4 Search Results

Abstract The non-equilibrium dynamics of electrons and nuclei govern the function of photoactive materials. Disentangling these dynamics remains a critical goal for understanding photoactive materials. Here we investigate the photoinduced dynamics of the [Fe(bmip) ₂ ] ²⁺ photosensitizer, where bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)-pyridine, with simultaneous femtosecond-resolution Fe Kα and Kβ X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS). This measurement shows temporal oscillations in the XES and XSS difference signals with the same 278 fs period oscillation. These oscillations originate from an Fe-ligand stretching vibrational wavepacket on a triplet metal-centered ( ³ MC) excited state surface. This ³ MC state is populated withmore » a 110 fs time constant by 40% of the excited molecules while the rest relax to a ³ MLCT excited state. The sensitivity of the Kα XES to molecular structure results from a 0.7% average Fe-ligand bond length shift between the 1 s and 2p core-ionized states surfaces.« less

Abstract Iron N‐heterocyclic carbene (NHC) complexes have received a great deal of attention recently because of their growing potential as light sensitizers or photocatalysts. We present a sub‐ps X‐ray spectroscopy study of an Fe ^II NHC complex that identifies and quantifies the states involved in the deactivation cascade after light absorption. Excited molecules relax back to the ground state along two pathways: After population of a hot ³ MLCT state, from the initially excited ¹ MLCT state, 30 % of the molecules undergo ultrafast (150 fs) relaxation to the ³ MC state, in competition with vibrational relaxation and cooling to the relaxedmore » ³ MLCT state. The relaxed ³ MLCT state then decays much more slowly (7.6 ps) to the ³ MC state. The ³ MC state is rapidly (2.2 ps) deactivated to the ground state. The ⁵ MC state is not involved in the deactivation pathway. The ultrafast partial deactivation of the ³ MLCT state constitutes a loss channel from the point of view of photochemical efficiency and highlights the necessity to screen transition‐metal complexes for similar ultrafast decays to optimize photochemical performance.« less

Iron N-heterocyclic carbene (NHC) complexes have received a great deal of attention recently because of their growing potential as light sensitizers or photocatalysts. Here, we present a sub-ps X-ray spectroscopy study of an Fe^IINHC complex that identifies absorption. Excited molecules relax back to the ground state along two pathways: After population of a hot ³MLCT state, from the initially excited ¹MLCT state, 30 % of the molecules undergo ultrafast (150 fs) relaxation to the ³MC state, in comand quantifies the states involved in the deactivation cascade after light petition with vibrational relaxation and cooling to the relaxed ³MLCT state. Themore » relaxed ³MLCT state then decays much more slowly (7.6 ps) to the ³MC state. The ³MC state is rapidly (2.2 ps) deactivated to the ground state. The ⁵MC state is not involved in the deactivation pathway. The ultrafast partial deactivation of the ³MLCT state constitutes a loss channel from the point of view of photochemical efficiency and highlights the necessity to screen transition-metal complexes for similar ultrafast decays to optimize photochemical performance.« less

The excited state dynamics of solvated [Fe(bpy)(CN)₄]^2-, where bpy = 2,2'-bipyridine, show significant sensitivity to the solvent Lewis acidity. Using a combination of optical absorption and X-ray emission transient spectroscopies, we have previously shown that the metal to ligand charge transfer (MLCT) excited state of [Fe(bpy)(CN)₄]^2- has a 19 picosecond lifetime and no discernable contribution from metal centered (MC) states in weak Lewis acid solvents, such as dimethyl sulfoxide and acetonitrile. Here, in the present work, we use the same combination of spectroscopic techniques to measure the MLCT excited state relaxation dynamics of [Fe(bpy)(CN)₄]^2- in water, a strong Lewis acidmore » solvent. The charge-transfer excited state is now found to decay in less than 100 femtoseconds, forming a quasi-stable metal centered excited state with a 13 picosecond lifetime. We find that this MC excited state has triplet (³MC) character, unlike other reported six-coordinate Fe(II)-centered coordination compounds, which form MC quintet (⁵MC) states. The solvent dependent changes in excited state non-radiative relaxation for [Fe(bpy)(CN)₄]^2- allows us to infer the influence of the solvent on the electronic structure of the complex. Lastly, the robust characterization of the dynamics and optical spectral signatures of the isolated ³MC intermediate provides a strong foundation for identifying ³MC intermediates in the electronic excited state relaxation mechanisms of similar Fe-centered systems being developed for solar applications.« less