Density Functional Physicality in Electronic Coupling Estimation: Benchmarks and Error Analysis

Antioxidative Triplet Excitation Energy Transfer in Bacterial Reaction Center Using a Screened Range Separated Hybrid Functional

Excess energy absorbed by photosystems (PSs) can result in photoinduced oxidative damage. Transfer of such energy within the core pigments of the reaction center in the form of triplet excitation is important in regulating and preserving the functionality of PSs. In the bacterial reaction center (BRC), the special pair (P) is understood to act as the electron donor in a photoinduced charge transfer process, triggering the charge separation process through the photoactive branch A pigments that experience a higher polarizing environment. At this work, triplet excitation energy transfer (TEET) in BRC is studied using a computational perspective to gain insights into the roles of the dielectric environment and interpigment orientations. We find in agreement with experimental observations that TEET proceeds through branch B. The TEET process toward branch B pigment is found to be significantly faster than the hypothetical process proceeding through branch A pigments with ps and ms time scales, respectively. Our calculations find that conformational differences play a major role in this branch asymmetry in TEET, where the dielectric environment asymmetry plays only a secondary role in directing the TEET to proceed through branch B. We also address TEET processes asserting the role of carotenoid as the final triplet energy acceptor and in a mutant form, where the branch pigments adjacent to P are replaced by bacteriopheophytins. The necessary electronic excitation energies and electronic state couplings are calculated by the recently developed polarization-consistent framework combining a screened range-separated hybrid functional and a polarizable continuum mode. The polarization-consistent potential energy surfaces are used to parametrize the quantum mechanical approach, implementing Fermi's golden rule expression of the TEET rate calculations.

Simulating Electron Transfer Reactions in Solution: Radical-Polar Crossover

Single-electron transfer (SET) promotes a wide variety of interesting chemical transformations, but modeling of SET requires a careful treatment of electronic and solvent effects to give meaningful insight. Therefore, a combined constrained density functional theory and molecular mechanics (CDFT/MM) tool is introduced specifically for SET-initiated reactions. Mechanisms for two radical-polar crossover reactions involving the organic electron donors tetrakis(dimethylamino)ethylene (TDAE) and tetrathiafulvalene (TTF) were studied with the new tool. An unexpected tertiary radical intermediate within the TDAE system was identified, relationships between kinetics and substitution in the TTF system were explained, and the impact of the solvent environments on the TDAE and TTF reactions were examined. The results highlight the need for including solvent dynamics when quantifying SET kinetics and thermodynamics, as a free energy difference of >20 kcal/mol was observed. Overall, the new method informs mechanistic analysis of SET-initiated reactions and therefore is envisioned to be useful for studying reactions in the condensed phase.

Theoretical Protocol Based on Long-Range Corrected Density Functional Theory and Tuning of Range-Split Parameter for Two-Electron Two-Proton Reduction of Phenylazocarboxylates

A theoretical protocol based on long-range corrected density functional theory is suggested for a highly accurate estimation of the two-electron two-proton (2e2p) reduction potential of ethyl 2-phenylazocarboxylate derivatives. Geometry optimization and single-point energy refinement with ωB97X-D are recommended. The impact of polarization and diffusion functions in the basis sets on the 2e2p reduction potential is discussed. Further improvements can be achieved by tuning the range-split parameter based on the linear relationship between the Hammett constant of phenyl substituents and the optimal ω value that most accurately reproduces the experiments. The suggested protocol can accurately predict the 2e2p reduction potential of five ethyl 2-phenylazocarboxylate derivatives. Based on these findings, 22 additional candidates are suggested to enlarge the electrochemical window and to increase the selectivity of 2e2p reactions. This study contributes to the development of a theoretical approach to accurately estimate the 2e2p reduction potential of azo groups.

How the Donor/Acceptor Spin States Affect the Electronic Couplings in Molecular Charge-Transfer Processes?

The electronic coupling matrix element H_AB is an essential ingredient of most electron-transfer theories. H_AB depends on the overlap between donor and acceptor wave functions and is affected by the involved states’ spin. We classify the spin-state effects into three categories: orbital occupation, spin-dependent electron density, and density delocalization. The orbital occupancy reflects the diverse chemical nature and reactivity of the spin states of interest. The effect of spin-dependent density is related to a more compact electron density cloud at lower spin states due to decreased exchange interactions between electrons. Density delocalization is strongly connected with the covalency concept that increases the spatial extent of the diabatic state’s electron density in specific directions. We illustrate these effects with high-level ab initio calculations on model direct donor–acceptor systems relevant to metal oxide materials and biological electron transfer. Obtained results can be used to benchmark existing methods for H_AB calculations in complicated cases such as spin-crossover materials or antiferromagnetically coupled systems.

Spin-Flip Density Functional Theory for the Redox Properties of Organic Photoredox Catalysts in Excited States

Photoredox catalysts (PCs) have contributed to the advancement of organic chemistry by accelerating conventional reactions and enabling new pathways through the use of reactive electrons in excited states. With a number of successful applications, chemists continue to seek new promising organic PCs to achieve their objectives. Instead of labor-intensive manual experimentation, quantum chemical simulations could explore the enormous chemical space more efficiently. The reliability and accuracy of quantum chemical simulations have become important factors for material screening. We designed a theoretical protocol capable of predicting redox properties in excited states with high accuracy for a selected model system of dihydroquinoxalino[2,3-b]quinoxaline derivatives. Herein, three factors were considered as critical to achieving reliable predictions with accurate physics: the solvent medium effect on excited-state geometries, an adequate amount of Hartree-Fock exchange (HFX), and the consideration of double-excitation character in excited states. We determined that it is necessary to incorporate solvent medium during geometry optimizations to obtain planar excited-state structures that are consistent with the experimentally observed modest Stokes shift. While density functionals belonging to the generalized gradient approximation family perform well for the prediction of photoelectrochemical properties, an incorrect description of exciton boundedness (spontaneous dissociation of excitons or extremely weak boundedness) on small organic molecules was predicted. The inclusion of an adequate amount of Hartree-Fock exchange was suggested as one approach to obtain bound excitons, which is physically reasonable. The last consideration is the double-excitation character in S₁ states. As revealed by the second-order algebraic diagrammatic construction theory, non-negligible double excitations exist in S₁ states in our model systems. Time-dependent density functional theory (TDDFT) is blind to doubly excited states, and this motivated us to use spin-flip DFT (SF-DFT). We established a theoretical protocol that could provide highly accurate estimations of photophysical properties and ground-/excited-state redox properties, focusing on the three factors mentioned above. Geometry optimization with DFT and TDDFT employing the B3LYP functional (20% HFX) in solution and energy refinement by SF-DFT reproduced the experimental redox properties in the excited and ground states remarkably well with mean signed deviations (MSDs) of 0.01 and -0.15 V, respectively. This theoretical protocol is expected to contribute to the understanding of exciton behavior in organic PCs and to the efficient design of new promising PC candidates.

How To Extract Quantitative Information on Electronic Transitions from the Density Functional Theory "Black Box"

Electronic couplings and vertical excitation energies are crucial determinants of charge and excitation energy transfer rates in a broad variety of processes ranging from biological charge transfer to charge transport through inorganic materials, from molecular sensing to intracellular signaling. Density Functional Theory (DFT) is generally used to calculate these critical parameters, but the quality of the results is unpredictable because of the semiempirical nature of the available DFT approaches. This study identifies a small set of fundamental rules that enables accurate DFT computation of electronic couplings and vertical excitation energies in molecular complexes and materials. These rules are applied to predict efficient DFT approaches to coupling calculations. The result is an easy-to-use guide for reliable DFT descriptions of electronic transitions.

Charge transport parameters for carbon based nanohoops and donor-acceptor derivatives

The effect of donor-acceptor (D-A) moieties on magnitudes such as reorganization energies and electronic couplings in cycloparaphenylene (CPP) carbon based nanohoops (i.e. conjugated organic molecules with cyclic topology) is highlighted via model computations and analysis of the available crystalline structure of N,N-dimethylaza[8]CPP. For the sake of comparison, intra-molecular and inter-molecular charge transport parameters are concomitantly modelled for the recently determined herringbone polymorph of [6]CPP, along with [8]CPP and [12]CPP. The peculiar contribution of low frequency vibrations to intramolecular reorganization energies is also disclosed by computing the Huang-Rhys factors for the investigated [n]CPPs and the N,N-dimethylaza derivative. In contrast with most planar organic semiconductors where the layer in which molecules are herringbone arranged identifies the high-mobility plane, nanohoops disclose inter-layer electronic couplings larger than the intra-layer counterparts. Charge transfer rate constants modelled with three different approaches (Marcus, Marcus-Levich-Jortner and spectral overlap) suggest that D-A nanohoops, owing to orbital localization, may be more efficient for charge transport than [n]CPPs for suitable solid phase arrangements.

Electron transfer characteristics of 2'-deoxy-2'-fluoro-arabinonucleic acid, a nucleic acid with enhanced chemical stability

The non-biological nucleic acid 2'-deoxy-2'-fluoro-arabinonucleic acid (2'F-ANA) may be of use because of its higher chemical stability than DNA in terms of resistance to hydrolysis and nuclease degradation. In order to investigate the charge transfer characteristics of 2'F-ANA, of relevance to applications in nucleic acid-based biosensors and chip technologies, we compare the electronic couplings for hole transfer between stacked nucleobase pairs in DNA and 2'F-ANA by carrying out density functional theory (DFT) calculations on geometries taken from molecular dynamics simulations. We find similar averages and distribution widths of the base-pair couplings in the two systems. On the basis of this result, 2'F-ANA is expected to have charge transfer properties similar to those of DNA, while offering the advantage of enhanced chemical stability. As such, 2'F-ANA may serve as a possible alternative to DNA for use in a broad range of nanobiotechnological applications. Furthermore, we show that the (experimentally observed) enhanced chemical stability resulting from the backbone modifications does not cause reduced fluctuations of the base-pair electronic couplings around the values found for "ideal" B-DNA (with standard step parameter values). Our study also supports the use of a DFT implementation, with the M11 functional, of the wave function overlap method to compute effective electronic couplings in nucleic acid systems.