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

Kunnus, Kristjan; Li, Lin; Titus, Charles J.; Lee, Sang Jun; Reinhard, Marco E.; Koroidov, Sergey; Kjær, Kasper S.; Hong, Kiryong; Ledbetter, Kathryn; Doriese, William B.; O'Neil, Galen C.; Swetz, Daniel S.; Ullom, Joel N.; Li, Dale; Irwin, Kent; Nordlund, Dennis; Cordones, Amy A.; Gaffney, Kelly J.

doi:10.1039/c9sc06272f

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

Journal Article · Thu Apr 16 00:00:00 EDT 2020 · Chemical Science

DOI:https://doi.org/10.1039/c9sc06272f· OSTI ID:1632150

^[1]; Li, Lin ^[1];

^[2];

^[3];

^[1]; Koroidov, Sergey ^[1];

^[1]; Hong, Kiryong ^[1]; Ledbetter, Kathryn ^[4]; Doriese, William B. ^[5];

^[5]; Swetz, Daniel S. ^[5]; Ullom, Joel N. ^[5]; Li, Dale ^[3]; Irwin, Kent ^[6]; Nordlund, Dennis ^[3]; Cordones, Amy A. ^[1]; Gaffney, Kelly J. ^[1]

SLAC National Accelerator Lab., Menlo Park, CA (United States). Photon Ultrafast Laser Science and Engineering Inst. (PULSE)
Stanford Univ., CA (United States). Dept. of Physics
SLAC National Accelerator Lab., Menlo Park, CA (United States)
SLAC National Accelerator Lab., Menlo Park, CA (United States). Photon Ultrafast Laser Science and Engineering Inst. (PULSE); Stanford Univ., CA (United States). Dept. of Physics
National Inst. of Standards and Technology (NIST), Boulder, CO (United States)
Stanford Univ., CA (United States). Dept. of Physics; SLAC National Accelerator Lab., Menlo Park, CA (United States)

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 (hν = 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.

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Research Organization:: SLAC National Accelerator Lab., Menlo Park, CA (United States)

Sponsoring Organization:: USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences & Biosciences Division; Knut and Alice Wallenberg (KAW) Foundation; Carlsberg Foundation; Danish Council for Independent Research

Grant/Contract Number:: AC02-76SF00515; KAW 2012.0359; KAW2014.0370

OSTI ID:: 1632150

Journal Information:: Chemical Science, Vol. 11, Issue 17; ISSN 2041-6520

Publisher:: Royal Society of ChemistryCopyright Statement

Country of Publication:: United States

Language:: English

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Cited by: 14 works

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