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Growing interest in the use of first-row transition metal complexes in a number of applied contexts—including but not limited to photoredox catalysis and solar energy conversion—underscores the need for a detailed understanding of their photophysical properties. A recent focus on ligand-field photocatalysis using cobalt(III) polypyridyls in particular has unlocked unprecedented excited-state reactivities. Photophysical studies on Co(III) chromophores in general are relatively uncommon, and so here we carry out a systematic study of a series of Co(III) polypyridyl complexes in order to delineate their excited-state dynamics. Compounds with varying ligand-field strengths were prepared and studied using variable-temperature ultrafast transient absorption spectroscopy. Analysis of the data establishes that the ground-state recovery dynamics are operating in the Marcus inverted region, in stark contrast to what is typically observed in other first-row metal complexes. The analysis has further revealed the underlying reasons driving this excited-state behaviour, thereby enabling potential advancements in the targeted use of the Marcus inverted region for a variety of photolytic applications.

Increasing interest in sustainable chemistry coupled with the quest to explore new reactivity has spurred research on first-row transition metal complexes for potential applications in a variety of settings. One of the more active areas of research is photoredox catalysis, where the synthetically tunable nature of their electronic structures provides a rich palette of options for tailoring their reactivity to a desired chemical transformation. Understanding the mechanism of excited-state reactivity is critical for the informed development of next-generation catalysts, which in turn requires information concerning the propensity of their electronic excited states to engage in the desired electron or energy transfer processes. Herein we provide direct evidence of the highly oxidizing nature of the lowest-energy ligand-field (LF) excited state of a first-row d6-low-spin Co(III) photosensitizer [Co(4,4′-Br2bpy)3]3+(where 4,4′-Br2bpy is 4,4′-dibromo-2,2′-bipyridine). The redox potential associated with the LF excited state of the Co(III) complex was bracketed by performing bimolecular quenching studies by using a series of simple organic electron donors. Time-resolved absorption spectroscopy confirmed a dynamic quenching process attributed to reductive quenching of the lowest-energy ligand-field excited state of the Co(III) chromophore. Analysis of the Stern–Volmer plots for each chromophore-quencher pair revealed a limiting value of Ered*∼ 1.25 V vs Fc/Fc+for the metal-centered excited state, which is significantly stronger than that of more commonly employed transition metal-based photoredox agents such as [Ru(bpy)3]2+(Ered*= 0.32 V vs Fc/Fc+) and [Ir(ppy)2(bpy)]+(Ered*= 0.27 V vs Fc/Fc+). These results suggest that this class of chromophores could find utility in applications requiring the activation of oxidatively resistant organic substrates for photoredox catalysis.

TiO2-based dye-sensitized solar cells employing Fe(2,2′-bipyridine-4,4′-dicarboxylic acid)2(CN)2(F2CA) have been studied by spectroscopic, electrochemical, photoelectrochemical, time-resolved spectroscopic, and computational methods in an effort to assess how intercalating and passivating electrolyte additives influence photovoltaic performance in this class of devices. Ru(2,2′-bipyridine-4,4′-dicarboxylic acid)2(NCS)2(N3)-sensitized titanium dioxide (TiO2) devices were also studied by similar methods to provide a comparative standard. Ground-state spectroscopic and electrochemical measurements show that the dye electronic structure was influenced in a nonsystematic way by the presence of various electrolyte components such as lithium (Li+), tetrabutylammonium (TBA+), and pyridyl additives, whereas the conduction band edge of TiO2was impacted in a systematic way. An order-of-magnitude increase in short-circuit current (Jsc) and cell efficiency (η) was observed for F2CA-sensitized devices with our selected electrolytes as compared to electrolyte compositions that are typically employed. These substantial gains in F2CA-sensitized device performance correlated with conduction band edge stabilization attributed to electrolyte cation intercalation. Time-resolved spectroscopic and computational studies explain how stabilization of the conduction band edge, which allows for better dye-conduction band overlap, increases the injection yield for F2CA–TiO2assemblies. These studies serve to elevate our understanding of how electrolyte composition may affect injection and photovoltaic performance in sensitizer assemblies with short-lived charge-separated excited states.

We have employed a range of ultrafast X-ray spectroscopies in an effort to characterize the lowest energy excited state of [Fe(dcpp)2]2+(where dcpp is 2,6-(dicarboxypyridyl)pyridine). This compound exhibits an unusually short excited-state lifetime for a low-spin Fe(II) polypyridyl complex of 270 ps in a room-temperature fluid solution, raising questions as to whether the ligand-field strength of dcpp had pushed this system beyond the 5T2/3T1crossing point and stabilizing the latter as the lowest energy excited state. Kα and Kβ X-ray emission spectroscopies have been used to unambiguously determine the quintet spin multiplicity of the long-lived excited state, thereby establishing the 5T2state as the lowest energy excited state of this compound. Geometric changes associated with the photoinduced ligand-field state conversion have also been monitored with extended X-ray absorption fine structure. The data show the typical average Fe-ligand bond length elongation of ∼0.18 Å for a 5T2state and suggest a high anisotropy of the primary coordination sphere around the metal center in the excited 5T2state, in stark contrast to the nearly perfect octahedral symmetry that characterizes the low-spin 1A1ground state structure. This study illustrates how the application of time-resolved X-ray techniques can provide insights into the electronic structures of molecules—in particular, transition metal complexes—that are difficult if not impossible to obtain by other means.

A study of a series of six-coordinate Co(III) complexes has been carried out to quantify spectroscopic parameters for a range of ligands that are commonly employed to realize strong charge-transfer absorptions in low-spin, d6systems. Identification of any three ligand-field transitions allows for the determination of the splitting parameter (10 Dq) as well as the Racah Band Cparameters for a given compound. The data revealed a relatively small spread in the magnitude of 10 Dq, ranging from ca. 23 000 cm–1in the case of [Co(pyrro-bpy)3]3+(where pyrro-bpy is 4,4′-dipyrrolidinyl-2,2′-bipyridine) to ca. 26 000 cm–1for [Co(terpy)2]3+(where terpy is 2,2′:6′,2″-terpyridine). Significantly, trends across the series suggest that polypyridyl ligands behave as net π-donors when interacting with Co(III), in contrast to the net π-accepting character they exhibit when bound to second- and third-row metals. The influence of strong σ donation associated with carbene-based ligands was evident from the data acquired for [Co(BMeImPy)2]3+(where BMeImPy is 3,3′-(pyridine-2,6-diyl)bis(1-methyl-1H-3-imidazolium)), where a 10 Dq value of ca. 30 000 cm–1was determined. Spectroscopic data were also analyzed for [Fe(bpy)3]2+using the results on [Co(bpy)3]3+as a reference point. A value for 10 Dq of 21 000 cm–1was estimated, indicating a reduction in the ligand-field strength of ca. 3000 cm–1upon replacing Co(III) with Fe(II). We suggest that this approach of taking advantage of the blueshift of the charge-transfer feature in Co(III) complexes to reveal otherwise obscured ligand-field bands can be a useful tool for the development of new ligand systems to expand the photofunctionality of first-row transition-metal-based chromophores.