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Issue 17, 2020
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Chemical control of competing electron transfer pathways in iron tetracyano-polypyridyl photosensitizers

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Abstract

Photoinduced intramolecular electron transfer dynamics following metal-to-ligand charge-transfer (MLCT) excitation of [Fe(CN)4(2,2′-bipyridine)]2− (1), [Fe(CN)4(2,3-bis(2-pyridyl)pyrazine)]2− (2) and [Fe(CN)4(2,2′-bipyrimidine)]2− (3) were investigated in various solvents with static and time-resolved UV-Visible absorption spectroscopy and Fe 2p3d resonant inelastic X-ray scattering (RIXS). This series of polypyridyl ligands, combined with the strong solvatochromism of the complexes, enables the 1MLCT vertical energy to be varied from 1.64 eV to 2.64 eV and the 3MLCT lifetime to range from 180 fs to 67 ps. The 3MLCT lifetimes in 1 and 2 decrease exponentially as the MLCT energy increases, consistent with electron transfer to the lowest energy triplet metal-centred (3MC) excited state, as established by the Tanabe–Sugano analysis of the Fe 2p3d RIXS data. In contrast, the 3MLCT lifetime in 3 changes non-monotonically with MLCT energy, exhibiting a maximum. This qualitatively distinct behaviour results from a competing 3MLCT → ground state (GS) electron transfer pathway that exhibits energy gap law behaviour. The 3MLCT → GS pathway involves nuclear tunnelling for the high-frequency polypyridyl breathing mode ( = 1530 cm−1), which is most displaced for complex 3, making this pathway significantly more efficient. Our study demonstrates that the excited state relaxation mechanism of Fe polypyridyl photosensitizers can be readily tuned by ligand and solvent environment. Furthermore, our study reveals that extending charge transfer lifetimes requires control of the relative energies of the 3MLCT and the 3MC states and suppression of the intramolecular distortion of the acceptor ligand in the 3MLCT excited state.

Graphical abstract: Chemical control of competing electron transfer pathways in iron tetracyano-polypyridyl photosensitizers

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Supplementary files

Article information


Submitted
11 Dec 2019
Accepted
15 Apr 2020
First published
16 Apr 2020

This article is Open Access
All publication charges for this article have been paid for by the Royal Society of Chemistry

Chem. Sci., 2020,11, 4360-4373
Article type
Edge Article

Chemical control of competing electron transfer pathways in iron tetracyano-polypyridyl photosensitizers

K. Kunnus, L. Li, C. J. Titus, S. J. Lee, M. E. Reinhard, S. Koroidov, K. S. Kjær, K. Hong, K. Ledbetter, W. B. Doriese, G. C. O'Neil, D. S. Swetz, J. N. Ullom, D. Li, K. Irwin, D. Nordlund, A. A. Cordones and K. J. Gaffney, Chem. Sci., 2020, 11, 4360
DOI: 10.1039/C9SC06272F

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