Triplet Concentration Research Articles

Evaluation of Probes to Measure Oxidizing Organic Triplet Excited States in Aerosol Liquid Water.

Oxidizing triplet excited states of organic matter (3C*) drive numerous reactions in fog/cloud drops and aerosol liquid water (ALW). Quantifying oxidizing triplet concentrations in ALW is difficult because 3C* probe loss can be inhibited by the high levels of dissolved organic matter (DOM) and copper in particle water, leading to an underestimate of triplet concentrations. In addition, illuminated ALW contains high concentrations of singlet molecular oxygen (1O2*), which can interfere with 3C* probes. Our overarching goal is to find a triplet probe that has low inhibition by DOM and Cu(II) and low sensitivity to 1O2*. To this end, we tested 12 potential probes from a variety of compound classes. Some probes are strongly inhibited by DOM, while others react rapidly with 1O2*. One of the probe candidates, (phenylthiol)acetic acid (PTA), seems well suited for ALW conditions, with mild inhibition and fast rate constants with triplets, but it also has weaknesses, including a pH-dependent reactivity. We evaluated the performance of both PTA and syringol (SYR) as triplet probes in aqueous extracts of particulate matter. While PTA is less sensitive to inhibition than SYR, it results in lower triplet concentrations, possibly because it is less reactive with weakly oxidizing triplets.

17.4: The Role of Dopant Materials on Current Efficiency Roll‐off in Organic Light‐Emitting Diodes

With the increase of user demand, higher brightness become a trend for the information display products, which makes efficiency roll‐off issue even more concerning in organic light‐emitting diodes (OLED). Here, we fabricated a set of red phosphorescent OLED structure for investigating the origin of roll‐off. By replacing different kinds of red dopant materials (RD‐1 and RD‐2) and red host materials, we found the main reason affecting efficiency roll‐off is dopant materials in emitting layer. Three key factors, including triplet‐triplet annihilation (TTA), triplet‐polaron quenching (TPQ), as well as the width of recombination zone were investigated. Firstly, the transient photoluminescence (PL) and electroluminescence (EL) results illustrated a longer excited state lifetime for RD‐2. Consequently, we demonstrated a relatively serious TPQ phenomenon in the unipolar device with RD‐2, which revealed a higher interaction probability between photon‐induced exciton and polaron. Finally, the OLED with RD‐2 showed apparently higher SCLC (space charge limited current) hole mobility in the hole‐only device. According to the recombination zone profiles and capacitance results, we proposed that a higher hole‐type space charge concentration in the device based on RD‐2 contributed to a higher polaron and triplet concentration in EML. Combined with a longer exciton diffusion length, a higher rate of TPQ was induced and resulted in relatively serious roll‐off phenomenon.

Sample volume effects in optical overhauser dynamic nuclear polarization

The optical dynamic nuclear polarization (DNP) method has been proposed as an alternative to microwave pumping as a hyperpolarization method for solution-state NMR studies. Using continuous laser illumination to photogenerate triplet states in the presence of a persistent radical produces chemically-induced dynamic electron polarization (CIDEP) via the radical-triplet pair mechanism (RTPM), with cross-relaxation transferring this to nuclear hyperpolarization via an Overhauser mechanism. Numerical simulations have previously indicated that reducing the sample volume while maintaining a constant optical density can significantly increase the NMR signal enhancement, due to the larger steady-state concentration of triplets obtained. Here we provide the first experimental confirmation of these effects, producing a nearly five-fold increase in the optical DNP enhancement factor just by reducing the sample volume with optimal dye and radical concentrations adjusted for each optical path length. The results are supported with an in depth analysis of volume effects in the numerical model, with which they are in good qualitative agreement. These important observations will impact on the future development of the technique, with particular significance for attempts to apply DNP methods to increase sensitivity for volume-limited biological samples.

A posteriori error analysis of mixed finite element methods for stress-assisted diffusion problems

We develop the a posteriori error analysis for mixed-primal and fully-mixed finite element methods approximating the stress-assisted diffusion of solutes in elastic materials. The systems are formulated in terms of stress, rotation and displacements for the elasticity equations, whereas the nonlinear diffusion is cast using either solute concentration (leading to a four-field mixed-primal formulation), or the triplet concentration – concentration gradient – and nonlinear diffusive flux (yielding the six-field fully-mixed variational formulation). We have addressed the well-posedness of these formulations in two recent works, also introducing discretisations based on PEERS or Arnold–Falk–Winther elements for the linear elasticity and either Lagrange, or Lagrange – Raviart-Thomas – Lagrange triplets for the approximation of the diffusion equation. Here we advocate the derivation of two efficient and reliable residual-based a posteriori error estimators focusing on the two-dimensional case. The proofs of reliability depend on adequately formulated inf–sup conditions in combination with a Helmholtz decomposition, and they also rely on the local approximation features of Clément and Raviart–Thomas interpolations. The efficiency of the estimators results from classical inverse and discrete trace inequalities together with localisation techniques based on edge- and triangle-bubble functions. The theoretical properties of these error indicators are confirmed through numerical tests, also serving to illustrate the performance of the adaptive mesh refinement.

Quantifying triplet formation in conjugated polymer/non-fullerene acceptor blends

Quantification of triplet state populations in non-fullerene acceptor blends has allowed us to correlate triplet and charge carrier concentrations, with important ramifications for organic photovoltaics.

Spectroscopic and Dynamic Properties of Electronically Excited Pendant Porphyrin Polymers with Backbones of Differing Flexibility

A zinc porphyrin-pendant norbornene polymer with a rigid backbone characterized by a 2:1 E/Z isomeric structure ratio has been synthesized, and its spectroscopic and photophysical properties are examined. Zinc tetraphenylporphyrin, the porphyrin-substituted norbornene monomer, and a previously reported zinc porphyrin-pendant polymer with a flexible polymethylene backbone have been used as comparators. Unlike its flexible counterpart, the rigid norbornene polymer exhibits clear exciton splitting of its Soret band, much more rapid relaxation rates of its excited singlet states, and a very small yield of an unusually short-lived triplet state. Unlike the flexible pendant polymer, which exhibits excimeric S2 fluorescence as a result of chromophore rotation, anti-Kasha emission from the norbornene polymer originates primarily from the unperturbed porphyrin E region. The low triplet yield in the polymer is attributed to greatly increased rates of competing internal conversion within the singlet manifold. Nevertheless, upconverted delayed fluorescence that is quenched by oxygen is observed upon intense steady-state Q-band excitation of degassed polymer solutions, signaling direct triplet involvement. Consistent with the polymer's rigid structure, this biexcitonic process is assigned to ultrafast singlet exciton migration and triplet-triplet annihilation following absorption of a second photon by the small steady-state concentration of polymer triplets.

Effect of energetic disorder on triplet-triplet annihilation in organic semiconductors

We present a model for the triplet-triplet annihilation in organic phosphorescent host-guest systems that considers the effect of the energetic disorder on the kinetics of the annihilation. Nonequilibrium (time-dependent) transport of the triplets that arises from the progressive relaxation of triplets in the energy landscape is taken into account in the model. Triplet excitons are considered to move between the guest sites (dye molecules) via the thermally activated tunneling mechanism; the annihilation step, however, takes place through the F\"orster mechanism. Based on the model developed, and by calculating the time evolution of the triplet concentration, we investigate the effect of the various parameters on the effective annihilation coefficient. At low dye concentrations, it is shown that relaxation of the triplets is not completed during their lifetimes, and therefore, the annihilation process occurs entirely in the nonequilibrium. We also address the competition between two pathways for the annihilation process: single-step, long-range annihilation, and diffusion-assisted annihilation. By a quantitative comparison with experimental data reported in the literature, we demonstrate certain conditions in which there is a considerable contribution from the diffusion to the total annihilation rate. The model presented in this work can also be used for the excitons that diffuse by the F\"orster mechanism.

Optically-generated Overhauser dynamic nuclear polarization: A numerical analysis.

Recently, an alternative approach to dynamic nuclear polarization (DNP) in the liquid state was introduced using optical illumination instead of microwave pumping. By exciting a suitable dye to the triplet state which undergoes a diffusive encounter with a persistent radical forming a quartet-doublet pair in the encounter complex, dynamic electron polarization (DEP) is generated via the radical-triplet pair mechanism. Subsequent cross-relaxation generates nuclear polarization without the need for microwave saturation of the electronic transitions. Here, we present a theoretical justification for the initial experimental results by means of numerical simulations. These allow investigation of the effects of various experimental parameters, such as radical and dye concentrations, sample geometry, and laser power, on the DNP enhancement factors, providing targets for experimental optimization. It is predicted that reducing the sample volume will result in larger enhancements by permitting a higher concentration of triplets in a sample of increased optical density. We also explore the effects of the pulsed laser rather than continuous-wave illumination, rationalizing the failure to observe the optical DNP effect under illumination conditions common to DEP experiments. Examining the influence of the illumination duty cycle, the conditions necessary to permit the use of pulsed illumination without compromising signal enhancement are determined, which may reduce undesirable laser heating effects. This first simulation of the optical DNP method therefore underpins the further development of the technology.

Two-photon-excited ultralong organic room temperature phosphorescence by dual-channel triplet harvesting.

Due to inefficient molecular design strategies, two-photon-excited ultralong organic room temperature phosphorescence (TPUOP) has not yet been reported in single-component materials. Herein, we present an innovative design method by dual-channel triplet harvesting to obtain the first bright TPUOP molecule with a lifetime of 0.84 s and a quantum efficiency of 16.6%. In compound o-Cz the donor and acceptor units are connected at the ortho position of benzophenone, showing intramolecular space charge transfer. Therefore, the two-photon absorption ability is improved due to the enhanced charge transfer character. Moreover, the small energy gap boosts dual-channel triplet harvesting via ultralong thermally activated delayed fluorescence and H-aggregation phosphorescence, which suppresses the long-lived triplet concentration quenching. Through two-photon absorption, a near-infrared laser (808 nm) is able to trigger the obvious ultralong emission under ambient conditions. This research work provides valuable guidance for designing near-infrared-excited ultralong organic room temperature phosphorescence materials.

“Trade‐Off” Hidden in Condensed State Solvation: Multiradiative Channels Design for Highly Efficient Solution‐Processed Purely Organic Electroluminescence at High Brightness

AbstractActualizing highly efficient solution‐processed thermally activated delayed fluorescent (TADF) organic light‐emitting diodes (OLEDs) at high brightness becomes significant to the popularization of purely organic electroluminescence. Herein, a highly soluble emitter benzene‐1,3,5‐triyltris((4‐(9,9‐dimethylacridin‐10(9H)‐yl)phenyl)methanone was developed, yielding high delayed fluorescence rate (kTADF > 105 s−1) ascribed to the multitransition channels and tiny singlet–triplet splitting energy (ΔEST ≈ 32.7 meV). The triplet locally excited state is 0.38 eV above the lowest triplet charge‐transfer state, assuring a solely thermal equilibrium route for reverse intersystem crossing. Condensed state solvation effect unveils a hidden “trade‐off”: the reverse upconversion and triplet concentration quenching processes can be promoted but with a reduced radiative rate from the increased dopant concentration and the more polarized surroundings. Striking a delicate balance, corresponding vacuum‐evaporated and solution‐processed TADF‐OLEDs realized maximum external quantum efficiencies (EQEs) of ≈26% and ≈22% with extremely suppressed efficiency roll‐off. Notably, the wet‐processed one achieves to date the highest EQEs of 20.7%, 18.5%, 17.1%, and 13.6%, among its counterparts at the luminance of 1000, 3000, 5000, and 10 000 cd m−2, respectively.

Investigation of electroluminescence mechanism in deep blue exciplex devices by utilizing magneto-electroluminescence

Recently, the electroluminescence (EL) mechanism in organic-light emitting diodes (OLEDs) based on blends of NPB:TPBi for deep blue light-emission has been reported in Advanced Materials by Jankus et al. in 2013. They have recorded EL and photoluminescence transients and fluorescence decays, demonstrating that the electroluminescence in their devices arises from enhanced exciplex emission via triplet-triplet annihilation (TTA) not the reverse intersystem crossing (RISC). In recent years, a large number of researches have showed that the magneto-electroluminescence (MEL) could be used as an effective measure to study the EL mechanism in OLEDs. And as a fingerprint tool MEL is sensitive to TTA and RISC. In this paper, we report similar exciplex-based OLEDs and show that no triplet-triplet annihilation and reverse intersystem crossing are observed at room temperature. TTA only occurs at low temperature and high current. We use O3 to treat the anode or replace the cathode with lower work function material, we haven’t observed TTA at room temperature yet. In order to increase the concentration of triplet excitons to facilitate the generation of TTA, we increase the injection current of the devices by over 1 order of magnitude, but we still haven’t observed the occurrence of TTA. The magneto-current of the device and its theoretical simulation show that triplet exciton interaction with hole polaron T1( ­­ )+P+( ¯ ) ® S1( ­¯ )+P+( ­ ) may have happened in the system, namely TPI (triplet-polaron interaction) process. This study will provide a reference for understanding the EL mechanism in the NPB:TPBi blending devices with deep blue light-emission.

Enhanced triplet-triplet annihilation in bicomponent organic systems by using a gap plasmon resonator.

The triplet-triplet annihilation (TTA) efficiency in bicomponent organic systems is investigated by employing a gap plasmon resonator. In our structure, strong absorption peaks arising from coupling between localized surface plasmons and surface plasmon polaritons closely overlap the Q band of porphyrin, leading to higher triplet concentrations within the film. We find that at ultralow excitation intensities on the order of watts per square centimeter (W cm(-2)), TTA becomes predominant for the organic system on a gap plasmon resonator. A strong surface-enhanced Raman scattering intensity is observed in this substrate, verifying the near-field enhancement.

Triplet Exciton Production and Lifetimes in Single-Wall Carbon Nanotubes

Despite a broad body of spectroscopic work on the photophysics of singlet excitons in SWNTs, triplet states and triplet–triplet interactions have mostly remained elusive. Here we report on the first spin sensitive study of triplet excitons in (6,5) SWNTs giving triplet lifetimes and the quantum yield of triplet exciton production using optically detected magnetic resonance (ODMR) [1]. SWNT samples were fabricated from aqueous suspensions and PFO-BPy dispersed CoMoCAT SWNTs. Triplet lifetimes of (30±10) µs were obtained from the modulation frequency dependence of ODMR signals at small excitation intensities. The yield of triplet exciton production of (5±2)% is determined using the intensity dependence of triplet-triplet annihilation, a process that competes with triplet decay as well as with non-radiative singlet decay. The results imply that steady state triplet concentrations in SWNTs are about 5 orders of magnitude higher than those of singlet excitons.[1] D. Stich, F. Späth, H. Kraus, A. Sperlich, V. Dyakonov and T. Hertel, Nature Photonics, in press (DOI: 10.1038/NPHOTON.2013.316)

Plasmon-Enhanced Triplet–Triplet Annihilation Using Silver Nanoplates

Photon upconversion processes have attracted substantial interest as a means of circumventing the Shockley–Queisser limit for single-junction photovoltaic devices. Despite this promise, the quantum yield of most upconversion processes is very low at the light intensities typical of solar radiation (∼100 mW/cm2). Additionally, bimolecular upconversion processes that rely on molecular diffusion (e.g., triplet–triplet annihilation) typically see further reductions in quantum yield when the upconverting chromophores are confined to a solid state or thin film matrix. Here we report a plasmon-based enhancement of the triplet–triplet annihilation process when silver nanoplates are embedded in poly(methyl methacrylate) thin films containing the upconverting materials palladium(II) octaethylporphyrin and 9,10-diphenylanthracene. The silver nanoplates are synthesized with localized surface plasmon resonance bands tailored to overlap strongly with the Q-band of the porphyrin, leading to enhanced light absorption within the film and higher overall triplet concentrations. Optimization of the silver nanoplate loading leads to a nearly 10-fold increase in the upconverted light intensity compared with control samples containing no silver.

Influence of triplet excitons on the lifetime of polymer-based organic light emitting diodes

In this work, the harmful effect of triplet excitons on the lifetime of poly(p-phenylene vinylene) (PPV)-based organic light emitting diodes is demonstrated. A model system has been designed in which the triplet population in a PPV derivative is increased by blending the polymer with the triplet sensitizer platinum (II) octaethylporphyrine ketone (PtOEPK). The increase in the triplet concentration of the polymer affects both the initial decay and the long-term evolution of luminance and leads to a drastic acceleration of device degradation, reflected in a dramatically shortened lifetime.

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.

Energy Upconversion via Triplet Fusion in Super Yellow PPV Films Doped with Palladium Tetraphenyltetrabenzoporphyrin: a Comprehensive Investigation of Exciton Dynamics

AbstractOne of the key issues concerning the development of efficient polymer solar cell technology is the lack of viable materials which absorb in the near‐infrared (NIR) region. This could be resolved by up‐converting energy from the NIR into visible using triplet fusion (TF) with an additional layer that is fabricated separately from the solar cell and deposited on top. Theoretically a maximum upconversion (UC) via TF efficiency of 50% could be obtained. Here, it is demonstrated that in a film of commercially available poly(para‐phenylene vinylene) copolymer “super yellow” (SY) doped with 4% palladium(meso‐tetraphenyl‐tetrabenzoporphyrin) (PdTPBP) sensitizer, an UC efficiency of 6% can be achieved. By using femtosecond and nanosecond spectroscopies it is shown that the main UC efficiency loss mechanism is due to triplet quenching in PdTPBP aggregates. The PdTPBP intersystem crossing rate constant is determined to be 1.8 × 1011 s−1 and the triplet energy transfer rate constant from PdTPBP to SY to be 109 s−1. Quenching in PdTPBP aggregates can account for a triplet concentration loss in the range of 76‐99%. As such, preventing sensitizer aggregation in NIR‐to‐visible upconverting films is crucial and may lead to substantial increase of UC efficiencies in films.

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.

Dynamics of triplet migration in films ofN, N′-diphenyl-N, N′-bis(1-naphthyl)-1, 1′-biphenyl-4, 4′′-diamine

We study triplet migration properties in NPB (N, N′-diphenyl-N, N′-bis(1-naphthyl)-1, 1′-biphenyl-4, 4′′-diamine) films using time resolved gated spectroscopy and dispersive migration theory asour main tools of analysis. We show that in NPB, a well-known hole transporterin organic light emitting diodes, at high excitation densities triplet migrationfollows two regimes—a dispersive non-equilibrium regime (distinguished by excitonenergetical relaxation within the distribution of hopping sites and as a consequence thehopping frequency being time dependent) that evolves into a second, non-dispersiveequilibrium regime. Further, we observe a third region, which we term acceleration.From the turning over time between dispersive and non-dispersive dynamics, wededuce the width of the triplet density of states (DOS). We observe how the DOSvariance changes when one decreases the thickness of the NPB film and notehow surface effects are becoming important. Furthermore, the DOS variance ofNPB changes when another organic layer is evaporated on top, namely Ir(piq)3(tris(1-phenylisoquinoline)iridium(III)). We believe that these changes are due tothe different polarizable media in contact with the NPB film, either vacuum orIr(piq)3. We also show in this paper that the triplet level when time approacheszero is much higher in energy than the relaxed triplet levels, as quoted in mostpublished papers; these values are thus incorrect for NPB. Lastly, it is possible thateven at room temperature, the dispersive regime might be important for tripletmigration at high initial triplet concentrations and might affect the diffusionlength of triplets to a certain extent. However, more experimentation needs to beperformed in order to address this question. Overall, we have characterized the tripletmigration dynamics of NPB fully and shown that it agrees with previously publishedobservations for other organic semiconductors and theoretical considerations.

The magnetic field effect on the transport and efficiency of group III tris(8-hydroxyquinoline) organic light emitting diodes

Magnetoresistance and efficiency measurements of organic light emitting diode structures based on the group III hydroxyquinolates (Mq3) have been made as a function of magnetic field and Mq3 thickness, where M=Al, Ga, and In. For all quinolates, independent of thickness, we observed very similar behavior for the efficiency of the devices, with large increases in efficiency occurring at low values of applied field, which rapidly saturate as the field is increased. The current through these devices is found to be a strong function of both the device thickness and the metal ion. For Alq3 based devices, the current changes appear to correlate strongly with the triplet population in the devices. For Gaq3 and Inq3 devices, the magnetoresistance is found not to correlate with the triplet concentration and this may be evidence that there is little energetic barrier for carrier trapping in these materials. For all materials, a further dependence of the magnetoresistance on applied field was observed, which needs closer investigation.