Ground and excited state isomeric forms of the anthracene-diethylaniline molecular exciplex

The potential energy surfaces for the molecular complex formed between anthracene (the electron acceptor) andN ,N-diethylaniline (DEA) (the electron donor) were computed as the quasi-adiabatic states resulting from the configuration interaction between the ground (AD), locally excited (A^*D) and charge-transfer (A⁻D⁺) excited electronic configurations. The results clearly indicate the existence of three geometrically and energetically different isomeric forms of the complex in the ground state. In the excited state, the potential energy surfaces reveal the existence of five well-defined equilibrium configurations separated by energy barriers and characterized by different admixtures of the (A^*D) and (A⁻D⁺) electronic configurations. Such a variety of equilibrium configurations in the ground and excited states is, in part, accounted for by the existence of two different conformational forms of DEA that can form complexes with anthracene, and are characterized by different balances between steric effects and interactions of electronic charge distributions in the complex components. The energies of transitions between the relevant ground and excited state equilibrium configurations were calculated and compared with spectroscopic data of a jet-cooled complex obtained in supersonic beam experiments. These transitions were successfully assigned to the observed resonance-like and exciplex-like spectra, and this enabled interpretation of observed changes in the fluorescence excitation and fluorescence spectra of the complex upon excess excitation energy.     

Effect of pressure on the formation of the intramolecular excited complex state in 4-(9-anthrylmethyl)-N,N-dimethylaniline

The formation of an intramolecular excited complex state in 4-(9-anthrylmethyl)-N,N-dimethylaniline was investigated at high pressures. A change in the kinetics from thermodynamic control to kinetic control was observed in ethyl ether solvent. In toluene the process is kinetically controlled over the whole pressure range. Appearance of this process depends upon the solvent viscosity, suggesting a conformational change in the course of the formation of the excited complex state.     

Remarkable stability and enhanced optical activity of a chiral supramolecular bis-porphyrin tweezer in both solution and solid state

Interaction of the achiral syn (face-to-face) conformer of the ethane-bridged bis(zinc octaethylporphyrin) with the enantiopure 1,2-diaminocyclohexane results in the exclusive formation of a supramolecular chiral tweezer. This 1:1 host-guest complex exhibits remarkable stability in both solution (even upon photoexcitation) and solid-state phases, with a high degree of optical activity arising from the two-point interaction mode and optimal spatial geometry.     

The photoinduced long-lived charge-separated state of Ru(bpy)3-methylviologen with cucurbit[8]uril in aqueous solution

A stable 1 : 1 inclusion complex of Ru(bpy)(3)-MV(2+) with cucurbit[8]uril (CB[8]) is formed in aqueous solution; upon light irradiation, a long lived (tau approximately 2 micros) charge-separated state Ru(3+)-MV(+ )-CB[8] is observed.     

Determination of the formation of dark state via depleted spontaneous emission in a complex solvated molecule

We present a general two-color two-pulse femtosecond pump-dump approach to study the specific population transfer along the reaction coordinate through the higher vibrational energy levels of excited states of a complex solvated molecule via the depleted spontaneous emission. The time-dependent fluorescence depletion provides the correlated dynamical information between the monitored fluorescence state and the SEP "dumped" dark states, and therefore allow us to obtain the dynamics of the formation of the dark states corresponding to the ultrafast photoisomerization processes. The excited-state dynamics of LDS 751 have been investigated as a function of solvent viscosity and solvent polarity, where a cooperative two-step isomerization process is clearly identified within LDS 751 upon excitation.     

Electronic and photophysical properties of a novel phenol bound dinuclear ruthenium complex: evidence for a luminescent mixed valence state

A novel dinuclear ruthenium(II) complex bridged by dianionic bridge 3-(2-phenol)-5-(pyridin-2-yl)-1,2,4-triazole in which the ruthenium metal atoms are bound through N,N coordination to the pyridine and triazole and O,N coordination to the triazole and phenolate is described. The electrochemical, spectroscopic and photophysical behaviour of the dimer is compared with its associated N,N- and O,N-coordinated mononuclear complexes. The mixed valence complex was prepared electrochemically and a weak inter-valence charge transfer transition is observed which from Hush theory provides an electronic coupling matrix element of 666 cm(-1), suggesting the complex is weakly coupled and valence trapped. In its native state the dinuclear compound is essentially non-emissive but upon the oxidation of the O,N moiety luminescence from the complex is reversibly switched on at 0.3 V and reversibly switched off by application of 1.3 or 0 V. To our knowledge this is the first report of a luminescent mixed valence ruthenium complex.     

Multidimensional solid state NMR of anisotropic interactions in peptides and proteins

Accurate determinations of chemical shift anisotropy (CSA) tensors are valuable for NMR of biological systems. In this review we describe recent developments in CSA measurement techniques and applications, particularly in the context of peptides and proteins. These techniques include goniometeric measurements of single crystals, slow magic-angle spinning studies of powder samples, and CSA recoupling under moderate to fast MAS. Experimental CSA data can be analyzed by comparison with ab initio calculations for structure determination and refinement. This approach has particularly high potential for aliphatic (13)C analysis, especially Calpha tensors which are directly related to structure. Carbonyl and (15)N CSA tensors demonstrate a more complex dependence upon hydrogen bonding and electrostatics, in addition to conformational dependence. The improved understanding of these tensors and the ability to measure them quantitatively provide additional opportunities for structure determination, as well as insights into dynamics.     

Mechanistic impact of water addition to SmI2: consequences in the ground and transition state

The mechanistic impact of water addition to SmI2 on the ground state and rate-limiting transition state structures in the reduction of benzyl bromide was determined using UV-vis spectroscopy, cyclic voltammetry, vapor pressure osmommetry, and stopped-flow spectrophotometric studies. The results obtained from these studies show that, upon addition of water, SmI2 in THF (or DME) becomes partially water-solvated by displacing metal-coordinated solvent. Further addition of water displaces remaining bound solvent and induces a monomer-dimer equilibrium of the SmI2-water complex. Concomitant with this process, a thermodynamically more powerful reductant is created. Rate studies on the reduction of benzyl bromide by SmI2-water are consistent with reaction occurring through a dimeric transition state with the assembly of the activated complex requiring an equivalent of water at low concentrations but not at higher concentrations. The mechanistic complexity of the SmI2-water system shows that simple empirical models describing the role of water in SmI2-mediated reductions are likely to contain a high degree of uncertainty.     

Effect of remote substituents on solid state packing interactions in macrocyclic crownophanes derived from 1,3,5-triaroylbenzenes

A series of four structurally related crownophanes has been prepared and characterized by X-ray crystallography. The crownophanes are based upon a 1,3,5-triaroylbenzene framework and were synthesized via enaminone/alkyne cyclotrimerization. The crownophanes differ in the identity of a peripheral substituent attached to a remote arene ring that is not part of the cyclophane macrocycle. Solid state structural characterization reveals that crownophanes with remote phenyl and phenol substituents self-assemble to form centrosymmetric dimers. Incorporation of remote alkoxy groups (methoxy or ethoxy) disrupts dimerization and leads to catameric networks. Each crownophane crystallized as an inclusion complex or a hydrate and, in one instance, water was found to occupy the macrocyclic cavity.     

Charge state effect on the zwitterion influence on stability of non-covalent interaction of single-stranded DNA with peptides

Negative ion ESI mass spectrometry was used to study the gas-phase stability and dissociation pathways of peptide-DNA complexes. We show that bradykinin and three modified peptides containing the basic residue arginine or lysine form stable interactions with single-stranded oligonucleotides. ESI-MS/MS of complexes of T(8) with PPGFSPFRR resulted in a major dissociation pathway through cleavage of the peptide covalent bond. The stability of the complex is due to electrostatic interaction between the negatively charged phosphate group and the basic side chain of the arginine and lysine residues as demonstrated by Vertes et al. and Woods et al. In fact, the present work establishes the role played by zwitterions on complex stabilisation. The presence of protons in nucleobase and/or amino acid contributes in reinforcing the strength of the salt bridge (SB) interaction. The zwitterionic form of the most basic of amino acid residues, arginine, is assumed to form a strong SB interaction to the negatively charged phosphate groups of DNA. This non-covalent complex is stable enough to withstand disruption of the non-covalent interaction and to first break the covalent bond. Moreover, the dependence of fragmentation patterns upon the complex charge state is explained by the fact that the net number of negative charges modulates the number of zwitterionic sites, which stabilise the complexes. Finally, the weak influence of the nucleobase is assumed by the existence of competition for proton addition between the nucleobase and the R/K side chain leading to a decrease in the stabilisation of the SB interaction.     

Interconvertible multiple photoluminescence color of a gold(i) isocyanide complex in the solid state: solvent-induced blue-shifted and mechano-responsive red-shifted photoluminescence

In this study, we report the interconvertible tetracolored solid state photoluminescence of gold(i) isocyanide complex 2 upon various external stimuli through solid state structural changes. Soaking complex 2 in acetone yields blue emission as a result of the formation of 2B. The subsequent removal of acetone yields 2G through a crystal-to-crystal phase transition, which exhibits green emission. This green-emitting solid 2G exhibits stepwise emission color changes to yellow and then to orange upon mechanical stimulation by ball-milling, which corresponds to the formation of 2Y and 2O, respectively. 2B could be recovered upon the addition of acetone to 2G, 2Y, and 2O. Thus, these four emitting solid states of 2 can be switched between repeatedly by means of acetone soaking and the application of mechanical stimulation. Importantly, single crystal and powder X-ray diffraction (PXRD) studies fully show the detailed molecular arrangements of 2B, 2G, and 2Y. This is the first mechanochromic compound to show interconvertible four color emission in the solid state. We also present the first example of using PXRD measurements and the Rietveld refinement technique for the structural analysis of a ground powder in a luminescence mechanochromism study. We obtained complete molecular-level structural information of the crystalline states of 2B, 2G, 2Y, and 2O. In comparison with a more solvophobic analogue 1, we suggest that the weak interaction of 2 with acetone in the solid state would allow a solvent inclusion/release mode, which is an important structural factor for the unprecedented multicolor mechanochromic luminescence.     

Jahn-Teller distortion in the phosphorescent excited state of three-coordinate Au(I) phosphine complexes

DFT calculations were used to optimize the phosphorescent excited state of three-coordinate [Au(PR3)3]+ complexes. The results indicate that the complexes rearrange from their singlet ground-state trigonal planar geometry to a T-shape in the lowest triplet luminescent excited state. The optimized structure of the exciton contradicts the structure predicted based on the AuP bonding properties of the ground-state HOMO and LUMO. The rearrangement to T-shape is a Jahn-Teller distortion because an electron is taken from the degenerate e' (5dxy, 5dx2-y2) orbital upon photoexcitation of the ground-state D3h complex. The calculated UV absorption and visible emission energies are consistent with the experimental data and explain the large Stokes' shifts while such correlations are not possible in optimized models that constrained the exciton to the ground-state trigonal geometry.     

In search of excited-state proton transfer in the lumichrome dimer in the solid state: theoretical and experimental approach

Quantum chemical density functional theory (DFT) calculations and spectral data were employed to investigate the possibility of the excited-state double proton transfer (ESDPT) in lumichrome crystals. The calculations in a lumichrome dimer predict a transfer of a proton in the first excited state, leading to a cation-anion pair. The presently reported X-ray structure of 1,3-dimethyllumichrome and its complex solid-state luminescence indicate that also in this molecule intermolecular hydrogen bonds might be involved in the photophysics. The long-wavelength emission in lumichrome crystals peaked at 530 nm is attributed to excited-state proton transfer, whereas a wider emission band in methylated lumichrome derivatives peaked at 560 nm is attributed to ions formed upon photoexcitation of the crystals.     

Complexation of Sc3N@C80 endohedral fullerene with cyclic Zn-bisporphyrins: solid state and solution studies

We report the synthesis of two cyclic β-pyrrole unsubstituted meso-tetraphenyl bisporphyrins in which the porphyrin units are connected by two 2,3-hexadiynyl-1,6-dioxo or two hexyl-1,6-dioxo spacers, respectively. Both cyclic porphyrin dimers exist in solution as mixtures of two conformational isomers. In the solid state, the receptor with diynyl spacers forms a 1:1 complex with the icosahedral (I(h)) isomer of the trimetallic nitride endohedral fullerene Sc(3)N@C(80). In this complex the receptor adopts a scoop-shaped conformation having a dihedral angle of 87.25° between the two porphyrin planes. The hexyl spaced analogue, however, adopts a similar conformation upon encapsulation of one molecule of Sc(3)N@C(80) in a self-assembled dimeric capsule. The capsular complexes pack in columns and render the fullerene units completely isolated. In toluene solution, (1)H NMR experiments indicate that the endohedral fullerene Sc(3)N@C(80) is exclusively bound by the expanded isomer of both dimers. UV-vis and fluorescence titration experiments confirmed the existence of strong π-π interactions between the fullerene Sc(3)N@C(80) and the flexible bisporphyrin dimer with hexyl spacers. At micromolar concentration, the flexible receptor forms only a 1:1 complex with the endohedral fullerene with stability constant value of K(a) = 2.6 ± 0.3 × 10(5) M(-1).     

Lipid analysis by thin-layer chromatography--a review of the current state

High-performance thin-layer chromatography (HPTLC) is a widely used, fast and relatively inexpensive method of separating complex mixtures. It is particularly useful for smaller, apolar compounds and offers some advantages over HPLC. This review gives an overview about the special features as well as the problems that have to be considered upon the HPTLC analysis of lipids. The term "lipids" is used here in a broad sense and comprises fatty acids and their derivatives as well as substances related biosynthetically or functionally to these compounds. After a short introduction regarding the stationary phases and the methods how lipids can be visualized on an HPTLC plate, the individual lipid classes will be discussed and the most suitable solvent systems for their separation indicated. The focus will be on lipids that are most abundant in biological systems, i.e. cholesterol and its derivates, glycerides, sphingo- and glycolipids as well as phospholipids. Finally, a nowadays very important topic, the combination between HPTLC and mass spectrometric (MS) detection methods will be discussed. It will be shown that this is a very powerful method to investigate the identities of the HPTLC spots in more detail than by the use of common staining methods. Future aspects of HPTLC in the lipid field will be also discussed.     

Bound state spectroscopy of NH-He

The NH-He van der Waals complex was characterized via laser excitation of bands associated with the NH A (3)Pi-X (3)Sigma(-) transition. It was demonstrated that the ground state supports a bound level with a rotational constant of B"=0.334(2) cm(-1). These results are in agreement with the predictions of recent high-level theoretical calculations. Spin-orbit predissociation of the excited complex was observed, and the spectra yield insights regarding the NH(A)+He potential energy surfaces.     

Plutonium valence state distributions

A method for ascertaining equilibrium valence state distributions of plutonium in acid solutions as a function of the plutonium oxidation number and the solution acidity is illustrated with an example. The method may be more practical for manual use than methods based upon polynomial equations. Mound Laboratory is operated by Monsanto Research Corporation for the U.S. Atomic Energy Commission under Contract No. AT-33-1-GEN-53.     

Mimicking heme enzymes in the solid state: metal-organic materials with selectively encapsulated heme

To carry out essential life processes, nature has had to evolve heme enzymes capable of synthesizing and manipulating complex molecules. These proteins perform a plethora of chemical reactions utilizing a single iron porphyrin active site embedded within an evolutionarily designed protein pocket. We herein report the first class of metal-organic materials (MOMs) that mimic heme enzymes in terms of both structure and reactivity. The MOMzyme-1 class is based upon a prototypal MOM, HKUST-1, into which catalytically active metalloporphyrins are selectively encapsulated in a "ship-in-a-bottle" fashion within one of the three nanoscale cages that exist in HKUST-1. MOMs offer unparalleled levels of permanent porosity and their modular nature affords enormous diversity of structures and properties. The MOMzyme-1 class could therefore represent a new paradigm for heme biomimetic catalysis since it combines the activity of a homogeneous catalyst with the stability and recyclability of heterogeneous catalytic systems within a single material.     

Molecular weight distributions in radiation-induced polymerization. IV. A comprehensive kinetic scheme for polymerization in the liquid state

A comprehensive kinetic scheme is proposed which takes into consideration γ-ray-initiated polymerization via free radical, cationic, or radical-cationic species, or their combinations, and the relative contribution of each depending upon experimental conditions. From this kinetic model, equations describing the kinetics of polymerization and the resulting molecular weight distributions have been derived. The resulting expressions are complex in nature, particularly for the case where a combination of mechanisms is involved. In view of this, a general approach to the problem of generating theoretical molecular weight distributions based on the application of high speed digital computers has been presented. The proposed technique is particularly powerful since no simplifying assumptions or mathematical approximations are required in order to obtain the molecular weight distribution.     

Dynamic study of excited state hydrogen-bonded complexes of harmane in cyclohexane-toluene mixtures

Photoinduced proton transfer reactions of harmane or 1-methyl-9H-pyrido[3,4-b]indole (HN) in the presence of the proton donor hexafluoroisopropanol (HFIP) in cyclohexane-toluene mixtures (CY-TL; 10% vol/vol of TL) have been studied. Three excited state species have been identified: a 1:2 hydrogen-bonded proton transfer complex (PTC), between the pyridinic nitrogen of the substrate and the proton donor, a hydrogen-bonded cation-like exciplex (CL*) with a stoichiometry of at least 1:3 and a zwitterionic exciplex (Z*). Time-resolved fluorescence measurements evidence that upon excitation of ground state PTC, an excited state equilibrium is established between PTC* and the cationlike exciplex, CL*, lambdaem approximately/= 390 nm. This excited state reaction is assisted by another proton donor molecule. Further reaction of CL* with an additional HFIP molecule produces the zwitterionic species, Z*, lambda(em) approximately/= 500 nm. From the analysis of the multiexponential decays, measured at different emission wavelengths and as a function of HFIP concentration, the mechanism of these excited state reactions has been established. Thus, three rate constants and three reciprocal lifetimes have been determined. The simultaneous study of 1,9-dimethyl-9H-pyrido[3,4-b]indole (MHN) under the same experimental conditions has helped to understand the excited state kinetics of these processes.