopy observables. The model deals with the evolution of a discrete statistical distribution of triplets in isolated polymer molecules. This approach should provide more accurate quantitative information on the dynamic processes involved, as compared to the previously used two-state model which assumes that a conjugated polymer cannot contain more than one triplet. In particular, it allows for determination of the triplet-triplet annihilation rate.

Absolute measurements of the triplet-triplet annihilation rate and the charge-carrier recombination layer thickness in working polymer light-emitting diodes based on polyspirobifluorene

The triplet exciton densities in electroluminescent devices prepared from two polyspirobifluorene derivatives have been investigated by means of time-resolved transient triplet absorption as a function of optical and electrical excitation power at 20 K. Because of the low mobility of the triplet excitons at this temperature, the triplet generation profile within the active polymer layer is preserved throughout the triplet lifetime and as a consequence the absolute triplet-triplet annihilation efficiency is not homogeneously distributed but depends on position within the active layer. This then gives a method to measure the charge-carrier recombination layer after electrical excitation relative to the light penetration depth, which is identical to the triplet generation layer after optical excitation. With the latter being obtained from ellipsometry, an absolute value of 5 nm is found for the exciton formation layer in polyspirobifluorene devices. This layer increases to 11 nm if the balance between the electron and the hole mobility is improved by chemically modifying the polymer backbone. Also, and consistent with previous work, triplet diffusion is dispersive at low temperature. As a consequence of this, the triplet-triplet annihilation rate is not a constant in the classical sense but depends on the triplet excitation dose. At 20 K and for typical excitation doses, absolute values of the latter rate are of the order of ${10}^{\ensuremath{-}14}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{3}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$.

Liquid and frozen multilayers of decanol in zeolites as microreactors for direct and oxygen mediated triplet‐triplet annihilation of porphyrin

The kinetics of triplet state decay and concomitant delayed fluorescence of tetraphenylporphyrin (TPP) incorporated into the multilayers of n‐decanol adsorbed on the external surface of zeolites at different temperatures were studied by the laser flash photolysis via monitoring the emission and diffuse‐reflectance. In deoxygenated samples, the kinetics of diffusion‐controlled bimolecular triplet‐triplet annihilation (TTA) has been used to probe the multilayers. A very large local concentration of reactants may be obtained using small bulk amounts. A fast TTA rate constant demonstrated the triplet energy migration in TPP assemblies formed at the liquid/solid interface. The TTA in aerated samples is mediated by molecular oxygen leading to 1O2 feedback—induced delayed fluorescence. Singlet oxygen works as an efficient mobile energy carrier between TPP triplets. The parameters of both TTA may be modified by multilayer, which acts as microreactor with supramolecular organization. The freezing of n‐decanol results in dramatic apparent acceleration of both direct and 1O2 mediated TTA due to the increase in local TPP concentration because of displacement of TPP molecules into the residual liquid domains in polycrystalline matrix. The concentration of TPP in those microreactors can be three orders of magnitude larger than the solubility limit in a particular neat liquid solvent demonstrating the unique features of solid solution, which seems to be due to the presence of crystal/liquid interfaces.

Direct and oxygen-mediated triplet–triplet annihilation of tetraphenylporphyrin in multilayers of decanol on the external surface of NaA zeolite

The kinetics of triplet state decay and concomitant delayed fluorescence (DF) of meso-tetraphenylporphyrin (TPP) adsorbed on the external surface of NaA zeolite covered by coadsorbed n-decanol (DEC) were studied by the laser flash photolysis. In deoxygenated solid samples of NaA with coadsorbed DEC, the diffusion-controlled bimolecular triplet–triplet annihilation (TTA) dominates in contrast to what is observed on the dry NaA surface. The TTA which results in DF can be described in terms of two reaction channels: (a) an energy transfer pathway with singlet excited state formation; (b) enhanced intersystem crossing in the triplet encounter complex. Whilst the TTA in multilayers occurs in DEC microphase, the reaction parameters change significantly when adsorbed ground-state molecules are involved. A fast TTA rate constant found at large TPP concentration in solid samples with DEC was attributed to triplet energy migration in TPP assemblies. Singlet oxygen works as an efficient mobile energy carrier between TPP triplets in both solid and liquid aerated samples. The parameters of this reaction may also be modified by changing the ratio [TPP]/[DEC] in multilayers, which in general act as microreactor systems with supramolecular organization of reactants.

Triplet-triplet interaction in a nearly one dimensional molecular crystal: Application to 1,4-dibromonaphtalene

The effect of a magnetic field (6 kG) on the delayed fluorescence in a 1,4-Dibromonaphtalene at 300 K and 20 K is analysed using a new approach of calculation of the triplet-triplet annihilation rate constant. The agreement of the best fit between experiment and theory allows reaching at 300 K and 20 K respectively the lifetimes and the interaction constant of the triplets pairs.

Evaluating Rates and Yields of Second-Order, Photoinitiated Reactions under Conditions of Gaussian-Profile Excitation

Under conditions of Gaussian radial profile excitation, a mixed-order kinetic model is used to interpret the rates and yields of photoinitiated reactions. This model is used to determine the triplet-triplet annihilation rate constant for benzophenone in acetonitrile and anthracene at room temperature. 34 refs., 9 figs.

Positive Magnetic Effect of Photo-Enhanced Current in anthracene. A Novel Determination of Triplet Triplet annihilation Rate Constant

Abstract It has been observed that under certain conditions (light intensity, field strength, etc.) the photoenhanced current (PEC) increases on applicationof external magnetic fields. This anomalous positive magnetic effect is accounted for in terms of a competition betweeen triplet-triplet and triplet-doubled reactions. Numerical simulation shows that the observed anomaly is possible for a rather limited range of parameter values. Hence, the analysis of the effect offers a good chance to check the magnitude of the triplet-triplet annihilation rate constant γTT. The currently accepted value of γTT = 2 × 10−11 cm3 s−1 is confirmed.

Temperature Dependence of the Delayed Fluorescence Lifetime in Anthracene Crystals with Triplet Exciton Traps

The lifetime of delayed fluorescence of anthracene crystals excited by the chopped-light of a He-Ne laser is investigated as a function of temperature over the range 110∼480 K. As compared with the fluorescence intensity, the lifetime is not influenced much by the temperature dependence of triplet-triplet annihilation rate and singlet-triplet absorption coefficient. Considering a trap saturation effect, an analysis has been made of the temperature dependence of the lifetime. In defect-induced crystals, three kinds of defects are observed by X-ray irradiation. Two are 0.33 and 0.16 eV in triplet trap depth, and the trapped triplet decay rate is ∼5×106 s-1 for both; the other is 0.29 eV and ∼1 s-1. The latter is of a different type from the former, namely, it is almost a pure trapping center which hardly quenches triplet excitons.

Interband transitions in molecular crystals

Absorptions from the triplet exciton bands in benzophenone, benzil and benzoin have been observed in crystals excited by 337 nm light from a N2 laser. In benzophenone and benzil, the crystal absorptions are much wider than those of the isolated molecules, showing that considerable mixing between the upper triplet exciton states and singlet and CT states is taking place. The phosphorescence spectrum of benzophenone crystals at high density excitation shows a (0, 0) band which is much weaker relative to the (0, 1) band than at low density excitation : this has been attributed to a high concentration of phonons in the crystal. The high excitation density also enabled the triplet-triplet exciton annihilation rates for benzophenone to be measured : γTT (295 K) = 1 × 10-18 m3 s-1 and γTT (77 K) = 6 × 10-9 m3s-1.

Magnetic field dependence of singlet exciton fission and fluorescence in crystalline tetracene at 300 K

The magnetic field dependence of singlet exciton fission and fluorescence was studied at 300 K in tetracene crystals in several crystal planes for different field intensities and orientations. The positions of the resonances at high field correspond to those predicted by the zero-field splitting tensor as obtained by EPR, indicating that the outcome of the singlet fission is a pair of free migrating triplet excitons. It is shown that the field dependence and the resonance lineshapes can only be fitted, to within the experimental error, with the kinematic theory of Suna assuming a nearly two-dimensional motion of triplet excitons, restricted to the (a'b) plane. With the assumption of an isotropic triplet exciton diffusion in the (a'b) plane reasonable sets of values can be deduced from the fit for the in-plane hopping rate, for the anisotropy of the diffusion, and for the nearest neighbour triplet-triplet annihilation rate. In many respects, triplet exciton dynamics in tetracene appears to be similar to that in anthracene, as expected from the calculation of the transfer matrix elements and the similarity of the crystal structures. The dependence of the effects on the exciting light intensity are also well accounted for using the usual kinetic equations for the different processes involved during the lifetime of the triplet excitons.

Behavior of Triplet Excitons in Platinum Phthalocyanine Single Crystals

Dependence of the phosphorescence emission of Platinum phthalocyanine single crystals around 971 nm on the magnetic field and on the incident laser intensity are investigated. The energy splitting of the spin sublevels of the lowest excited triplet state is about 8.5 cm -1 in zero field and about 12.4 cm -1 in the external magnetic field of 50 kG. The T–T (triplet-triplet) annihilation rate is estimated to be about 1×10 -2 cm 3 /sec and the magnetic field dependence of the T–T annihilation reveals that the | 0> spin component of the triplet exciton plays the most important role. The temperature dependence of the phosphorescence at the low temperature range is also studied.

Mutual annihilation of triplet excitons in a nearly one-dimensional system: 1,4-dibromonaphthalene

The analysis of the variation with incident flux of the time dependence of the delayed fluorescence in conjunction with the determination of the absolute ground state-first excited triplet absorption coefficients at room temperature, yields the value of γ tot = (5.5 ± 2.0) × 10 −12 cm 3 s −1, for the total triplet-triplet annihilation rate constant in 1,4-dibromonaphthalene crystals. The one-dimensional mutual annihilation rate constant for the triplet exciton motion restricted to linear chains along the crystal c axis is γ 1 tot = (1.0 ± 0.4) × 10 3 cm s −1. The results are discussed in terms of recent theories of mutual annihilation of triplets in one-dimensional systems.

Carrier-Exciton Interactions in Crystalline Anthracene

Quenching of delayed fluorescence by injected holes and electrons is observed when either monomolecular or bimolecular exciton processes afford the dominant exciton decay mechanism. With small triplet exciton densities, injected carriers reduce the triplet exciton lifetime, and hence the delayed fluorescence intensity. With high triplet exciton densities this effect is unimportant; however, the over-all triplet-triplet annihilation rate constant is found to increase while the singlet component of the annihilation rate constant decreases in the presence of charge carriers. These effects are discussed in terms of triplet pair formation and annihilation.