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Results for “charge transfer state”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Metastable charge-transfer state of californium( iii ) compounds

Among a series of anomalous physical and chemical properties of Cf(III) compounds revealed by recent investigations, the present work addresses the characteristics of the optical spectra of An(HDPA) 3 ·H 2 O (An = Am, Cm, and Cf), especially the broadband photoluminescence from Cf(HDPA) 3 ·H 2 O induced by ligand-to-metal charge transfer (CT). As a result of strong ion–ligand interactions and the relative ease of reducing Cf(III) to Cf(II), a CT transition occurs at low energy (<3 eV) via the formation of a metastable Cf(II) state. It is shown that the systematic trend in CT transitions of the lanthanide series is not paralleled by actinide elements lighter than Cf(III), and californium represents a turning point in the periodicity of the actinide series. Analyses and modeling of the temperature-dependent luminescence dynamics indicate that the metastable Cf(II) charge-transfer state undergoes radiative and non-radiative relaxations. Broadening of the CT transition arises from strong vibronic coupling and hole–charge interactions in the valence band. As a result, the non-radiative relaxation of the metastable CT state results from a competition between phonon-relaxation and thermal tunneling that populates the excited states of Cf(III).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Heavily doped n-type PbSe and PbS nanocrystals using ground-state charge transfer from cobaltocene

Colloidal nanocrystals (NCs) of lead chalcogenides are a promising class of tunable infrared materials for applications in devices such as photodetectors and solar cells. Such devices typically employ electronic materials in which charge carrier concentrations are manipulated through “doping;” however, persistent electronic doping of these NCs remains a challenge. Here, we demonstrate that heavily doped n-type PbSe and PbS NCs can be realized utilizing ground-state electron transfer from cobaltocene. This allows injecting up to eight electrons per NC into the band-edge state and maintaining the doping level for at least a month at room temperature. Doping is confirmed by inter- and intra-band optical absorption, as well as by carrier dynamics. Finally, FET measurements of doped NC films and the demonstration of a p-n diode provide additional evidence that the developed doping procedure allows for persistent incorporation of electrons into the quantum-confined NC states.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Natural Charge-Transfer Analysis: Eliminating Spurious Charge-Transfer States in Time-Dependent Density Functional Theory via Diabatization, with Application to Projection-Based Embedding

For many types of vertical excitation energies, linear-response time-dependent density functional theory (LR-TDDFT) offers a useful degree of accuracy combined with unrivaled computational efficiency, although charge-transfer excitation energies are often systematically and dramatically underestimated, especially for large systems and those that contain explicit solvent. As a result, low energy electronic spectra of solution-phase chromophores often contain tens to hundreds of spurious charge-transfer states, making LR-TDDFT needlessly expensive in bulk solution. More nefariously, intensity borrowing by the low-energy charge-transfer states can affect intensities of the valence excitations even if those excitation energies are accurate. At higher excitation energies, it is difficult to distinguish spurious CT states from genuine charge-transfer-to-solvent (CTTS) excitations. In this work, we introduce an automated diabatization scheme that enables fast and effective screening of the CTTS acceptor space in bulk solution. Our procedure introduces the concept of “natural charge-transfer orbitals”, which provide a means to isolate characteristic pairs of orbitals that are most likely to participate in a CTTS excitation. The projection of these orbitals onto solvent-centered virtual orbitals provides a criterion for defining the most important solvent molecules in a given excitation. We apply this method to analyze an ab initio molecular dynamics (MD) trajectory of I-(aq) and report the lowest-energy CTTS band in the absorption spectrum. Our results are in excellent agreement with experimental measurements for bulk I-(aq), and only one-third of the water molecules in the I-(H2O)96 simulation cell need to be described with LR-TDDFT in order to obtain excitation energies that are converged to < 0.1 eV. The tools introduced herein will improve the accuracy, efficiency, and usability of LR-TDDFT in solution-phase environments

Carter-Fenk, Kevin D.↗
DOE OSTI2023

Excited-State Charge Transfer in Push–Pull Platinum(II) π-Extended Porphyrins Fused with Pentacenequinone

Platinum(II) π-extended porphyrins fused with pentacenequinone and dihydropentacene have been successfully synthesized. These porphyrins were investigated using various techniques including absorption, steady-state, and time-resolved phosphorescence spectroscopy and differential pulse voltammetry. UV–vis absorption spectra of pentacenequinone-fused porphyrins (SW-Pt1 and SW-Pt2) showed unusually broad and nontypical absorption patterns. Phosphorescence spectra of SW-Pt1, SW-Pt2, and SW-Pt3 displayed similar emissions in the 704–706 nm region indicating electronic transitions of similar origin; however, the triplet lifetimes were found to be quenched in the case of both SW-Pt1 and SW-Pt2, suggesting the occurrence of excited-state events. Facile reductions were obtained for both the pentacene-quinone-fused monomer (SW-Pt2) and dimer (SW-Pt1) and were identified to be located at the pentacenequinone components. Here, the observed orbital segregations for SW-Pt2 and SW-Pt1 from DFT calculations supported the possibility of charge transfer in these push–pull systems. Interestingly, the established energy level diagram revealed that the charge transfer from the triplet excited Pt porphyrin is thermodynamically an uphill process. Systematic studies involving both femtosecond and nanosecond transient absorption techniques revealed that the singlet excited Pt porphyrins undergo an intermediate charge transfer state prior to populating the triplet state, providing a plausible explanation for phosphorescence quenching. The lifetime of the intermediate charge transfer states was found to be 25.9 and 5.68 ps, respectively, for SW-Pt1 and SW-Pt2.

Charge transfer↗
DOE OSTI2026

Highly-destabilized ligand field excited states of iron carbene complexes and their relation to charge transfer state lifetimes

Lifetimes of photoexcited charge transfer (CT) states in transition metal chromophores are influenced by low-lying ligand field (LF) excited states, especially for 3d metal complexes. To manipulate interactions between LF and CT states, it is important to be able to control LF excited state energies using tunable synthetic variables. In this report, we use Fe 2p3d L 3 -edge resonant inelastic X-ray scattering (RIXS) to measure LF excited state energies of three homoleptic iron chromophores coordinated by strong-field N-heterocyclic carbenes (NHCs). We investigate the effect of oxidation state and ligand scaffold on LF energies and covalency parameters. A cyclometalated bis(NHC) ligand affords both high LF excited state energies (and thus high 10 Dq) as well as high metal–ligand covalency compared to other iron complexes with very strong-field ligands. However, for the set of complexes investigated, we do not observe meaningful correlation between the LF excited state energies and the CT excited state lifetimes. These results illustrate that targeting long-lived CT excited states necessitates control of multiple molecular excited state properties, with destabilization of the LF excited state energies proving necessary, but insufficient, to control the CT excited state lifetime in Fe carbene complexes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Influence of Disorder and State Filling on Charge-Transfer-State Absorption and Emission Spectra

We conduct comprehensive temperature-dependent measurements of the charge-transfer-(CT) state photocurrent and emission spectra for two organic small molecule donor:fullerene (C 60 ) acceptor bulk heterojunction solar cells. We reveal that the CT spectral width and position are affected by static energetic disorder in the blend, especially evident at low temperatures. The relative contributions of the static and dynamic disorder broadening in the CT spectra are effectively extracted through consideration of a Gaussian CT energetic distribution. However, electroluminescence (EL) spectra can only be interpreted when injected carriers reach thermal equilibrium sites within the disordered density of states and emission occurs from the lowest possible CT energy. For the blend with the smaller energetic disorder, this is the case near room temperature; for the other blend with larger static disorder, carriers fail to reach thermal equilibrium sites even at room temperature and EL spectra need to be interpreted with care. For example, in the latter case, the effect of energetic disorder might not be apparent from EL spectra because the lowest energy sites are not participating. Nonetheless, these states contribute to the photocurrent generation-recombination and energy-loss processes and thus demand accurate characterization, which we show is feasible through temperature-dependent external quantum-efficiency measurements.

14 SOLAR ENERGY↗
DOE OSTI2013

Magneto-Dielectric Effects Induced by Optically-Generated Intermolecular Charge-Transfer States in Organic Semiconducting Materials

Traditionally, magneto-dielectric effects have been developed by combining ferroelectric and magnetic materials. Here, we show a magneto-dielectric effect from optically-generated intermolecular charge-transfer states in an organic semiconducting donor:acceptor (PVK:TCNB) system. We observe in magnetic field effects of photoluminescence that a magnetic field can change singlet/triplet population ratio in intermolecular charge-transfer states. Furthermore, our theoretical analysis and experimental evidence indicate that the singlets and triplets in charge-transfer states have stronger and weaker electrical polarizations, respectively. Therefore, the observed magneto-dielectric effect can be attributed to magnetically-dependent singlet/triplet ratio in intermolecular charge-transfer states. In principle, a magneto-dielectric effect can be generated through two different channels based on magneto-polarization and magneto-current effects when the singlet/triplet ratio in intermolecular charge-transfer states is changed by a magnetic field. We find, from the simulation of dielectric effects, that magneto-polarization and magneto-current effects play primary and secondary roles in the generation of magneto-dielectric effect.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Predicting Excitation Energies of Twisted Intramolecular Charge-Transfer States with the Time-Dependent Density Functional Theory: Comparison with Experimental Measurements in the Gas Phase and Solvents Ranging from Hexanes to Acetonitrile

Electronically excited states characterized by intramolecular charge transfer play an essential role in many biological processes and optical devices. The ability to make quantitative ab initio predictions of the relative energetics involved is a challenging yet desirable goal, especially for large molecules in solution. In this work, we present a data set of 61 experimental measurements of absorption and emission processes, both in the gas phase and in solvents representing a broad range of polarities, which involve intramolecular charge transfer mediated by a nonzero, "twisted" dihedral angle between one or more donor and acceptor subunits. Among a variety of density functionals investigated within the framework of linear-response theory, the "optimally tuned" LRC-ωPBE functional, which utilizes a system-specific yet nonempirical procedure to specify the range-separation parameter, emerges as the preferred choice. For the entire set of excitation energies, involving changes in dipole moment ranging from 4 to >20 Debye, the mean signed and absolute errors are 0.02 and 0.18 eV, respectively (compared, e.g., to -0.30 and 0.30 for PBE0, 0.44 and 0.47 for LRC-ωPBEh, 0.83 and 0.83 for ωB97X-V). We analyze the performance of polarizable continuum solvation models available in Q-Chem that partition the solvent response into fast and slow time scales, and clear trends emerge when measurements corresponding to the four small 4-(dimethylamino)benzonitrile (DMABN)-like molecules and a charged species are excluded. We make the case that the large errors found only for small molecules in the gas phase and weak solvents cannot be expected to improve via the optimal tuning procedure, which enforces a condition that is exact only in the well-separated donor-acceptor limit, and present empirical evidence implicating the outsized importance for small donor-acceptor systems of relaxation effects that cannot be accounted for by the linear-response time-dependent density functional theory within the adiabatic approximation. Finally, we demonstrate the utility of the optimally tuned density functional approach by targeting the charge-transfer states of a large biomimetic model system for light-harvesting structures in Photosystem II.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Effect of ground-state charge transfer on photoexcited charge transfer in transition metal dichalcogenide heterostructures

We report an experimental investigation on the effect of ground-state charge transfer and its induced electric field on photoexcited charge transfer in van der Waals heterostructures. Two heterostructure samples were fabricated by stacking an undoped WSe 2 monolayer with either a Nb-doped or undoped MoSe 2 monolayer. While no ground-state charge transfer is expected in the MoSe 2 /WSe 2 heterostructure, the doped holes in the MoSe 2 :Nb/WSe 2 heterostructure can transfer to WSe 2 , creating a space-charge electric field. By comparing the photoluminescence and time-resolved differential reflectance of the two heterostructures, we find that photoexcited hole transfer from MoSe 2 to WSe 2 is largely blocked by this field, whereas photoexcited electron transfer from WSe 2 to MoSe 2 is less affected. Furthermore, these results provide insight into the impact of doping on the charge-transfer performance of van der Waals heterostructures.

2D materials↗
DOE OSTI2026

Chirality transfer from chiral perovskite to molecular dopants via charge transfer states

Chiral perovskites are semiconductors with broken mirror symmetries. Their photo responses are often constrained in the UV range. In this work, we demonstrate that doping 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane in the chiral perovskite matrix introduces a visible light absorption feature through the emerging charge-transfer electronic states. These charge-transfer states exhibits circular dichroism inherited from the chiral host, indicating effective chirality transfer from host to guest component via electronic coupling. Quantum-chemical modeling identifies a strong wave function overlap between an electron and a hole of the guest-host in a closely packed crystal configuration promoting the charge transfer state’s optical activity. We further integrate the doped chiral perovskite film into photodetectors and demonstrate a selective detection of circularly polarized light in both UV and visible regions. Our results suggest a universal approach of introducing visible photo absorption states to the chiral matrix to broaden the optical active range while enhancing the electrical conductivity.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Revealing the Full Charge Transfer State Absorption Spectrum of Organic Solar Cells

Anisotropic charge transfer (CT) state absorption originating from the natural transition dipole alignment at planar donor–acceptor interfaces can be exploited to reveal the full CT state absorption spectrum in organic solar cells. Application of this method reveals the existence of previously unobserved, higher-lying CT absorption buried beneath the excitonic background.

14 SOLAR ENERGY↗
DOE OSTI2024

Sub-100-fs energy transfer in coenzyme NADH is a coherent process assisted by a charge-transfer state

Excitation energy transfer (EET) is a key photoinduced process in biological chromophoric assemblies. Here we investigate the factors which can drive EET into efficient ultrafast sub-ps regimes. We demonstrate how a coherent transport of electronic population could facilitate this in water solvated NADH coenzyme and uncover the role of an intermediate dark charge-transfer state. High temporal resolution ultrafast optical spectroscopy gives a 54 ± 11 fs time constant for the EET process. Nonadiabatic quantum dynamical simulations computed through the time-evolution of multidimensional wavepackets suggest that the population transfer is mediated by photoexcited molecular vibrations due to strong coupling between the electronic states. The polar aqueous solvent environment leads to the active participation of a dark charge transfer state, accelerating the vibronically coherent EET process in favorably stacked conformers and solvent cavities. Our work demonstrates how the interplay of structural and environmental factors leads to diverse pathways for the EET process in flexible heterodimers and provides general insights relevant for coherent EET processes in stacked multichromophoric aggregates like DNA strands.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Molecular Excited States: Accurate Calculation of Relative Energies and Electronic Coupling Between Charge Transfer and Non-Charge Transfer States

We show for a series of six small donor–acceptor dyads that the energy difference between non-charge transfer (non-CT) and charge transfer (CT) excited states, as well as the squares of the electronic couplings between these states, can be predicted from first-principles using variational orbital adapted configuration interaction singles (VOA-CIS) theory. VOA-CIS correctly predicts the observed experimental trends in these values and provides quantitative accuracy roughly on par with a modern long-range corrected density functional, ωB97X. Using VOA-CIS and ωB97X, the experimental energy difference between the non-CT and CT excited states is predicted with root mean squared errors of 0.22 eV and 0.21 eV, respectively. The square of the electronic coupling between these states is predicted with root mean squared errors of 0.08 eV 2 and 0.07 eV 2 , respectively. Here, orbital optimized CIS (OO-CIS) and CIS(D), two perturbative corrections to CIS, provide a significant correction to the errant relative energies predicted by CIS, but the correction is insufficient to recover the experimentally observed trend.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Nitrile Vibration Reports Induced Electric Field and Delocalization of Electron in the Charge-Transfer State of Aryl Nitriles

An electric field is created upon photoinduced charge separation in electron donor–acceptor (D–A) molecules. The photophysics of a prototypical D–A molecule, 4-(dimethylamino)-benzonitrile (DMABN), has been under extensive investigation for decades. Here in this paper, by using the framework of the vibrational Stark effect (VSE), we show that the nitrile vibration quantifies a significant induced electric field in the intramolecular charge-transfer state of DMABN. We further demonstrate that such a phenomenon can be observed in a structurally similar aryl nitrile and that the VSE depends on solvent polarity due to dielectric screening. Finally, our current work shows how the superb sensitivity of the nitrile vibration can be used to identify the nature of electron delocalization and quantify the induced electric field in photoinduced charge transfer processes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗