Search results for "Triplet"
showing 10 items of 159 documents
Ab Initio Studies of Triplet-State Properties for Organic Semiconductor Molecules
2012
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 proc…
Tuning the photophysical properties of cationic iridium(iii) complexes containing cyclometallated 1-(2,4-difluorophenyl)-1H-pyrazole through function…
2012
Four new heteroleptic iridium(III) complexes in the family [Ir(dfppz)(2)((NN)-N-boolean AND)](+), where Hdfppz = 1-(2,4-difluorophenyl)-1H-pyrazole and (NN)-N-boolean AND = 6-phenyl-2,2'-bipyridine (1), 4,4'-(di-tert-butyl)-6-phenyl-2,2'-bipyridine (2), 4,4'-(di-tert-butyl)-6,6'-diphenyl-2,2'-bipyridine (3) and 4,4'-bis(dimethylamino)-2,2'-bipyridine (4), have been synthesized as the hexafluoridophosphate salts and fully characterized. Single crystal structures of ligand 3 and the precursor [Ir-2(dfppz)(4)(mu-Cl)(2)] have been determined, along with the structures of the complexes 4{[Ir(dfppz)(2)(1)][PF6]}center dot 3CH(2)Cl(2), [Ir(dfppz)(2)(3)][PF6]center dot CH2Cl2 and [Ir(dfppz)(2)(4)][…
Biexciton formation and exciton coherent coupling in layered GaSe.
2015
Nonlinear two-dimensional Fourier transform (2DFT) and linear absorption spectroscopy are used to study the electronic structure and optical properties of excitons in the layered semiconductor GaSe. At the 1s exciton resonance, two peaks are identified in the absorption spectra, which are assigned to splitting of the exciton ground state into the triplet and singlet states. 2DFT spectra acquired for co-linear polarization of the excitation pulses feature an additional peak originating from coherent energy transfer between the singlet and triplet. At cross-linear polarization of the excitation pulses, the 2DFT spectra expose a new peak likely originating from bound biexcitons. The polarizati…
Regio(ir)regular naphthalenediimide- and perylenediimide-bithiophene copolymers: How MO localization controls the bandgap
2016
Absorption spectra of regio(ir)regular naphthalenediimide (NDI)- and perylenediimide (PDI)-bithiophene (2T) donor/acceptor (D/A) copolymers are surprisingly similar despite cross-conjugation in the regioirregular structures. This result is traced back to largely localized frontier molecular orbitals (FMOs) as revealed by (time-dependent) DFT calculations. Interestingly, while the FMOs of the P(PDI-2T) copolymer are localized solely in the PDI units, they are predominantly localized in the respective D/A units of the P(NDI-2T) copolymer. The pronounced CT character of the lowest singlet state in P(NDI-2T) should give rise to a close lying CT triplet state, generating small singlet–triplet ga…
Fetus acardius: two- and three-dimensional ultrasonographic diagnoses.
2001
Objective. To determine whether ultrasonographic detection of acardiac fetuses and diagnostic accuracy of related malformations improve with complementary use of two-dimensional ultrasonography, three-dimensional ultrasonography, and Doppler scanning. Methods. Three pregnant women with multifetal gestations who were found to have discordant fetuses on initial two-dimensional ultrasonographic scanning were subsequently scanned with three-dimensional ultrasonography and color Doppler ultrasonography. Results. Although the possibility of acardiac fetuses was entertained in all cases after two-dimensional ultrasonographic scanning, the diagnosis was confirmed, and the accuracy and extent of fet…
Scope and Limitations of Baird's Theory on Triplet State Aromaticity: Application to the Tuning of Singlet–Triplet Energy Gaps in Fulvenes
2007
Utilizing Baird's theory on triplet state aromaticity, we show that the singlet-triplet energy gaps (DeltaE(ST)) of pentafulvenes are easily varied through substitution by as much as 36 kcal mol(-1). This exploits the fact that fulvenes act as aromatic chameleons in which the dipoles reverse on going from the singlet ground state (S(0)) to the lowest pipi* triplet state (T1); thus, their electron distributions are adapted so as to achieve some aromaticity in both states. The results are based on quantum chemical calculations with the OLYP density functional theory method and the CASPT2 ab initio method, as well as spectroscopic determination of DeltaE(ST) by triplet sensitization. The findi…
Role of vitreous matrix on the optical activity of Ge-doped silica
2003
Abstract We report an experimental study on the relationship between the optical activity of Ge-oxygen deficient centers and dynamic properties and conformational heterogeneity of vitreous matrix in silica. We focus our attention on the absorption band at ∼5.2 eV (B 2β ) and on the two related emissions at ∼4.2 eV (α E ) and at ∼3.1 eV (β). From the temperature dependence of B 2β band we estimate a mean energy value of 26 meV for local vibrational modes coupled to the electronic transition, suggesting that the chromophore and its surrounding have access to low frequency dynamics. From the thermal behavior of the two emissions we distinguish the two competitive relaxation processes from the …
A theory ofnonverticaltriplet energy transfer in terms of accurate potential energy surfaces: The transfer reaction from π,π* triplet donors to 1,3,5…
2004
Triplet energy transfer (TET) from aromatic donors to 1,3,5,7-cyclooctatetraene (COT) is an extreme case of "nonvertical" behavior, where the transfer rate for low-energy donors is considerably faster than that predicted for a thermally activated (Arrhenius) process. To explain the anomalous TET of COT and other molecules, a new theoretical model based on transition state theory for nonadiabatic processes is proposed here, which makes use of the adiabatic potential energy surfaces (PES) of reactants and products, as computed from high-level quantum mechanical methods, and a nonadiabatic transfer rate constant. It is shown that the rate of transfer depends on a geometrical distortion paramet…
LogDet divergence-based metric learning with triplet constraints and its applications.
2014
How to select and weigh features has always been a difficult problem in many image processing and pattern recognition applications. A data-dependent distance measure can address this problem to a certain extent, and therefore an accurate and efficient metric learning becomes necessary. In this paper, we propose a LogDet divergence-based metric learning with triplet constraints (LDMLT) approach, which can learn Mahalanobis distance metric accurately and efficiently. First of all, we demonstrate the good properties of triplet constraints and apply it in LogDet divergence-based metric learning model. Then, to deal with high-dimensional data, we apply a compressed representation method to learn…
Highly Stable and Efficient Light-Emitting Electrochemical Cells Based on Cationic Iridium Complexes Bearing Arylazole Ancillary Ligands.
2017
A series of bis-cyclometalated iridium(III) complexes of general formula [Ir(ppy)2(N∧N)][PF6] (ppy− = 2-phenylpyridinate; N∧N = 2-(1H-imidazol-2-yl)pyridine (1), 2-(2-pyridyl)benzimidazole (2), 1-methyl-2-pyridin-2-yl- 1H-benzimidazole (3), 2-(4′-thiazolyl)benzimidazole (4), 1- methyl-2-(4′-thiazolyl)benzimidazole (5)) is reported, and their use as electroluminescent materials in light-emitting electrochemical cell (LEC) devices is investigated. [2][PF6] and [3][PF6] are orange emitters with intense unstructured emission around 590 nm in acetonitrile solution. [1][PF6], [4][PF6], and [5][PF6] are green weak emitters with structured emission bands peaking around 500 nm. The different photoph…