Transition-Based Constrained DFT for the Robust and Reliable Treatment of Excitations in Supramolecular Systems

Highly Accurate and Robust Constraint-Based Orbital-Optimized Core Excitations

We adapt our recently developed constraint-based orbital-optimized excited-state method (COOX) for the computation of core excitations. COOX is a constrained density functional theory (cDFT) approach based on excitation amplitudes from linear-response time-dependent DFT (LR-TDDFT), and has been shown to provide accurate excitation energies and excited-state properties for valence excitations within a spin-restricted formalism. To extend COOX to core-excited states, we introduce a spin-unrestricted variant which allows us to obtain orbital-optimized core excitations with a single constraint. Using a triplet purification scheme in combination with the constrained unrestricted Hartree-Fock formalism, scalar-relativistic zero-order regular approximation corrections, and a semiempirical treatment of spin-orbit coupling, COOX is shown to produce highly accurate results for K- and L-edge excitations of second- and third-period atoms with subelectronvolt errors despite being based on LR-TDDFT, for which core excitations pose a well-known challenge. L- and M-edge excitations of heavier atoms up to uranium are also computationally feasible and numerically stable, but may require more advanced treatment of relativistic effects. Furthermore, COOX is shown to perform on par with or better than the popular ΔSCF approach while exhibiting more robust convergence, highlighting it as a promising tool for inexpensive and accurate simulations of X-ray absorption spectra.

A Constraint-Based Orbital-Optimized Excited State Method (COOX)

In this work, we present a novel method to directly calculate targeted electronic excited states within a self-consistent field calculation based on constrained density functional theory (cDFT). The constraint is constructed from the static occupied-occupied and virtual-virtual parts of the excited state difference density from (simplified) linear-response time-dependent density functional theory calculations (LR-TDDFT). Our new method shows a stable convergence behavior, provides an accurate excited state density adhering to the Aufbau principle, and can be solved within a restricted SCF for singlet excitations to avoid spin contamination. This also allows the straightforward application of post-SCF electron-correlation methods like MP2 or direct RPA methods. We present the details of our constraint-based orbital-optimized excited state method (COOX) and compare it to similar schemes. The accuracy of excitation energies will be analyzed for a benchmark of systems, while the quality of the resulting excited state densities is investigated by evaluating excited state nuclear forces and excited state structure optimizations. We also investigate the performance of the proposed COOX method for long-range charge transfer excitations and conical intersections with the ground-state.

Sulfur Oxidation State and Substituents Influenced Multifunctional Organic Luminophores in BTP Core for OLEDs: A Computational Study on RTP, TADF Emitter and Sensitizer

The exploration of triplet excitons in thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP) molecules has become a subject of significant attention and interest in recent studies. This study employed density functional theory (DFT) and time-dependent DFT theoretical methods to delve into the intricate relationship between the molecular structure and properties of molecules designed with the oxidation of sulfur atoms (S, SO, and SO2) in benzothiazinophenothiazine (BTP) core units. The calculations revealed that as the oxidation state of the sulfur atom increased, the BTP derivatives exhibited elevated ionization potentials (IPs), electron affinities (EAs), and triplet energies (ET), accompanied by reduced reorganization energies (λ), singlet energies (ES), and a S1-T1 energy gap (ΔEST). Additionally, the decrease in the exchange energy prompts a shift in the excited-state properties of molecules, transitioning them from hybridized local and charge transfer (HLCT) to charge transfer (CT) in the S1 state while maintaining their HLCT character in the T1 state. The sulfur oxidation process systematically decreases spin-orbit coupling magnitudes in the S1-T1 and T1-S0 pathways while increasing the KRISC rate, signifying a reduced propensity for phosphorescence radiative decay in oxidized molecules. Thorough investigations have explored the screening effect and orbital mixing of lone pair electrons in sulfur atoms, satisfying the desired criteria for a multifunctional RTP, TADF emitter and sensitizer.