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Combining Experiment and Computation to Solve Complex Problems in Photochemistry
No full textLight-driven reactions are an attractive method of accessing novel chemistry. Their broad applicability to industrial processes and renewable energy research makes understanding their mechanisms extremely important. Steady-state and time-resolved spectroscopic experiments provide a powerful window into the complex behavior of photoactive chemicals. Those results can be made even more impactful by rationalizing the quantum-mechanical nature of the observed phenomena using computational modeling. This thesis discusses three projects in the field of photochemistry: (1) computational and experimental description of novel emission photophysics in a Re(4-Pam) model system for photoinduced PCET (2) computational elucidation of a series of Pd-biladienes� excitation wavelength-dependent photophysics and (3) computational and experimental investigation of several azo dyes� excited-state dynamics and proton-dependent photochemical properties. A novel Re(I) bipyridine complex, which is being developed as a model system for photoinduced proton-coupled electron transfer (PCET), was found to exhibit an unexpected biexponential photoluminescence decay. Spectroscopic and computational results show that this phenomenon occurs due to the presence of two discrete rotamers that contribute differently to the compound�s bulk emission. A deeper understanding of the role of protons in this complex provides key insight for its use in future PCET studies. Three Pd(II) biladiene complexes that were developed as candidate drugs for photodynamic therapy (PDT) were observed to produce excitation wavelength-dependent photophysics. The Pd(II) complexes decay with two lifetimes upon lower-energy excitation, whereas at higher-energy excitation a third lifetime is observed. Computational studies were employed to rationalize these results. The behavior arises from unusually high spin-orbit coupling between high-energy singlet and triplet (>T9) excited states that facilitate intersystem crossing into the high-energy triplet excited states, thereby opening an alternate pathway for excited-state deactivation. Such pathways are not accessible in analogous porphyrins, which do not show excitation wavelength-dependent dynamics. Access to high-energy triplet states is rare and can enable new and interesting possibilities for these molecules� use in addition to PDT. Several sets of synthetically modified azo dyes were interrogated to systematically quantify the role of various electron donating and withdrawing substituents, and the impact of heteroaryl or polyaryl groups on their photophysical properties. Two anthracene-based azo dyes were investigated for their ability to undergo excited-state PCET. Using a mix of computational and experimental techniques it was found that, while PCET could be achieved with as little as 1 eq of trifluoroacetic acid under electrochemical potential, full protonation of the methoxy-substituted dye required four orders of magnitude more of a stronger acid. Another study on a set of four naphthalene-based azo dyes revealed that altering a phenyl ring to have increasingly electron-donating substituents in the position para to the azo bond reduces the lifetime of the cis photoisomer. Protonation at the pyridyl nitrogen site of three pyridine-based azo dyes unexpectedly disabled their ability to photoisomerize, despite the protonation site being distant from the azo bond around which the isomerization occurs. DFT calculations revealed that significant changes to the dyes� potential energy landscapes after protonation results in an inability for the dyes to reach the cis conformation after illumination. Two factors were primary contributors to the differing photophysics of the protonated species (1) dramatic reduction of the energy barrier between cis and trans geometries on the dyes� ground state potential energy curves and (2) a change in the S1 potential energy minimum to a geometry more closely resembling the trans-isomer. Increasing the understanding of azo dye photophysics is crucial for industries such as textile pigmentation and wastewater treatment
The transition from Elasto‐Hydrodynamic to Mixed Regimes in Lubricated Friction of Soft Solid Surfaces
No full textLubricated contacts in soft materials are common in various engineering and natural settings, such as tires, haptic applications, contact lenses, and the fabrication of soft electronic devices. Two major regimes are elasto‐hydrodynamic lubrication (EHL), in which solid surfaces are fully separated by a fluid film, and mixed lubrication (ML), in which there is partial solid‐to‐solid contact. The transition between these regimes governs the minimum sliding friction achievable and is thus very important. Generally, the transition from EHL to ML regimes is believed to occur when the thickness of the lubricant layer is comparable with the amplitude of surface roughness. Here, it is reported that in lubricated sliding experiments on smooth, soft, poly(dimethylsiloxane) substrates, the transition can occur when the thickness of the liquid layer is much larger than the height of the asperities. Direct visualization of the "contact" region shows that the transition corresponds to the formation of wave‐like surface wrinkles at the leading contact edge and associated instabilities at the trailing contact edge, which are believed to trigger the transition to the mixed regime. These results change the understanding of what governs the important EHL–ML transition in the lubricated sliding of soft solids
Thank You to Our 2022 Peer Reviewers
No full textThe editorial board of AGU Advances thanks the individuals who reviewed for the journal in 2022. , Plain Language Summary Thank you to the 131 people who reviewed manuscripts for AGU Advances in 2022. , Key Points The editors thank the 2022 peer reviewer
Leader subjective ambivalence
No full textIn this article, we investigate the effects of leader subjective ambivalence on team performance. Integrating the ambivalence literature and social learning theory, we propose a multi‐level model of whether, when, and why team leaders\u27 subjective ambivalence enhances team performance outcomes. The results of two laboratory experiments (Studies 1 and 2) demonstrate initial support for the relationship between leader subjective ambivalence and information‐seeking behaviors. The results of a longitudinal field study (Study 3) based on 164 projects (164 leaders and 725 subordinates) show that leader subjective ambivalence has a positive indirect effect on team task performance first through leader information‐seeking behaviors and later through team information‐seeking behaviors. Our results further indicate that project complexity is a boundary condition for the proposed conditional indirect effect of leader subjective ambivalence on team performance outcomes. We discuss the theoretical and practical implications of these findings
Crosstalk between tumour and stroma modifies CLIC4 cargo in extracellular vesicles
No full textMouse models of breast cancer have revealed that tumour‐bearing hosts must express the oxidoreductase CLIC4 to develop lung metastases. In the absence of host CLIC4, primary tumours grow but the lung premetastatic niche is defective for metastatic seeding. Primary breast cancer cells release EVs that incorporate CLIC4 as cargo and circulate in plasma of wildtype tumour‐bearing hosts. CLIC4‐deficient breast cancer cells also form tumours in wildtype hosts and release EVs in plasma, but these EVs lack CLIC4, suggesting that the tumour is the source of the plasma‐derived EVs that carry CLIC4 as cargo. Paradoxically, circulating EVs are also devoid of CLIC4 when CLIC4‐expressing primary tumours are grown in CLIC4 knockout hosts. Thus, the incorporation of CLIC4 (and perhaps other factors) as EV cargo released from tumours involve specific signals from the surrounding stroma determined by its genetic composition. Since CLIC4 is also detected in circulating EVs from human breast cancer patients, future studies will address its association with disease
Evolving information complexity of coarsening materials microstructures
No full textAbstractThe temporal evolution of microstructural features in metals and ceramics has been the subject of intense investigation over many years because deviations from normal grain growth behavior are ubiquitous and strongly dictate observed mechanical and magnetic properties. To distinguish among different grain growth scenarios, we examine the time evolution of the information content of both synthetic and experimental coarsening microstructures as quantified by both a computable information density (CID) and a spectral entropy along with selected metrics and measures of shared information and interaction strength. In these approaches, microstructural evolution is described in terms of two time series representations, namely: (1) strings and their compressed counterparts that reflect the information contained in the configuration of a system over time, and (2) the spectra of graph Laplacians that embody the information contained in a coarsening grain network. These approaches permit one to characterize dynamically evolving microstructures and to identify correlation times associated with different coarsening scenarios. Moreover, as the information content of a system is a proxy for the entropy, a thermodynamic description of grain growth is also described.</jats:p
A brain-wide analysis maps structural evolution to distinct anatomical module
No full textThe vertebrate brain is highly conserved topologically, but less is known about neuroanatomical variation between individual brain regions. Neuroanatomical variation at the regional level is hypothesized to provide functional expansion, building upon ancestral anatomy needed for basic functions. Classically, animal models used to study evolution have lacked tools for detailed anatomical analysis that are widely used in zebrafish and mice, presenting a barrier to studying brain evolution at fine scales. In this study, we sought to investigate the evolution of brain anatomy using a single species of fish consisting of divergent surface and cave morphs, that permits functional genetic testing of regional volume and shape across the entire brain. We generated a high-resolution brain atlas for the blind Mexican cavefish Astyanax mexicanus and coupled the atlas with automated computational tools to directly assess variability in brain region shape and volume across all populations. We measured the volume and shape of every grossly defined neuroanatomical region of the brain and assessed correlations between anatomical regions in surface fish, cavefish, and surface √ó cave F2 hybrids, whose phenotypes span the range of surface to cave. We find that dorsal regions of the brain are contracted, while ventral regions have expanded, with F2 hybrid data providing support for developmental constraint along the dorsal-ventral axis. Furthermore, these dorsal-ventral relationships in anatomical variation show similar patterns for both volume and shape, suggesting that the anatomical evolution captured by these two parameters could be driven by similar developmental mechanisms. Together, these data demonstrate that A. mexicanus is a powerful system for functionally determining basic principles of brain evolution and will permit testing how genes influence early patterning events to drive brain-wide anatomical evolution.</jats:p
Melting point of dried gold nanoparticles prepared with ultrasonic spray pyrolysis and lyophilisation
No full textAbstract
A coupled process of ultrasonic spray pyrolysis and lyophilisation was used for the synthesis of dried gold nanoparticles. Two methods were applied for determining their melting temperature: uniaxial microcompression and differential scanning calorimetry (DSC) analysis. Uniaxial microcompression resulted in sintering of the dried gold nanoparticles at room temperature with an activation energy of 26–32.5 J/g, which made it impossible to evaluate their melting point. Using DSC, the melting point of the dried gold nanoparticles was measured to be around 1064.3°C, which is close to pure gold. The reason for the absence of a melting point depression in dried gold nanoparticles was their exothermic sintering between 712 and 908.1°C.</jats:p
Reduction of<i>Paraoxonase</i>Expression Followed by Inactivation across Independent Semiaquatic Mammals Suggests Stepwise Path to Pseudogenization
No full textAbstractConvergent adaptation to the same environment by multiple lineages frequently involves rapid evolutionary change at the same genes, implicating these genes as important for environmental adaptation. Such adaptive molecular changes may yield either change or loss of protein function; loss of function can eliminate newly deleterious proteins or reduce energy necessary for protein production. We previously found a striking case of recurrent pseudogenization of the Paraoxonase 1 (Pon1) gene among aquatic mammal lineages—Pon1 became a pseudogene with genetic lesions, such as stop codons and frameshifts, at least four times independently in aquatic and semiaquatic mammals. Here, we assess the landscape and pace of pseudogenization by studying Pon1 sequences, expression levels, and enzymatic activity across four aquatic and semiaquatic mammal lineages: pinnipeds, cetaceans, otters, and beavers. We observe in beavers and pinnipeds an unexpected reduction in expression of Pon3, a paralog with similar expression patterns but different substrate preferences. Ultimately, in all lineages with aquatic/semiaquatic members, we find that preceding any coding-level pseudogenization events in Pon1, there is a drastic decrease in expression, followed by relaxed selection, thus allowing accumulation of disrupting mutations. The recurrent loss of Pon1 function in aquatic/semiaquatic lineages is consistent with a benefit to Pon1 functional loss in aquatic environments. Accordingly, we examine diving and dietary traits across pinniped species as potential driving forces of Pon1 functional loss. We find that loss is best associated with diving activity and likely results from changes in selective pressures associated with hypoxia and hypoxia-induced inflammation.</jats:p