Spectroscopy of isolated, mass-selected tryptophan-Ag3 complexes: a model for photoabsorption enhancement in nanoparticle-biomolecule hybrid systems

We wish to show that gas-phase studies of small metal cluster-biomolecule complexes provide fundamental insights into the mechanism leading to enhanced optical absorption in nanoparticle-biomolecular hybrid systems. Here we present, for the first time, a joint experimental and theoretical study of photoabsorption and photofragmentation of the silver trimer-tryptophan cation complex ([Trp.Ag3]+). We demonstrate that binding the metal cluster to a biomolecule leads to a remarkably high optical absorption as compared to the bare tryptophan or the [Trp.Ag]+ complex. As calculations show this arises due to coupling between the excitations in the metallic subunit with a charge transfer excitation to the tryptophan molecule.     

Signature OH absorption spectrum from cluster models of solvation: a solvent-to-solute charge transfer state

Ab initio electronic structure theory calculations on cluster models support the characterization of the signature absorption spectrum of a solvated hydroxyl OH radical as a solvent-to-solute charge transfer state modulated by the hydrogen-bonding environment. Vertical excited states in OH(H2O)n clusters (n = 0-7, 16) calculated at the TDDFT level of theory (with companion calculations at the EOM-CCSD level of theory for n 相似文献   

Interaction of guanine, its anions, and radicals with lysine in different charge states

Modification in DNA or protein structure can severely affect DNA-protein interactions and the functioning of biological systems. Some new insights into radiation-induced effects of guanine-lysine interactions have been obtained here by theoretical investigations. Geometries of zwitterionic and non-zwitterionic lysine in different charge states (neutral, radical cation, and protonated cation) were optimized employing the B3LYP/6-31G** and B3LYP/AUG-cc-pVDZ levels of hybrid density functional theory (DFT) and using the second-order M?ller-Plesset perturbation theory along with the 6-31G** basis set. In the case of neutral lysine in the gas phase, no zwitterionic structure was obtained. The non-zwitterionic structures of lysine in radical and protonated cationic forms are appreciably more stable than the corresponding zwitterionic structures in the gas phase as obtained at all levels of theory employed here. Binding of guanine and different dehydrogenated guanine radicals with lysine in different charge states was studied at the B3LYP/6-31G** level of DFT. When guanine makes a complex with the lysine radical cation, large amounts of spin and positive charge densities are transferred from the lysine radical cation to guanine and the guanine is thus converted from its normal form to the radical cationic form. Complexation of the lysine radical cation with the H1-hydrogen-abstracted guanine radical leads to CO2 liberation and proton transfer from lysine. These results are compared with the available experimental ones.     

Ligand-exchange dependent properties of hybrid nanocomposites based on luminescent colloidal CdSe nanocrystals in P3HT matrix

In this work, a suitable ligand-exchange process has been proposed which elucidates the possibility to modulate charge/energy transfer rate between polymer and semiconductor quantum dots. The photoluminescence studies of CdSe emission as well as transient absorption measurements confirm mainly electron transfer for P3HT:CdSe (TOPO) and electron/energy transfer for P3HT:CdSe (OA) nanocomposites, respectively. The dominance of charge transfer for P3HT:CdSe (TOPO) as compared to P3HT:CdSe (OA) nanocomposites can be attributed to complete and partial removal of the surface ligands (TOPO, OA) upon ligand exchange with pyridine as elucidated from FTIR results. The various characterization techniques viz. Fourier transform infrared (FTIR), Raman, photoluminescence (PL), optical and transient absorption (UV-Vis and TA) spectroscopies used in this work provide an insight into the charge separation, charge accumulation and/or trapping of charge carriers for the better understanding of hybrid organic-inorganic photovoltaics. Composites of CdSe (OA) quantum dots in particular with P3HT polymer owing to its higher crystallinity and ordered morphology provide a new and promising direction toward developing effective light energy harvesting strategies in organic photovoltaics.     

Is Our Way of Thinking about Excited States Correct? Time‐Resolved Dispersive IR Study on p‐Nitroaniline

Low‐lying excited electronic states of an important class of molecules known as push–pull chromophores are central to understanding their potential nonlinear optical properties. Here we report that a combination of high‐sensitivity nanosecond time‐resolved dispersive IR spectroscopy and DFT calculations on p‐nitroaniline (PNA), a prototypical push–pull molecule, reveals that PNA in the lowest excited triplet state has a partial quinoid structure. In this structure, the quinoid configuration is restricted to a part of the phenyl ring adjacent to the NO₂ group. The partial quinoid structure of PNA cannot be explained by a commonly used hybrid of a neutral form and a zwitterionic charge‐transfer form. Our findings not only cast doubt on the general applicability of the classical way of looking at excited states, based exclusively on characteristic resonance structures, but also provide deeper insights into excited‐state structure of highly polarizable molecular systems.     

Adsorption of small molecules on silver clusters

We report investigations of adsorption of N(2) and O(2) molecules on silver cluster cations. We have first revisited structures of small silver clusters based on first-principles calculations within the framework of density functional theory with hybrid functional. The 2D to 3D transition for the neutral clusters occurs from n = 6 to 7 and for cations, in agreement with experiments, from n = 4 to 5. With the refined structures, adsorption energies of N(2) and O(2) molecules have been calculated. We have identified characteristic drops in the adsorption energies of N(2) that further link our calculations and experiments, and confirm the reported 2D-3D transition for cations. We have found that perturbations caused by physisorbed molecules are small enough that the structures of most Ag clusters remain unchanged, even though physisorption stabilizes the 3D Ag(7)(+) structure slightly more than the 2D counterpart. Results for pure O(2) adsorption indicate that charge transfer from Ag(n)(+) to O(2) occurs when n > 3. Below that size oxygen essentially physisorbes such as nitrogen to the cluster. We interpret the experimentally observed mutually cooperative co-adsorption of oxygen and nitrogen using results from density functional theory with generalized gradient approximations. The key to the enhancement is N(2)-induced increase in charge transfer from Ag(n)(+) cations to O(2).     

Electron and hydrogen transfer in small hydrogen fluoride anion clusters

A new stable structure has been found for the anion clusters of hydrogen fluoride. The ab initio method was used to optimize the structures of the (HF)(3)(-), (HF)(4)(-), (HF)(5)(-), and (HF)(6)(-) anion clusters with an excess "solvated" electron. Instead of the well-known "zig-zag" (HF)(n)(-) structure, a new form, (HF)(n-1)F(-)···H, was found with lower energy. In this new form, the terminal hydrogen atom in the (HF)(n)(-) chain is separated from the other part of the cluster and the inner hydrogens transfer along the hydrogen bonds toward the outside fluoride. The negative charge also transfers from the terminal HF molecule of the chain to the center fluoride atoms. The (HF)(n)(-) clusters for n = 4, 5, and 6 have not yet been observed experimentally. These results should assist in the search for these systems and also provide a possible way to study the proton and electron transfer in some large hydrogen bonding systems.     

Gas Phase H/D Exchange of Sodiated Amino Acids: Why Do We See Zwitterions?

The gas-phase interaction of sodiated amino acids and sodiated amino acid methyl esters with various deuterium donors is investigated by combining results of H/D exchange reactions with those from density functional theory and molecular dynamics calculations. Discrepancy between experimentally and theoretically obtained structures for sodium cationized amino acids is explained by deuterium donor caused perturbation of the most stable amino acid conformation. Detailed study of H/D exchange mechanism on sodiated amino acids shows that the H/D exchange reaction is preceded by a multistep quasi-isoenergetic transition (perturbation) from a charge solvated to zwitterionic structure in the amino acid. Although the computation refers to the system AlaNa(+) and D(2)O, these mechanisms apply to all amino acids, except those where a functional side-chain group takes part in the perturbation process. The suggested perturbation mechanism applies also for other deuterium donors such as CD(3)OD or even ND(3) and indicates that a single water molecule suffices to convert the sodiated amino acid from charge solvated to zwitterionic form.     

Electronic properties of the silver-silver chloride cluster interface

The objective of this study was to gain insight into the electronic structure of silver-silver chloride cluster composites and especially into the metal-semiconductor interface. For this purpose a theoretical study of (AgCl)(n) (n=4, 32, 108, 192, and 256), of Ag(m) (m=1-9, 30, 115, 276, and 409), and of the cluster composites Ag(115)-(AgCl)(192) and Ag(409)-(AgCl)(192) has been carried out. Density of levels (DOL), local density of levels (l-DOL), and projection of surface states, as well as projection of properties of individual atoms or groups of atoms obtained in molecular orbital calculations, are shown to be powerful tools for gaining deep insight into the properties of these large systems. The Ag(115)-(AgCl)(192) aggregate, consisting of a cubic Ag(115) cluster without corner atoms on top of a cubic (AgCl)(192) cluster, was found to be remarkably stable with a cluster-to-cluster distance of about 280 pm, and a geometry in which the number of bonding interactions between the silver atoms of Ag(115) and the chloride ions of (AgCl)(192) is at its maximum. A sharp jump in charge distribution occurs at the Ag(115)-(AgCl)(192) composite interface. The first AgCl slab picks up negative charge from the two adjacent silver slabs, so that in total the silver cluster is positively charged. In addition, the core of the silver cluster is positively charged with respect to its outermost layer. The main reason for the charge transfer from the silver cluster to the silver chloride is the newly formed MIGS (metal induced gap states) in the energy-gap range of the silver chloride and the MIdS (metal induced d states) in the d-orbital region. Their wave functions mix with orbitals of the silver cluster and with both the orbitals of the silver and the chloride ions of the silver chloride. The MIGS and the MIdS are of a quite localized nature. In them, nearest neighbor interactions dominate, with the exception of close-lying silver chloride surface states-which mix in to a large extent. We conclude that especially the MIGS not only influence the photochemical properties of silver chloride, but that their existence might be probed by appropriate spectroscopic measurements.     

Ligand field photofragmentation spectroscopy of [Ag(L)N]2+ complexes in the gas phase: experiment and theory

Experiments have been undertaken to record photofragmentation spectra from a series of [Ag(L)N]2+ complexes in the gas phase. Spectra have been obtained for silver(II) complexed with the ligands (L): acetone, 2-pentanone, methyl-vinyl ketone, pyridine, and 4-methyl pyridine (4-picoline) with N in the range of 4-7. A second series of experiments using 1,1,1,3-fluoroacetone, acetonitrile, and CO2 as ligands failed to show any evidence of photofragmentation. Interpretation of the experimental data has come from time-dependent density functional theory (TDDFT), which very successfully accounts for trends in the spectra in terms of subtle differences in the properties of the ligands. Taking a sample of three ligands, acetone, pyridine, and acetonitrile, the calculations show all the spectral transitions to involve ligand-to-metal charge transfer, and that wavelength differences (or lack of spectra) arise from small changes in the energies of the molecular orbitals concerned. The calculations account for an absence in the spectra of any effects due to Jahn-Teller distortion, and they also reveal structural differences between complexes where the coordinating atom is either oxygen or nitrogen that have implications for the stability of silver(II) compounds. Where possible, comparisons have also been made with the physical properties of condensed phase silver(II) complexes.     

Electronic excitation spectra of the [Ir(ppy)2(bpy)]+ photosensitizer bound to small silver clusters Ag(n) (n = 1-6)

The changes in nature and order of the excited electronic states of the photosensitizer [Ir(ppy)(2)(bpy)](+) upon binding to small silver clusters, Ag(n) (n = 1-6), were studied theoretically using the linear response TDDFT method with the range-separated LC-BLYP functional. Binding energies and localization of HOMO and LUMO orbitals are found to oscillate with the number of silver atoms. Special emphasis is put on the discussion of long-range charge transfer transitions between the photosensitizer and the silver cluster. The energies of these transitions were found to be only slightly dependent on the relative orientations of both fragments, but strongly dependent on the intermolecular distance. The absorption spectrum of the combined system does not show a systematic trend with respect to cluster size, but it is strongly modified by the charge transfer transitions. Possible photophysical processes of the systems containing larger clusters are discussed.     

Photofragmentation of phase-transferred gold nanoparticles by intense pulsed laser light

Gold nanoparticles with an average diameter of approximately 20 nm were prepared in an aqueous solution by a wet chemistry method. The parent gold nanoparticles were then capped with a 4-aminothiophenol protecting layer and transferred into toluene by tuning the surface charge of the modified nanoparticles. Gold nanoparticles before and after phase transfer were subjected to photofragmentation by a pulsed 532 nm laser. The effects of solvent properties and surface chemistry on the photofragmentation of the gold nanoparticles have been investigated. Fast photofragmentation has been observed in the organic solvent in which the dielectric constant, heat capacity, and thermal conductivity are lower. The results suggest new approaches for the preparation of very small gold clusters from gold nanoparticles.     

Ultrafast Relaxation Processes of Conjugated Polymer Nanoparticles in the Presence of Au Nanoparticles

Considering the importance of conjugated polymer nanoparticles, major emphasis has been given for designing and understanding the energy transfer and charge transfer processes of organic‐inorganic hybrids for light harvesting applications. In the present study, we have designed an aqueous solution‐based light harvesting system using conjugated polymer nanoparticles (poly[2‐methoxy‐5‐(2‐ethylhexyloxy)‐1,4‐phenylenevinylene], MEH‐PPV) and Au nanoparticles. The change in photo‐induced processes in the presence of metal nanoparticles are studied by steady‐state absorption, time‐resolved emission, time‐resolved fluorescence up‐conversion, ultrafast anisotropy and femtosecond transient absorption spectroscopy. Global and target analysis of transient absorption data validate the creation of a collective delocalized state in polymer nanoparticles, and the time scale for excitation energy funnelling from S₁ state to low lying collective delocalized state (CL_s) is 18 ps. Then, the electron transfer from the CL_s state to Au NP occurs with a time constant of 150 ps. The 815 ps long lived charge transfer (CT) state signifies the charge transfer from the CL_s state of polymer nanoparticles to Au NP. Such basic understanding of relaxation processes in hybrid systems is very important for designing inorganic‐organic hybrid light‐harvesting systems.     

Quantum chemical study on excited states and charge transfer of oxazolo[4,5-b]-pyridine derivatives

The excitedstate intramolecular charge transfer of four oxazolo[4,5-b]pyridine derivatives with different electron donating and electron withdrawing groups was investigated using the time-dependent density functional theory. The vertical excitation energies and the electronic structures were explored. Their distinct properties of absorption and fluorescence spectra in solvent phase were explained according to the electronic coupling matrix elements calculated by the Mulliken-Hush theory. The sub-stituent on the oxazolo[4,5-b]pyridines will remarkably change their spectra properties and increase the first excited-state dipole moments. The effect of protonation on the absorption and fluorescence spectra was also investigated systematically. Our study suggests that the present method is feasible to explain charge transfer excitation and predict the properties of absorption and emission spectra in the studied systems.     

Mechanism of Silver(I)‐Catalyzed Enantioselective Synthesis of Axially Chiral Allenes Based on Propargylamines

The silver(I)‐catalyzed synthesis picture of axially chiral allenes based on propargylamines has been outlined using density functional theory (DFT) method for the first time. Our calculations find that, the coordination of silver(I) into triple bond of propargylamines at anti‐position of nitrogen shows a stronger activation on the triple bond than that at cis‐position, which is favorable for the subsequent hydrogen transfer. The NBO charge analysis for the hydrogen transfer affirms the experimental speculation that this step is a hydride transfer process. The energy barrier of the anti‐periplanar elimination of vinyl‐silver is 26.9 kJ·mol^?1 lower than that of the syn‐periplanar elimination, supporting that (?)‐allene is the main product of this reaction. In a word, the most possible route for this reaction is that the silver(I) coordinates into the triple bond of propargylamines at anti‐position of nitrogen, then the formed silver(I) complex undergoes a hydride transfer to give a vinyl‐silver, finally the vinyl‐silver goes through an anti‐periplanar elimination to give (?)‐allene. The hydride transfer with the energy barrier of 44.8 kJ·mol^?1 is the rate‐limiting step in whole catalytic process. This work provides insight into why this reaction has a very high enantioselectivity.     

Electron solvation in water clusters following charge transfer from iodide

The dynamics following charge transfer to solvent from iodide to a water cluster are studied using time-resolved photoelectron imaging of I-(H2O)n and I-(D2O)n clusters with n< or =28. The results show spontaneous conversion, on a time scale of approximately 1 ps, from water cluster anions with surface-bound electrons to structures in which the excess electron is more strongly bound and possibly more internalized within the solvent network. The resulting dynamics provide valuable insight into the electron solvation dynamics in water clusters and the relative stabilities between recently observed isomers of water cluster anions.     

Investigation of two-photon absorption properties in branched alkene and alkyne chromophores

Novel alkene and alkyne branched structures have been synthesized, and their two-photon absorption (2PA) properties are reported. This series of alkene and alkyne trimer systems tests the mechanistic approach for enhancing the 2PA process which is usually dictated by the pi-bridging, delocalization length, and corresponding charge transfer on the 2PA cross sections. The results suggest that alkene branched systems have higher 2PA cross sections. While steady-state absorption and emission measurements were not successful in predicting the observed trend of 2PA cross sections, time-resolved measurements have explained the trends observed. It was found that, upon photoexcitation, there is an ultrafast charge localization to an intramolecular charge-transfer (ICT) state, followed by the presence of a solvent and conformationally relaxed ICT state in these branched systems.     

Gas phase hydrogen/deuterium exchange of arginine and arginine dipeptides complexed with alkali metals

The hydrogen/deuterium (H/D) exchange of protonated and alkali-metal cationized Arg-Gly and Gly-Arg peptides with D(2)O in the gas phase was studied using electrospray ionization quadropole ion trap mass spectrometry. The Arg-Gly and Gly-Arg alkali metal complexes exchange significantly more hydrogens than protonated Arg-Gly and Gly-Arg. We propose a mechanism where the peptide shifts between a zwitterionic salt bridge and nonzwitterionic charge solvated conformations. The increased rate of H/D exchange of the alkali metal complexes is attributed to the peptide metal complexes' small energy difference between the salt-bridge conformation and the nonzwitterionic charge-solvated conformation. Implications for the applicability of this mechanism to other zwitterionic systems are discussed.     

Coordination chemistry of the solvated silver(I) ion in the oxygen donor solvents water, dimethyl sulfoxide, and N,N'-dimethylpropyleneurea

The hydrated and dimethyl sulfoxide and N,N'-dimethylpropyleneurea solvated silver(I) ions have been characterized structurally in solution by means of extended X-ray absorption fine structure (EXAFS) and large-angle X-ray scattering (LAXS). The coordination chemistry of the hydrated and dimethyl sulfoxide solvated silver(I) ions has been reevaluated because of different results from the EXAFS and LAXS methods reported previously. Consistent results are obtained with a linearly distorted tetrahedral model with two short and approximately two long Ag-O bond distances: mean Ag-O bond lengths of 2.32(1) and 2.54(1) A for the hydrate, 2.31(1) and 2.48(2) A for the dimethyl sulfoxide solvate, and 2.31(1) and 2.54(2) A for the N,N'-dimethylpropyleneurea solvate, in solution.