Bridged μ-oxo iron porphyrins serve as photocatalysts for oxidative organic transformations, but suffer from low photon-to-product efficiency. This low photochemical quantum yield is most commonly attributed to the short lifetime of a disproportionated TPPFe(II)/TPPFe(IV)=O state, but an alternate hypothesis suggests that the majority photoproduct is a catalytically inactive ligand-centered TPPFe(III)+/TPPFe(III)–O – ion pair. We use femtosecond optical and extreme ultraviolet (XUV) spectroscopy to investigate the early photodynamics of the μ-oxo iron bisporphyrin (TPPFe) 2 O and identify the primary loss mechanism. XUV spectroscopy probes 3p → 3d transitions, corresponding to M 2,3 -edge XANES spectra, and is a distinctive probe of the metal oxidation state. Excitation of the mixed π–π*/ligand-to-metal charge transfer (LMCT) band results in the formation of an iron(II)/iron(III) LMCT state in tens of femtoseconds. This state decays on a subpicosecond timescale to the ligand-centred iron(III) ion pair state, and no TPPFe(IV)=O species is observed within the sensitivity of the measurement. Finally, the lack of an iron(II)/iron(IV) XANES spectrum suggests that preferential formation of the inactive iron(III) ion pair state is a main cause of the low quantum yield of this and similar bisporphyrins.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2022

Toward Improved Charge Separation through Conformational Control in Copper Coordination Complexes

The continued development of solar energy as a renewable resource necessitates the design and study of new approaches to sustaining photodriven charge separation (CS). To this end, we present a bioinspired approach in which triggered conformational changes are used to control electron transfer (ET) events. We report, photo-induced conformational rearrangements of a ligand are translated into changes in the coordination geometry and environment about a bound metal ion. Taking advantage of the differential coordination properties of Cu I and Cu II , these dynamics facilitate intramolecular ET from Cu I to the ligand to create a CS state. The synthesis and photophysical characterization of CuCl(dpaa R ) (dpaa = dipicolylaminoacetophenone, with R = H and OMe) is presented. These ligands incorporate a fluorophore into their framework that gives rise to a twisted intramolecular charge transfer (TICT) excited state. Excited state ligand twisting provides a tetragonal coordination geometry capable of capturing Cu II in the CS state when an internal ortho-OMe binding site is present (as in dpaaOMe). We employ NMR, IR, EPR, and optical spectroscopies, X-ray diffraction, and electrochemical methods to establish the ground state properties of the Cu I and Cu II complexes. We then investigate the photophysical dynamics of these Cu I complexes via time-resolved photoluminescence (TR-PL), and optical transient absorption (OTA) spectroscopies. We show that relative to controls lacking a TICT-active ligand, the lifetime of the CS state is enhanced ~1000-fold. Further, the presence of the ortho-OMe substituent greatly enhances the lifetime of the TICT* state and biases the coordination environment toward Cu II . The presence of Cu I decreases photoinduced degradation from 14 to <2% but does not result in significant quenching via ET. Factors affecting CS in these systems are discussed, laying the groundwork for our strategy toward solar energy conversion.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2021

Ballistic Δ S = 2 intersystem crossing in a cobalt cubane following ligand-field excitation probed by extreme ultraviolet spectroscopy

We report femtosecond M 2,3 -edge X-ray absorption near-edge structure (XANES) spectroscopy is used to probe the excited-state dynamics of the cobalt cubane [Co III 4 O 4 ](OAc) 4 (py) 4 (OAc = acetate, py = pyridine), a model for water oxidation catalysts. After ligand-field excitation, intersystem crossing (ISC) to a metal-centered quintet occurs in 38 fs. 30% of the hot quintet undergoes ballistic back-ISC directly to the singlet ground state, with the remainder relaxing to a long-lived triplet.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2021

Carrier-Specific Hot Phonon Bottleneck in CH 3 NH 3 PbI 3 Revealed by Femtosecond XUV Absorption

Femtosecond carrier cooling in the organohalide perovskite semiconductor CH 3 NH 3 PbI 3 is measured using extreme ultraviolet (XUV) and optical transient absorption spectroscopy. XUV absorption between 44 and 58 eV measures transitions from the I 4d core to the valence and conduction bands and gives distinct signals for hole and electron dynamics. The core-to-valence-band signal directly maps the photoexcited hole distribution and provides a quantitative measurement of the hole temperature. The combination of XUV and optical probes reveals that upon excitation at 400 nm, the initial hole distribution is 3.5 times hotter than the electron distribution. At an initial carrier density of 1.4 × 10 20 cm –3 both carriers are subject to a hot phonon bottleneck, but at 4.2 × 10 19 cm –3 the holes cool to less than 1000 K within 400 fs. Finally, this result places significant constraints on the use of organohalide perovskites in hot-carrier photovoltaics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2021

Polymer thin films as universal substrates for extreme ultraviolet absorption spectroscopy of molecular transition metal complexes

Polystyrene and polyvinyl chloride thin films are explored as sample supports for extreme ultraviolet (XUV) spectroscopy of molecular transition metal complexes. Thin polymer films prepared by slip-coating are flat and smooth, and transmit much more XUV light than silicon nitride windows. Analytes can be directly cast onto the polymer surface or co-deposited within it. The M-edge XANES spectra (40–90 eV) of eight archetypal transition metal complexes (M = Mn, Fe, Co, Ni) are presented to demonstrate the versatility of this method. The films are suitable for pump/probe transient absorption spectroscopy, as shown by the excited-state spectra of Fe(bpy) 3 2+ in two different polymer supports.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2021

Synthesis of a series of M( ii ) (M = Mn, Fe, Co) chloride complexes with both inter- and intra-ligand hydrogen bonding interactions

Hydrogen bonding networks are vital for metallo-enzymes to function; however, modeling these systems is non-trivial. The development of 1st-row transition metal chloride complexes with intramolecular hydrogen-bonding interactions are detailed herein.

Leahy, Clare A.↗