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.

Experimental tools capable of monitoring both atomic and electronic structure on ultrafast (femtosecond to picosecond) time scales are needed for investigating photophysical processes fundamental to light harvesting, photocatalysis, energy and data storage, and optical display technologies. Time-resolved hard x-ray (>3 keV) spectroscopies have proven valuable for these measurements due to their elemental specificity and sensitivity to geometric and electronic structures. Here, we present the first tabletop apparatus capable of performing time-resolved x-ray emission spectroscopy. The time resolution of the apparatus is better than 6 ps. By combining a compact laser-driven plasma source with a highly efficient array of microcalorimeter x-ray detectors, we are able to observe photoinduced spin changes in an archetypal polypyridyl iron complex ½Feð2; 2 0 -bipyridineÞ 3 2þ and accurately measure the lifetime of the quintet spin state. Our results demonstrate that ultrafast hard x-ray emission spectroscopy is no longer confined to large facilities and now can be performed in conventional laboratories with 10 times better time resolution than at synchrotrons. Our results are enabled, in part, by a 100-to 1000-fold increase in x-ray collection efficiency compared to current techniques.

We observe photocatalyst-based regioselectivity distinctions in Diels–Alder reactions using an electron-poor dienophile. We find that one complex, [Cr­(Ph2phen)3]­(BF4)3, offers significantly higher selectivity than other Cr- and Ru-containing photosensitizers. Computational results, supported by excited-state quenching and reaction quantum yield studies, indicate that similar electron- and energy-transfer pathways are available to all photosensitizers studied, but [Cr­(Ph2phen)3]3+ prefers an energy-transfer pathway that selectively orients the electron-poor alkene for cycloaddition, while an unselective outer-sphere radical-cation/electron-transfer pathway is favored for other photosensitizers.