@article{cesana_li_shepard_ting_hart_olson_alvarado_son_steiman_castellano_et al._2022, title={A biohybrid strategy for enabling photoredox catalysis with low-energy light}, volume={8}, ISSN={["2451-9294"]}, DOI={10.1016/j.chempr.2021.10.010}, abstractNote={Natural systems drive the high-energy reactions of photosynthesis with efficient and broadband energy capture. Transition-metal photocatalysts similarly convert light into chemical reactivity, and yet suffer from light-limited operation and require blue-to-UV excitation. In photosynthesis, both light capture and reactivity have been optimized by separation into distinct sites. Inspired by this modular architecture, we synthesized a biohybrid photocatalyst by covalent attachment of the photosynthetic light-harvesting protein R-phycoerythrin (RPE) to the transition-metal photocatalyst tris(2,2′-bipyridine)ruthenium(II) ([Ru(bpy)3]2+). Spectroscopic investigation found that absorbed photoenergy was efficiently funneled from RPE to [Ru(bpy)3]2+. The utility of the biohybrid photocatalyst was demonstrated via an increase in yields for a thiol-ene coupling reaction and a cysteinyl-desulfurization reaction, including recovered reactivity at red wavelengths where [Ru(bpy)3]2+ alone does not absorb.}, number={1}, journal={CHEM}, author={Cesana, Paul T. and Li, Beryl X. and Shepard, Samuel G. and Ting, Stephen I and Hart, Stephanie M. and Olson, Courtney M. and Alvarado, Jesus I. Martinez and Son, Minjung and Steiman, Talia J. and Castellano, Felix N. and et al.}, year={2022}, month={Jan}, pages={174–185} }
 @article{dill_portillo_shepard_shores_rappé_damrauer_2020, title={Long-Lived Mixed 2MLCT/MC States in Antiferromagnetically Coupled d3 Vanadium(II) Bipyridine and Phenanthroline Complexes}, volume={59}, url={http://dx.doi.org/10.1021/acs.inorgchem.0c01950}, DOI={10.1021/acs.inorgchem.0c01950}, abstractNote={Exploration of [V(bpy)3]2+ and [V(phen)3]2+ (bpy = 2,2'-bipyridine; phen = 1,10-phenanthroline) using electronic spectroscopy reveals an ultrafast excited-state decay process and implicates a pair of low-lying doublets with mixed metal-to-ligand charge-transfer (MLCT) and metal-centered (MC) character. Transient absorption (TA) studies of the vanadium(II) species probing in the visible and near-IR, in combination with spectroelectrochemical techniques and computational chemistry, lead to the conclusion that after excitation into the intense and broad visible 4MLCT ← 4GS (ground-state) absorption band (ε400-700 nm = 900-8000 M-1 cm-1), the 4MLCT state rapidly (τisc < 200 fs) relaxes to the upper of two doublet states with mixed MLCT/MC character. Electronic interconversion (τ ∼ 2.5-3 ps) to the long-lived excited state follows, which we attribute to formation of the lower mixed state. Following these initial dynamics, GS recovery ensues with τ = 430 ps and 1.6 ns for [V(bpy)3]2+ and [V(phen)3]2+, respectively. This stands in stark contrast with isoelectronic [Cr(bpy)3]3+, which rapidly forms a long-lived doublet metal-centered (2MC) state following photoexcitation and lacks strong visible GS absorption character. 2MLCT character in the long-lived states of the vanadium(II) species produces geometric distortion and energetic stabilization, both of which accelerate nonradiative decay to the GS compared to [Cr(bpy)3]3+, where the GS and 2MC are well nested. These conclusions are significant because (i) long-lived states with MLCT character are rare in first-row transition-metal complexes and (ii) the presence of a 2MLCT state at lower energy than the 4MLCT state has not been previously considered. The spin assignment of charge-transfer states in open-shell transition-metal complexes is not trivial; when metal-ligand interaction is strong, low-spin states must be carefully considered when assessing reactivity and decay from electronic excited states.}, number={20}, journal={Inorganic Chemistry}, publisher={American Chemical Society (ACS)}, author={Dill, Ryan D. and Portillo, Romeo I. and Shepard, Samuel G. and Shores, Matthew P. and Rappé, Anthony K. and Damrauer, Niels H.}, year={2020}, month={Oct}, pages={14706–14715} }
 @article{fayad_bui_shepard_castellano_2020, title={Photochemical Upconversion in Water Using Cu(I) MLCT Excited States: Role of Energy Shuttling at the Micellar/Water Interface}, volume={3}, ISSN={2574-0962 2574-0962}, url={http://dx.doi.org/10.1021/acsaem.0c02492}, DOI={10.1021/acsaem.0c02492}, abstractNote={Photochemical upconversion (UC) through triplet–triplet annihilation (TTA), which employs a visible absorbing triplet photosensitizer and an annihilator, is a process that generates a high energy photon from two lower energy photons. TTA-UC has been largely developed in pure organic solvents and solid-state polymeric constructs while featuring near exclusive use of rare and expensive metals within the photosensitizer. In this current investigation, we demonstrate that TTA-UC from the long lifetime earth-abundant photosensitizer [Cu(dsbtmp)2](PF)6 (dsbtmp = 2,9-di(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline), abbreviated as Cu-PS, functions in water through encapsulation within a cationic-based assembly. Cetyltrimethylammonium bromide was the surfactant of choice as it electrostatically binds the negatively charged water-soluble 10-phenylanthracene-9-carboxylate (PAC) acceptor/annihilator and ultimately facilitates energy transfer across the interface. Efficient triplet–triplet energy transfer (TTET) from Cu-PS to the PAC acceptor was achieved in this aqueous assembly. Unfortunately, the hindered mobility of the PAC moieties ultimately hampered the annihilation process, and this was reflected in attenuated TTA rates and efficiencies. The combined experimental data illustrated that the water-soluble PAC acceptor was able to vectorially deliver the excited-state energy stored in Cu-PS across the interface into the bulk aqueous solution by engaging in excited-state electron transfer with methyl viologen acceptors. These results are important for remotely operating photoredox reactions in water while rendering a photosensitizer spatially isolated in the hydrophobic core of a micelle.}, number={12}, journal={ACS Applied Energy Materials}, publisher={American Chemical Society (ACS)}, author={Fayad, Remi and Bui, Anh Thy and Shepard, Samuel G. and Castellano, Felix N.}, year={2020}, month={Dec}, pages={12557–12564} }
 @article{fatur_shepard_higgins_shores_damrauer_2017, title={A Synthetically-Tunable System to Control MLCT Excited-State Lifetimes and Spin States in Iron(II) Polypyridines}, volume={3}, url={https://doi.org/10.1021/jacs.7b00700}, DOI={10.1021/jacs.7b00700}, abstractNote={2,2':6',2″-Terpyridyl (tpy) ligands modified by fluorine (dftpy), chlorine (dctpy), or bromine (dbtpy) substitution at the 6- and 6″-positions are used to synthesize a series of bis-homoleptic Fe(II) complexes. Two of these species, [Fe(dctpy)2]2+ and [Fe(dbtpy)2]2+, which incorporate the larger dctpy and dbtpy ligands, assume a high-spin quintet ground state due to substituent-induced intramolecular strain. The smaller fluorine atoms in [Fe(dftpy)2]2+ enable spin crossover with a T1/2 of 220 K and a mixture of low-spin (singlet) and high-spin (quintet) populations at room temperature. Taking advantage of this equilibrium, dynamics originating from either the singlet or quintet manifold can be explored using variable wavelength laser excitation. Pumping at 530 nm leads to ultrafast nonradiative relaxation from the singlet metal-to-ligand charge transfer (1MLCT) excited state into a quintet metal centered state (5MC) as has been observed for prototypical low-spin Fe(II) polypyridine complexes such as [Fe(tpy)2]2+. On the other hand, pumping at 400 nm excites the molecule into the quintet manifold (5MLCT ← 5MC) and leads to the observation of a greatly increased MLCT lifetime of 14.0 ps. Importantly, this measurement enables an exploration of how the lifetime of the 5MLCT (or 7MLCT, in the event of intersystem crossing) responds to the structural modifications of the series as a whole. We find that increasing the amount of steric strain serves to extend the lifetime of the 5,7MLCT from 14.0 ps for [Fe(dftpy)2]2+ to the largest known value at 17.4 ps for [Fe(dbtpy)2]2+. These data support the design hypothesis wherein interligand steric interactions are employed to limit conformational dynamics and/or alter relative state energies, thereby slowing nonradiative loss of charge-transfer energy.}, number={12}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Fatur, Steven M and Shepard, Samuel G and Higgins, Robert F. and Shores, Matthew P. and Damrauer, Niels H.}, year={2017}, month={Mar}, pages={4493–4505} }
 @article{shepard_fatur_rappé_damrauer_2016, title={Highly Strained Iron(II) Polypyridines: Exploiting the Quintet Manifold To Extend the Lifetime of MLCT Excited States}, volume={138}, DOI={10.1021/jacs.5b13524}, abstractNote={Halogen substitution at the 6 and 6″ positions of terpyridine (6,6″-Cl2-2,2:6',2″-terpyridine = dctpy) is used to produce a room-temperature high-spin iron(II) complex [Fe(dctpy)2](BF4)2. Using UV-vis absorption, spectroelectrochemistry, transient absorption, and TD-DFT calculations, we present evidence that the quintet metal-to-ligand charge-transfer excited state ((5)MLCT) can be accessed via visible light absorption and that the thermalized (5,7)MLCT is long-lived at 16 ps, representing a > 100 fold increase compared to the (1,3)MLCT within species such as [Fe(bpy)3](2+). This result opens a new strategy for extending iron(II) MLCT lifetimes for potential use in photoredox processes.}, number={9}, journal={J. Am. Chem. Soc.}, publisher={American Chemical Society (ACS)}, author={Shepard, Samuel G. and Fatur, Steven M. and Rappé, Anthony K. and Damrauer, Niels H.}, year={2016}, month={Mar}, pages={2949–2952} }
 @article{higgins_fatur_shepard_stevenson_boston_ferreira_damrauer_rappé_shores_2016, title={Uncovering the Roles of Oxygen in Cr(III) Photoredox Catalysis}, volume={138}, DOI={10.1021/jacs.6b02723}, abstractNote={A combined experimental and theoretical investigation aims to elucidate the necessary roles of oxygen in photoredox catalysis of radical cation based Diels-Alder cycloadditions mediated by the first-row transition metal complex [Cr(Ph2phen)3](3+), where Ph2phen = bathophenanthroline. We employ a diverse array of techniques, including catalysis screening, electrochemistry, time-resolved spectroscopy, and computational analyses of reaction thermodynamics. Our key finding is that oxygen acts as a renewable energy and electron shuttle following photoexcitation of the Cr(III) catalyst. First, oxygen quenches the excited Cr(3+)* complex; this energy transfer process protects the catalyst from decomposition while preserving a synthetically useful 13 μs excited state and produces singlet oxygen. Second, singlet oxygen returns the reduced catalyst to the Cr(III) ground state, forming superoxide. Third, the superoxide species reduces the Diels-Alder cycloadduct radical cation to the final product and reforms oxygen. We compare the results of these studies with those from cycloadditions mediated by related Ru(II)-containing complexes and find that the distinct reaction pathways are likely part of a unified mechanistic framework where the photophysical and photochemical properties of the catalyst species lead to oxygen-mediated photocatalysis for the Cr-containing complex but radical chain initiation for the Ru congener. These results provide insight into how oxygen can participate as a sustainable reagent in photocatalysis.}, number={16}, journal={J. Am. Chem. Soc.}, publisher={American Chemical Society (ACS)}, author={Higgins, Robert F. and Fatur, Steven M. and Shepard, Samuel G. and Stevenson, Susan M. and Boston, David J. and Ferreira, Eric M. and Damrauer, Niels H. and Rappé, Anthony K. and Shores, Matthew P.}, year={2016}, month={Apr}, pages={5451–5464} }