Triplet Contribution Research Articles

Theoretical studies on the excited-state properties of di(pyridinyl)methanone-based emitters with efficient thermally activated delayed fluorescence

The excited-state properties of seven thermally activated delayed fluorescence (TADF) emitters with di(pyridinyl)methanone acceptor were investigated using DFT and TD-DFT methods. The results indicate that the species and number of donors strikingly impact the excited-state nature. The mono-substituted donor increases the local-excited singlet (1LE) and local-excited triplet (3LE) contribution, promoting the radiative decay processes and strengthening spin–orbit coupling (SOC) between S1 and T1 states. The decreased charge transfer (CT) contribution of S1 gives rise to a much smaller ΔEST, and the mixed CT-LE nature of T1 guarantees the strong SOC for the designed emitters 1–4 and 6, endowing them with high reverse intersystem crossing rates.

Progress and Perspective toward Continuous-Wave Organic Solid-State Lasers.

A continuous-wave (CW) organic solid-state laser is highly desirable for spectroscopy, sensing, and communications, but is a significant challenge in optoelectronics. The accumulation of long-lived triplet excitons and relevant excited-state absorptions, as well as singlet-triplet annihilation, are the main obstacles to CW lasing. Here, progress in singlet- and triplet-state utilizations in organic gain media is reviewed to reveal the issues in working with triplets. Then, exciton behaviors that inhibit light oscillations during long excitation pulses are discussed. Further, recent advances in increasing organic lasing pulse widths from microseconds toward the indication of CW operation are summarized with respect to molecular designs, advanced resonator architectures, triplet scavenging, and potential triplet contribution strategies. Finally, future directions and perspectives are proposed for achieving stable CW organic lasers with significant triplet contribution.

Some new observations for the Georgi-Machacek scenario with triplet Higgs scalars

The Georgi-Machacek model, introducing a complex and a real scalar triplet as additional components of the electroweak symmetry breaking sector, enables substantial triplet contributions to the weak gauge boson masses, subject to the equality of the complex and the real triplet vacuum expectation values (vev) via a custodial SU(2) symmetry. We present an updated set of constraints on this scenario, from collider data (including those from 137/139~fb$^{-1}$ of luminosity at the Large Hadron Collider), available data on the 125-GeV scalar, indirect limits and also theoretical restrictions from vacuum stability and unitarity. It is found that some bounds get relaxed, and the phenomenological potential of the scenario is more diverse, if the doubly charged scalar in the spectrum can decay not only into two like-sign $W$'s but also into one or two singly charged scalars. Other interesting features are noticed in a general approach, such as substantial $\gamma\gamma$ and $Z\gamma$ branching ratios of the additional custodial singlet scalar, and appreciable strength of the trilinear interaction of a charged scalar, the $W$ and the $Z$. Finally,, we take into account the possibility of custodial SU(2) breaking, resulting in inequality of the real and the complex scalar vevs which too in principle may allow large triplet contribution to weak boson masses. Illustrative numerical results on the modified limits and predictions are presented, once more taking into account all the constraints mentioned above.

Relativistic two-electron atomic and molecular energies using LS coupling and double groups: Role of the triplet contributions to singlet states.

The triplet contribution is computed to the 1 and 2 S0e1 states of the He atom, to the 1S0e1 state of the Li+ and Be2+ ions, and to the X1Σg + ground state of the H2 molecule by extensive use of double-group symmetry (equivalent to LS coupling for the atomic systems) during the course of the variational solution of the no-pair Dirac-Coulomb-Breit (DCB) wave equation. The no-pair DCB energies are converged within sub-parts-per-billion relative precision, using an explicitly correlated Gaussian basis optimized to the non-relativistic energies. The α fine-structure constant dependence of the triplet sector contribution to the variational energy is α4Eh at leading order, in agreement with the formal perturbation theory result available from the literature.

Photon-upconverters for blue organic light-emitting diodes: a low-cost, sky-blue example.

In the research ecosystem's quest towards having deployable organic light-emitting diodes with higher-energy emission (e.g., blue light), we advocate focusing on fluorescent emitters, due to their relative stability and colour purity, and developing design strategies to significantly improve their efficiencies. We propose that all triplet–triplet annihilation upconversion (TTA-UC) emitters would make good candidates for triplet fusion-enhanced OLEDs (“FuLEDs”), due to the energetically uphill nature of the photophysical process, and their common requirements. We demonstrate this with the low-cost sky-blue 1,3-diphenylisobenzofuran (DPBF). Having satisfied the criteria for TTA-UC, we show DPBF as a photon upconverter (Ith 92 mW cm−2), and henceforth demonstrate it as a bright emitter for FuLEDs. Notably, the devices achieved 6.5% external quantum efficiency (above the ∼5% threshold without triplet contribution), and triplet-exciton-fusion-generated fluorescence contributes up to 44% of the electroluminescence, as shown by transient measurements. Here, triplet fusion translates to a quantum yield (ΦTTA-UC) of 19%, at an electrical excitation of ∼0.01 mW cm−2. The enhancement is meaningful for commercial blue OLED displays. We also found DPBF to have decent hole mobilities of ∼0.08 cm2 V−1 s−1. This additional finding can lead to DPBF being used in other capacities in various printable electronics.

Эффект сверхпроводящего спинового клапана в структурах со слоями ферромагнитного сплава Гейслера

The transport properties of two types of spin valves are analyzed, in which the Heusler alloy Co2Cr1-xFexAly was used as one of the two ferromagnetic layers in the F1/F2/S structures. The Heusler alloy layer was used: 1) as a weak ferromagnet, in the case of the F2 layer; 2) as a halfmetal, in the case of the F1 layer. In the first case, a large classical effect of the superconducting spin valve ΔTc was obtained, which was facilitated by a significant triplet contribution to the effect of the superconducting spin valve ΔTctrip. In the second case, a gigantic effect value ΔTctrip was found reaching 0.5 K.

Superconducting spin-valve effect in heterostructures with ferromagnetic Heusler alloy layers

The transport properties of two types of spin valves are analyzed, in which the Heusler alloy Co2Cr1-xFexAly was used as one of the two ferromagnetic layers in the F1/F2/S structures. The Heusler alloy layer was used: 1) as a weak ferromagnet, in the case of the F2 layer; 2) as a halfmetal, in the case of the F1 layer. In the first case, a large classical effect of the superconducting spin valve Delta Tc was obtained, which was facilitated by a significant triplet contribution to the effect of the superconducting spin valve Delta Tctrip. In the second case, a gigantic effect value Delta Tctrip was found reaching 0.5 K. Keywords: superconductivity, ferromagnetism, thin films, superconducting spin valve.

Excited lepton triplet contribution to electroweak observables at one loop level

In this paper, we present the one-loop radiative corrections to the electroweak precision observable Delta rho coming from the I_W=1 multiplet excited leptons. We have calculated the couplings of the exotic lepton triplet to the vector bosons and ordinary leptons using the effective Lagrangian approach. These couplings are then used to estimate the excited lepton triplet contribution to the Delta rho parameter. The mass degenerate excited lepton contribution to Delta rho is small and can be neglected. However, if the excited leptons are non-degenerate, their contribution can be large which can result in more stringent constraints on the excited fermion parameter space compared to the constraints from present experimental searches and perturbative unitarity condition.

Superconducting Spin-Valve Effect in Structures with a Ferromagnetic Heusler Alloy Layer

We present comparative analysis of superconducting properties of two types of spin valves containing Heusler alloy Co2Cr1 – xFexAly as one of ferromagnetic layers (F1 or F2) in the F1/F2/S structures. We have used the Heusler alloy layer (i) as a weak ferromagnet in the case of the F2 layer and (ii) as a half-metal in the case of F1 layer. In the former case, large classical effect ΔTc of the superconducting spin valve is obtained; this is facilitated by a substantial triplet contribution Δ $$T_{c}^{{{\text{trip}}}}$$ to the superconducting spin valve effect. In the latter case, giant value of Δ $$T_{c}^{{{\text{trip}}}}$$ reaching 0.5 K is observed.

Observation of Nonradiative Deactivation Behavior from Singlet and Triplet States of Thermally Activated Delayed Fluorescence Emitters in Solution.

Excited states of emissive organic molecules undergo various kinds of quenching phenomena such as vibration-coupled quenching depending on their environmental conditions. Because bright singlet excitons in purely organic molecules exhibiting thermally activated delayed fluorescence (TADF) can access dark triplet excited states, photogenerated singlet excitons can decay nonradiatively through both singlet and triplet excited states. Here, we investigated nonradiative decay behavior, including internal and external exciton quenching processes, of various types of TADF materials in solution. Under air-saturated conditions, both the lowest singlet and triplet excited states of almost all of the TADF materials showed oxygen quenching. We considered the effect of oxygen quenching for both spin states to develop a method for determination of the triplet contribution to the total photoluminescence quantum yield from the transient photoluminescence profiles. Furthermore, we observed a clear energy gap law for the internal nonradiative processes.

Modelling and fitting the Polaron Pair Magnetoconductance model to obtain a realistic local hyperfine field in Tris-(8-hydroxyquinoline)aluminium based diodes

The Polaron Pair (PP) model has been successfully applied to magnetoconductance (MC) in organic semiconductor devices under ultra-small magnetic fields (USMFE). We report µT resolution MC measurements carried out with high sensitivity (better than 10−6) on the common organic semiconductor tris-(8-hydroxyquinoline)aluminium in the range ±500 µT displaying clear minima at ~±240 µT. Unlike traditional approaches, where device MC is simply evaluated using the PP model using nominal parameters for microscopic quantities such as the local hyperfine magnetic field, we have carried out actual fitting of the PP MC model to the experimentally obtained data. The fitting procedure yields physically realistic values for the polaron pair decay rate, local hyperfine magnetic field and triplet contribution to dissociation namely: k = 28.6 ± 9.7 MHz, {B}_{hf} = 0.34 ± 0.04 mT and {delta }_{TS} = 0.99 ± 0.01 respectively. The local hyperfine field obtained by fitting is in excellent agreement with independently calculated values for this system and is reproducible across different devices and independent of drive conditions. This demonstrates the applicability of the fitting approach to any organic USMFE MC data for obtaining microscopic parameter values.

Connecting nonzero θ13, Dirac CP phase and leptogenesis through spontaneous CP violation

We have analyzed a Type-I+II seesaw scenario to generate neutrino masses and mixing in a A4 based flavor symmetric framework. The Standard Model (SM) particle content is extended by three singlet right handed neutrinos, an additional higgs triplet along with few SM singlet flavon fields. In this set-up, the pure type-I contribution to the neutrino mass matrix exhibits a tribimaximal (TBM) of lepton mixing where the triplet contribution generates θ13. Complex vev of one flavon provides a unique source of spontaneous CP violation. Here it turns out that the triplet contribution provides a common source for θ13, Dirac CP phase (δ) and CP violation required for leptogenesis.

Superconducting spin-valve and triplet superconductivity

Recent experimental results on the superconducting spin-valve effect and generation of the long-range triplet superconductivity in a F1/F2/S structure are reviewed (here, F1 and F2 are uncoupled ferromagnetic layers, and S is the superconducting layer). The main results are the following: (i) the maximum of the magnitude of the superconducting spin-valve effect increases with decreasing the exchange field h in the ferromagnetic layer; (ii) a full switching between the normal and superconducting states may be realized with the aid of the triplet contribution to the spin-valve effect.

Experimental investigation of the role of the triplet pairing in the superconducting spin-valve effect

An important role of the morphology of a superconducting layer in the superconducting spin-valve effect has been established. The triplet pairing induced by the superconductor/ferromagnet proximity effect has been experimentally investigated for samples CoO x /Py1/Cu/Py2/Cu/Pb (where Py = Ni0.81Fe0.19) with a smooth superconducting layer. The optimization of the parameters of this structure has demonstrated a complete switching between the normal and superconducting states with a change in the relative orientation of magnetizations of the ferromagnetic layers from the antiparallel to orthogonal orientation. A pure triplet contribution has been observed for the sample with a permalloy layer thickness at which the superconducting spin-valve effect vanishes. A direct comparison of the experimental data with the theoretical calculation of the temperature of the transition to the superconducting state has been performed for the first time.

Excited-state dynamics of oxazole: A combined electronic structure calculations and dynamic simulations study

In the present work, the combined electronic structure calculations and surface hopping simulations have been performed to investigate the excited-state decay of the parent oxazole in the gas phase. Our calculations show that the S2 state decay of oxazole is an ultrafast process characterized by the ring-opening and ring-closure of the five-membered oxazole ring, in which the triplet contribution is minor. The ring-opening involves the OC bond cleavage affording the nitrile ylide and airine intermediates, while the ring-closure gives rise to a bicyclic species through a 25 bond formation. The azirine and bicyclic intermediates in the S0 state are very likely involved in the phototranspositions of oxazoles. This is different from the previous mechanism in which these intermediates in the T1 state have been proposed for these phototranspositions.

Strong electroweak phase transition from Supersymmetric Custodial Triplets

The Supersymmetric Custodial Triplet Model, a supersymmetric generalization of the Georgi-Machacek model, has proven to be an interesting modification of the MSSM. It extends the MSSM Higgs sector by three extra SU(2)L triplets in such a way that approximate custodial invariance is preserved and rho-parameter deviations are kept under control. By means of a sizeable triplet contribution to electroweak breaking the model is able to generate a barrier at tree level between the false vacuum and the electroweak one. This will result in a strong first order phase transition for an important region of the parameter space. We also look at the gravitational waves that could be generated as a result of the phase transition and show how future interferometers could be used as a probe of the model.

The scalar triplet contribution to lepton flavour violation and neutrinoless double beta decay in Left-Right Symmetric Model

We analyse in detail the scalar triplet contribution to the low-energy lepton flavour violating (LFV) and lepton number violating (LNV) processes within a TeV-scale left-right symmetric framework. We show that in both type-I and type-II seesaw dominance for the light neutrino masses, the triplet of mass comparable to or smaller than the largest right-handed neutrino mass scale can give sizeable contribution to the LFV processes, except in the quasi-degenerate limit of light neutrino masses, where a suppression can occur due to cancellations. In particular, a moderate value of the heaviest neutrino to scalar triplet mass ratio $r\lesssim {\cal O}(1)$ is still experimentally allowed and can be explored in the future LFV experiments. Similarly, the contribution of a relatively light triplet to the LNV process of neutrinoless double beta decay could be significant, disfavouring a part of the model parameter space otherwise allowed by LFV constraints. Nevertheless, we find regions of parameter space consistent with both LFV and LNV searches, for which the values of the total effective neutrino mass can be accessible to the next generation ton-scale experiments. Such light triplets can also be directly searched for at the LHC, thus providing a complementary probe of this scenario. Finally, we also study the implications of the triplet contribution for the left-right symmetric model interpretation of the recent diboson anomaly at the LHC.

SpontaneousCPviolation in lepton-sector: A common origin forθ13, the DiracCPphase, and leptogenesis

A possible interplay between the two terms of the general type-II seesaw formula is exercised which leads to the generation of nonzero ${\ensuremath{\theta}}_{13}$. The specific flavor structure of the model, guided by the ${A}_{4}\ifmmode\times\else\texttimes\fi{}{Z}_{4}\ifmmode\times\else\texttimes\fi{}{Z}_{3}$ symmetry and accompanied with the Standard Model singlet flavons, yields the conventional seesaw contribution to produce the tribimaximal lepton mixing which is further corrected by the presence of the $SU(2{)}_{L}$ triplet contribution to accommodate ${\ensuremath{\theta}}_{13}$. We consider the $CP$ symmetry to be spontaneously broken by the complex vacuum expectation value (vev) of a singlet field $S$. While the magnitude of its complex vev is responsible for generating ${\ensuremath{\theta}}_{13}$, its phase part induces the low energy $CP$ violating phase ($\ensuremath{\delta}$) and the $CP$ violation required for leptogenesis. Hence the triplet contribution, although subdominant, plays a crucial role in providing a common source for nonzero ${\ensuremath{\theta}}_{13}$, $\ensuremath{\delta}$ and $CP$-violation required for leptogenesis. We find that the recent hint for $\ensuremath{\delta}$ close to $3\ensuremath{\pi}/2$ is somewhat favored in this setup though it excludes the exact equality with $3\ensuremath{\pi}/2$. We also discuss the generation of lepton asymmetry in this scenario.

Dirac triplet extension of the MSSM

In this paper we explore extensions of the Minimal Supersymmetric Standard Model involving two $SU(2)_L$ triplet chiral superfields that share a superpotential Dirac mass yet only one of which couples to the Higgs fields. This choice is motivated by recent work using two singlet superfields with the same superpotential requirements. We find that, as in the singlet case, the Higgs mass in the triplet extension can easily be raised to $125\,\text{GeV}$ without introducing large fine-tuning. For triplets that carry hypercharge, the regions of least fine tuning are characterized by small contributions to the $\mathcal T$ parameter, and light stop squarks, $m_{\tilde t_1} \sim 300-450\,\text{GeV}$; the latter is a result of the $\tan\beta$ dependence of the triplet contribution to the Higgs mass. Despite such light stop masses, these models are viable provided the stop-electroweakino spectrum is sufficiently compressed.

34.1: Invited Paper: Effect of Singlet Triplet Recycling in the Charge Transfer State Manifold and Molecular Geometry on Thermally Activated Delayed Fluorescence

Detailed photophysical measurements of intramolecular charge transfer states have been made both in solution and solid state. Temperature dependent emission and delayed emission are used to map the energy levels involved in molecule decay, and through detailed kinetic modeling of the thermally activated processes observed, true electron exchange energies, and energy barrier for reverse intersystem crossing of the systems determined. From this it is possible to simply determine many rate constants for the TADF mechanism and the magnitude of triplet contribution to overall emission.