l0pt Figure Self-assembling of organic chromophoric systems into elegant supramolecular architectures with emergent properties has received prodigious attention in recent years.¹ In the first example (Figure), a naphthalene-naphthalimide donor-acceptor (D-A) dyad assembled into segregated D-A stacks in the crystalline state. The photo-induced charge separated state in the aggregate state lasts 10,000 times longer than the monomeric dyad. The femtosecond transient absorption spectra depicted the spectroscopic signature for naphthalene dimer radical cation indicating the migration of charges through the stacks.¹ In the second example, we report the crystalline evidence for Greek cross-dipole (α=90^°) stacking of 1,7-dibromoperylene-3,4,9,10-tetracarboxylic tetrabutylester (PTE-Br2) displaying null excitonic coupling and thereby monomeric optical behavior.² In the third example, we showcase a radial assembly of 1,8-dibromonaphthalene(2,6-diisopropylphenyl)imide (NIBr2) in crystalline phase driven by hexabromine synthon.⁴ NIBr2 exhibits ultrafast intersystem crossing³⁻⁵ and solid-state room temperature phosphorescence. References: ¹ A. R. Mallia, P. S. Salini, M. Hariharan, J. Am. Chem. Soc., 137 (2015) 15604. ² E. Sebastian, A. M. Philip, A. Benny, M. Hariharan, Angew. Chem., Int. Ed., 57 (2018) 15696. ³ M. A. Niyas, R. Ramakrishnan, V. Vijay, E. Sebastian, M. Hariharan, J. Am. Chem. Soc, 141 (2019) 4536. ⁴ K. Nagarajan, A. R. Mallia, K. Muraleedharan, M. Hariharan, Chem. Sci., 8 (2017) 1776. ⁵ M. P. Lijina, A. Benny, R. Ramakrishnan, N. Nair, M. Hariharan, J. Am. Chem. Soc. 41 (2020), 17393.

Designing earth-abundant photosensitizers to replace rare-earth complexes is of high importance for the efficient collection and utilization of solar energy. The primary obstacle to using first-row transition metal complexes is the ultrafast dissipation of collected energy due to low energy metal-centered states. Here, we study a series of Fe(II) complexes with different ligand frameworks in hopes to alter the intersection between various metal-centered states. Using femtosecond M-edge XANES, the involvement of triplet and quintet metal-centered states are identified. By decoupling the axial and equatorial stretching modes of an Fe(II) complex, we find that the lifetime of a triplet metal-centered state is significantly increased due to Jahn-Teller distortion in the excited state. This behavior is not observed in prototypical Fe(II) polypyridyl complexes, and provides a method to reduce energy losses in earth-abundant photosensitizers.

The ultrafast electron dynamics of transition metal oxide clusters provides information about the stability and reactivity of their bulk material counterparts. Nickel and chromium oxides are widely used catalytic materials which may be improved with the detailed atomic precision of their stable conformations and low energy electron properties. In our experiments, neutral Ni_nO_m and Cr_nO_m clusters are produced with small amounts of oxygen and their mass spectra show a primary stable stoichiometry of m = n - 1 for Ni clusters and m = 2n - 2 for Cr clusters. I will present our ultrafast pump-probe spectroscopy measurements on the direct ionization, dissociation, formation, and excitation-relaxation properties for these metal oxide cluster systems extending up to n = 9. Nickel oxides show an increase in the lifetime with the decrease in oxygen content, owing to the change in geometry and increase in relaxation pathways. In contrast, chromium oxides generally show a decreased lifetime with less oxygen atoms from the stoichiometric (CrO₂)_n cluster. In both cluster systems, long-lived lifetimes (over 2 ps) are observed in the suboxide clusters. Overall, it is shown that the ultrafast dynamics observed for these clusters depend strongly on their metal composition, size, and oxidation state.

Monolayer protected nanoclusters (MPCs) are structurally-precise, quantum-confined metal nanostructures that include three structural components that influence their optical and electronic properties. These components include (i) an all-metal atom core, (ii) an organometallic semiring of alternating Au-S staples, and (iii) organic passivating ligands. The high degree of structural precision afforded by MPCs provides a well-defined platform to investigate structure-dependent energy relaxation in quantum-confined metals. In this presentation, ultrafast electronic relaxation and charge transfer dynamics will be described. Femtosecond two-dimensional electronic spectroscopy (2DES) provides spectrally resolved excitation-detection correlation maps while preserving the high time resolution of ultrafast spectroscopy by spreading the excitation-detection transient signals over two frequency axes. Here, 2DES was used to distinguish multiple electronic fine-structure states that comprise a charge-transfer resonance excited using laser pulses spanning 1.85 to 2.10 eV. These results show that electronically excited charge carriers relax through internal conversion via intramolecular vibrational relaxation (IVR) processes within 200 fs. The IVR process manifests as spectral diffusion in Au₃₈(SC₆H₁₃)₂₄ nanoclusters, which was monitored by resolving the excitation-detection frequency-frequency correlation function (FFCF) decay obtained from the 2DES measurements. The 2DES-detected FFCF method was applied to understand solvent dependences of the charge carriers, which undergo interfacial ligand-to-metal charge transfer, prior to thermalization. The effects of different ligands were also investigated. These results demonstrate the ability of 2DES combined with FFCF to map energy flow through nearly degenerate states, and quantify rapid internal conversion of excited charge carriers in structurally-precise metal clusters.

Transition metal oxides are widely used as catalysts for industrial processes. Notably, TiO₂ is used in applications such as water splitting, photovoltaics, and environmental degradation of organic pollutants. Despite the wide usage of TiO₂, there is not a clear understanding of the mechanism that underpins its reactivity. The study of ultrafast excitation/relaxation dynamics in neutral titanium oxide clusters offers atomic level insight into the driving forces in bulk reactions. Specifically, we utilize pump-probe spectroscopy to reveal the influence of both size and oxidation affect the excited state lifetimes of neutral metal oxides. All clusters exhibit a rapid relaxation lifetime of  30 fs, followed by a sub-picosecond lifetime that we attribute to carrier recombination. The excited state lifetimes oscillate with size in agreement with the cluster stability, coordination of metal atoms, and localization of electron excitation. These experimental results, when paired with TD-DFT, demonstrate the importance of subtle changes in the atomic positions and geometry over the cluster dynamics. Such revelations relate directly to the significance of defect sites in bulk materials. This presentation will focus on both experimental and computational analysis of sub-nanometer TiO₂ clusters and how the lifetimes of excited states can be understood through analysis of electron/hole dynamics. A thorough discussion of how properties of neutral clusters change with the addition or subtraction of a single atom will be used to provide insight into the properties that make TiO₂ such a favorable material.

Ultrafast relaxation dynamics of neutral aluminum oxide clusters (Al_nO_m, n,m < = 20) in the gas phase were studied by combining two-color femtosecond spectroscopy and time-of-flight mass spectrometry. The clusters are pumped using the second harmonic of a Ti:Sapphire femtosecond (fs) laser system (400 nm = 3.1eV) and subsequently ionized through strong field ionization of the the fundamental beam (800 nm = 1.55 eV) with a time resolution of < 35 fs. The excited states explored here are high binding energy excited states, which are mainly of oxygen 2p character into the Al sp-type orbitals. Changes in the ultrafast dynamics are observed to be strongly influenced by the stoichiometry and size of these clusters. Neutral clusters are ideal mimics of the true active sites of condensed phase and surface chemistry. We have developed an approach that involves the calculation of several topological descriptors to interpret the influence of the structural differences between clusters on the excited state lifetimes. Time-dependent density functional theory (TDDFT) calculations reveal the energies and structures of the excited states at both ground and excited state structure. These studies on molecular scale clusters provide an ideal arena for probing chemical activities and mechanisms in an unperturbed environment that can assist in the analysis of condensed phase catalysis systems. Using the atomic precision of clusters, we identify preferred bonding arrangements, electronic structure, and oxidation states that lead to increased excited state lifetimes and therefore enhance photochemical transformation.

r0pt Figure The Co₄O₄ cubane and its oxidized derivatives are molecular models of heterogeneous cobalt-based oxygen evolution reaction catalysts and are structural cognates of the oxygen evolving center in plant life. Insight into these latter systems is garnered through study of the high-valent states achieved in oxidized cubane. The oxidized cubane species is generated transiently through photoinduced electron transfer to a laser-excited pendant perylene bisimide chromophore and interrogated via extreme ultraviolet (XUV) spectroscopy. The XUV probe effects transitions at the M_2,3 edge (3p→3d) in the cobalt centers and is sensitive to changes in e.g. oxidation and spin states. However, spectroscopic characterization of the high-valent states achieved in through this charge separation is hindered by the concurrent population of metal-centered excited states via energy transfer. Once populated, these states undergo further evolution via intersystem crossing and/or relaxation back into the charge separated state, imparting significant ambiguity into the observed XUV signals. This work clarifies these results through further studies into the ultrafast dynamics of excited cubane molecules, as well as synthetic strategies toward manipulation of the electronics and energetics of the pendant chromophore. As probed by our tabletop high-harmonic generation (HHG) XUV instrument, the excitation energy in this system is followed as it cascades through metal-centered states in the cubane manifold vs. the formation of charge separated states.

Photoinjection studies of two wide bandgap semiconductors, 4H-SiC and GaN, are conducted with fs infrared reflectivity measurements in order to characterize how the relative penetration depths of UV-pump/IR-probe light affects the modulation of the IR reflectivity spectrum. The infrared spectrum of these materials is dominated by the high reflectivity reststrahlen band region that occurs between the longitudinal optical and transverse optical phonons. The injection of free carriers shifts this metal-like region to higher frequencies via coupling of the longitudinal optical phonon to the free carrier plasma (LOPC effect). The result of this LOPC active tuning is strongly perturbed by the charge carrier spatial distribution, and is thus highly sensitive to the means of carrier generation. We probe the effects of charge carrier spatial distribution on the photomodulated reflectivity of two promising wide bandgap semiconductors, indirect bandgap 4H-SiC and direct bandgap GaN, by comparing the transient reflectivity following photoexcitation with light of short and long penetration depths relative to the IR probe depth. This work shows sensitivity of bulk electronic properties to the charge carrier distribution that is critical to understanding the contributions of these materials to complex nanophotonic devices.

Transient extreme ultraviolet (XUV) spectroscopy is becoming well developed for the study of transition metal oxides like those used in solar fuel generation. Si and Ge compounds, other popular solar energy semiconductors, have been used as test cases for the ab-initio prediction of XUV edges and their photoexcited change. Here, we use transient XUV spectroscopy to study the ultrafast carrier dynamics of ZnTe, both as a CO₂ reduction material of interest to the LiSA (Liquid Sunlight Alliance) initiative as well as a test of our group’s ab-initio methods. XUV spectra were taken in a grazing reflection geometry to enhance the sensitivity to carrier dynamics at the ZnTe surface. The Te 4d core-to-valence transition is then measured after broad and narrow band excitation replicating solar or UV light, respectively. Electron and hole energies are observed as a function of time by state-filling effects. A fast electronic response is observed within the ZnTe bandgap and is compared to theoretical signatures of photoexcited renormalization or surface states.

Hybrid organic-inorganic halide perovskites, such as CH₃NH₃PbBr₃ (MAPbBr₃) have emerged as outstanding light absorbing and emitting materials. MAPbBr₃ has a bandgap around 2.3 eV and a high open circuit voltage, making it suitable for tandem solar cell applications. However, detailed studies on photoexcited carrier dynamics of MAPbBr₃ are limited compared to its iodine counterpart. We show that tabletop high-harmonic extreme ultraviolet (XUV) transient absorption spectroscopy measures transition from Br 3d_5/2 and 3d_3/2 to the valence and conduction band of photoexcited MAPbBr₃ perovskite, enabling us to observe carrier specific dynamics. Taking advantage of a straightforward, simple parabolic model incorporating band filling and band gap renormalization, we also study the time evolution of the band gap renormalization effect. Our work highlights the application of femtosecond XUV spectroscopy for understanding non-equilibrium ultrafast photophysics of complex semiconductor materials by distinguishing competing processes: carrier cooling, band gap renormalization, band filling and recombination.

Breaking and making of chemical bonds in molecules attached to a substrate constitute the elementary steps of a surface chemical reaction and occur on the ultrafast time scale of nuclear motion. Monitoring these elementary steps is crucial for a detailed understanding of the mechanism of surface photoinduced and photocatalytic reactions. In this work, an experimental methodology that combines surface mass spectrometry with femtosecond pump-probe technique and ultraviolet multiphoton or extreme ultraviolet single photon ionization is employed to decipher the mechanism of surface photoinduced and photocatalytic reactions. The technique relies on the detection of highly unstable intermediate species and final products with time-, mass-, and energy resolution. In the first part of the talk, the photoinduced reaction dynamics of methyl iodide adsorbed on an amorphous cerium oxide ultrathin film is investigated to understand the effect of a non-crystalline surface on the reaction dynamics. In the second part of the talk, the reaction dynamics that leads to the formation of methane and methanol from methyl iodide and water co-adsorbed on a TiO2(110) surface will be presented. The results presented here demonstrate the viability of the employed experimental methodology to decipher complex surface photoinduced and photocatalytic reaction mechanisms.

X-rays promote electrons from the core levels to vacant valence orbitals, thus endowing them with a unique element specificity. Moreover, the core level transitions can easily sense the shifts in the electron density in the proximity of the probed element. We produce soft X-rays around 280 eV driving high harmonics in a helium gas target with 1470 nm pulses. This table-top broad band X-ray source allows us to investigate the ultrafast dynamics in photoexcited pyrazine (C₄H₄N₂) with a new view of the electronic states involved. In addition to the previously characterized ¹B_2u (ππ*) (S₂) and ¹B_3u (nπ*) (S₁) states, the participation of the optically dark ¹A_u (nπ*) state is assigned by a combination of experimental X-ray core-to-valence spectroscopy, electronic structure calculations and nonadiabatic dynamics simulations. The ¹A_u (nπ*) state is populated about 200 femtoseconds after electronic excitation and plays a key role in the relaxation of pyrazine to the ground state.

Third-row transition metal complexes, particularly those containing platinum and iridium, are ubiquitous photosensitizers and catalysts, and transient x-ray absorption near edge structure (XANES) spectroscopy at the metals’ L-edges has been increasingly used to study their photodynamics. While synchrotron XANES provides high photon flux and ultrafast time resolution, beamtime is unfortunately limited. We have examined 5d metal complexes using a tabletop XANES spectrometer which generates femtosecond XUV pulses (30-100 eV, 30 fs IRF) via high-harmonic generation (HHG), where the N_6,7-edge (4f-5d) and O_2,3-edge (5p-5d) of these metals appear. We show the ground state N-edge and O-edge XUV spectra of several platinum and iridium complexes. We will use transient XUV spectroscopy to demonstrate that these edges can be used to track oxidation state changes at the metal center after excitation of a MLCT transition in Ir(ppy)₃. Overall, this will provide information on the metal electronic structure and allow for parsing out the impact of metal vs ligands in the excited states when combined with other transient spectroscopies.