A5013747717- Spectroscopy and Quantum Chemical Studies
- Advanced Chemical Physics Studies
- Quantum, superfluid, helium dynamics
- Organic Light-Emitting Diodes Research
- Organic Electronics and Photovoltaics
- Molecular Junctions and Nanostructures
- Photochemistry and Electron Transfer Studies
- Advanced Physical and Chemical Molecular Interactions
- Luminescence and Fluorescent Materials
- Perovskite Materials and Applications
- Conducting polymers and applications
- Cold Atom Physics and Bose-Einstein Condensates
- Advanced Thermodynamics and Statistical Mechanics
- Crystallization and Solubility Studies
- X-ray Diffraction in Crystallography
- Photochromic and Fluorescence Chemistry
- Analytical chemistry methods development
- Advanced Fluorescence Microscopy Techniques
- Machine Learning in Materials Science
- Coordination Chemistry and Organometallics
- Spacecraft and Cryogenic Technologies
- Nonlinear Optical Materials Research
- Electron Spin Resonance Studies
- Quantum Mechanics and Applications
- Electron and X-Ray Spectroscopy Techniques
University College London
2021-2024
Cornell University
2016-2022
University of Cambridge
2013-2021
Ithaca College
2016
We have designed a series of pentacene dimers separated by homoconjugated or nonconjugated bridges that exhibit fast and efficient intramolecular singlet exciton fission (iSF). These materials are distinctive among reported iSF compounds because they exist in the unexplored regime close spatial proximity but weak electronic coupling between triplet pair states. Using transient absorption spectroscopy to investigate photophysics these molecules, we find display desirable excited-state...
We show that a single change in the derivation of linearized semiclassical-initial value representation (LSC-IVR or "classical Wigner approximation") results classical dynamics which conserves quantum Boltzmann distribution. rederive (standard) LSC-IVR approach by writing (exact) time-correlation function terms normal modes free ring-polymer (i.e., discrete imaginary-time Feynman path), taking limit number polymer beads N → ∞, such lowest normal-mode frequencies take their "Matsubara"...
We recently obtained a quantum-Boltzmann-conserving classical dynamics by making single change to the derivation of "Classical Wigner" approximation. Here, we show that further approximation this "Matsubara dynamics" gives rise two popular heuristic methods for treating quantum Boltzmann time-correlation functions: centroid molecular (CMD) and ring-polymer (RPMD). CMD is mean-field Matsubara dynamics, discarding (classical) fluctuations around centroid, RPMD result term in Liouvillian which...
Surprisingly, there exists a quantum flux-side time-correlation function which has non-zero t → 0+ limit and thus yields rigorous generalization of classical transition-state theory (TST). In this Part I two articles, we introduce the new derive its limit. The ingredient is generalized Kubo transform allows flux side dividing surfaces to be same path-integral space. Choosing single point gives identical an expression introduced on heuristic grounds by Wigner in 1932; however, does not give...
Singlet fission in organic semiconductors causes a singlet exciton to decay into pair of triplet excitons and holds potential for increasing the efficiency photovoltaic devices. In this combined experimental theoretical study, we reveal that covalent dimer semiconductor tetracene undergoes activated by qualitatively different mechanisms depending on solvent environment. We show intramolecular vibrations are an integral part mechanism, giving rise mixing between charge transfer excitations....
Electrically injected charge carriers in organic light-emitting devices (OLEDs) undergo recombination events to form singlet and triplet states a 1:3 ratio, representing fundamental hurdle for achieving high quantum efficiency. Dopants based on thermally activated delayed fluorescence (TADF) have emerged as promising candidates addressing the spin statistics issue OLEDs. In these materials, reverse singlet-triplet intersystem crossing (rISC) becomes efficient, thereby activating luminescence...
Organic light-emitting diodes (OLEDs) with doublet-spin radical emitters have emerged as a new route to efficient display technologies. In contrast standard organic semiconductors, materials unpaired electrons. This feature results in the most well-known examples of radicals being where they are reactive species chemical reactions. Stabilized can be used optoelectronic applications, which exploit their optical and spin properties, allowing up 100% internal quantum efficiency (IQE) for...
Organic light-emitting diodes (OLEDs) must be engineered to circumvent the efficiency limit imposed by 3:1 ratio of triplet singlet exciton formation following electron-hole capture. Here we show spin nature luminescent radicals such as TTM-3PCz allows direct energy harvesting from both and excitons through transfer, with subsequent rapid efficient light emission doublet excitons. This is demonstrated a model Thermally-Activated Delayed Fluorescence (TADF) organic semiconductor, 4CzIPN,...
Clathrate hydrates hold considerable promise as safe and economical materials for hydrogen storage. Here we present a quantum mechanical study of H$_2$ D$_2$ diffusion through hexagonal face shared by two large cages clathrate over wide range temperatures. Path integral molecular dynamics simulations are used to compute the free-energy profiles function temperature. Ring polymer rate theory, incorporating both exact statistics approximate dynamical effects, is utilized in calculations rates...
In Part I [J. Chem. Phys. 138, 084108 (2013)] we derived a quantum transition-state theory by taking the t->0+ (short-time) limit of new form flux-side time-correlation function containing ring-polymer dividing surface. This appears to be unique in giving positive-definite Boltzmann statistics, and is identical molecular dynamics (RPMD) TST. Here, show that TST (i.e. RPMD-TST) exact if there no recrossing (by real-time dynamics) surface, nor any surface orthogonal it space describing...
Abstract Carbene‐metal‐amides (CMAs) are a promising family of donor–bridge–acceptor molecular charge‐transfer (CT) emitters for organic light‐emitting diodes. A universal approach is demonstrated to tune the energy their CT emission. blueshift up 210 meV achievable in solid state via dilution polar host matrix. The origin this shift has two components: constraint thermally‐activated triplet diffusion, and electrostatic interactions between guest host. This allows emission mid‐green CMA...
We derive an exact quantum propagator for nonadiabatic dynamics in multi-state systems using the mapping variable representation, where classical-like Cartesian variables are used to represent both continuous nuclear degrees of freedom and discrete electronic states. The resulting Liouvillian is a Moyal series that, when suitably approximated, can allow use classical efficiently model large systems. demonstrate that different truncations lead existing approximate semiclassical mixed...
Recent synthetic studies on the organic molecules tetracene and pentacene have found certain dimers oligomers to exhibit an intense absorption in visible region of spectrum which is not present monomer or many previously-studied dimers. In this article we combine experimental synthesis with electronic structure theory spectral computation show that arises from otherwise dark charge-transfer excitation 'borrowing intensity' UV excitation. Further, by characterizing role relevant molecular...
We investigate the mechanisms of condensed phase proton-coupled electron transfer (PCET) using Mapping-Variable Ring Polymer Molecular Dynamics (MV-RPMD), a recently developed method that employs an ensemble classical trajectories to simulate nonadiabatic excited state dynamics. Here, we construct series system-bath model Hamiltonians for PCET, where four localized electron-proton states are coupled thermal bath via single solvent mode, and employ MV-RPMD population Specifically, each model,...
It was shown recently that there exists a true quantum transition-state theory (QTST) corresponding to the t->0+ limit of (new form of) flux-side time-correlation function. Remarkably, this QTST is identical ring-polymer molecular dynamics (RPMD) TST. Here we provide evidence which suggests very strongly (= RPMD-TST) unique, in sense any other function gives either non-positive-definite statistics or zero. We introduce generalized includes all (known) functions as special limiting cases....
We obtain thermostatted ring polymer molecular dynamics (TRPMD) from exact quantum via Matsubara dynamics, a recently-derived form of linearization which conserves the Boltzmann distribution. Performing contour integral in complex distribution replacement imaginary Liouvillian results with Fokker-Planck term gives TRPMD. thereby provide error terms between TRPMD and predict systems they are likely to be small. Using harmonic analysis we show that careful addition friction causes correct...
The discovery of molecules with tailored optoelectronic properties, such as specific frequency and intensity absorption or emission, is a major challenge in creating next-generation organic light-emitting diodes (OLEDs) photovoltaics. This raises the following question: How can we predict potential chemical structure from these properties? Approaches that attempt to tackle this inverse design problem include virtual screening, active machine learning, genetic algorithms. However, approaches...
We extend the Mixed Quantum-Classical Initial Value Representation (MQC-IVR), a semiclassical method for computing real-time correlation functions, to electronically nonadiabatic systems using Meyer-Miller-Stock-Thoss (MMST) Hamiltonian treat electronic and nuclear degrees of freedom (dofs) within consistent dynamic framework. introduce an efficient symplectic integration scheme, MInt algorithm, numerical time-evolution phase space variables as well Monodromy matrix, under non-separable MMST...
The first page of this article is displayed as the abstract.
In a previous article [T. J. H. Hele and S. C. Althorpe, Chem. Phys. 138, 084108 (2013)], we showed that the t → 0+ limit of ring-polymer molecular dynamics (RPMD) rate-theory is also new type quantum flux-side time-correlation function, in which dividing surfaces are invariant to imaginary-time translation; other words, RPMD transition-state theory (RMPD-TST) (QTST). Recently, Jang Voth [J. 144, 084110 (2016)] rederived this claimed it gives instead centroid-density approximation. Here show...
The energies of molecular excited states arise as solutions to the electronic Schr\"{o}dinger equation and are often compared experiment. At same time, nuclear quantum motion is known be important induce a red-shift state energies. However, it thus far unclear whether incorporating in calculations leads systematic improvement their predictive accuracy, making further investigation necessary. Here we present such an by employing two first-principles methods for capturing effect fluctuations...
A common strategy to simulate mixed quantum-classical dynamics is by propagating classical trajectories with mapping variables, often using the Meyer-Miller-Stock-Thoss (MMST) Hamiltonian or related spin-mapping approach. When quantum subsystem, coupled reduce a set of equations motion integrate. Several numerical algorithms have been proposed, but thorough performance comparison appears be lacking. Here, we compare three time-propagation for MMST Hamiltonian: Momentum Integral (MInt) (
We apply Thermostatted Ring Polymer Molecular Dynamics (TRPMD), a recently proposed approximate quantum dynamics method, to the computation of thermal reaction rates. Its short-time transition-state theory limit is identical rigorous theory, and we find that its long-time independent location dividing surface. TRPMD rate then applied one-dimensional model systems, atom-diatom bimolecular reactions H + H2, D MuH, F prototypical polyatomic CH4. Above crossover temperature, virtually invariant...