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

Abstract

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.

Visible-Light Organophotoredox-Mediated [3 + 2] Cycloaddition of Arylcyclopropylamine with Structurally Diverse Olefins for the Construction of Cyclopentylamines and Spiro[4.n] Skeletons

“…Even if the amount of QXPT-NPh was reduced to 0.1 mol %, the yield could still reach 75% (Table , entry 19). It should be noted that in Xu and other’s research, − as photoredox catalysts, QXPT-NPh and related photocatalysts were used as reductants to initiate single-electron transfer, while in our research such type of photocatalysts were used as oxidants to initiate single-charge transfer for the first time, in which the photoredox process was initiated by the SET from substrates to the photocatalysts.…”

“…As one type of hetero-polyaromatic hydrocarbons (PAHs), quinoxaline-fused phenazine/phenoxazine/phenothiazine/carbazole are initially designed as materials and biological molecules. Such hetero PAHs are first applied on photopolymerization and are introduced by Xu et al − into visible-light-mediated organic reactions. N -Phenyl quinoxaline-phenothiazine ( QXPT-NPh ) can be easily prepared. , Herein, we report our recent results on the intermolecular [3 + 2] cycloadditions of cyclopropylanilines with olefins, using QXPT-NPh as a highly efficient organic photoredox catalyst under irradiation of blue light-emitting diodes (LEDs) (Scheme ).…”

Isocyanides as Catalytic Electron Acceptors in the Visible Light Promoted Oxidative Formation of Benzyl and Acyl Radicals

“…Density Functional Theory (DFT) was used for all the ground-state calculations while its time-dependent version (TD-DFT) [15][16][17][18][19][20][21][22][23][24][25] was employed for computing excited state properties, according to the procedure described in the Computational Methods section. The calculated reduction potentials in acetonitrile are shown in Table 1 along with those measured by cyclic voltammetry (CV).…”

Excited States by Coupling Piris Natural Orbital Functionals with the Extended Random-Phase Approximation

“…Excited states , are important for the description of photochemical and electrochemical processes, fluorescence and phosphorescence phenomena, spectroscopy, and chemical reaction mechanisms, , with a variety of cutting-edge chemical applications such as the development of new materials for organic solar cells and batteries. − The energy of these states can be calculated using the configuration interaction (CI) method; however, this becomes too expensive even for low levels of CI, although some variations have been developed to address this issue . There are other electronic structure tools that allow studying excited species at a more affordable cost, such as the equation-of-motion coupled-cluster (EOM-CC) − approach, the time-dependent density functional theory (TD-DFT), and the random-phase approximation, but the accuracy achieved is not as good as desired and the picture becomes more complex when it comes to static correlation, since multireference methods are required. , …”