We present a detailed study of the time-dependent spectroscopy, photophysics and photochemistry following excitation of single vibronic levels of a single, known conformation of the two protonated pentapeptides, Leu-enkephalin (Tyrosine-Glycine-Glycine-Phenylalanine-Leucine, YGGFL) and its chromophore-swapped analog FGGYL, carried out under cryo-cooled conditions in the gas phase. Using UV-IR double resonance, we recorded excited state IR spectra over the 2400-3800 cm⁻¹ region and used them to identify unique tyrosine (Tyr) OH stretch transitions that report on the excited state present at a given delay between UV and IR laser pulses. Photofragment mass spectra that reflect the relative product yields were recorded out of the S1 origin (UV only) and following IR excitation at different delays between UV and IR excitation. Three competing site-specific fragmentation pathways were discovered involving peptide backbone cleavage, Tyr sidechain loss, and N-terminal NH₃ loss mediated by electron transfer. In YGGFL, IR excitation in the S1 state promotes electron transfer (ET) from the aromatic ring to the N-terminal R-NH₃⁺ group leading to loss of neutral NH₃. This product channel is missing in FGGYL due to the larger distance for ET from Y(4) to NH₃⁺. Selective loss of the Tyr sidechain occurs out of an excited state process that is enhanced by IR excitation in S1 and T1(v) states of both YGGFL and FGGYL. Finally, IR excitation out of S1 selectively produces backbone fragmentation at the amide(4) group, producing the b₄⁺ cation. We postulate that this selective fragmentation results from intersystem crossing to produce vibrationally excited triplet state parent ions with enough energy to launch the proton along a proton conduit present in the known starting structure, irrespective of the position of initial site-specific electronic excitation.

Molecules containing two electronic chromophores that are chemically distinct from one another (e.g., with one serving as ‘donor’ and the other as ‘acceptor’) can undergo selective excitation by choice of excitation wavelengths that are often separated by tens or hundreds of nanometers. This talk focuses on the fascinating circumstance in which the two chromophores are chemically identical, differing only in the local environments in which they reside. We use the protonated pentapeptide Tyrosine-Glycine-Glycine-Tyrosine-Leucine (Tyr-Gly-Gly-Tyr-Leu, YGGYL) as a scaffold for placing tyrosine and/or deuterated tyrosine (‘dY’) in well-defined positions. We study this protonated peptide in the gas phase under cryo-cooled conditions ( ∼ 5 K). Using IR-UV double resonance, we record the infrared spectrum of YGGYL in the ground electronic state and determine its 3D structure in which the two Tyr chromophores are separated by 11 Å on opposite sides of the peptide scaffold. Due to their different local environments on the scaffold, we prove that the two Tyr chromophores have distinct electronic origins and vibronic structure that can nevertheless be selectively excited with a tunable UV laser. Since electronic absorption of the ions is observed by photofragmentation, mass spectra of the photofragments report directly on the competing photochemical pathways occurring out of the excited electronic states accessed by the laser. We synthesize versions of the peptide selectively deuterated on one or the other Tyr ring, YGG(dY)L and (dY)GGYL. In so doing, we can distinguish between competing photofragmentation channels involving loss of the tyrosyl sidechain from either chromophore (·CH₂-Ph-OH or ·CH₂-Ph(d4)-OH). We also recorded IR spectra following UV excitation of Y(1) or Y(4), thereby obtaining not only the infrared spectral signatures of the excited states but also accessing vibrational states higher up in the electronic manifold that can influence the photochemistry. Three enhanced fragmentation channels are discovered, which involved peptide backbone cleavage, loss of the two tyrosyl sidechain (Y(1) and Y(4)) and loss of N-terminal NH₃. We find that terminal tyrosyl sidechain loss (Y(1)) is enhanced preferentially over Y(4) loss. We will discuss a photochemical scheme that accounts for key aspects of the competing photochemical channels.

r0pt Figure First experimental observations and theoretical explanations of rotational and vibrational magnetism in molecules date back over 50 years, as they have been mostly investigated in the golden era of microwave spectroscopy.R.E. Moss and A.J. Perry; The vibrational Zeeman effect, Molecular Physics, 1973 25, 1121-1134W.H. Flygare; Magnetic interactions in molecules and an analysis of molecular electronic charge distribution from magnetic parameters, Chemical Reviews, 1974, 74, 653-687. Yet, a comprehensive theoretical description of this phenomenon is still missing. In this talk, different models and approaches will be presented, and their validity is assessed on selected molecules. In the light of most recent interest in `molecular magnets', typically represented by transition-metal complexes, we will put special emphasis on the metal phthalocyanines, a highly versatile class of aromatic, planar, macrocyclic molecules with a chelated central metal ion. The excitation of twofold degenerate molecular vibrations, with a relative phase shift of π/2 between the two modes, causes a pseudo-rotational movement of each individual nucleus. While the latter is easily translated into a contribution to the overall magnetic field through electrodynamics (e.g. as the dipole field of a current loop), the description of the electronic contribution is less straight-forward. Possible ways to describe both parts will be discussed and compared, ranging from partial charge models to more tedious quantum-mechanical treatments of this effect.R. Willhelmer, M. Diez, J.K. Krondorfer and A.W. Hauser; Molecular pseudorotation in phthalocyanines as a tool for magnetic field control at the nanoscale, Journal of the Americal Chemical Society, submitted, 2024html:<hr /><h3>Footnotes: R.E. Moss and A.J. Perry; The vibrational Zeeman effect, Molecular Physics, 1973 25, 1121-1134

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Footnotes:

R. Willhelmer, M. Diez, J.K. Krondorfer and A.W. Hauser; Molecular pseudorotation in phthalocyanines as a tool for magnetic field control at the nanoscale, Journal of the Americal Chemical Society, submitted, 2024

Charge migration in molecules, which refers to the movement of electron density driven by pure electronic excitation, has attracted significant attention in the past decades. studying migration can help our understanding of coupled electron-nuclei motion and has the potential to allow us to predict and control chemical reactions at the most fundamental level. Propiolic acid (HCCCOOH), which is primarily used in organic synthesis and chemical research, has recently been a subject of interest in attosecond spectroscopy. Theoretical studies on this polyatomic system suggest strong charge migration dynamics between the carbon-carbon triple bond and carbon-oxygen double bond. However, there is not yet any experimental work on the subject. Not much is known about its strong field ionization and dissociation dynamics either. We are interested in applying the two-electron angular streaking technique to study the charge migration dynamics in propiolic acid. The two-electron angular streaking (2eAS) technique is based on strong field ionization and measuring the 3D momentum of ejected photoelectrons. The attosecond time resolution is achieved by measuring the relative ejection angles between the two electrons. In this preliminary study, using velocity map imaging (VMI) in coincidence mode, we identified three fragmentation channels from strong-field double ionization of propiolic acid: HCCCO⁺ (mass 53) and OH⁺ (mass 17), HCCO⁺ (mass 41) and HOC⁺ (mass 29), HOCO⁺ (mass 45) and HCC⁺ (mass 25), and measured the branching ratios. At least one channel (53, 17) shows a clear angle dependence of the ionization rate. This angle dependence, which arises from the molecular symmetry and the orientation of its electronic orbitals could potentially offer insights into the charge migration pathways. This suggests propiolic acid to be a good system for an attosecond two-electron angular streaking experiment.

Mg⁺(benzene) is generated in a supersonic molecular beam using laser vaporization of a magnesium rod in a pulsed expansion of argon. Ions are analyzed and mass-selected using a reflectron time-of-flight mass spectrometer. Photodissociation results in a dissociative charge transfer process producing benzene⁺. DC slice Velocity Map Imaging (VMI) experiments were performed using 355 nm (3.49 eV) and 266 nm (4.66 eV). The velocity distribution of benzene⁺ at 355 nm and 266 nm was used to determine an upper limit on the dissociation energy. Photodissociation spectra for the Mg⁺ and the benzene⁺ fragments were measured in the visible and UV region using a tunable UV-VIS OPO. The resonances observed provide insight on the electronic structure which helps with the interpretation of the VMI experiment. Results are then compared to B3LYP/Def2TZVP calculations and a published CID value of the dissociation energy.

Iron bisporphyrin molecules are commonly used as photocatalysts for oxidation reactions, but their quantum yield is very low, on the order of 10⁻⁴. To determine the underlying mechanisms preventing a high quantum yield, femtosecond Fe K-edge x-ray absorption and emission spectroscopy experiments were performed at the Linac Coherent Light Source x-ray free electron laser to explore the excited state dynamics of bis-μ-oxo Fe(III) tetraphenylporphyrin. These experiments reveal that upon photoexcitation with 400 nm light, the catalytically active Fe(IV) state is formed in miniscule amounts.

Multi-metallic transition metal complexes (TMCs) represent a step towards supramolecular assemblies that can ultimately enable control of the charge flow in solution-based photo-redox processes. However, the development of design guidelines for such molecular assemblies is impeded by a lack of understanding of the interplay of TMCs with their liquid environment upon electron transfer. Therefore, we herein use an X-ray free electron laser as an element-specific probe of ultrafast electronic and atomic motion^1,2 in a trimetallic cyanide-bridged iron-ruthenium complex (FeRuFe) to evaluate the effect of solute-solvent hydrogen bonding interactions upon intramolecular electron transfer. The experiment is performed in a pump-probe geometry on a liquid jet, where a visible/near-infrared pump induces a metal-to-metal charge transfer (MMCT) between Ru and Fe which is followed by an ultrafast back-electron-transfer. To resolve and correlate electronic and structural dynamics upon MMCT excitation, we simultaneously perform ultrafast X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS) measurements in a series of solvents with different hydrogen bonding properties. XES is sensitive to the oxidation and spin state of the Fe centres and XSS provides information on atomic motion in solute and solvent, as demonstrated recently.³ The resulting molecular level understanding of solvent reorganization coupled to electron transfer highlights that the strength and type of solute-solvent interactions are a central factor in determining the outcome of photoinduced charge transfer processes in TMCs. References: [1] Schoenlein, R. et al. Philos. Trans. R. Soc. Math. Phys. Eng. Sci. 377, 20180384 (2019). [2] Gaffney, K. J. Chem. Sci. 12, 8010–8025 (2021). [3] Biasin, E. et al. Nat. Chem. 13, 343–349 (2021).

Localized plasmon-induced electrochemical reactivity at gold nanorod surfaces has drawn considerable attention for potential applications in catalysis and sensing. In this work, we utilized single-particle dark-field hyperspectral imaging to investigate the electrodissolution of gold nanorods under dark or laser illumination. We discovered that the blue shift in the plasmon resonance of gold nanorods under illumination indicated the location of d-band holes is primarily responsible for the preferential dissolution of the tip region. Furthermore, we investigated the competition between hot carrier location and surface electrochemistry on reactive hotspots in gold nanorods electrodissolution, leading to a more nuanced picture of the nanolocalized reactivity of gold nanorods. The effects of applied potential, electrolyte concentration, laser power density, and laser wavelength on the dissolution behavior of gold nanorods were investigated, all of which were found to affect the balance between hot carrier location dominating and surface electrochemistry dominating electrodissolution. This work provides important insights into the fundamental mechanisms underlying the nanolocalized reactivity of gold nanorods and opens up new avenues for the design of improved catalysts. The understanding gained from this study will be useful for the rational design of efficient plasmon-driven catalytic systems for various applications in energy conversion and environmental remediation.

Plasmon decay at the metal interface generates energetic carriers. However, effectively harnessing these hot carriers presents a significant challenge. At metal-liquid interface, hot electrons generated from plasmonic nanoparticles can inject into the solution forming solvated electrons. These solvated electrons have excellent reducing power, making them ideal homogeneous catalysts for redox chemistry. While their non-plasmonic generation typically requires harsh physical and chemical conditions that can also create other reactive species, limiting their applicability, plasmon-induced solvated electron production occurs selectively under mild light excitation. However, the photoemission quantum yields of such studies [1,2] remain low. Through careful fabrication of nanostructured electrodes and tuning of electrochemical parameters, electric fields can be increased leading to enhanced photoemission quantum yields. One of such strategies would be to switch to organic solvents, where threshold energy can be lowered, thereby increasing the photoemission yield. The long lifetimes of solvated electrons, ranging from microseconds in water up to seconds in organic solvents, make them ideal candidates for initiating reduction reactions. One of the few organic solvents that can stabilize solvated electrons at room temperature is hexamethylphosphoramide (HMPA), a highly relevant solvent for petrochemicals. In this work, we are employing 100 nm silver nanosphere decorated electrodes to generate solvated electrons in HMPA under low intensity one-photon excitation conditions using near UV irradiation. To test the reactivity of plasmon generated solvated electrons we are conducting an in-situ reduction of 2-methyl cyclopentanone, a cyclic ketone. To accurately quantify the reduction reaction yield, we have established a routine quantitative NMR (qNMR) protocol. 1. Mechanism for plasmon-generated solvated electrons, PNAS, 120, 3, e2217035120 (2023) 2. Plasmon-Generated Solvated Electrons for Chemical Transformations, J. Am. Chem. Soc, 2022, 144, 44, 20183–20189.

Research has been conducted on nonfullerene acceptors, (NFAs), as an approach to increase the efficiency of organic photovoltaics (OPVs). In this study, the investigated NFAs are called bis(naphthalene-imide)arylenelidenes (BINAs) and these molecules have an acceptor-donor-acceptor structural framework. Based on the donor groups, the BINAs are classified as either fused-ring electron acceptors (FREAs) or non-fused-ring electron acceptors (non-FREAs). The BNIA with a non-fused ring donor group (NITV) has roughly 2x less efficiency compared to NIDT and NIBT (FREAs). Femtosecond transient absorption (fs-TA) is utilized to investigate the exciton dynamics of the BNIAs. The fs-TA results show the exciton decay of NIBT is approximately 6x slower than the decays of the other BNIAs. In addition, the results show the reorganization energy, between exciton and charge transfer states, of NIBT is roughly one-third smaller compared to the energies of NITV and NIDT. Even though NITV and NIDT have different donor groups, these BNIAs have similar reorganization energies and exciton dynamics. The results of this study show the additional of pi-bridges on the donor group of NIBT leads to an efficient intramolecular charge generation and OPV device due to strong exciton dynamics. Ideally, these results have the potential to increase the knowledge of structure-function relationship for NFAs and OPVs.