@article{barth_pyrch_mccormick_danilov_castellano_2024, title={Excited State Bond Homolysis of Vanadium(V) Photocatalysts for Alkoxy Radical Generation}, volume={8}, ISSN={["1520-5215"]}, url={https://doi.org/10.1021/acs.jpca.4c04250}, DOI={10.1021/acs.jpca.4c04250}, abstractNote={Advancements in photocatalysis have transformed synthetic organic chemistry, using light as a powerful tool to drive selective chemical transformations. Recent approaches have focused on metal-halide ligand-to-metal charge transfer (LMCT) photoactivated bond homolysis reactions leveraged by earth-abundant elements to generate valuable synthons for radical-mediated cross-coupling reactions. Of recent utility, oxovanadium(V) LMCT photocatalysts exhibit selective alkoxy radical generation from aliphatic alcohols upon blue light (UVA) irradiation under mild conditions. The selective photochemical liberation of alkoxy radicals is valuable for applying late-stage fragmentation approaches in organic synthesis and depolymerization strategies for nonbiodegradable polymers. Steady-state and time-resolved spectroscopy were used to assign the electronic structure of three well-defined V(V) photocatalysts in their ground and excited states. We assign the excited state for this transformation at earth-abundant vanadium(V), interrogating the electronic structure using static UV–visible absorption, ultrafast transient absorption, and electron paramagnetic resonance spectroscopy coupled to computational approaches. These findings afford assignments of the short-lived excited state intermediates that dictate selective homolytic bond cleavage in metal alkoxides, illustrating the valuable insight gleaned from fundamental investigations of the molecular photochemistry responsible for light-escalated chemical transformations.}, journal={JOURNAL OF PHYSICAL CHEMISTRY A}, author={Barth, Alexandra T. and Pyrch, Austin J. and McCormick, Conor T. and Danilov, Evgeny O. and Castellano, Felix N.}, year={2024}, month={Aug} }
 @article{sridhar_khansari_o'donnell_barth_danilov_castellano_maggard_dougherty_2024, title={Ligand field exciton annihilation in bulk CrCl<sub>3</sub>}, volume={161}, ISSN={["1089-7690"]}, url={https://doi.org/10.1063/5.0223772}, DOI={10.1063/5.0223772}, abstractNote={The layered van der Waals material CrCl3 exhibits very strongly bound ligand field excitons that control optoelectronic applications and are connected with magnetic ordering by virtue of their d-orbital origin. Time-resolved photoluminescence of these exciton populations at room temperature shows that their relaxation is dominated by exciton–exciton annihilation and that the spontaneous decay lifetime is very long. These observations allow the rough quantification of the exciton annihilation rate constant and contextualization in light of a recent theory of universal scaling behavior of the annihilation process.}, number={11}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Sridhar, Samanvitha and Khansari, Ario and O'Donnell, Shaun and Barth, Alexandra T. and Danilov, Evgeny O. and Castellano, Felix N. and Maggard, Paul A. and Dougherty, Daniel B.}, year={2024}, month={Sep} }
 @article{barth_fajardo jr_sattler_winkler_gray_2023, title={Electronic Structures and Photoredox Chemistry of Tungsten(0) Arylisocyanides}, ISSN={["1520-4898"]}, DOI={10.1021/acs.accounts.3c00184}, abstractNote={ConspectusThe high energy barriers associated with the reaction chemistry of inert substrates can be overcome by employing redox-active photocatalysts. Research in this area has grown exponentially over the past decade, as transition metal photosensitizers have been shown to mediate challenging organic transformations. Critical for the advancement of photoredox catalysis is the discovery, development, and study of complexes based on earth-abundant metals that can replace and/or complement established noble-metal-based photosensitizers.Recent work has focused on redox-active complexes of 3d metals, as photosensitizers containing these metals most likely would be scalable. Although low lying spin doublet ("spin flip") excited states of chromium(III) and metal-to-ligand charge transfer (MLCT) excited states of copper(I) have relatively long lifetimes, the electronic excited states of many other 3d metal complexes fall on dissociative potential energy surfaces, owing to the population of highly energetic σ-antibonding orbitals. Indeed, we and other investigators have shown that low lying spin singlet and triplet excited states of robust closed-shell metal complexes are too short-lived at room temperature to engage in bimolecular reactions in solutions. In principle, this problem could be overcome by designing and constructing 3d metal complexes containing strong field π-acceptor ligands, where thermally equilibrated MLCT or intraligand charge transfer excited states might fall well below the upper surfaces of dissociative 3d-3d states. Notably, such design elements have been exploited by investigators in very recent work on redox-active iron(II) systems. Another approach, one we have actively pursued, is to design and construct closed-shell complexes of earth-abundant 5d metals containing very strong π-acceptor ligands, where vertical excitation of 5d-5d excited states at the ground state geometry would require energies far above minima in the potential surfaces of MLCT excited states. As this requirement is met by tungsten(0) arylisocyanides, these complexes have been the focus of our work aimed at the development of robust redox-active photosensitizers.In the following Account, we review recent work on homoleptic tungsten(0) arylisocyanides. Originally reported by our group 45 years ago, W(CNAr)6 complexes have exceptionally large one- and two-photon absorption cross-sections. One- or two-photon excitation produces relatively long-lived (hundreds of nanoseconds to microsecond) MLCT excited states in high yields. These MLCT excited states, which are very strong reductants with E°(W+/*W0) = -2.2 to -3.0 V vs Fc[+/0], mediate photocatalysis of organic reactions with both visible and near-infrared (NIR) light. Here, we highlight design principles that led to the development of three generations of W(CNAr)6 photosensitizers; and we discuss likely steps in the mechanism of a prototypal W(CNAr)6-catalyzed base-promoted homolytic aromatic substitution reaction. Among the many potential applications of these very bright luminophores, two-photon imaging and two-photon-initiated polymerization are ones we plan to pursue.}, journal={ACCOUNTS OF CHEMICAL RESEARCH}, author={Barth, Alexandra T. and Fajardo Jr, Javier and Sattler, Wesley and Winkler, Jay R. and Gray, Harry B.}, year={2023}, month={Jun} }