Michael Sachs

All Publications

• Revealing Parallel Inter- and Intra-ligand Charge Transfer Dynamics in [Ru(L)2(dppz)]2+ Molecular Lightswitch with N K-edge X-ray Absorption Spectroscopy. Angewandte Chemie (International ed. in English) Ryland, E. S., Yang, X., Garratt, D., Henke, W. C., Kahraman, A., Taub, M., Sachs, M., Biasin, E., Hampton, C. Y., Hoffman, D. J., Coslovich, G., Kunnus, K., Dakovski, G. L., Mara, M. W., Chen, L. X., Mulfort, K. L., Li, X., Cordones, A. A. 2025: e202509496

  Abstract

  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)2dppz]2+ (aqueous [RutheniumII(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.

  View details for DOI 10.1002/anie.202509496

  View details for PubMedID 40638863

• Metal-centred states control carrier lifetimes in transition metal oxide photocatalysts. Nature chemistry Sachs, M., Harnett-Caulfield, L., Pastor, E., Davies, B., Sowood, D. J., Moss, B., Kafizas, A., Nelson, J., Walsh, A., Durrant, J. R. 2025

  Abstract

  Efficient sunlight-to-energy conversion requires materials that can generate long-lived charge carriers upon illumination. However, the targeted design of semiconductors possessing intrinsically long lifetimes remains a key challenge. Here using a series of transition metal oxides, we establish a link between carrier lifetime and electronic configuration in transition metal-based semiconductors. We identify a subpicosecond relaxation mechanism via metal-centred ligand field states that compromise quantum yields in open d-shell transition metal oxides (for example, Fe2O3, Co3O4, Cr2O3 and NiO), which is more reminiscent of molecular complexes than crystalline semiconductors. We found that materials with spin-forbidden ligand field transitions could partially mitigate this relaxation pathway, explaining why Fe2O3 achieves higher photoelectrochemical activity than other visible light-absorbing transition metal oxides. However, achieving high yields of long-lived charges requires transition metal oxides with d0 or d10 electronic configurations (for example, TiO2 and BiVO4), where ligand field states are absent. These trends translate to transition metal-containing semiconductors beyond oxides, enabling the design of photoabsorbers with better-controlled recombination channels in photovoltaics, photocatalysis and communication devices.

  View details for DOI 10.1038/s41557-025-01868-y

  View details for PubMedID 40603604

  View details for PubMedCentralID 4210134

• Unravelling the effects of active site density and energetics on the water oxidation activity of iridium oxides NATURE CATALYSIS Liang, C., Rao, R. R., Svane, K. L., Hadden, J. H. L., Moss, B., Scott, S. B., Sachs, M., Murawski, J., Frandsen, A., Riley, D., Ryan, M. P., Rossmeisl, J., Durrant, J. R., Stephens, I. E. L. 2024

  View details for DOI 10.1038/s41929-024-01168-7

  View details for Web of Science ID 001242129100001

• Correlating activities and defects in (photo)electrocatalysts using in-situ multi-modal microscopic imaging. Nature communications Mesa, C. A., Sachs, M., Pastor, E., Gauriot, N., Merryweather, A. J., Gomez-Gonzalez, M. A., Ignatyev, K., Giménez, S., Rao, A., Durrant, J. R., Pandya, R. 2024; 15 (1): 3908

  Abstract

  Photo(electro)catalysts use sunlight to drive chemical reactions such as water splitting. A major factor limiting photocatalyst development is physicochemical heterogeneity which leads to spatially dependent reactivity. To link structure and function in such systems, simultaneous probing of the electrochemical environment at microscopic length scales and a broad range of timescales (ns to s) is required. Here, we address this challenge by developing and applying in-situ (optical) microscopies to map and correlate local electrochemical activity, with hole lifetimes, oxygen vacancy concentrations and photoelectrode crystal structure. Using this multi-modal approach, we study prototypical hematite (α-Fe2O3) photoelectrodes. We demonstrate that regions of α-Fe2O3, adjacent to microstructural cracks have a better photoelectrochemical response and reduced back electron recombination due to an optimal oxygen vacancy concentration, with the film thickness and extended light exposure also influencing local activity. Our work highlights the importance of microscopic mapping to understand activity, in even seemingly homogeneous photoelectrodes.

  View details for DOI 10.1038/s41467-024-47870-9

  View details for PubMedID 38724495

  View details for PubMedCentralID 5333116