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Many linked processes occur concurrently in strongly excited semiconductors, such as interband and intraband absorption, scattering of electrons and holes by the heated lattice, Pauli blocking, bandgap renormalization and the formation of Mahan excitons. In this work, we disentangle their dynamics and contributions to the optical response of a ZnO thin film. Using broadband pump-probe ellipsometry, we can directly and unambiguously obtain the real and imaginary part of the transient dielectric function which we compare with first-principles simulations. We find interband and excitonic absorption partially blocked and screened by the photo-excited electron occupation of the conduction band and hole occupation of the valence band (absorption bleaching). Exciton absorption turns spectrally narrower upon pumping and sustains the Mott transition, indicating Mahan excitons. Simultaneously, intra-valence-band transitions occur at sub-picosecond time scales after holes scatter to the edge of the Brillouin zone. Our results pave new ways for the understanding of non-equilibrium charge-carrier dynamics in materials by reliably distinguishing between changes in absorption coefficient and refractive index, thereby separating competing processes. This information will help to overcome the limitations of materials for high-power optical devices that owe their properties from dynamics in the ultrafast regime., Comment: 21 pages plus supplement

The orange carotenoid protein (OCP) is a structurally and functionally modular photoactive protein involved in cyanobacterial photoprotection. Recently, based on bioinformatic analysis and phylogenetic relationships, new families of OCP have been described, OCP2 and OCPx. The first characterization of the OCP2 showed both faster photoconversion and back-conversion, and lower fluorescence quenching of phycobilisomes relative to the well-characterized OCP1. Moreover, OCP2 is not regulated by the fluorescence recovery protein (FRP). In this work, we present a comprehensive study combining ultrafast spectroscopy and structural analysis to compare the photoactivation mechanisms of OCP1 and OCP2 from Tolypothrix PCC 7601. We show that despite significant differences in their functional characteristics, the spectroscopic properties of OCP1 and OCP2 are comparable. This indicates that the OCP functionality is not directly related to the spectroscopic properties of the bound carotenoid. In addition, the structural analysis by X-ray footprinting reveals that, overall, OCP1 and OCP2 have grossly the same photoactivation mechanism. However, the OCP2 is less reactive to radiolytic labeling, suggesting that the protein is less flexible than OCP1. This observation could explain fast photoconversion of OCP2.

Physical chemistry, chemical physics 20(9), 6274 - 6286 (2018). doi:10.1039/C7CP04880G, The structural dynamics of charge-transfer states of nitrogen-ligated copper complexes has been extensively investigated in recent years following the development of pump–probe X-ray techniques. In this study we extend this approach towards copper complexes with sulfur coordination and investigate the influence of charge transfer states on the structure of a dicopper(I) complex with coordination by bridging disulfide ligands and additionally tetramethylguanidine units [Cu$^{I}$$_{2}$(NSSN)$_2$]$^{2+}$. In order to directly observe and refine the photoinduced structural changes in the solvated complex we applied picosecond pump–probe X-ray absorption spectroscopy (XAS) and wide-angle X-ray scattering (WAXS). Additionally, the ultrafast evolution of the electronic excited states was monitored by femtosecond transient absorption spectroscopy in the UV-Vis probe range. DFT calculations were used to predict molecular geometries and electronic structures of the ground and metal-to-ligand charge transfer states with singlet and triplet spin multiplicities, i.e. S$_0$, $^1$MLCT and $^3$MLCT, respectively. Combining these techniques we elucidate the electronic and structural dynamics of the solvated complex upon photoexcitation to the MLCT states. In particular, femtosecond optical transient spectroscopy reveals three distinct timescales of 650 fs, 10 ps and >100 ps, which were assigned as internal conversion to the ground state (S$_n$ → S$_0$), intersystem crossing $^1$MLCT → $^3$MLCT, and subsequent relaxation of the triplet to the ground state, respectively. Experimental data collected using both X-ray techniques are in agreement with the DFT-predicted structure for the triplet state, where coordination bond lengths change and one of the S–S bridges is cleaved, causing the movement of two halves of the molecule relative to each other. Extended X-ray absorption fine structure spectroscopy resolves changes in Cu–ligand bond lengths with precision on the order of 0.01 Å, whereas WAXS is sensitive to changes in the global shape related to relative movement of parts of the molecule. The results presented herein widen the knowledge on the electronic and structural dynamics of photoexcited copper–sulfur complexes and demonstrate the potential of combining the pump–probe X-ray absorption and scattering for studies on photoinduced structural dynamics in copper-based coordination complexes., Published by RSC Publ., Cambridge

The flashes of light communicated by fireflies conceal a rich and exciting solution spectrochemistry that utilize the ground state of the heterocyclic acid D-luciferin as a substrate for enzymatic (protein luciferase) multistep oxidation to generate oxyluciferin in its first excited state. Subsequent deexcitation produces visible photon, which is observed as bioluminescence.

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The mysterious flashes of light communicated by fireflies conceal a rich and exciting solution spectrochemistry that utilize the ground state of the heterocyclic acid D-luciferin as a substrate for enzymatic (protein luciferase) multistep oxidation to generate oxyluciferin in its first excited state. Subsequent deexcitation produces visible photon, which is observed as bioluminescence. Due to alleged instability and luminescence lifetime about few nanoseconds for one single molecule of OxyLH2, the spectrochemistry and structure of OxyLH2 within luciferase in its exited state have still remained unexplored. A triple chemical equilibrium by double deprotonation and keto-enol tautomerism turns this simple molecule into an intricate case where the relative spectral contributions of six chemical species combine over a physiologically relevant pH range, rendering physical isolation and spectral characterization of most of the species unmanageable. To disentangle the individual spectral contributors, we prepared selectively chemically modified oxyluciferin derivatives where some of the chemical equilibria have been intentionally blocked. A multivariate curve resolution– alternating least squares (MCR-ALS) procedure was applied simultaneously to an extensive set of pH-dependent spectroscopic data for oxyluciferin and the target derivatives. The analysis provided, for the first time, the stationary UV-VIS absorption spectra of the pure individual components free of contributions from the other forms, their pH-dependent profiles and distributions, and the most accurate to date values for the three equilibrium constants. To understand dynamics of the emitter we then undertook thoroughly stationary and ultrafast time resolved (fluorescence, absorption, infrared) studies of oxyluciferin and its derivatives in different environment and different state (solution and crystals). The obtained results represent the fundamental benchmark for further in situ spectroscopic studies of the fast processes that occur in the luciferase during the bioluminescence reaction. Moreover, thorough understanding of photochemical properties of oxyluciferin opens interesting perspectives in designing of several irreplaceable tools for microscopic imaging of cells and tissues in vivo.

Despite being of critical relevance for the bioanalytical performance, two key aspects of the firefly bioluminescence (BL) have long remained a subject of speculation and dispute. First, the exact chemical identity of oxyluciferin (OxyLH2) complexed with the luciferase (Luc) at various stages of the BL reaction sequence and at various conditions has not been determined yet. A second thoroughly debated issue is the molecular origin of natural or artificially (point-mutation) induced tuning of the emission energy. In this work, the most recent results from our steady-state and time-resolved spectroscopy of OxyLH2 and its derivatives will be described. These results provide direct insight into the structure and fate of different forms of the emitter in solvated form and when complexed with Luc. By studying the photochemical behavior of several synthetic derivatives, we have assessed the effects of its functional groups and support the assignments by two non-natural fluorophores. The natural emitter OxyLH2 has been studied intensively, and it behaves as a typical photoacid with an estimated pKa* of 0.5. Despite the common belief that the hydroxyl of the thiazoline- substituted phenolic group serves as a source of photogenerated protons, we were very surprised to observe excited-state proton transfer (ESPT) from HOxy, which has only one enolic hydroxyl group. Moreover, the photoacidity of HOxy was stronger than of the parent OxyLH2. Usually the HO- photoacids are based on the phenolic compounds, and the examples of non-traditional photoacids are extremely rare. We are tempted to conclude that the ESPT reactivity associated with the phenol photodissociation was misinterpreted. Based on these observations, we interpreted the photoiduced behavior of OxyLH2 in water and in the complex with Luc as a complex ESPT cascade. Depending on pH, the emitting species included keto- and enol tautomers in various protonation steps. Our spectral assignments could be directly utilized in the analysis of the multiband Luc chemiluminescence spectra.