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.

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)] and [Fe(dbtpy)], 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)] enable spin crossover with a T 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 (MLCT) excited state into a quintet metal centered state (MC) as has been observed for prototypical low-spin Fe(II) polypyridine complexes such as [Fe(tpy)]. On the other hand, pumping at 400 nm excites the molecule into the quintet manifold (MLCT ← MC) 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 theMLCT (or MLCT, 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 theMLCT from 14.0 ps for [Fe(dftpy)] to the largest known value at 17.4 ps for [Fe(dbtpy)]. 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.

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.

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 4 MLCT ← 4 GS (ground-state) absorption band (ε 400−700 nm = 900−8000 M −1 cm −1 ), the 4 MLCT state rapidly (τ isc < 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 ( 2 MC) state following photoexcitation and lacks strong visible GS absorption character. 2 MLCT 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 2 MC are well nested. These conclusions are significant because (i) long-lived states with MLCT character are rare in firstrow transition-metal complexes and (ii) the presence of a 2 MLCT state at lower energy than the 4 MLCT 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.

Photocatalysts convert light into potent reactivity. Here, we report a biohybrid catalyst in which a photosynthetic protein performs broad-spectrum light absorption and subsequent energy transfer to a conjugated photocatalyst, leading to improved yields in test reactions. This strategy has the potential to be generalized for applications in industrial and biological catalysis.