Making Optical Excitations Visible - an Exciton Wavefunction Extension to the Time-dependent Configuration Interaction Method

Oligothiophene-Based Photovoltaic Materials for Organic Solar Cells: Exciton Properties by the CNDOL Fockian Approach

In this study, we apply the CNDOL/2SS approximate Fockian with the configuration interaction of singles (CIS) method to explore the excitonic properties of oligothiophene-based materials for organic solar cells. Our calculations of the excited states and charge density distributions of isolated chromophores and a donor-acceptor pair align closely with experimental data. The methodology used is reliable and useful for addressing complex donor-acceptor systems and their eventual design. The prediction of exciton binding energy using the Coulomb and exchange (ECE) term of the CIS energy transitions, combined with charge density difference maps to visualize the electronic structure of excitons, aids in distinguishing charge transfer states between multiple transitions present in the molecular aggregates representing the donor-acceptor pair. Our results indicate that the donor-acceptor blend Tz6T:eC9-4F exhibits strong low-energy light absorption and a state alignment that enables barrier-free charge transport. The sandwich-type arrangement of this pair reveals a charge transfer (CT) state characterized by low exciton binding energy (low ECE term), highlighting its potential for optimizing organic solar cell performance. In contrast, less-ordered arrangements of the donor-acceptor pair show CT states at higher energies, which may compete with other deactivation processes and reduce the efficiency. This study provides a cost-effective approach to predicting and interpreting the feasibility of charge transfer in molecular aggregate designs for solar cells.

Probing Electronic Symmetry Reduction during Charge Migration via Time-Resolved X-Ray Diffraction

The present work focuses on probing ultrafast charge migration after symmetry-breaking excitation using ultrashort laser pulses. LiCN is chosen as prototypical system because it can be oriented in the laboratory frame and it possesses optically-accessible charge transfer states at low energies. The charge migration is simulated within the hybrid time-dependent density functional theory/configuration interaction framework. Time-resolved electronic current densities and simulated time-resolved x-ray diffraction signals are used to unravel the mechanism of charge migration. Our simulations demonstrate that specific choices of laser polarization lead to a control over the symmetry of the induced charge migration. Moreover, time-resolved x-ray diffraction signals are shown to encode transient symmetry reduction at intermediate times.

Quantum-Compute Algorithm for Exact Laser-Driven Electron Dynamics in Molecules

In this work, we investigate the capability of known quantum computing algorithms for fault-tolerant quantum computing to simulate the laser-driven electron dynamics of excitation and ionization processes in small molecules such as lithium hydride, which can be benchmarked against the most accurate time-dependent full configuration interaction (TD-FCI) calculations. The conventional TD-FCI wave packet propagation is reproduced using the Jordan-Wigner transformation for wave function and operators and the Trotter product formula for expressing the propagator. In addition, the time-dependent dipole moment, as an example of a time-dependent expectation value, is calculated using the Hadamard test. To include non-Hermitian operators in the ionization dynamics, a similar approach to the quantum imaginary time evolution (QITE) algorithm is employed to translate the propagator, including a complex absorption potential, into quantum gates. The computations are executed on a quantum computer simulator. By construction, all quantum computer algorithms, except for the QITE algorithm used only for ionization but not for excitation dynamics, would scale polynomially on a quantum computer with fully entangled qubits. In contrast, TD-FCI scales exponentially. Hence, quantum computation holds promises for substantial progress in the understanding of electron dynamics of excitation processes in increasingly large molecular systems, as has already been witnessed in electronic structure theory.

Early dynamics of the emission of solvated electrons from nanodiamonds in water

Solvated electrons are among the most reductive species in an aqueous environment. Diamond materials have been proposed as a promising source of solvated electrons, but the underlying emission process in water remains elusive so far. Here, we show spectroscopic evidence for the emission of solvated electrons from detonation nanodiamonds upon excitation with both deep ultraviolet (225 nm) and visible (400 nm) light using ultrafast transient absorption. The crucial role of surface termination in the emission process is evidenced by comparing hydrogenated, hydroxylated and carboxylated nanodiamonds. In particular, a transient response that we attribute to solvated electrons is observed on hydrogenated nanodiamonds upon visible light excitation, while it shows a sub-ps recombination due to trap states when excited with deep ultraviolet light. The essential role of surface reconstructions on the nanodiamonds in these processes is proposed based on density functional theory calculations. These results open new perspectives for solar-driven emission of solvated electrons in an aqueous phase using nanodiamonds.