ChemPhysChem

The cover picture illustrates the technique of time-resolved X-ray liquidography (solution scattering), a direct tool to probe structural dynamics and spatiotemporal kinetics of solution-phase reactions. Picosecond optical pulses (red line) trigger a chemical reaction in a solution and 100 ps long X-ray pulses (cyan line) from a synchrotron probe the evolving structures of transient species over picoseconds to microseconds. Molecules ranging from small molecules to organometallic complexes, nanoparticles and proteins are studied. In the corners are shown time-dependent difference scattering patterns for the iodine elimination reaction of 1,2-diiodoethane (C₂H₄I₂) in a methanol solution, along with the associated major transient structures identified via data analysis. On page 1958 H. Ihee et al. review experimental and theoretical approaches, application examples and future perspectives for femtosecond applications of this technique.

Reversible molecular logic with a cocktail of two parallel operating fluorophores is shown. Combinations of two chemical inputs induce four different fluorescence outputs as seen in the photograph. This corresponds to one-to-one mapping of input and output vectors, implemented by a Feynman logic gate, as described by E. Pérez-Inestrosa, U. Pischel et al on p. 2004.

Time and motion: Time-resolved X-ray liquidography (see picture) can directly probe the structural dynamics and spatiotemporal kinetics of the liquid phase with unprecedented spatial and temporal resolution. Recent developments in this method, including technical innovations, theoretical analysis, and several examples ranging from simple molecules to protein solutions, are reviewed.

Exciton migration: Intra-molecular electronic energy transfer, EET (scheme a), in a conjugated polymer (extended MEH-PPV in a good solvent) exhibits coherence in 200 fs. For inter-molecular EET (scheme b, in collapsed MEH-PPV nanoparticle), uncorrelated fluctuations lead to rapid de-coherence (<10 fs).

Vibrational circular dichroism: Normally, a femtosecond pulse would be too short to produce a spectrum. This may be possible with the heterodyned Fourier transform spectral interferometry and a pump–probe pulse scheme. The new technique already provided spectra of limonene (see picture), so far without time resolution.

With a twist: The rotational barrier of ethane (see picture) is analyzed in terms of atomic and diatomic contributions of the energy. No charge-localized reference state is necessary. Hyperconjugation effects give rise to different individual energy components that cancel almost completely. The picture of attributing the barrier to the repulsion between the hydrogen atoms is legitimate.

A phototriggered system for metal-ion release is developed using a phenolic Schiff-base derivative as ligand (see scheme). It is found that Zn²⁺ can be completely released from the ligand by using a phototrigger. The metal-releasing system is also active for other bivalent metal ions.

Photochromic reactions occurring in the solid state are studied by using solid-state ¹³C NMR and IR spectroscopy. Electrical switching of small-molecule photochromic compounds is observed on single crystals and thin films. The figure shows the voltage–current characteristics of a thin film of a dithienylethene derivative before (○) and after (•) UV irradiation.

A prototype photothermal procedure: Alkylsiloxane monolayers on surface-oxidized silicon substrates are processed in gaseous bromine using a focused Ar⁺-laser operated at λ=514 nm. Monolayer bromination remains confined to a micrometer-sized reaction zone, where laser irradiation results in a local temperature rise (see figure).

Return trip included! The first molecular realization of a reversible logic operation with a simple cocktail of two classical fluorophores is demonstrated (see figure). Protons and anions, which serve as inputs, trigger independent fluorescence answer signals. Thus, XOR and YES gates are straightforward implemented, resulting in reversible logic behavior.

Molecular networks: Chiral and metallized Salen molecules on a Cu(111) are investigated using local probe techniques (see figure). Whereas for the parent Co-Salen molecule no self-assembly is observed, in the metal–organic complexes the growth of large and regular molecular networks is achieved through a target-oriented synthetic design of the local electrostatic dipolar molecular fields.

Blu-ray balls: Au₁₁ clusters were synthesized in the presence of carboxylic-acid-terminated thiolated molecules. Due to the protection of the monodisperse clusters from quenchers in the solution by the high-density ligands, the Au₁₁ clusters exhibit blue luminescence with a QY of 8.6 % (see picture). They have great potential for applications of novel fluorophores.

Key signatures: Vibrational Raman optical activity (VROA) backward spectra are simulated for the 1,1′-binaphthyl compound in its cisoid (left in figure) and transoid (right) conformations. Both conformations have distinct VROA patterns, as illustrated by their atomic contributions to the VROA intensity for the mode at 1350 cm⁻¹ represented by a red impulse.

The color purple: Low-temperature scanning tunneling microscopy is applied to investigate the adsorption and self-assembly of Violet Lander molecules on native and KBr-passivated InSb c(8×2) reconstructed surfaces (see picture). Several different molecular conformations, adjusted to the anisotropic structure of the substrate, are observed. Tip-induced lateral manipulation of large molecules with spacer groups is proved to be feasible and controllable.

Interesting interactions: FTIR spectroscopy is applied to study the interactions between different nonaqueous polar solvents and sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT) in nonaqueous AOT/n-heptane reverse micelles. The figure shows a schematic representation of various micelle interfaces obtained with different polar solvents (GY=glycerol; EG=ethylene glycol; DMF=dimethylformamide).

Optical force sensor: To explore the use of fluorophores as optical sensors for local mechanical forces, a custom-tailored chromophore is synthesized and aligned in a flexible polyvinyl chloride matrix by stretching (see picture). Applying tensile stress causes a decrease in the fluorescence lifetime by 2.5 % and a blue-shift of the fluorescence spectrum of 1.2 nm, consistent with the predictions from semiempirical calculations.

A quantum-chemical approach for the determination of modes with maximum Raman and Raman optical activity (ROA) intensity is presented. The intensity-carrying modes are hypothetical modes that provide information about those molecular distortions leading to intense bands in the spectrum. Modes with maximum Raman optical activity intensity are presented for Λ-tris(propane-1,3-dionato)cobalt(III) [see picture].

Stable mates: Gold nanoparticles protected by a novel π-conjugated polymer show an exceptional nonlinear absorption effect (see picture), which simultaneously contains saturated absorption resulting from third-order nonlinearity and large reverse-saturated absorption resulting from fifth-order nonlinearity.

Artificial motors: Solvent-driven molecular shuttles containing a pyrene-connected macrocycle and an intramolecular charge-transfer (ICT) chromophore stopper (TDPD) are constructed. The macrocycle is located close to or far from the chromophore in apolar and polar solvents, respectively, which alters the fluorescent emission of the pyrene probe in the macrocycle (see figure).

New two-photon fluorescent probe: Comprehensive photophysical and two-photon absorption properties of a new fluorene derivative containing the 2-(2′-hydroxyphenyl)benzothiazole terminal construct are investigated (see figure). The potential use of this fluorenyl probe in bioimaging is demonstrated via one- and two-photon fluorescence imaging of COS-7 cells.

Enhanced gas solubility in nanoliquids, that is, in solvents confined in mesopores of solids, is directly measured by volumetry for H₂ in five solvents. Gas solubilities in liquids are enhanced up to 15-fold over the corresponding bulk values for nanoliquid sizes smaller than 15 nm (see bar chart). Previous observations by ¹H NMR spectroscopy are confirmed, and the role of surface excess concentrations at mesoconfined gas/liquid interfaces in enhancing the gas solubility is discussed.

Probing bistable systems: X-ray absorption near-edge spectroscopy (XANES) is successfully applied to probe the thermally and light-induced valence tautomeric interconversion in a 1:1 Co–dioxolene complex (see picture).

Thermally versus non-thermally assisted exciton migration: The population of the excited state of a “red” emitting trap is controlled by exciton migration in the copolymer host, showing an energy barrier of 20 meV that turns over to a non-activated regime below 200 K (see figure).

The other non-bonding interaction: Raman and infrared spectra of solutions in cryogenic solvents reveal the formation of halogen-bonded complexes of trifluoromethyl chloride, bromide and iodide with dimethylether (see picture), with strengths varying between 7 and 15 kJ mol⁻¹.

Banded particle films: Convective assembly of particles from dilute suspensions can produce discrete film morphologies in the form of bands. The spacing between such bands, when they are in a monolayer, is a strong function of the particle size. A geometric model considering the intersection of the particles with a stretched liquid meniscus can quantitatively predict these spacings with varying particle diameter (see picture).

Unbalanced: An enantioselective autocatalytic reaction coupled to the exergonic and fast formation of the heterochiral dimer may lead to a kinetically controlled spontaneous mirror symmetry breaking. The key feature is the non-linear effect arising from the mutual inhibition step (heterochiral dimerization) coupled to the enantioselective autocatalysis (see graph).

Summing up: A theoretical methodology simulates vibrational sum-frequency generation (SFG) spectra for different combinations of polarizations and is applied to p-nitrothiophenol and 2,4-dinitroaniline. The IR vectors and Raman tensors are calculated with DFT. Comparison of such simulations with experiment allows determination of the molecular orientation (see picture).

Highly stable and efficient polyfluorene: Triphenylamine (TPA) introduced as a lateral fluorene substituent of polyfluorene (PFO) prevents keto-defect formation, which leads to improved performances and colour stability of devices. Ad hoc synthesized model oligomers are used to monitor energy transfer between fluorene and fluorenone moieties (see figure).