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Establishing the origin of Marcus-inverted-region behaviour in the excited-state dynamics of cobalt(III) polypyridyl complexes

Nature Chemistry volume 16, pages 1665–1672 (2024)Cite this article

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A Publisher Correction to this article was published on 13 August 2024

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Abstract

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.

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Fig. 1: Structure, ground-state and excited-state properties.
Fig. 2: Schematic representation of the energy gap law and Marcus theory.
Fig. 3: Variable-temperature transient absorption studies.
Fig. 4: Origin of Marcus inverted region in Co(III).
Fig. 5: Proposed model.

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All the data that support the findings of this study are provided via Figshare at https://doi.org/10.6084/m9.figshare.25803085 (ref. 64). Source data are provided with this paper.

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Acknowledgements

We thank E. Jakubikova and M. Deegbey from North Carolina State University for helpful discussions and suggestions. We also thank C. Larsen from University of Auckland for providing a useful new perspective on the Marcus analysis that we allude to in the main text and incorporated into the ESI. This work was supported in part through computational resources and services provided by the Institute for Cyber-Enabled Research at Michigan State University. The research was generously supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, US Department of Energy under grant no. DE-FG02-01ER15282.

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  1. Department of Chemistry, Michigan State University, East Lansing, MI, USA

    Atanu Ghosh, Jonathan T. Yarranton & James K. McCusker

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Contributions

A.G. performed the transient absorption experiments and analysed the data, performed the DFT calculations and created all the figures. J.T.Y synthesized the compounds. All authors contributed to the writing of the. J.K.M directed the project.

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Correspondence to James K. McCusker.

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Nature Chemistry thanks Christopher Larsen and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Computational studies to estimate reorganization energy.

(a) Tanabe-Sugano diagram appropriate for [Co(pyrro-bpy)3](PF6)3 based on the ligand-field analysis described in ref. 29. The diagram was constructing using the experimentally determined Racah B and C parameter values of 480 cm-1 and 3430 cm-1, respectively. The vertical dashed line corresponds to the value of 10 Dq found for [Co(pyrro-bpy)3](PF6)3. It should be emphasized that these diagrams reflect vertical transition energies, not the zero-point energy of a given excited state that ultimately determines which state lies lowest in energy. Inset: An expanded view of the region near the crossing point between the 5T2 and 3T1 ligand-field excited states. (b) Comparison between time-resolved absorption spectra obtained from a singular value decomposition analysis of the experimental transient absorption data and TD-DFT-computed excited-state absorption spectra for the structurally relaxed, lowest-energy S=1 (3MC) excited state. Inset: Spin density associated with the 3MC ligand-field excited state derived from DFT calculations carried out at the optimized equilibrium geometry, showing localization of the spin density predominantly on the metal center. Positive (excess α) and negative (excess β) spin density contributions are shown as green and orange isosurfaces, respectively (isovalue = 0.003). (c) DFT-predicted 1A1→1ES (pink) and 1A1→3MC (blue) vertical transition energies compared with their experimentally determined values (dashed lines). The black triangles at the bottom of the plot correspond to the energy difference between triplet energy at singlet optimized geometry and triplet optimized geometry (black triangles) as a function of % HF exchange.

Source data

Supplementary information

Supplementary Information

Supplementary Discussion.

Supplementary Data 1

Computational data and DFT coordinates of the optimized geometries.

Source data

Source Data Fig. 1

Data used to generate the lifetime traces plot.

Source Data Fig. 3

Kinetic data as a function of temperature and numeric values used to generate the Marcus-type plot shown in Fig. 3d.

Source Data Extended Data Fig. 1

Experimental transient absorption spectra data along with time-dependent DFT-computed excited-state absorption spectra and single-point energies used for benchmarking DFT methods.

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Ghosh, A., Yarranton, J.T. & McCusker, J.K. Establishing the origin of Marcus-inverted-region behaviour in the excited-state dynamics of cobalt(III) polypyridyl complexes. Nat. Chem. 16, 1665–1672 (2024). https://doi.org/10.1038/s41557-024-01564-3

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