Energy transfer and triplet exciton confinement in polymeric electrophosphorescent devices

Energy transfer and triplet exciton confinement in polymer/phosphorescent dopant systems have been investigated. Various combinations of host‐guest systems have been studied, consisting of two host polymers, poly(vinylcarbazole) (PVK) and poly[9,9‐bis(octyl)‐fluorene‐2,7‐diyl] (PF), blended with five different phosphorescent iridium complexes with different triplet energy levels. These combinations of hosts and dopants provide an ideal situation for studying the movement of triplet excitons between the host polymers and dopants. The excitons either can be confined at the dopant sites or can flow to the host polymers, subject to the relative position of the triplet energy levels of the material. For PF, because of its low triplet energy level, the exciton can flow back from the dopants to PF when the dopant has a higher triplet energy and subsequently quench the device efficiency. In contrast, efficient electrophosphorescence has been observed in doped PVK films because of the high triplet energy level of PVK. Better energy transfer from PVK to the dopants, as well as triplet exciton confinement on the dopants, leads to higher device performance than found in PF devices. Efficiencies as high as 16, 8.0, and 2.6 cd/A for green, yellow, and red emissions, respectively, can be achieved when PVK is selected as the host polymer. The results in this study show that the energy transfer and triplet exciton confinement have a pronounced influence on the device performance. In addition, this study also provides material design and selection rules for the efficient phosphorescent polymer light‐emitting diodes. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2681–2690, 2003     

One-dimensional exciton diffusion in perylene bisimide aggregates

The dynamics and mobility of excitons in J-aggregates of perylene bisimides are investigated by transient absorption spectroscopy with a time resolution of 50 fs. The transient spectra are compatible with an exciton delocalization length of two monomers and indicate that vibrational and configurational relaxation processes are not relevant for the spectroscopic properties of the aggregates. Increasing the pump pulse energy and in that way the initial exciton density results in an accelerated signal decay and pronounced exciton-exciton annihilation dynamics. Modeling the data by assuming a diffusive exciton motion reveals that the excitons cannot migrate freely in all three directions of space but their mobility is restricted to one dimension. The observed anisotropy supports this picture and points against direct Fo?rster-transfer-mediated annihilation between the excitons. A diffusion constant of 1.29 nm(2)/ps is deduced from the fitting procedure that corresponds to a maximal exciton diffusion length of 96 nm for the measured exciton lifetime of 3.6 ns. The findings indicate that J-aggregates of perylene bisimides are promising building blocks to facilitate directed energy transport in optoelectronic organic devices or artificial light-harvesting systems.     

Radiative and radiationless triplet exciton decay in benzophenone

The EPR of triplet excitons and of shallow triplet traps in crystalline benzophenone is observed by an optical detection technique. The predominantly radiationless mode of de-excitation of the traps is demonstrated and shown to account for most of the quenching of electronic excitation energy in this system.     

Electron transfer between colloidal ZnO nanocrystals

Colloidal ZnO nanocrystals capped with dodecylamine and dissolved in toluene can be charged photochemically to give stable solutions in which electrons are present in the conduction bands of the nanocrystals. These conduction-band electrons are readily monitored by EPR spectroscopy, with g* values that correlate with the nanocrystal sizes. Mixing a solution of charged small nanocrystals (e(-)(CB):ZnO-S) with a solution of uncharged large nanocrystals (ZnO-L) caused changes in the EPR spectrum indicative of quantitative electron transfer from small to large nanocrystals. EPR spectra of the reverse reaction, e(-)(CB):ZnO-L + ZnO-S, showed that electrons do not transfer from large to small nanocrystals. Stopped-flow kinetics studies monitoring the change in the UV band-edge absorption showed that reactions of 50 μM nanocrystals were complete within the 5 ms mixing time of the instrument. Similar results were obtained for the reaction of charged nanocrystals with methyl viologen (MV(2+)). These and related results indicate that the electron-transfer reactions of these colloidal nanocrystals are quantitative and very rapid, despite the presence of ~1.5 nm long dodecylamine capping ligands. These soluble ZnO nanocrystals are thus well-defined redox reagents suitable for studies of electron transfer involving semiconductor nanostructures.     

Reversible triplet energy transfer between neo-alloocimene and anthracene

The rate constants for triplet energy transfer between neo-alloocimene and anthracene have been redetermined by a combination of pulsed laser photolysi     

On triplet exciton trapping in 1,4-dibromonaphthalene

From the study of triplet exciton trapping in 1,4-dibromonaphthalene it appears that several optically accessible excitons can exist with excitation energies within 1 cm^?1 and lifetimes differing by two orders of magnitude. Furthermore trap-like states exist ≈ cm^?1 below the exciton band. Delayed fluorescence arises from annihilation between an exciton and the very shallow trap-like state.     

Triplet-singlet radiationless energy transfer between benzophenone and perylene in vitreous solution

We have measured at 77 K the “critical radius” R₀ of the resonant intermolecular energy transfer from the triplet state of benzophenone to the singlet state of perylene, these two comporents being in vitreous solution.     

Sensitive triplet exciton detection in polyfluorene using Pd-coordinated porphyrin

We developed a sensitive spectroscopic method to probe triplet concentration in thin films of polyfluorene (PF) at room temperature. The energy of photoexcited triplet excitons is transferred to the guest metal-organic complex, meso-tetratolylporphyrin-Pd (PdTPP), and detected as phosphorescent emission. The phosphorescence intensity of PdTPP-PF blends is proportional to the independently measured triplet concentration using photoinduced absorption experiments. The high sensitivity of this method allows room temperature detection of triplet excitons in spin-coated polymer films as thin as 10 nm. We found that the triplet lifetime is independent of PdTPP concentration and therefore this method is nearly non-perturbing for the triplet population.     

Evidence against reversible triplet energy transfer between acetophenone and norbornene

Kinetic absorption and emission spectroscopy has been used to monitor the decay of triplet acetophenone produced in benzene by both pulse radiolysis and pulse laser photolysis techniques. The dependence of this decay on the concentrations of norbornene and acetophenone shows that, in contrast to a previous claim, triplet energy transfer between acetophenone and norbornene is not reversible. These data, together with quantum yield measurements, indicate that in this system ground state acetophenone quenches triplet acetophenone and either triplet norbornene or a ketone triplet—olefin exciplex by processes which do not involve energy transfer.     

Experimental study of triplet exciton diffusion in diphenyl and benzophenone crystals

Biphenyl crystals to which have been added to 10^?2 mole/mole of benzophenone and a various concentration of naphthalene, excited by radiation absorbed only by benzophenone molecules give naphthalene phosphorescence. This emission is interpreted as a consequence of energy migration from benzophenone molecules to the naphthalene molecules through the triplet exciton band of biphenyl. From the study, at 120 K, of the intensity of the naphthalene phosphorescence dependence on concentrations, a coefficient of diffusion of triplet excitons of biphenyl is measured (D ≈ 10^?6 cm² s^?1). Analogous kinetics applied to binary mixed crystals, naphthalene-benzophenone, give a coefficient of diffusion of triplet excitions for benzophenone (D ≈ 10^?7 cm² s^?1).     

Method for accurate experimental determination of singlet and triplet exciton diffusion between thermally activated delayed fluorescence molecules

Understanding triplet exciton diffusion between organic thermally activated delayed fluorescence (TADF) molecules is a challenge due to the unique cycling between singlet and triplet states in these molecules. Although prompt emission quenching allows the singlet exciton diffusion properties to be determined, analogous analysis of the delayed emission quenching does not yield accurate estimations of the triplet diffusion length (because the diffusion of singlet excitons regenerated after reverse-intersystem crossing needs to be accounted for). Herein, we demonstrate how singlet and triplet diffusion lengths can be accurately determined from accessible experimental data, namely the integral prompt and delayed fluorescence. In the benchmark materials 4CzIPN and 4TCzBN, we show that the singlet diffusion lengths are (9.1 ± 0.2) and (12.8 ± 0.3) nm, whereas the triplet diffusion lengths are negligible, and certainly less than 1.0 and 1.2 nm, respectively. Theory confirms that the lack of overlap between the shielded lowest unoccupied molecular orbitals (LUMOs) hinders triplet motion between TADF chromophores in such molecular architectures. Although this cause for the suppression of triplet motion does not occur in molecular architectures that rely on electron resonance effects (e.g. DiKTa), we find that triplet diffusion is still negligible when such molecules are dispersed in a matrix material at a concentration sufficiently low to suppress aggregation. The novel and accurate method of understanding triplet diffusion in TADF molecules will allow accurate physical modeling of OLED emitter layers (especially those based on TADF donors and fluorescent acceptors).

A method for measuring triplet diffusion between TADF molecules is presented, and implications of limited triplet diffusion for OLEDs discussed.     

Triplet—triplet annihilation and exciton—exciton interactions

A theoretical discussion is presented on the nature of direct and indirect interactions between two triplet excitons. It is shown that the direct exciton interaction through the coulombic interaction of electrons is important when there is a large change in the polarizability of the molecule during its excitation. The indirect exciton—exciton interaction is mainly due to the exciton—phonon interaction arising from the variation in the D-shift term. It is independent of temperature and spin substate. It is attractive in nature for excitons of narrow band-width, and if the exciton—acoustic phonon coupling is inducing the indirect interaction, it falls as R⁻³. It is suggested that a kinematic theory of the triplet—triplet annihilation should incorporate the effects of such interactions.     

The dynamics of triplet exciton trapping in 1,2,4,5- tetrachlorobenzene

The phosphorescence intensifies and decay times of the triplet exciton and X₂-trap emission in neat and doped 1,2,4,5-tetrachlorobenzene crystals were measured as a function of temperature. The comparison of the results found on neat and doped crystals allows the statement, that exciton trapping is not limited by the motion of the excitons towards the traps but by the trapping step on the trap molecule solely. For the individual trapping step a rate of about 3 × 10⁶ S^?1 was determined.     

The exciton—phonon interaction for triplet exciton bands of organic solids: An experimental and theoretical study

Very high resolution optical data on the temperature dependences of the Davydov component absorption profiles and polarization dependent one-phonon structures associated with the lowest triplet exciton band of anthracene are presented along with a theoretical framework for their interpretation. The interpretation of the one-particle exciton—phonon transitions involving both one-phonon creation and annihilation (cold and hot phonon transitions) is entirely consistent with the analysis of the T-dependent dephasing of the lowest zero-phonon Davydov transition in terms of two mechanisms: delocalized exciton—delocalized phonon scattering operative for the low frequency (< 30 cm^?) phonons which undergo no lattice distortion and: Raman like phonon scattering operative for the higher frequency phonons which do. It is the latter which leads to the identical T² linewidth dependences of the two Davydov components in the high T limit. The former scattering is dominant at the lowest temperatures. In addition, the, marked and polarization dependent mirror symmetry breakdown between the hot and cold one-particle transitions can be nicely understood in terms of interferences occurring between the Condon and phononic (from the dependence of the pure exciton transition dipoles on phonon coordinate displacement) contributions to the one-particle transition dipoles. It is argued that our findings for anthracene should prove useful for triplet exciton bands in other organic solids.     

Photoinduced electron transfer between triplet erythrosin dianion and highly charged ionic quenchers

Abstract  

The lifetime of the lowest triplet state of the dianion erythrosin B depends on its concentration because of self-quenching. The self-quenching rate constants vary with the solution viscosity at room temperature. Dextrose was used to change the viscosity of aqueous solutions in the range 1 ≤ η ≤ 5.31 cP. Photoinduced electron transfer reactions between the triplet state of the erythrosin dianion and the highly charged ionic quenchers K₄[Fe(CN)₆] and K₄[Mo(CN)₈] were investigated in aqueous borate buffer solutions at pH 9.2 using flash photolysis. Electron transfer rates vary from 3.0 × 10⁸ to 1.4 × 10⁸ M⁻¹ s⁻¹ depending on viscosity.     

Negative polaron and triplet exciton diffusion in organometallic "molecular wires"

The dynamics of negative polaron and triplet exciton transport within a series of monodisperse platinum (Pt) acetylide oligomers is reported. The oligomers consist of Pt-acetylide repeats, [PtL(2)-C≡C-Ph-C≡C-](n) (where L = PBu(3) and Ph = 1,4-phenylene, n = 2, 3, 6, and 10), capped with naphthalene diimide (NDI) end groups. The Pt-acetylide segments are electro- and photoactive, and they serve as conduits for transport of electrons (negative polaron) and triplet excitons. The NDI end groups are relatively strong acceptors, serving as traps for the carriers. Negative polaron transport is studied by using pulse radiolysis/transient absorption at the Brookhaven National Laboratory Laser-Electron Accelerator Facility (LEAF). Electrons are rapidly attached to the oligomers, with some fraction initially residing upon the Pt-acetylide chains. The dynamics of transport are resolved by monitoring the spectral changes associated with transfer of electrons from the chain to the NDI end group. Triplet exciton transport is studied by femtosecond-picosecond transient absorption spectroscopy. Near-UV excitation leads to rapid production of triplet excitons localized on the Pt-acetylide chains. The excitons transport to the chain ends, where they are annihilated by charge separation with the NDI end group. The dynamics of triplet transport are resolved by transient absorption spectroscopy, taking advantage of the changes in spectra associated with decay of the triplet exciton and rise of the charge-separated state. The results indicate that negative polarons and excitons are transported rapidly, on average moving distances of ~3 nm in less than 200 ps. Analysis of the dynamics suggests diffusive transport by a site-to-site hopping mechanism with hopping times of ~27 ps for triplets and <10 ps for electrons.     

Models for intramolecular triplet exciton migration applied to vinyl aromatic polymers

Models for intramolecular triplet exciton migration have been developed for vinyl aromatic polymers. A one-dimensional model which allows only neighbor-to-neighbor migrations yields frequencies which are several orders of magnitude smaller than those predicted either by exchange or dipole-dipole mechanisms. An intramolecular model permitting three translational degrees of freedom predicts triplet exciton hop frequencies on the order of 10⁴ s⁻ in reasonable agreement with either exchange or dipole-dipole mechanisms.     

The neutral exciton model for triplet factor-group splittings in aromatic crystals

The calculation of triplet factor-group splitting in benzene, naphthalene and anthracene crystals has been reexamined. Substantially revised values of the two-centre exchange integrals in the neutral exciton model give much improved agreement with experiment in benzene; the need for refinements such as ion-pair contributions is marginal. In naphthalene and anthracene the new values are little altered. Thus, in benzene and naphthalene the neutral exciton model appears to be adequate; in anthracene the previously noted disagreement with experiment remains.Multicentre intermolecular integrals can be neglected in the examples studied, and are unlikely to be important in other aromatic crystals.