Dynamical evolution of the Schottky barrier as a determinant contribution to electron-hole pair stabilization and photocatalysis of plasmon-induced hot carriers.

The harnessing of plasmon-induced hot carriers promises to open new avenues for the development of clean energies and chemical catalysis. The extraction of carriers before thermalization and recombination is of fundamental importance to obtain appealing conversion yields. Here, hot carrier injection in the paradigmatic Au-TiO2 system is studied by means of electronic and electron-ion dynamics. Our results show that pure electronic features (without considering many-body interactions or dissipation to the environment) contribute to the electron-hole separation stability. These results reveal the existence of a dynamic contribution to the interfacial potential barrier (Schottky barrier) that arises at the charge injection pace, impeding electronic back transfer. Furthermore, we show that this charge separation stabilization provides the time needed for the charge to leak to capping molecules placed over the TiO2 surface triggering a coherent bond oscillation that will lead to a photocatalytic dissociation. We expect that our results will add new perspectives to the interpretation of the already detected long-lived hot carrier lifetimes and their catalytical effect, and concomitantly to their technological applications.

Spin-Polarized Tunable Photocurrents.

Harnessing the unique features of topological materials for the development of a new generation of topological based devices is a challenge of paramount importance. Using Floquet scattering theory combined with atomistic models we study the interplay among laser illumination, spin, and topology in a two-dimensional material with spin-orbit coupling. Starting from a topological phase, we show how laser illumination can selectively disrupt the topological edge states depending on their spin. This is manifested by the generation of pure spin photocurrents and spin-polarized charge photocurrents under linearly and circularly polarized laser illumination, respectively. Our results open a path for the generation and control of spin-polarized photocurrents.

Plasmon-induced hot-carrier generation differences in gold and silver nanoclusters.

In the last thirty years, the study of plasmonic properties of noble metal nanostructures has become a very dynamic research area. The design and manipulation of matter in the nanometric scale demands a deep understanding of the underlying physico-chemical processes that operate in this size regimen. Here, a fully atomistic study of the spectroscopic and photodynamic properties of different icosahedral silver and gold nanoclusters has been carried out by using a Time-Dependent Density Functional Tight-Binding (TD-DFTB) model. The optical absorption spectra of different icosahedral silver and gold nanoclusters of diameters between 1 and 4 nanometers have been simulated. Furthermore, the energy absorption process has been quantified by means of calculating a fully quantum absorption cross-section using the information contained in the reduced single-electron density matrix. This approach allows us take into account the quantum confinement effects dominating in this size regime. Likewise, the plasmon-induced hot-carrier generation process under laser illumination has been explored from a fully dynamical perspective. We have found noticeable differences in the energy absorption mechanisms and the plasmon-induced hot-carrier generation process in both metals which can be explained by their respective electronic structures. These differences can be attributed to the existence of ultra-fast electronic dissipation channels in gold nanoclusters that are absent in silver nanoclusters. To the best of our knowledge, this is the first report that addresses this topic from a real time fully atomistic time-dependent approach.

Solvation of barium atoms and singly charged cations in acetonitrile clusters.

The size distributions of neutral and cationic Ba x (CH3CN) n (x = 0, +1; n ≤ 7) clusters, as produced by a standard laser vaporization-supersonic expansion pick-up source, were determined from molecular beam experiments. The size distribution for cations is in the range of n = 1-7, whereas only the n = 1 complex is observed for neutral clusters, and these two features are unaffected by the variables controlling the performance of the cluster source. The distinct behavior is compatible with the expected charge-dipole interactions in the ionic species, which are stronger than the dipole induced-dipole interactions at play in neutral clusters, and it is corroborated by the relative magnitude of the theoretical successive binding energies (SBEs) for the lowest-lying isomers of cationic and neutral clusters with n = 1-5, as computed at the density functional theory level. The theoretical results also allow for the rationalization of the bimodal Ba+(CH3CN)1-7 size distribution, featuring an apparent minimum at n = 3, in terms of chiefly 6s-5d σ hybridization of the Ba+ ions, which ultimately leads to a relatively small third SBE for the Ba+(CH3CN)3 complex, as compared to those for n = 1, 2, and 4. Additional Born-Oppenheimer molecular dynamics simulations on the Ba+(CH3CN)2-4 clusters suggest that all of the ligands are coordinated to the Ba+ ion and prevent considering completion of the first solvent shell as responsible for the bimodal size distribution.

Optical properties and charge distribution in rod-shape DNA-silver cluster emitters.

While the atomic structure of DNA_Agn clusters remains unknown many efforts have been made to understand the photophysical properties of this type of systems. It is known that partial oxidation of the silver cluster is necessary for generation of fluorescent emitters. In this sense, the rod-shape model proposed by Gwinn and coworkers (D. Schultz, K. Gardner, S. S. R. Oemrawsingh, N. Markesevic, K. Olsson, M. Debord, D. Bouwmeester, and E. Gwinn, Adv. Mater., 2013, 25, 2797-2803), based on the idea that a neutral rod is generated with Ag+ acting as a "glue" in between the neutral rod and the DNA bases, is a good approximation in order to explain experimental results. With the aim to shed light towards the understanding of these systems, we explore the electronic dynamics and charge distribution in zigzag rod-shape DNA_Agn clusters, using the Ag0/Ag+ stoichiometry found experimentally.

Fingerprints of Both Watson-Crick and Hoogsteen Isomers of the Isolated (Cytosine-Guanine)H+ Pair.

 Gas phase protonated guanine-cytosine (CGH+) pair was generated using an electrospray ionization source from solutions at two different pH (5.8 and 3.2). Consistent evidence from MS/MS fragmentation patterns and differential ion mobility spectra (DIMS) point toward the presence of two isomers of the CGH+ pair, whose relative populations depend strongly on the pH of the solution. Gas phase infrared multiphoton dissociation (IRMPD) spectroscopy in the 900-1900 cm-1 spectral range further confirms that the Watson-Crick isomer is preferentially produced (91%) at pH = 5.8, while the Hoogsteen isomer predominates (66%) at pH = 3.2). These fingerprint signatures are expected to be useful for the development of new analytical methodologies and to trigger isomer selective photochemical studies of protonated DNA base pairs.

DNA-protected silver emitters: charge dependent switching of fluorescence.

The relationship between the state of charge and spectroscopy of DNA-protected silver emitters is not yet well understood. This remains one of the major issues to unveil in order to fully disentangle the spectroscopic features of these novel systems. It is a well known fact that a fluorescence response arises upon chemical reduction of silver cations attached to DNA, leading to neutral (or partially oxidized) "bright" clusters. It is important to note that the absence of fluorescence in completely ionic complexes is universal in the sense that it does not depend on any experimental variable. This suggests that its origin may be founded on the nature of the interaction between DNA bases and silver cations. Nevertheless, to the best of our knowledge, no explanation exists for this charge dependent switching between dark completely ionic complexes and bright (neutral or partially oxidized) clusters. In this brief report we address this experimental fact on the basis of the electronic structure of the complex as a function of its charge and quantum dynamical simulations of the processes following photoexcitation. These data provide a dynamical picture of the correlation between charge and fluorescence.

Communication: UV photoionization of cytosine catalyzed by Ag(+).

The photo-induced damages of DNA in interaction with metal cations, which are found in various environments, still remain to be characterized. In this paper, we show how the complexation of a DNA base (cytosine (Cyt)) with a metal cation (Ag(+)) changes its electronic properties. By means of UV photofragment spectroscopy of cold ions, it was found that the photoexcitation of the CytAg(+) complex at low energy (315-282) nm efficiently leads to ionized cytosine (Cyt(+)) as the single product. This occurs through a charge transfer state in which an electron from the p orbital of Cyt is promoted to Ag(+), as confirmed by ab initio calculations at the TD-DFT/B3LYP and RI-ADC(2) theory level using the SV(P) basis set. The low ionization energy of Cyt in the presence of Ag(+) could have important implications as point mutation of DNA upon sunlight exposition.

On the Ag(+)-cytosine interaction: the effect of microhydration probed by IR optical spectroscopy and density functional theory.

The gas-phase structures of cytosine-Ag(+) [CAg](+) and cytosine-Ag(+)-H2O [CAg-H2O](+) complexes have been studied by mass-selected infrared multiphoton dissociation (IRMPD) spectroscopy in the 900-1800 cm(-1) spectral region using the Free Electron Laser facility in Orsay (CLIO). The IRMPD experimental spectra have been compared with the calculated IR absorption spectra of the different low-lying isomers (computed at the DFT level using the B3LYP functional and the 6-311G++(d,p) basis set for C, H, N and O atoms and the Stuttgart effective core potential for Ag). For the [CAg](+) complex, only one isomer with cytosine in the keto-amino (KA) tautomeric form and Ag(+) interacting simultaneously with the C(2)[double bond, length as m-dash]O(7) group and N(3) of cytosine was observed. However, the mono-hydration of the complex in the gas phase leads to the stabilization of a two quasi-isoenergetic structure of the [CAg-H2O](+) complex, in which Ag(+) interacts with the O atom of the water molecule and with the N(3) or C(2)[double bond, length as m-dash]O(7) group of cytosine. The relative populations of the two isomers determined from the IRMPD kinetics plot are in good agreement with the calculated values. Comparison of these results with those of protonated cytosine [CH](+) and its mono-hydrated complex [CH-H2O](+) shows some interesting differences between H(+) and Ag(+). In particular, while a single water molecule catalyzes the isomerization reaction in the case of [CH-H2O](+), it is found that in the case of [CAg-H2O](+) the addition of water leads to the stabilization of two isomers separated by small energy barrier (0.05 eV).

Excited states of proton-bound DNA/RNA base homodimers: pyrimidines.

We are presenting the electronic photofragment spectra of the protonated pyrimidine DNA base homodimers. Only the thymine dimer exhibits a well structured vibrational progression, while the protonated monomer shows broad vibrational bands. This shows that proton bonding can block some nonradiative processes present in the monomer.

Analysis of illicit drugs by direct ablation of solid samples.

Analysis of illicit drugs arises as an important field of work given the high social impacts presented by drugs in the modern society. Direct laser ablation of solid compounds allows their analysis without sampling or preparation procedures. For that purpose, an experimental set-up that combines laser ablation with time-of- flight mass spectrometry has been constructed very recently to perform studies on the mass spectra of such drugs as 3,4-methylenedioxy-N-methylamphetamine, commonly known as MDMA or ecstasy. Analysis of the observed fragmentation pattern in mass spectra may elucidate the ablation-induced photofragmentation phenomena produced, which differ from those previously observed with conventional ionization methods.

The shape of D-glucosamine.

The bioactive amino monosaccharide D-glucosamine has been generated in the gas phase via laser ablation of D-glucosamine hydrochloride. Three cyclic α-(4)C1 pyranose forms have been identified using Fourier transform microwave techniques. Stereoelectronic hyperconjugative forces - essentially linked with the anomeric or gauche effect - and cooperative OH···O, OH···N and NH···O chains, extended along the entire molecule, are found to be the main factors driving the conformational behavior. The orientation of the NH2 group within each conformer has been determined by the values of the nuclear quadrupole coupling constants. The results have been compared with those recently obtained for the archetypical D-glucose.

Gas Phase Structure of Metal Mediated (Cytosine)2Ag+ Mimics the Hemiprotonated (Cytosine)2H+ Dimer in i-Motif Folding.

The study of metal ion-DNA interaction aiming to understand the stabilization of artificial base pairing and a number of noncanonical motifs is of current interest, due to their potential exploitation in developing new technological devices and expanding the genetic code. A successful strategy has been the synthesis of metal-mediated base pairs, in which a coordinative bond to a central metal cation replaces a H-bond in a natural pair. In this work, we characterized, for the first time, the gas phase structure of the cytosine···Ag+···cytosine (C-Ag+-C) complex by means of InfraRed-MultiPhoton-Dissociation (IR-MPD) spectroscopy and theoretical calculation. The IR-spectrum was confidently assigned to one structure with the Ag+ acting as a bridge between the heteronitrogen atoms in each cytosine (both in the keto-amino form). This structure is biologically relevant since it mimics the structure of the hemiprotonated C-H+-C dimer responsible for the stabilization of the i-motif structure in DNA, with the replacement of the NH···N bond by a stronger N···Ag+···N bond. Moreover, since the structure of the C-Ag+-C complex is planar, it allows an optimum intercalation between pairs of the two antiparallel strand duplex in the DNA i-motif structure.

Excited states of protonated DNA/RNA bases.

The very fast relaxation of the excited states to the ground state in DNA/RNA bases is a necessary process to ensure the photostability of DNA and its rate is highly sensitive to the tautomeric form of the bases. Protonation of the bases plays a crucial role in many biochemical and mutagenic processes and it can result in alternative tautomeric structures, thus making important the knowledge of the properties of protonated DNA/RNA bases. We report here the photofragmentation spectra of the five protonated DNA/RNA bases. In most of the cases, the spectra exhibit well resolved vibrational structures, with broad bands associated with very short excited state lifetimes. The similarity between the electronic properties, e.g. excitation energy and very short excited state lifetimes for the canonical tautomers of protonated and neutral DNA bases, suggests that the former could also play an important role in the photostability mechanism of DNA.

Effect of Ag(+) on the Excited-State Properties of a Gas-Phase (Cytosine)2Ag(+) Complex: Electronic Transition and Estimated Lifetime.

Recently, DNA molecules have received great attention because of their potential applications in material science. One interesting example is the production of highly fluorescent and tunable DNA-Agn clusters with cytosine (C)-rich DNA strands. Here, we report the UV photofragmentation spectra of gas-phase cytosine···Ag(+)···cytosine (C2Ag(+)) and cytosine···H(+)···cytosine (C2H(+)) complexes together with theoretical calculations. In both cases, the excitation energy does not differ significantly from that of isolated cytosine or protonated cytosine, indicating that the excitation takes place on the DNA base. However, the excited-state lifetime of the C2H(+) (τ = 85 fs), estimated from the bandwidth of the spectrum, is at least 2 orders of magnitude shorter than that of the C2Ag(+) (τ > 5000 fs). The increased excited-state lifetime upon silver complexation is quite unexpected, and it clearly opens the question about what factors are controlling the nonradiative decay in pyrimidine DNA bases. This is an important result for the expanding field of metal-mediated base pairing and may also be important to the photophysical properties of DNA-templated fluorescent silver clusters.