Triplet Doublet Research Articles

Efficient near-infrared organic light-emitting diodes with emission from spin doublet excitons

The development of luminescent organic radicals has resulted in materials with excellent optical properties for near-infrared emission. Applications of light generation in this range span from bioimaging to surveillance. Although the unpaired electron arrangements of radicals enable efficient radiative transitions within the doublet-spin manifold in organic light-emitting diodes, their performance is limited by non-radiative pathways introduced in electroluminescence. Here we present a host–guest design for organic light-emitting diodes that exploits energy transfer with up to 9.6% external quantum efficiency for 800 nm emission. The tris(2,4,6-trichlorophenyl)methyl-triphenyl-amine radical guest is energy-matched to the triplet state in a charge-transporting anthracene-derivative host. We show from optical spectroscopy and quantum-chemical modelling that reversible host–guest triplet–doublet energy transfer allows efficient harvesting of host triplet excitons.

Triplet Combination or Doublet Regimen Versus Chemoradiation as Neoadjuvant Therapy for Locally Advanced Rectal Cancer

Triplet Combination or Doublet Regimen Versus Chemoradiation as Neoadjuvant Therapy for Locally Advanced Rectal Cancer

Role of triplet-doublet quenching in organic magnetoconductance

Abstract The role of triplet-doublet (T-D) quenching in the magnetoconductance (MC), at low temperature (30 K) and 1 V applied voltage, of an organic light emitting diode (OLED) is analysed thoroughly. This diode is based on an anthracene containing poly (p-phenylene-ethynylene)-alt-poly (p-phenylene-venylene) (PPE-PPV) named (AnE-PV stat) and shows a negative MC. The stochastic Liouville equation is applied to interpret the experimental results. We demonstrate that the evolution of the doublet character of the T-D pair states, as a function of the applied magnetic field, is behind the line shape of the MC. We also show that the T-D quenching probability ( P q ) has a maximum value when the doublet character (DC) is shared by the most T-D pair states. Through the best fitting of the experimental data, we determine the zero-field splitting (ZFS) parameters of the triplet exciton; D z f s = − 1.9 × 10 − 5 eV and E z f s = 2 × 10 − 8 eV. The dissociation rate k − 1 of a T-D pair and the recombination rate k 2 are in the vicinity of 10 6 s−1.

Modulation of magnetotransport in asymmetrically coupled double quantum dot system

We study the transport properties in double quantum dots asymmetrically coupled to leads in magnetic field. We focus on the situation in which the second dot (QD2) couples with the leads with a weak hybridization function. The results shows that by tuning the energy level [Formula: see text] of QD2 one can control the conductance and its spin polarization of the system. In the absence of magnetic field [Formula: see text], with increasing [Formula: see text], the conductance shows a dip structure. This behavior of conductance results from a continuous triplet–doublet quantum phase transition. In the presence of magnetic field [Formula: see text], we obtain a perfect spin filtering with a fully-polarized conductance of up-spin or down-spin.

Time-Resolved Continuous-Wave and Pulse EPR Investigation of Photoinduced States of Zinc Porphyrin Linked with an Ethylenediamine Copper Complex

Electron spin dynamics of the photoexcited state of a new porphyrin system built of a zinc porphyrin molecule with a linked complex of copper ion (ZnPCu) was studied using electron paramagnetic resonance (EPR) methods. The time-resolved continuous-wave and echo-detected EPR data show that electron spins of the ZnPCu system are polarized. The shape of the EPR spectrum and the echo-detected nutation spectroscopy data indicate that there is a relatively small spin–spin interaction between two subunits, namely, less than the zero-field splitting parameter of the excited triplet ZnP molecule. Results have been interpreted assuming that the S = 1 Zn porphyrin subunit electron spins are polarized via the triplet mechanism and this polarization is transferred to the S = 1/2 copper subunit via a flip-flop process induced by the exchange interaction between these subunits. The formation of the integral spin polarization in the triplet–doublet system as a function of the value and type of the triplet–doublet interaction was analyzed.

Gate-controlled transitions in triple dots with interdot repulsion and magnetic field

We study the quantum phase transition (QPT) and electronic transport in triple quantum dots for a wide range of the gate voltage ϵ. We focus on the effect of the interdot repulsion V and the magnetic field B. In the case of particle–hole (p–h) symmetry and B = 0, we find the local quadruplet–doublet transition of first order when V increases to a critical point Vc ≈ U, where U is the on-site repulsion. Beyond the p–h symmetry, the sequence of the QPTs depends on ϵ. For small ϵ, a first order doublet–singlet transition is observed. For middle ϵ, we find the quadruplet–triplet transition of first order at Vc1 and the triplet–singlet transition of the Kosterlitz–Thouless type at Vc2. For large ϵ, there are two kinds of first order QPT with phase sequence quadruplet–triplet–doublet. The magnetic field B compensates for the effect of V. For V > U, as B increases we find a first order or second order QPT from a low-spin state to a high-spin state. The restoring of the Kondo effect and a perfect spin filtering is realized in the appropriate regime of the magnetic field.

Stimulated Electron Spin Polarization in Strongly Coupled Triplet–Doublet Spin Pairs

The possibility of stimulating electron spin polarization in a system consisting of a stable paramagnetic center and a chromophore that can be excited into its triplet state is discussed. In such systems, the doublet state of the paramagnetic center couples to the excited triplet state of the chromophore and if the coupling is larger than the difference in the precession frequencies of the doublet and triplet, the eigenstates of the coupled system are quartet and doublet states. The quartet state is usually the lowest energy excited state. Following light excitation, the initial electronic relaxation to the quartet state generates strong multiplet polarization if it is governed by the spin–orbit coupling that follows the molecular symmetry. It is shown that application of a selective π-pulse to the ±3/2 ↔ ±1/2 transitions of the quartet converts this multiplet polarization into net polarization. The magnitude and orientation dependence of the generated polarization is estimated on the basis of a simple analytical model. The experimental conditions required for this net polarization to be retained in the ground state after decay of the quartet state are discussed. The viability of using this as a method to enhance the signal strength of a spin label or metal center in selective excitation experiments is considered.

Application of the short and long consecutive pairs model to the triplet-doublet interaction in molecular crystals

We have adapted the model of two consecutive pairs to the study of the triplet-doublet (T-D) interaction in molecular crystals. We have applied this model to the modulation of the photoconductivity in crystalline anthracene by a static magnetic field (MFE) and a microwave field (PDMR). We were able to reproduce, for the first time, quite perfectly two types of experiments with the same set of kinetic constants.

ChemInform Abstract: The Excited Multiplet States of Metalloporphyrins and Metallophthalocyanines Coordinated or Linked to Nitroxide Radicals

Abstract We have reviewed several time-resolved electron paramagnetic resonance (TREPR) studies focusing on influences of nitroxide radicals on the photophysical properties of metalloporphyrins and metallophthalocyanines, which are coordinated or linked to nitroxide radicals. This paper summarizes TREPR studies on the excited multiplet states of metalloporphyrins and metallophthalocyanines coordinated by several kinds of nitroxide radicals, where the excited doublet and quartet states generated by the interaction between the excited triplet chromophore and doublet radical have been analyzed in detail. Selective population from the excited multiplet states to the triplet ground state is also discussed for phthalocyaninatosilicon covalently linked to two TEMPO radicals in terms of a new concept for controlling magnetic properties by photoexcitation.

Spin-multiplicity of a moderately coupled triplet–doublet spin pair in a biphenylene-linked porphyrin dimer

Electron spin transient nutation spectroscopy has been applied to a moderately coupled excited triplet and doublet spin-pair. In a biphenylene-linked copper(II)-free base porphyrin dimer, laser irradiation of either the free base half or the copper half, leads to excitation to the excited states and relaxation to the lowest excited state of the dimer. In this lowest excited state, the free base moiety in the lowest triplet state is coupled to the copper porphyrin ground doublet state, and thus forming a triplet–doublet spin pair. Time-resolved electron paramagnetic resonance (TREPR) spectra of the dimer in frozen solution at 40 K show much wider spectral width and a prominent band in the centre of the spectra than either of those for the free base monomer triplet or that of the copper(II) monomer porphyrin. These features are quite different from the strongly-coupled and weakly-coupled triplet–doublet spin pairs. Transient nutation method by pulsed EPR is used to examine the spin-multiplicity of the excited state in the dimer at three magnetic field positions. It was found that spin-multiplicity depends largely on the magnetic field. This is rationalized in terms of an intermediate size of coupling. In such intermediate coupling cases, the degree of mixing of the triplet and doublet basis wavefunctions depends greatly on the relative orientation between magnetic field and molecular axis because the ratio of the coupling size to an energy difference between the unperturbed species, triplet and doublet, varies with magnetic field direction. A splitting of TREPR spectra in partially oriented samples suggests that exchange coupling |J| is around 10 mT, which reasonably matches the results of the pulsed experiments.

Electron spin dynamics of triplet and doublet molecules in room temperature ionic liquids studied by a time-resolved EPR method

A time-resolved (TR-) EPR method was applied to measure the triplet spectra of porphyrins such as ZnTPP (TPP, tetraphenylporphyrin) and H4TPP2+ in room temperature ionic liquids (RTIL). The spectra were successfully obtained at 298 K with clear canonical peaks of the triplet sublevels. Magneto-photo selection experiments were carried out for ZnTPP in NR4BF4 (NR4: N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium) and H4TPP2+ in BmimPF6 (Bmim: 1-butyl-3-methylimidazolium). In these systems, the intensity ratio of the canonical peaks depends on the orientation between the laser polarization and the external magnetic field at the delay time on the order of submicro-seconds. This experimental finding suggests that there are very slow tumbling motions of ZnTPP and H4TPP2+ in RTILs. Electron spin polarization created in the triplet–doublet system is also successfully observed in RTILs by the TR-EPR method. Based on the analysis of the time evolution curves of both the triplet and doublet molecules, spin polarization mechanisms in the RTILs were discussed.

Spatially Compact Solutions and Stabilization in Einstein-Yang-Mills-Higgs Theories

New solutions to the static, spherically symmetric Einstein-Yang-Mills-Higgs equations with the Higgs field in the triplet (doublet) representation are presented. They form continuous families parametrized by alpha = M(W)/M(Pl) [M(W) (M(Pl)) denoting the W boson (the Planck) mass]. The corresponding space-times are regular and have spatially compact sections. A particularly interesting class with the Yang-Mills amplitude being nodeless is exhibited and is shown to be linearly stable with respect to spherically symmetric perturbations. For some solutions with nodes of the Yang-Mills amplitude a new stabilization phenomenon is found, according to which their unstable modes disappear as alpha increases (for the triplet) or decreases (for the doublet).

Neutrino masses induced by R-parity violation in a SUSY SU(5) model with additional [formula omitted

Within the framework of an SU(5) SUSY GUT model, a possible general form of the neutrino mass matrix induced by R-parity violation is investigated. The model has matter fields 5̄′L+5′L in addition to the ordinary matter fields 5̄L+10L and Higgs fields Hu+H̄d. The R-parity violating terms are given by 5̄L5̄L10L, while the Yukawa interactions are given by H̄d5̄′L10L. Since the matter fields 5̄′L and 5̄L are different from each other at the unification scale, the R-parity violation effects at a low energy scale appear only through the 5̄′L↔5̄L mixings. In order to make this R-parity violation effect harmless for proton decay, a discrete symmetry Z3 and a triplet–doublet splitting mechanism analogous to that in the 5-plet Higgs fields are assumed.

Electron spin polarization of the excited quartet state of strongly coupled triplet-doublet spin systems.

The electron spin polarization associated with electronic relaxation in molecules with trip-quartet and trip-doublet excited states is calculated. Such molecules typically relax to the lowest trip-quartet state via intersystem crossing from the trip doublet, and it is shown that when spin-orbit coupling provides the main mechanism for this relaxation pathway it leads to spin polarization of the trip quartet. Analytical expressions for this polarization are derived using first- and second-order perturbation theory and are used to calculate powder spectra for typical sets of magnetic parameters. It is shown that both net and multiplet contributions to the polarization occur and that these can be separated in the spectrum as a result of the different orientation dependences of the +/-1/2<-->+/-3/2 and +1/2<-->-1/2 transitions. The net polarization is found to be localized primarily in the center of the spectrum, while the multiplet contribution dominates in the outer wings. Despite the fact that the multiplet polarization is much stronger than the net polarization for individual orientations of the spin system, the difference in orientation dependence of the transitions leads to comparable amplitudes for the two contributions in the powder spectrum. The influence of this difference on the line shape is investigated in simulations of partially ordered samples. Because the initial nonpolarized state of the spin system is not conserved for the proposed mechanism, the net polarization can survive in the doublet ground state following electronic relaxation of the triplet part of the system.

Light-induced electron spin polarization in rigidly linked, strongly coupled triplet–doublet spin pairs

Perturbation theory is used to predict the light-induced spin polarization associated with the quartet state of rigid, strongly coupled triplet–doublet spin pairs. It is shown that spin–orbit coupling mixes the trip–doublet and trip–quartet states and that if the mixing determines the rate of ISC, it leads to net and multiplet spin polarization of the quartet state. Analytical expressions for the contributions to the spin polarization are derived using first-order perturbation theory. Using these expressions, the sign, magnetic field dependence and orientation dependence of the spin polarization are discussed in terms of the exchange coupling, zero-field splitting and spin–orbit coupling within the system.

Theoretical study of hyperfine related CIDEP of radical–triplet pairs

Considering the interaction between excited triplet molecule and doublet radical, based on the second-order perturbation theory and the motion equation of density matrix, the spin polarization intensity on the radical was theoretically calculated. The results of calculation indicate that the polarization was hyperfine related. With the overpopulation on doublet spin states of radical–triplet pairs as initial condition, the CIDEP on the radicals were A + A/E pattern and with the overpopulation on quartet spin states of radical–triplet pairs, the CIDEP was E + E/A pattern.

Optimal magnetization in liquids, generated by triplet—doublet interaction

This study is an extension of a recent theoretical treatment for calculating the magnetization generated by the encounters of radicals and photoexcited triplets in solution. Whereas the previous study employed a restricted analytical approach to the problem, the present study takes into account a general numerical formulation for the solution of the stochastic Liouville equation to calculate the electron spin polarization generated in the radical, following its encounter with the triplet. This method considers the efficiency of triplet quenching by the radical, which is an important factor in determining the radical polarization and the triplet lifetime in the solution. In addition, numerical calculation of the diffusion process is used to obtain the overall magnetization of the radical and its time dependence. The theory presented here complies with the experimental results and allows for efficient optimization of the magnetization in terms of magnitude and overall lifetime. Such an optimization is accomplished by the proper choice of the chemical system, which is exposed to light excitation, solvent properties and temperature. The ultimate goal of this study is to achieve photo-controlled high magnetization, which can be used in a variety of novel microwave applications.

Fermion mass hierarchy in six-dimensional SO(10) grand unified theory on a T2/Z2 orbifold

We suggest a simple supersymmetric SO (10) grand unified theory in 6 dimensions which produces the suitable fermion mass hierarchies. The 5th- and 6th-dimensional coordinates are compactified on a T 2 / Z 2 orbifold. The gauge and Higgs fields propagate in 6 dimensions while ordinal chiral matter fields are localized in 4 dimensions. The orbifolding and boundary conditions realize the gauge symmetry reduction, SU (3) C × SU (2) L × U (1) Y × U (1) X , and the triplet–doublet mass splitting. We introduce extra three sets of vector-like heavy fields: two sets propagate in 6 dimensions and chiral fields which couple to them are defined as the 1st generation, and one set propagates in 5 dimensions and chiral fields which couple to them are defined as the 2nd generation. The suitable fermion mass hierarchies are generated by integrating out these vector-like heavy fields.

Modeling of a Rotaxane-based Molecular Device

ABSTRACTA computational procedure is presented for investigating photo-induced switchable rotaxanes and demonstrated for a known system. This procedure starts with the generation of more than 104 chemically reasonable rotaxane conformations based on an empirical intramolecular potential energy function. Single-point energy calculations at the semi-empirical (AM1) level are carried out for each structure in the singlet (ground), triplet, and anionic doublet states. The structural features are assigned and then correlated with energy for each state. What emerges is a profile of the structure-energy relationship that captures the salient features of the system that endow it with device-like character. Full geometry optimization of a subset of co-conformations (∼1%) demonstrates that the procedure based on single-point calculations is sufficient to obtain a profile of the relationship of structural features to energy that is consistent with experiments, at greatly reduced computational cost.

Modeling of a Rotaxane-based Molecular Device

ABSTRACTA computational procedure is presented for investigating photo-induced switchable rotaxanes and demonstrated for a known system. This procedure starts with the generation of more than 104 chemically reasonable rotaxane conformations based on an empirical intramolecular potential energy function. Single-point energy calculations at the semi-empirical (AM1) level are carried out for each structure in the singlet (ground), triplet, and anionic doublet states. The structural features are assigned and then correlated with energy for each state. What emerges is a profile of the structure-energy relationship that captures the salient features of the system that endow it with device-like character. Full geometry optimization of a subset of co-conformations (∼1%) demonstrates that the procedure based on single-point calculations is sufficient to obtain a profile of the relationship of structural features to energy that is consistent with experiments, at greatly reduced computational cost.