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Showing papers on "Excited state published in 2014"


Journal ArticleDOI
TL;DR: Strong but unconventional electron-hole interactions are expected to be ubiquitous in atomically thin materials using a microscopic theory in which the nonlocal nature of the effective dielectric screening modifies the functional form of the Coulomb interaction.
Abstract: We have experimentally determined the energies of the ground and first four excited excitonic states of the fundamental optical transition in monolayer ${\mathrm{WS}}_{2}$, a model system for the growing class of atomically thin two-dimensional semiconductor crystals. From the spectra, we establish a large exciton binding energy of 0.32 eV and a pronounced deviation from the usual hydrogenic Rydberg series of energy levels of the excitonic states. We explain both of these results using a microscopic theory in which the nonlocal nature of the effective dielectric screening modifies the functional form of the Coulomb interaction. These strong but unconventional electron-hole interactions are expected to be ubiquitous in atomically thin materials.

1,910 citations


Journal ArticleDOI
TL;DR: The result reveals significantly reduced and nonlocal dielectric screening of Coulomb interactions in 2D semiconductors and will have a significant impact on next-generation photonics and optoelectronics applications based on 2D atomic crystals.
Abstract: Exciton binding energy and excited states in monolayers of tungsten diselenide (WSe(2)) are investigated using the combined linear absorption and two-photon photoluminescence excitation spectroscopy. The exciton binding energy is determined to be 0.37 eV, which is about an order of magnitude larger than that in III-V semiconductor quantum wells and renders the exciton excited states observable even at room temperature. The exciton excitation spectrum with both experimentally determined one- and two-photon active states is distinct from the simple two-dimensional (2D) hydrogenic model. This result reveals significantly reduced and nonlocal dielectric screening of Coulomb interactions in 2D semiconductors. The observed large exciton binding energy will also have a significant impact on next-generation photonics and optoelectronics applications based on 2D atomic crystals.

1,044 citations


Journal ArticleDOI
TL;DR: Large normal-mode splitting between a magnetostatic mode (the Kittel mode) in a ferromagnetic sphere of yttrium iron garnet and a microwave cavity mode is demonstrated.
Abstract: We demonstrate large normal-mode splitting between a magnetostatic mode (the Kittel mode) in a ferromagnetic sphere of yttrium iron garnet and a microwave cavity mode. Strong coupling is achieved in the quantum regime where the average number of thermally or externally excited magnons and photons is less than one. We also confirm that the coupling strength is proportional to the square root of the number of spins. A nonmonotonic temperature dependence of the Kittel-mode linewidth is observed below 1 K and is attributed to the dissipation due to the coupling with a bath of two-level systems.

697 citations


Journal ArticleDOI
TL;DR: For a wide range of photovoltaic devices based on polymer:fullerene, small-molecule:C60 and polymer:polymer blends, the study reveals that the internal quantum efficiency is essentially independent of whether or not D, A or CT states with an energy higher than that of CT1 are excited.
Abstract: Interfaces between organic electron-donating (D) and electron-accepting (A) materials have the ability to generate charge carriers on illumination. Efficient organic solar cells require a high yield for this process, combined with a minimum of energy losses. Here, we investigate the role of the lowest energy emissive interfacial charge-transfer state (CT1) in the charge generation process. We measure the quantum yield and the electric field dependence of charge generation on excitation of the charge-transfer (CT) state manifold via weakly allowed, low-energy optical transitions. For a wide range of photovoltaic devices based on polymer:fullerene, small-molecule:C60 and polymer:polymer blends, our study reveals that the internal quantum efficiency (IQE) is essentially independent of whether or not D, A or CT states with an energy higher than that of CT1 are excited. The best materials systems show an IQE higher than 90% without the need for excess electronic or vibrational energy.

685 citations


Journal ArticleDOI
TL;DR: In this paper, a twisting donor-acceptor triphenylamine-thiadiazol molecule (TPA-NZP) exhibits fluorescent emission through a hybridized local and charge-transfer excited state (HLCT), which is demonstrated from both fluorescent solvatochromic experiment and quantum calculations.
Abstract: In principle, the ratio (Φ) of the maximum quantum efficiencies for electroluminescence (EL) to photoluminescence (PL) can be expected to approach unity, if the exciton (bound electron–hole pair) generated from the recombination of injected electrons and holes in OLEDs has a sufficiently weak binding energy. However, seldom are examples of Φ > 25% reported in OLEDs because of the strongly bound excitons for most organic semiconductors in nature. Here, a twisting donor–acceptor triphenylamine-thiadiazol molecule (TPA-NZP) exhibits fluorescent emission through a hybridized local and charge-transfer excited state (HLCT), which is demonstrated from both fluorescent solvatochromic experiment and quantum chemical calculations. The HLCT state possesses two combined and compatible characteristics: a large transition moment from a local excited (LE) state and a weakly bound exciton from a charge transfer (CT) state. The former contributes to a high-efficiency radiation of fluorescence, while the latter is responsible for the generation of a high fraction of singlet excitons. Using TPA-NZP as the light-emitting layer in an OLED, high Φ values of 93% (at low brightness) and 50% (at high brightness) are achieved, reflecting sufficient employment of the excitons in the OLED. Characterization of the EL device shows a saturated deep-red emission with CIE coordinates of (0.67, 0.32), accompanied by a rather excellent performance with a maximum luminance of 4574 cd m−2 and a maximum external quantum efficiency (ηext) of ∼2.8%. The HLCT state is a new way to realize high-efficiency of EL devices.

486 citations


Journal ArticleDOI
15 May 2014-Nature
TL;DR: It is demonstrated that femtosecond resolution X-ray fluorescence spectroscopy, with its sensitivity to spin state, can elucidate the spin crossover dynamics of [Fe(2,2′-bipyridine)3]2+ on photoinduced metal-to-ligand charge transfer excitation.
Abstract: Crucial to many light-driven processes in transition metal complexes is the absorption and dissipation of energy by 3d electrons. But a detailed understanding of such non-equilibrium excited-state dynamics and their interplay with structural changes is challenging: a multitude of excited states and possible transitions result in phenomena too complex to unravel when faced with the indirect sensitivity of optical spectroscopy to spin dynamics and the flux limitations of ultrafast X-ray sources. Such a situation exists for archetypal polypyridyl iron complexes, such as [Fe(2,2'-bipyridine)3](2+), where the excited-state charge and spin dynamics involved in the transition from a low- to a high-spin state (spin crossover) have long been a source of interest and controversy. Here we demonstrate that femtosecond resolution X-ray fluorescence spectroscopy, with its sensitivity to spin state, can elucidate the spin crossover dynamics of [Fe(2,2'-bipyridine)3](2+) on photoinduced metal-to-ligand charge transfer excitation. We are able to track the charge and spin dynamics, and establish the critical role of intermediate spin states in the crossover mechanism. We anticipate that these capabilities will make our method a valuable tool for mapping in unprecedented detail the fundamental electronic excited-state dynamics that underpin many useful light-triggered molecular phenomena involving 3d transition metal complexes.

372 citations


Journal ArticleDOI
TL;DR: In this paper, a detailed analysis of the gas temperature determination from rotational spectra is performed, and a large range of conditions for which non-equilibrium occurs are identified.
Abstract: The gas temperature in non-equilibrium plasmas is often obtained from the plasma-induced emission by measuring the rotational temperature of a diatomic molecule in its excited state. This is motivated by both tradition and the availability of low budget spectrometers. However, non-thermal plasmas do not automatically guarantee that the rotational distribution in the monitored vibrational level of the diatomic molecule is in equilibrium with the translational (gas) temperature. Often non-Boltzmann rotational molecular spectra are found in non-equilibrium plasmas. The deduction of a gas temperature from these non-thermal distributions must be done with care as clearly the equilibrium between translational and rotational degrees of freedom cannot be achieved. In this contribution different methods and approaches to determine the gas temperature are evaluated and discussed. A detailed analysis of the gas temperature determination from rotational spectra is performed. The physical and chemical background of non-equilibrium rotational population distributions in molecular spectra is discussed and a large range of conditions for which non-equilibrium occurs are identified. Fitting procedures which are used to fit (non-equilibrium) rotational distributions are analyzed in detail. Lastly, recommendations concerning the conditions for which the gas temperatures can be obtained from diatomic spectra are formulated.

366 citations


Journal ArticleDOI
TL;DR: In this article, a zero-dimensional kinetic model of CO2 splitting in non-equilibrium plasmas is presented, which includes a description of the CO2 vibrational kinetics (25 vibrational levels up to the dissociation limit of the molecule), taking into account state specific VT and VV relaxation reactions and the effect of vibrational excitation on other chemical reactions.
Abstract: We present a zero-dimensional kinetic model of CO2 splitting in non-equilibrium plasmas. The model includes a description of the CO2 vibrational kinetics (25 vibrational levels up to the dissociation limit of the molecule), taking into account state-specific VT and VV relaxation reactions and the effect of vibrational excitation on other chemical reactions. The model is applied to study the reaction kinetics of CO2 splitting in an atmospheric-pressure dielectric barrier discharge (DBD) and in a moderate-pressure microwave discharge. The model results are in qualitative agreement with published experimental works. We show that the CO2 conversion and its energy efficiency are very different in these two types of discharges, which reflects the important dissociation mechanisms involved. In the microwave discharge, excitation of the vibrational levels promotes efficient dissociation when the specific energy input is higher than a critical value (2.0 eV/molecule under the conditions examined). The calculated energy efficiency of the process has a maximum of 23%. In the DBD, vibrationally excited levels do not contribute significantly to the dissociation of CO2 and the calculated energy efficiency of the process is much lower (5%).

340 citations


Journal ArticleDOI
TL;DR: In this paper, a series of twisting donor-acceptor (D-A) molecules are designed and synthesized, and their HLCT state characters are verified by both fluorescent solvatochromic experiments and quantum chemical calculations.
Abstract: For a donor–acceptor (D–A) molecule, there are three possible cases for its low-lying excited state (S1): a π–π* state (a localized electronic state), a charge-transfer (CT) state (a delocalized electronic state), and a mixed or hybridized state of π–π* and CT (named here as the hybridized local and charge transfer (HLCT) state). The HLCT state is an important excited state for the design of next-generation organic light-emitting diode (OLED) materials with both high photoluminescence (PL) efficiency and a large fraction of singlet exciton generation in electroluminescence (EL). According to the principle of state mixing in quantum chemistry, a series of twisting D–A molecules are designed and synthesized, and their HLCT state characters are verified by both fluorescent solvatochromic experiments and quantum chemical calculations. The CT components in the HLCT state, which greatly affect the molecular optical properties, are found to be enhanced with a decrease of the twist angle of the D–A segment or an increase of the D–A intensity in these twisting D–A molecules. In OLEDs, using these HLCT compounds as the emitting layer, the maximum exciton utilization efficiency is harvested up to 93%. Surprisingly, an exception of Kasha's rule is revealed in some HLCT compounds: restricted internal-conversion (IC) from the high-lying triplet state (T2) to the low-lying triplet T1, and a reopened path of reverse intersystem crossing (RISC) from T2 to S1 or S2, based on the analysis of the excited-state energy levels and the measurement of the low-temperature spectrum. RISC from T2 to S1 (S2) as a “hot exciton” channel is believed to contribute to the large proportion of the radiative singlet excitons.

321 citations


Journal ArticleDOI
TL;DR: The optical transistor is used to demonstrate the nondestructive detection of a single Rydberg atom with a fidelity of 0.72(4) and a record switch contrast of 40% for a coherent gate input with mean photon number one.
Abstract: Researchers have used interactions between highly excited atoms to make an optical transistor that can be activated by a single photon.

313 citations


Journal ArticleDOI
TL;DR: In this paper, the authors introduce the concept of collective electron pairs (CEP) through a unitary transformation of electron pairs, which achieves bosonic commutation relations at the dilute limit, being able to accumulate many of them at a single quantum state.
Abstract: Among quantum phenomena in solids at low temperatures, the superconductivity and Bose–Einstein condensation (BEC) are representatives of arising from coherent macroscopic quantum states. In this article, we discuss possible correlations between these two phenomena. It is well known that the Cooper pairs are not true bosons and then, we introduce the concept of collective electron pairs (CEP) through a unitary transformation of electron pairs. The CEP accomplish bosonic commutation relations at the dilute limit, being able to accumulate many of them at a single quantum state, in contrast to the standard Cooper pairs. An exact solution of all single CEP eigenstates is found by means of the Richardson’s equation within a multishell model. The obtained energy spectrum is used to determine the BEC temperature of CEP. In addition, we present an alternative approach to calculate the superconducting critical temperature by using the BEC formalism for a system composed by ground-state CEP, excited pairs and unpaired electrons.

Journal ArticleDOI
TL;DR: In this article, a D-A charge transfer molecule in the solid state, can emit not only via an intramolecular charge transfer (ICT) excited state, but also from exciplex states, formed between the molecule and the host material.
Abstract: New emitters that can harvest both singlet and triplet excited states to give 100% internal conversion of charge into light, are required to replace Ir based phosphors in organic light emitting diodes (OLEDs). Molecules that have a charge transfer (CT) excited state can potentially achieve this through the mechanism of thermally activated delayed fluorescence (TADF). Here, it is shown that a D–A charge transfer molecule in the solid state, can emit not only via an intramolecular charge transfer (ICT) excited state, but also from exciplex states, formed between the molecule and the host material. OLEDs based on a previously studied D–A–D molecule in a host TAPC achieves >14% external electroluminescence yield and shows nearly 100% efficient triplet harvesting. In these devices, it is unambiguously established that the triplet states are harvested via TADF, but more interestingly, these results are found to be independent of whether the emitter is the ICT state or the D–A–D/host exciplex.

Journal ArticleDOI
TL;DR: In this article, the authors proposed an approach for the measurement of electric fields based on the interaction of radio frequency (RF) fields with alkali atoms excited to Rydberg states via the Autler-Townes effect and detect the splitting via electromagnetically induced transparency.
Abstract: We discuss a fundamentally new approach for the measurement of electric fields that will lead to the develop- mentofabroadband,directSI-traceable,compact,self-calibrating -field probe (sensor). This approach is based on the interaction of radio frequency (RF) fields with alkali atoms excited to Rydberg states. The RF field causes an energy splitting of the Rydberg states via the Autler-Townes effect and we detect the splitting via electromagnetically induced transparency. In effect, alkali atoms placed in a vapor cell act like an RF-to-optical transducer, converting an RF -field strength measurement to an optical frequency measurement. We demonstrate the broadband nature of this approach by showing that one small vapor cell can be used to measure -field strengths over a wide range of frequencies: 1 GHz to 500 GHz. The technique is validated by comparing experimental data to both numerical simulations and far-field calculations for various frequencies. We also discuss various applications, including: a direct traceable measurement, the ability to measure both weak and strong field strengths, compact form factors of the probe, and sub-wavelength imaging and field mapping. Index Terms—Atom based metrology, Autler-Townes splitting, broadband sensor and probe, electrical field measurements and sensor, electromagnetically induced transparency (EIT), Rydberg atoms, sub-wavelength imaging.

Journal ArticleDOI
TL;DR: This review discusses how quantum coherence manifests in photosynthetic light harvesting and its implications, and examines the concept of an exciton, an excited electronic state delocalized over several spatially separated molecules, which is the most widely available signature of Quantum coherence in light harvesting.
Abstract: Photosynthesis begins with light harvesting, where specialized pigment–protein complexes transform sunlight into electronic excitations delivered to reaction centres to initiate charge separation. There is evidence that quantum coherence between electronic excited states plays a role in energy transfer. In this review, we discuss how quantum coherence manifests in photosynthetic light harvesting and its implications. We begin by examining the concept of an exciton, an excited electronic state delocalized over several spatially separated molecules, which is the most widely available signature of quantum coherence in light harvesting. We then discuss recent results concerning the possibility that quantum coherence between electronically excited states of donors and acceptors may give rise to a quantum coherent evolution of excitations, modifying the traditional incoherent picture of energy transfer. Key to this (partially) coherent energy transfer appears to be the structure of the environment, in particular the participation of non-equilibrium vibrational modes. We discuss the open questions and controversies regarding quantum coherent energy transfer and how these can be addressed using new experimental techniques.

Journal ArticleDOI
16 Oct 2014-Nature
TL;DR: The existence of Rydberg excitons in the copper oxide Cu2O, with principal quantum numbers as large as n = 25, is demonstrated, which may allow the formation of ordered exciton phases or the sensing of elementary excitations in their surroundings on a quantum level.
Abstract: Rydberg excitons (condensed-matter analogues of hydrogen atoms) are shown to exist in single-crystal copper oxide with principal quantum numbers as large as n = 25 and giant wavefunctions with extensions of around two micrometres; this has implications for research in condensed-matter optics. Excitons, electron–hole pairs that play an essential role in the optical properties of semiconductors, can be viewed as condensed-matter analogues of hydrogen atoms, with a similar excitation spectrum. Dietmar Frohlich and colleagues extend the series of excitations from the previous record of principal quantum number n = 12, to n = 25 for excitons in single crystal cuprous oxide. At such high quantum numbers, the wave function of the excitons becomes giant, around 2 micrometres, and it is expected that these giant excitons (also called Rydberg excitons) strongly interact with each other. The authors observe evidence for a blockade effect where the presence of an exciton prevents excitation of another exciton in its vicinity. This work opens new research directions for optics in condensed matter. A highly excited atom having an electron that has moved into a level with large principal quantum number is a hydrogen-like object, termed a Rydberg atom. The giant size of Rydberg atoms1 leads to huge interaction effects. Monitoring these interactions has provided insights into atomic and molecular physics on the single-quantum level. Excitons—the fundamental optical excitations in semiconductors2, consisting of an electron and a positively charged hole—are the condensed-matter analogues of hydrogen. Highly excited excitons with extensions similar to those of Rydberg atoms are of interest because they can be placed and moved in a crystal with high precision using microscopic energy potential landscapes. The interaction of such Rydberg excitons may allow the formation of ordered exciton phases or the sensing of elementary excitations in their surroundings on a quantum level. Here we demonstrate the existence of Rydberg excitons in the copper oxide Cu2O, with principal quantum numbers as large as n = 25. These states have giant wavefunction extensions (that is, the average distance between the electron and the hole) of more than two micrometres, compared to about a nanometre for the ground state. The strong dipole–dipole interaction between such excitons is indicated by a blockade effect in which the presence of one exciton prevents the excitation of another in its vicinity.

Journal ArticleDOI
18 Dec 2014-Nature
TL;DR: It is shown that a correlated two-electron wave packet can be reconstructed from a 1.2-femtosecond quantum beat among low-lying doubly excited states in helium, and multidimensional spectroscopy experiments of the type reported here will provide benchmark data for testing fundamental few-body quantum dynamics theory in more complex systems.
Abstract: The concerted motion of two or more bound electrons governs atomic and molecular non-equilibrium processes including chemical reactions, and hence there is much interest in developing a detailed understanding of such electron dynamics in the quantum regime. However, there is no exact solution for the quantum three-body problem, and as a result even the minimal system of two active electrons and a nucleus is analytically intractable. This makes experimental measurements of the dynamics of two bound and correlated electrons, as found in the helium atom, an attractive prospect. However, although the motion of single active electrons and holes has been observed with attosecond time resolution, comparable experiments on two-electron motion have so far remained out of reach. Here we show that a correlated two-electron wave packet can be reconstructed from a 1.2-femtosecond quantum beat among low-lying doubly excited states in helium. The beat appears in attosecond transient-absorption spectra measured with unprecedentedly high spectral resolution and in the presence of an intensity-tunable visible laser field. We tune the coupling between the two low-lying quantum states by adjusting the visible laser intensity, and use the Fano resonance as a phase-sensitive quantum interferometer to achieve coherent control of the two correlated electrons. Given the excellent agreement with large-scale quantum-mechanical calculations for the helium atom, we anticipate that multidimensional spectroscopy experiments of the type we report here will provide benchmark data for testing fundamental few-body quantum dynamics theory in more complex systems. They might also provide a route to the site-specific measurement and control of metastable electronic transition states that are at the heart of fundamental chemical reactions.

Journal ArticleDOI
TL;DR: Study of the ground-state and finite-density optical response of molybdenum disulfide by solving the semiconductor Bloch equations, using ab initio band structures and Coulomb interaction matrix elements reveals a redshift of the excitonic ground- state absorption.
Abstract: We study the ground-state and finite-density optical response of molybdenum disulfide by solving the semiconductor Bloch equations, using ab initio band structures and Coulomb interaction matrix elements. Spectra for excited carrier densities up to 1013 cm–2 reveal a redshift of the excitonic ground-state absorption, whereas higher excitonic lines are found to disappear successively due to Coulomb-induced band gap shrinkage of more than 500 meV and binding-energy reduction. Strain-induced band variations lead to a redshift of the lowest exciton line by ∼110 meV/% and change the direct transition to indirect while maintaining the magnitude of the optical response.

Journal ArticleDOI
TL;DR: Gold clusters, Au18GSH14, were found to have the highest potential as a photosensitizer on the basis of the quantum yield of electron transfer and good visible light absorption properties.
Abstract: Glutathione-protected gold clusters exhibit size-dependent excited state and electron transfer properties. Larger-size clusters (e.g., Au25GSH18) with core-metal atoms display rapid (<1 ps) as well as slower relaxation (∼200 ns) while homoleptic clusters (e.g., Au10–12GSH10–12) exhibit only slower relaxation. These decay components have been identified as metal–metal transition and ligand-to-metal charge transfer, respectively. The short lifetime relaxation component becomes less dominant as the size of the gold cluster decreases. The long-lived excited state and ability to participate in electron transfer are integral for these clusters to serve as light-harvesting antennae. A strong correlation between the ligand-to-metal charge-transfer excited state lifetime and photocatalytic activity was evidenced from the electron transfer to methyl viologen. The photoactivity of these metal clusters shows increasing photocatalytic reduction yield (0.05–0.14) with decreasing cluster size, Au25 < Au18 < Au15 < Au10–...

Journal ArticleDOI
TL;DR: In this article, a material containing a phenothiazine (PTZ) electron donor unit and 2,4,6-triphenyl-1,3,5-triazine (TRZ) acceptor unit, PTZ-TRZ, which exhibits thermally activated delayed fluorescence (TADF) was developed.
Abstract: A material containing a phenothiazine (PTZ) electron donor unit and 2,4,6-triphenyl-1,3,5-triazine (TRZ) electron acceptor unit, PTZ-TRZ, which exhibits thermally activated delayed fluorescence (TADF) was developed. Density functional theory calculations revealed the existence of two ground-state conformers with different energy gaps between the lowest singlet excited state and lowest triplet excited state (1.14 and 0.18 eV), which resulted from the distortion of PTZ, as confirmed by X-ray structure analysis. PTZ-TRZ in toluene solution showed two broad, structureless emissions, confirming the existence of two different excited states. From detailed analyses of the absorption and photoluminescence spectra, we determined that both emissions were intramolecular charge-transfer (ICT) fluorescence. Therefore, the excited-state conformers of PTZ-TRZ resulted in dual ICT fluorescence. Because previously reported dual fluorescence from single molecules involves locally excited and ICT fluorescence, the dual ICT ...

Journal ArticleDOI
TL;DR: In this paper, the density matrix renormalization group (DMRG) theory and its symmetrization scheme for quantum chemistry applied to calculate the excited states structure was used to understand the nature of the low-lying excited state structure.

Journal ArticleDOI
04 Sep 2014-ACS Nano
TL;DR: A RRS study of samples of WSe2 with one, two, and three layers, as well as bulk 2H-WSe2, using up to 20 different laser lines covering the visible range shows that Raman enhancement is much stronger for the excited A' and B' states.
Abstract: Resonant Raman spectroscopy (RRS) is a very useful tool to study physical properties of materials since it provides information about excitons and their coupling with phonons. We present in this work a RRS study of samples of WSe2 with one, two, and three layers (1L, 2L, and 3L), as well as bulk 2H-WSe2, using up to 20 different laser lines covering the visible range. The first- and second-order Raman features exhibit different resonant behavior, in agreement with the double (and triple) resonance mechanism(s). From the laser energy dependence of the Raman intensities (Raman excitation profile, or REP), we obtained the energies of the excited excitonic states and their dependence with the number of atomic layers. Our results show that Raman enhancement is much stronger for the excited A' and B' states, and this result is ascribed to the different exciton-phonon coupling with fundamental and excited excitonic states.

Journal ArticleDOI
TL;DR: It was found that the emission of 2 at ambient temperature represents a thermally activated delayed fluorescence (TADF) which renders the compound to be a good candidate for singlet harvesting in OLEDs and a reduction of nonradiative deactivation and thus an increase of emission quantum yield.
Abstract: The complexes [Cu(I)(POP)(dmbpy)][BF4] (1) and [Cu(I)(POP)(tmbpy)][BF4] (2) (dmbpy = 4,4′-dimethyl-2,2′-bipyridyl; tmbpy = 4,4′,6,6′-tetramethyl-2,2′-bipyridyl; POP = bis[2-(diphenylphosphino)-phenyl]ether) have been studied in a wide temperature range by steady-state and time-resolved emission spectroscopy in fluid solution, frozen solution, and as solid powders. Emission quantum yields of up to 74% were observed for 2 in a rigid matrix (powder), substantially higher than for 1 of around 9% under the same conditions. Importantly, it was found that the emission of 2 at ambient temperature represents a thermally activated delayed fluorescence (TADF) which renders the compound to be a good candidate for singlet harvesting in OLEDs. The role of steric constraints within the complexes, in particular their influences on the emission quantum yields, were investigated by hybrid-DFT calculations for the excited triplet state of 1 and 2 while manipulating the torsion angle between the bipyridyl and POP ligands. Bo...

Journal ArticleDOI
TL;DR: By defining an oscillator strength measure of the coherent population of the multiexcitonic diabat, essential to singlet fission, it is found this population can, in principle, be increased by small compression along a specific crystal direction.
Abstract: We present a detailed study of pentacene monomer and dimer that serves to reconcile extant views of its singlet fission. We obtain the correct ordering of singlet excited- state energy levels in a pentacene molecule (E (S1 )< E (D)) from multireference calculations with an appropriate active orbital space and dynamical correlation being incorporated. In order to understand the mechanism of singlet fission in pentacene, we use a well-developed diabatization scheme to characterize the six low-lying singlet states of a pentacene dimer that approximates the unit cell structure of crystalline pentacene. The local, single-excitonic diabats are not directly coupled with the important multiexcitonic state but rather mix through their mutual couplings with one of the charge-transfer configurations. We analyze the mixing of diabats as a function of monomer separation and pentacene rotation. By defining an oscillator strength measure of the coherent population of the multiexcitonic diabat, essential to singlet fission, we find this population can, in principle, be increased by small compression along a specific crystal direction.

Journal ArticleDOI
TL;DR: A new dicalcium silicate phosphor, Ca(2-x)Eu(x)SiO4, which emits red light in response to blue-light excitation, is reported, which is promising materials for next-generation, white-light-emitting diode applications.
Abstract: We report a new dicalcium silicate phosphor, Ca2−xEuxSiO4, which emits red light in response to blue-light excitation. When excited at 450 nm, deep-red emission at 650 nm was clearly observed in Ca1.2Eu0.8SiO4, the external and internal quantum efficiencies of which were 44 % and 50 %, respectively. The red emission from Ca2−xEuxSiO4 was strongly related to the peculiar coordination environments of Eu2+ in two types of Ca sites. The red-emitting Ca2SiO4:Eu2+ phosphors are promising materials for next-generation, white-light-emitting diode applications.

Journal ArticleDOI
TL;DR: In this article, the size-dependent excited state optical properties of Ag2S QDs are systematically investigated by photoluminescence (PL), PL excitation (PLE), and time-resolved PL spectroscopy.
Abstract: Ag2S quantum dots (QDs) have attracted increasing attention due to their appealing optical properties in the near-infrared regime. However, a full understanding of the quantum confinement effect of Ag2S QDs has not been achieved so far. Herein, for the first time, the size-dependent excited state optical properties of Ag2S QDs are systematically investigated by photoluminescence (PL), PL excitation (PLE), and time-resolved PL spectroscopy. Experimentally, we determine the exciton Bohr radius of Ag2S QDs as 2.2 nm, which is highly consistent with theoretical results.

Journal ArticleDOI
TL;DR: This work extends previous work on singlet exciton fission in isolated dimers to the case of crystalline materials, focusing on pentacene as a canonical and concrete example, demonstrating significant charge-transfer character in the low-lying excited states.
Abstract: We extend our previous work on singlet exciton fission in isolated dimers to the case of crystalline materials, focusing on pentacene as a canonical and concrete example. We discuss the proper interpretation of the character of low-lying excited states of relevance to singlet fission. In particular, we consider a variety of metrics for measuring charge-transfer character, conclusively demonstrating significant charge-transfer character in the low-lying excited states. The impact of this electronic structure on the subsequent singlet fission dynamics is assessed by performing real-time master-equation calculations involving hundreds of quantum states. We make direct comparisons with experimental absorption spectra and singlet fission rates, finding good quantitative agreement in both cases, and we discuss the mechanistic distinctions that exist between small isolated aggregates and bulk systems.

Journal ArticleDOI
TL;DR: In this paper, phonon induced electronic dynamics in the ground and excited states of the negatively charged silicon-vacancy ($\mathrm{SiV}^-$) centre in diamond were investigated for the temperature range 4-350 K.
Abstract: We investigate phonon induced electronic dynamics in the ground and excited states of the negatively charged silicon-vacancy ($\mathrm{SiV}^-$) centre in diamond. Optical transition line widths, transition wavelength and excited state lifetimes are measured for the temperature range 4-350 K. The ground state orbital relaxation rates are measured using time-resolved fluorescence techniques. A microscopic model of the thermal broadening in the excited and ground states of the $\mathrm{SiV}^-$ centre is developed. A vibronic process involving single-phonon transitions is found to determine orbital relaxation rates for both the ground and the excited states at cryogenic temperatures. We discuss the implications of our findings for coherence of qubit states in the ground states and propose methods to extend coherence times of $\mathrm{SiV}^-$ qubits.

Journal ArticleDOI
TL;DR: 2D electronic–vibrational spectroscopy is developed, capable of correlating the electronic and vibrational degrees of freedom, and applied to the study of the 4-(di-cyanomethylene)-2-methyl-6-p-(dimethylamino)styryl-4H-pyran (DCM) laser dye in deuterated dimethyl sulfoxide and its excited state relaxation pathways.
Abstract: Multidimensional nonlinear spectroscopy, in the electronic and vibrational regimes, has reached maturity. To date, no experimental technique has combined the advantages of 2D electronic spectroscopy and 2D infrared spectroscopy, monitoring the evolution of the electronic and nuclear degrees of freedom simultaneously. The interplay and coupling between the electronic state and vibrational manifold is fundamental to understanding ensuing nonradiative pathways, especially those that involve conical intersections. We have developed a new experimental technique that is capable of correlating the electronic and vibrational degrees of freedom: 2D electronic–vibrational spectroscopy (2D-EV). We apply this new technique to the study of the 4-(di-cyanomethylene)-2-methyl-6-p-(dimethylamino)styryl-4H-pyran (DCM) laser dye in deuterated dimethyl sulfoxide and its excited state relaxation pathways. From 2D-EV spectra, we elucidate a ballistic mechanism on the excited state potential energy surface whereby molecules are almost instantaneously projected uphill in energy toward a transition state between locally excited and charge-transfer states, as evidenced by a rapid blue shift on the electronic axis of our 2D-EV spectra. The change in minimum energy structure in this excited state nonradiative crossing is evident as the central frequency of a specific vibrational mode changes on a many-picoseconds timescale. The underlying electronic dynamics, which occur on the hundreds of femtoseconds timescale, drive the far slower ensuing nuclear motions on the excited state potential surface, and serve as a excellent illustration for the unprecedented detail that 2D-EV will afford to photochemical reaction dynamics.

Journal ArticleDOI
TL;DR: In the comparison with several other methods previously used for dynamics simulations of adenine, ADC(2) has the best performance, providing the most consistent results so far, and TDDFT based on a long-range corrected functional fails to predict the ultrafast deactivation.
Abstract: Surface hopping dynamics methods using the coupled cluster to approximated second order (CC2), the algebraic diagrammatic construction scheme to second order (ADC(2)), and the time-dependent density functional theory (TDDFT) were developed and implemented into the program system Newton-X. These procedures are especially well-suited to simulate nonadiabatic processes involving various excited states of the same multiplicity and the dynamics in the first excited state toward an energetic minimum or up to the region where a crossing with the ground state is found. 9H-adenine in the gas phase was selected as the test case. The results showed that dynamics with ADC(2) is very stable, whereas CC2 dynamics fails within 100 fs, because of numerical instabilities present in the case of quasi-degenerate excited states. ADC(2) dynamics correctly predicts the ultrafast character of the deactivation process. It predicts that C2-puckered conical intersections should be the preferential pathway for internal conversion f...

Journal ArticleDOI
19 Dec 2014-Science
TL;DR: It is demonstrated how the outcome of light-induced ET can be radically altered by mode-specific infrared (IR) excitation of vibrations that are coupled to the ET pathway, effectively switching a charge separation pathway off.
Abstract: Electron transfer (ET) from donor to acceptor is often mediated by nuclear-electronic (vibronic) interactions in molecular bridges. Using an ultrafast electronic-vibrational-vibrational pulse-sequence, we demonstrate how the outcome of light-induced ET can be radically altered by mode-specific infrared (IR) excitation of vibrations that are coupled to the ET pathway. Picosecond narrow-band IR excitation of high-frequency bridge vibrations in an electronically excited covalent trans-acetylide platinum(II) donor-bridge-acceptor system in solution alters both the dynamics and the yields of competing ET pathways, completely switching a charge separation pathway off. These results offer a step toward quantum control of chemical reactivity by IR excitation.