Ligand manipulation of charge transfer excited state relaxation and spin crossover in [Fe(2,2′-bipyridine)2(CN)2]

We have used femtosecond resolution UV-visible and Kβ x-ray emission spectroscopy to characterize the electronic excited state dynamics of [Fe(bpy)2(CN)2], where bpy=2,2′-bipyridine, initiated by metal-to-ligand charge transfer (MLCT) excitation. The excited-state absorption in the transient UV-visible spectra, associated with the 2,2′-bipyridine radical anion, provides a robust marker for the MLCT excited state, while the transient Kβ x-ray emission spectra provide a clear measure of intermediate and high spin metal-centered excited states. From these measurements, we conclude that the MLCT state of [Fe(bpy)2(CN)2] undergoes ultrafast spin crossover to a metal-centered quintet excited state through a short lived metal-centered triplet transient species. These measurements of [Fe(bpy)2(CN)2] complement prior measurement performed on [Fe(bpy)3]2+ and [Fe(bpy)(CN)4]2− in dimethylsulfoxide solution and help complete the chemical series [Fe(bpy)N(CN)6–2N]2N-4, where N = 1–3. The measurements confirm that simple ligand modifications can significantly change the relaxation pathways and excited state lifetimes and support the further investigation of light harvesting and photocatalytic applications of 3d transition metal complexes.

[1]  M. Nielsen,et al.  Coherent structural trapping through wave packet dispersion during photoinduced spin state switching , 2017, Nature Communications.

[2]  Lisa A. Fredin,et al.  A low-spin Fe(iii) complex with 100-ps ligand-to-metal charge transfer photoluminescence , 2017, Nature.

[3]  D. Sokaras,et al.  Charge and Spin-State Characterization of Cobalt Bis(o-dioxolene) Valence Tautomers Using Co Kβ X-ray Emission and L-Edge X-ray Absorption Spectroscopies. , 2017, Inorganic chemistry.

[4]  Lisa A. Fredin,et al.  transfer excited state relaxation and spin crossover in iron coordination complexes with ligand substitution † , 2016 .

[5]  V. Sundström,et al.  Fe N-Heterocyclic Carbene Complexes as Promising Photosensitizers. , 2016, Accounts of chemical research.

[6]  Tadesse A. Assefa,et al.  Femtosecond X-Ray Scattering Study of Ultrafast Photoinduced Structural Dynamics in Solvated [Co(terpy)_{2}]^{2+}. , 2016, Physical review letters.

[7]  T. Penfold,et al.  High-Efficiency Iron Photosensitizer Explained with Quantum Wavepacket Dynamics. , 2016, The journal of physical chemistry letters.

[8]  S. Haacke,et al.  A new record excited state (3)MLCT lifetime for metalorganic iron(ii) complexes. , 2016, Physical chemistry chemical physics : PCCP.

[9]  A. Rappé,et al.  Highly Strained Iron(II) Polypyridines: Exploiting the Quintet Manifold To Extend the Lifetime of MLCT Excited States. , 2016, Journal of the American Chemical Society.

[10]  M. Chergui,et al.  Sub-50-fs photoinduced spin crossover in [Fe(bpy)₃]²⁺. , 2015, Nature chemistry.

[11]  Wenkai Zhang,et al.  Mechanistic studies of photoinduced spin crossover and electron transfer in inorganic complexes. , 2015, Accounts of chemical research.

[12]  F. Neese,et al.  Kβ Mainline X-ray Emission Spectroscopy as an Experimental Probe of Metal–Ligand Covalency , 2014, Journal of the American Chemical Society.

[13]  Kelly J. Gaffney,et al.  Tracking excited-state charge and spin dynamics in iron coordination complexes , 2014, Nature.

[14]  C. de Graaf,et al.  Ultrafast deactivation mechanism of the excited singlet in the light-induced spin crossover of [Fe(2,2'-bipyridine)3]2+. , 2013, Chemistry.

[15]  V. Sundström,et al.  Towards longer-lived metal-to-ligand charge transfer states of iron(II) complexes: an N-heterocyclic carbene approach. , 2013, Chemical communications.

[16]  M. Nielsen,et al.  Spin-state studies with XES and RIXS: From static to ultrafast. , 2013 .

[17]  Jianzhang Zhao,et al.  Triplet photosensitizers: from molecular design to applications. , 2013, Chemical Society reviews.

[18]  Kristoffer Haldrup,et al.  Femtosecond X-ray absorption spectroscopy at a hard X-ray free electron laser: application to spin crossover dynamics. , 2013, The journal of physical chemistry. A.

[19]  Sébastien Boutet,et al.  Energy-dispersive X-ray emission spectroscopy using an X-ray free-electron laser in a shot-by-shot mode , 2012, Proceedings of the National Academy of Sciences.

[20]  M. Chergui.  On the interplay between charge, spin and structural dynamics in transition metal complexes. , 2012, Dalton transactions.

[21]  C. Sousa,et al.  On the role of the metal‐to‐ligand charge transfer states in the light‐induced spin crossover in FeII (bpy)3 , 2011 .

[22]  J. McCusker,et al.  Femtosecond Soft X-ray Spectroscopy of Solvated Transition-Metal Complexes: Deciphering the Interplay of Electronic and Structural Dynamics. , 2011, The journal of physical chemistry letters.

[23]  Frank Neese,et al.  Probing valence orbital composition with iron Kbeta X-ray emission spectroscopy. , 2010, Journal of the American Chemical Society.

[24]  C. de Graaf,et al.  Study of the light-induced spin crossover process of the [Fe(II)(bpy)3]2+ complex. , 2010, Chemistry.

[25]  F. van Mourik,et al.  Vibrational coherences and relaxation in the high-spin state of aqueous [Fe(II)(bpy)3]2+. , 2009, Angewandte Chemie.

[26]  S. Johnson,et al.  Femtosecond XANES Study of the Light-Induced Spin Crossover Dynamics in an Iron(II) Complex , 2009, Science.

[27]  Christian Bressler,et al.  Ultrafast nonadiabatic dynamics of [Fe(II)(bpy)(3)](2+) in solution. , 2007, Journal of the American Chemical Society.

[28]  Benjamin G. Levine,et al.  Isomerization through conical intersections. , 2007, Annual review of physical chemistry.

[29]  A. Shukla,et al.  Probing the 3d spin momentum with X-ray emission spectroscopy: the case of molecular-spin transitions. , 2006, The journal of physical chemistry. B.

[30]  J. McCusker.  Femtosecond absorption spectroscopy of transition metal charge-transfer complexes. , 2003, Accounts of chemical research.

[31]  G. Meyer,et al.  Charge-transfer studies of iron cyano compounds bound to nanocrystalline TiO(2) surfaces. , 2002, Inorganic chemistry.

[32]  F. D. Groot,et al.  High-Resolution X-ray Emission and X-ray Absorption Spectroscopy , 2001 .

[33]  D Murphy,et al.  Highly phosphorescent bis-cyclometalated iridium complexes: synthesis, photophysical characterization, and use in organic light emitting diodes. , 2001, Journal of the American Chemical Society.

[34]  P. Liska,et al.  Engineering of efficient panchromatic sensitizers for nanocrystalline TiO(2)-based solar cells. , 2001, Journal of the American Chemical Society.

[35]  J. McCusker,et al.  Femtosecond Excited-State Dynamics of an Iron(II) Polypyridyl Solar Cell Sensitizer Model , 2000 .

[36]  S. Ferrere.  New Photosensitizers Based upon [Fe(L)2(CN)2] and [Fe(L)3] (L = Substituted 2,2‘-Bipyridine): Yields for the Photosensitization of TiO2 and Effects on the Band Selectivity , 2000 .

[37]  T. Meyer,et al.  Medium Effects on Charge Transfer in Metal Complexes , 1998 .

[38]  T. Meyer,et al.  Medium Effects on Charge Transfer in Metal Complexes. , 1998, Chemical reviews.

[39]  S. Ferrere,et al.  Photosensitization of TiO2 by [FeII(2,2‘-bipyridine-4,4‘-dicarboxylic acid)2(CN)2]: Band Selective Electron Injection from Ultra-Short-Lived Excited States , 1998 .

[40]  R. Peacock,et al.  Electronic absorption spectra of the iron(II) complexes of 2,2'-bipyridine, 2,2'-bipyrimidine, 1,10-phenanthroline, and 2,2':6',2''-terpyridine and their reduction products , 1992 .

[41]  P. S. Braterman,et al.  Spectroelectrochemistry of aromatic ligands and their derivatives. 1. Reduction products of 4,4'-bipyridine, 2,2'-bipyridine, 2,2'-bipyrimidine, and some quaternized derivatives , 1991 .

[42]  C. Creutz,et al.  Solvent tuning of the excited-state properties of (2,2'-bipyridine)tetracyanoferrate(II): direct observation of a metal-to-ligand charge-transfer excited state of iron(II) , 1987 .

[43]  H. Toma,et al.  Spectroscopic studies of preferential and asymmetric solvation in substituted cyanoiron(II) complexes , 1983 .

[44]  T. Netzel,et al.  Lifetimes, spectra, and quenching of the excited states of polypyridine complexes of iron(II), ruthenium(II), and osmium(II) , 1980 .

[45]  A. A. Schilt.  Mixed Ligand Complexes of Iron(II) and (III) with Cyanide and Aromatic Di-imines , 1960 .

[46]  Uwe Bergmann,et al.  High resolution 1s core hole X-ray spectroscopy in 3d transition metal complexes—electronic and structural information , 2005 .

[47]  E. Henry,et al.  [8] Singular value decomposition: Application to analysis of experimental data , 1992 .

[48]  J. Winkler,et al.  Lifetimes and spectra of the excited states of cis-dicyanobis(2,2'-bipyridine)iron(II) and -ruthenium(II) in solution , 1987 .