Amy Cordones-Hahn

The electrochemical reduction of CO2 has received significant scientific interest over the past two decades as a key step in the synthesis of CO2 into combustible fuels. Cobalt phthalocyanine (CoPc) adsorbed on carbon nanotubes has risen as a rare electrocatalyst that reduces CO2 beyond two electrons, specifically to methanol. Recent efforts have improved the efficiency of this process and established that methanol production proceeds by reduction of a free CO intermediate, however, many aspects of the catalytic pathway remain unclear, which may underpin persistent issues of stability and selectivity. Here we employ electrochemistry, spectroelectrochemistry, X-ray adsorption spectroscopy (XAS), synthesis of catalytic intermediates, and density functional theory (DFT) calculations to understand the catalytic mechanism and identify distinguishing molecular features. We find that CoPc undergoes three sequential reductions between 0 and −2.5 vs NHE; the first reduction occurring at the Co center, and the second two reductions populating the Pc ring. XAS and theory show the Pc ligand to be noninnocent, interacting with the electronic structure of the Co center and causing Co to adopt a Co(II) state as the complex undergoes the second and third reductions. Consistent with previous studies, [CoPc]2– is found to be active for CO2 reduction, however, we find that the catalyst must be reduced further to [CoPc]3– to bind CO at the Co site. Carbonylation of [CoPc]3– under aprotic conditions leads to fast decomposition, forming [Co(CO)4]−. However, we show that [CoPc]3–, if ring protonated by weak acid to form [CoPc-H]2–, reacts with CO to generate a formyl without decomposition, the first step in methanol synthesis. DFT calculations of the mechanism indicate that the reaction of [CoPc-H]2– with CO to generate the formyl in solution proceeds by an intermolecular proton transfer between the Pc ring of [CoPc-H]2– and the Co–CO of [Co-COPc-H]2–. Overall, this study points to the importance of the Pc ligand in determining the catalytic behavior of CoPc and provides the first examples of several isolated reduced CoPc complexes and catalytic intermediates that we hope will guide future work.

Proton-coupled electron transfer (PCET) is foundational to catalysis, bioenergetics, and energy conversion, yet directly observing the interplay between electronic redistribution, protonation, and solvent reorganization remains challenging. We combine femtosecond optical spectroscopy, ultrafast N K-edge X-ray absorption spectroscopy, and time-resolved X-ray solution scattering to capture the steps of a sequential PCET reaction in water with atomic-site specificity. Using a ruthenium polypyridyl model complex, we resolve the electron redistribution upon photoinduced metal-to-ligand charge transfer and subsequent ( ~ 460 ps) protonation at a ligand nitrogen, as well as the concomitant rearrangement of the first-solvation-shell. Combined with advanced electronic structure and molecular dynamics simulations, our measurements reveal a marked localization of the excited-state electron density at the protonated N site, together with a switch from N···HO to NH···O hydrogen-bonds. These results establish a multimodal X-ray framework for mechanistic insight into PCET and its control in catalysis, artificial photosynthesis, and biological energy flow.

This work identifies the impact of covalent metal–ligand interactions on the excited states of planar [M(diimine)(dithiolate)] complexes (M = Pt, Ni) with bipyridyl (bpy) and benzene-1,2-dithiolate (bdt) ligands. The long-lived excited states of the Pt(II) complexes have long been surmised to have metal-and-dithiolate ligand → diimine ligand charge transfer (MLL’CT) character due to disproportionately strong metal–dithiolate orbital mixing, whereas the nature of the shorter-lived excited states of the nonluminescent Ni(II) complexes has not been investigated. Time-resolved Pt 2p (L3-edge) X-ray absorption near-edge structure (XANES) spectroscopy is presented herein to definitively prove that strong orbital overlap between the metal center and dithiolate ligand yields the MLL’CT character of the long-lived excited state in [Pt(bpy)(bdt)]. For [Ni(bpy)(bdt)], ultrafast transient Ni 1s (K-edge) XANES is used to resolve the electronic and structural relaxation dynamics following MLL’CT excitation. Combined with electronic structure calculations, the results show that the low-lying Ni 3dyz orbital in [Ni(bpy)(bdt)] facilitates subpicosecond formation of a metastable triplet excited state with ligand field state symmetry (3LF). Tetrahedral structural relaxation dynamically alters the differential Ni–diimine and Ni–dithiolate covalencies and thereby modulates the resulting excited state charge distribution. The Ni-and-bpy → bdt charge transfer character of the fully relaxed 3LF state is found to reverse the directionality of the ligand-to-ligand charge transfer established for the 3MLL’CT state of the Pt congener. Altogether, these results demonstrate that inequivalent metal–ligand covalencies in mixed-ligand donor–acceptor complexes could be applied to control excited state charge distributions within photoactive transition metal complexes.

Understanding the key parameters that govern the photophysical and photochemical properties of transition metal complexes is essential for the development of efficient photosensitizers for photocatalytic applications. Achieving this objective necessitates clear and detailed investigations of their electronic excited states, for which time-resolved metal Kβ X-ray emission spectroscopy (XES) has proven highly effective. Here, we present a time-resolved Fe Kβ XES study of a heteroleptic Fe(II) polypyridyl carbene complex, [Fe(phen)2(C4H10N4)]2+ (1; phen = 1,10-phenanthroline), utilizing both the valence-to-core and Kβ mainline spectral regions, complemented by variable-temperature transient optical absorption (VT-TA) spectroscopy. Detailed analysis of the time-resolved Kβ XES data, supported by density functional theory (DFT) calculations and an Eyring analysis of the VT-TA data, reveals parallel excited state relaxation dynamics that support an assignment of the long-lived excited state to a triplet metal-centered state. Placing these results in the context of prior studies of heteroleptic Fe(II) polypyridyl cyanide complexes motivated a series of DFT calculations to investigate the effects of ligand structural flexibility and arrangement. These calculations reinforce the experimentally derived conclusion that constraining structural flexibility with multidentate ligands significantly impacts the excited state relaxation dynamics. Furthermore, our study emphasizes that the arrangement of strong field ligands in heteroleptic complexes substantially affects the energy of Jahn–Teller active triplet metal-centered states in low-spin d6 metal complexes. Together, these findings provide synthetic design principles for extending metal-to-ligand charge transfer excited state lifetimes of heteroleptic Fe complexes.

The ultrafast excited state pathways and dynamics of Ni^II-bipyridine complexes influence the yield of photochemical processes involved in their catalytic cross-coupling reactions. Here we present ultrafast Ni Kβ X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS) of a Ni^II-bipyridine aryl halide complex, [Ni(^t-Bubpy)(o-tol)Br], to quantify the excited state population dynamics and structural changes of the precatalyst. Due to the local spin-sensitivity of XES, the population dynamics of metal-to-ligand charge transfer (MLCT) and metal-centered (MC) excited states is established. A rapid ground state recovery pathway is newly identified, representing a significant deactivation pathway during photocatalysis. Furthermore, the pseudotetrahedral structure of the long-lived MC excited state is unambiguously identified and refined by XSS. The results advance our understanding of the ultrafast relaxation mechanisms that impact the photocatalytic mechanism and yield for Ni^II-bipyridine aryl halide cross-coupling catalysts.

Lifetimes of photoexcited charge transfer (CT) states in transition metal chromophores are influenced by low-lying ligand field (LF) excited states, especially for 3d metal complexes. To manipulate interactions between LF and CT states, it is important to be able to control LF excited state energies using tunable synthetic variables. In this report, we use Fe 2p3d L_{3 -}edge resonant inelastic X-ray scattering (RIXS) to measure LF excited state energies of three homoleptic iron chromophores coordinated by strong-field N-heterocyclic carbenes (NHCs). We investigate the effect of oxidation state and ligand scaffold on LF energies and covalency parameters. A cyclometalated bis(NHC) ligand affords both high LF excited state energies (and thus high 10Dq) as well as high metal-ligand covalency compared to other iron complexes with very strong-field ligands. However, for the set of complexes investigated, we do not observe meaningful correlation between the LF excited state energies and the CT excited state lifetimes. These results illustrate that targeting long-lived CT excited states necessitates control of multiple molecular excited state properties, with destabilization of the LF excited state energies proving necessary, but insufficient, to control the CT excited state lifetime in Fe carbene complexes.

The possible mechanistic roles of bridging ligands in chromophore-catalyst assemblies are examined to assess if the bridge can play an active role in light-driven electron storage towards multi-redox photocatalysis, as opposed to its traditional role facilitating electron transfer (ET) from chromophore-to-catalyst. In a dyad linking a Ru polypyridyl chromophore and Rh proton reduction catalyst, [Ru-Rh]3+, the central azaacene motif in the tetrapyridophenazine (tpphz) bridging ligand is shown here to accumulate electrons, rather than supporting ET to the catalyst. Ultrafast time-resolved X-ray absorption spectroscopies probing the chromophore (Ru L3-edge) and catalyst (Rh L3- and Cl K-edges) atoms demonstrate that photoexcitation induces ET away from the chromophore, but not to the catalyst motif. Ab initio calculations show that the spectral features are consistent with electron accumulation on tpphz. An alternative photocatalytic mechanism is identified, in which tpphz stores electrons and the multi-electron Rh catalyst becomes activated by a disproportionation mechanism. The results indicate a broader role for bridging ligands in photocatalytic assemblies to be considered in their future design.

In photoactive metal complexes the localization of photoexcited charges dictates the site of chemical reactivity, but few studies measure the charge redistribution in these systems with spatial precision. Herein, we track the inter‐ and intra‐ligand charge transfer processes that underpin light‐driven charge separation in the well‐studied “molecular lightswitch” [Ru(bpy) ₂ dppz] ²⁺ (aqueous [Ruthenium ^II (2,2′‐bipyridine)2(dipyrido[3,2‐a:2′,3′‐c]phenazine)] ²⁺ [Cl ⁻ ] ₂ ) by probing the electronic structure of ligand nitrogen atoms in real‐time using ultrafast X‐ray absorption spectroscopy and first principles calculations. We confirm the localization of excited electron density on the phenazine N atoms of dppz and we newly identify two parallel electron transfer pathways to populate this state. Sub‐70 fs electron transfer to the phenazine portion of dppz is observed and attributed to intra‐ligand electron transfer following Ru‐to‐dppz metal‐to‐ligand charge transfer (MLCT) excitation. This fast charge transfer was not reported in prior ultrafast studies. The slower (ca. 2 ps) charge transfer reported extensively in time‐resolved optical absorption and emission studies is reassigned here to inter‐ligand electron “hopping” between nearly isoenergetic ligand moieties following Ru‐to‐bpy MLCT excitation. The results demonstrate much faster charge separation than previously identified in this well‐studied system, highlighting how extended azaacene ligand motifs promote the competitive charge transfer processes needed to drive light‐driven electron transfer chemistry.

In photoactive metal complexes the localization of photoexcited charges dictates the site of chemical reactivity, but few studies measure the charge redistribution in these systems with spatial precision. Herein, we track the inter‐ and intra‐ligand charge transfer processes that underpin light‐driven charge separation in the well‐studied “molecular lightswitch” [Ru(bpy) ₂ dppz] ²⁺ (aqueous [Ruthenium ^II (2,2′‐bipyridine)2(dipyrido[3,2‐a:2′,3′‐c]phenazine)] ²⁺ [Cl ⁻ ] ₂ ) by probing the electronic structure of ligand nitrogen atoms in real‐time using ultrafast X‐ray absorption spectroscopy and first principles calculations. We confirm the localization of excited electron density on the phenazine N atoms of dppz and we newly identify two parallel electron transfer pathways to populate this state. Sub‐70 fs electron transfer to the phenazine portion of dppz is observed and attributed to intra‐ligand electron transfer following Ru‐to‐dppz metal‐to‐ligand charge transfer (MLCT) excitation. This fast charge transfer was not reported in prior ultrafast studies. The slower (ca. 2 ps) charge transfer reported extensively in time‐resolved optical absorption and emission studies is reassigned here to inter‐ligand electron “hopping” between nearly isoenergetic ligand moieties following Ru‐to‐bpy MLCT excitation. The results demonstrate much faster charge separation than previously identified in this well‐studied system, highlighting how extended azaacene ligand motifs promote the competitive charge transfer processes needed to drive light‐driven electron transfer chemistry.