Contours of constant phonon energies in thebc-plane of sapphire (Al2O3) by means of inelastic neutron scattering

By means of inelastic neutron scattering the contoursv=const. have been determined for the transverse acoustic phonon branch which is polarized in the mirror plane (bc-plane). The results show that in all directions effects of dispersion can be neglected forq0.5q _max.     

Bis(merocyanine) Hetero-Folda-Dimers: Evaluation of Exciton Coupling between Different Types of π-Stacked Chromphores

Exciton coupling between different types of chromophores has been rarely investigated. Herein, a systematic study on the exciton coupling between merocyanine chromophores of different conjugation length with varying excited state energies is presented. In this work well-defined hetero-dimer stacks were obtained upon folding of bis(merocyanine) dyes in nonpolar solvents. They show distinctly different absorption properties in comparison with the spectra of the single chromophores, revealing a significant coupling between the different chromophores. The simulated absorption spectra obtained from time-dependent density functional theory (TD-DFT) calculations are in good agreement with the experimental spectra. Our theoretical analysis based on an extension of Kasha's exciton theory discloses strong coupling between the dyes’ transition dipole moments despite of an excited-state energy difference of 0.60 eV between the chromophores.     

Exciton‐Vibrational Couplings in Homo‐ and Heterodimer Stacks of Perylene Bisimide Dyes within Cyclophanes: Studies on Absorption Properties and Theoretical Analysis

The optical properties of a series of three cyclophanes comprising either identical or different perylene bisimide (PBI) chromophores were studied by UV/Vis absorption spectroscopy and their distinctive spectral features were analyzed. All the investigated cyclophanes show significantly different absorption features with respect to the corresponding constituent PBI monomers indicating strong coupling interactions between the PBI units within the cyclophanes. DFT calculations suggest a π‐stacked arrangement of the PBI units at close van der Waals distance in the cyclophanes with rotational displacement. Simulations of the absorption spectra based on time‐dependent quantum mechanics properly reproduced the experimental spectra, revealing exciton‐vibrational coupling between the chromophores both in homo‐ and heterodimer stacks. The PBI cyclophane comprising two different PBI chromophores represents the first example of a PBI heterodimer stack for which the exciton coupling has been investigated. The quantum dynamics analysis reveals that exciton coupling in heteroaggregates is indeed of similar strength as for homoaggregates.     

Spacer‐Modulated Differentiation Between Self‐Assembly and Folding Pathways for Bichromophoric Merocyanine Dyes

We have synthesized a large series of bis(merocyanine) dyes with varying spacer unit and investigated in detail their self‐organization behavior by concentration‐ as well as solvent‐dependent UV/Vis spectroscopy. Our in‐depth studies have shown that the self‐organization of the present bis(merocyanine) dyes is subtly influenced by the nature of the spacer unit. The utilization of rigid spacers results in the formation of self‐associated bimolecular complexes with high binding strength, while flexible spacers drive the respective bichromophoric dyes to intramolecular folding. Our thorough investigations on the impact of alkyl spacer chain length on the folding tendency of the present series of bis(merocyanine) dyes revealed a biphasic behavior, that is, a steep increase of the folding tendency for the dyes containing C4 to C7 chains and then a gentle decrease for dyes with longer alkyl spacer chains as evidenced by free energy (ΔG) values for the folding of these dyes. Furthermore, analyses of aggregates’ optical properties based on exciton theory as well as quantum chemical calculations suggest a bimolecular aggregate structure for the dye possessing a rigid spacer and a rotationally twisted pleated structure for the bis(merocyanine) dyes having spacer units with less than seven carbon atoms, while the application of longer alkyl chain linkers (≥C7) provides enough flexibility to orient the chromophores in electrostatically most favored antiparallel fashion.     

Thiophene-functionalized isoindigo dyes bearing electron donor substituents with absorptions approaching the near infrared region

A series of six new, highly soluble N,N'-dialkylated isoindigo derivatives bearing different electron donating thiophene units at the 6,6'-positions were synthesized by Stille cross-coupling reaction. The optical and electrochemical properties of these dyes were studied by UV-vis spectroscopy and cyclic voltammetry, revealing a good tunability of their electronic properties by peripheral substituents with amino groups leading to strong absorption reaching the NIR region. The DFT calculations of the frontier molecular orbitals of these dyes corroborate the observed substituent effect on absorption and redox properties.     

Alpha and gamma spectrometry for waste management studies

Automated alpha and gamma spectrometry systems have been fabricated to analyze large numbers of samples in batches and to perform on-line analysis for a number of experiments concurrently. Specially designed sample changers for alpha and gamma spectrometry and gamma scanners for used fuel and in-situ glass leach experiments have been built. These communicate through digital interfaces, microprocessors, microcomputers and programmable controllers to a Nuclear Data model ND6700 computer/spectrometer. Application software for data entry and reports has been written to interact with Nuclear Data software to pemit easy and efficient operation.     

Early evolution of transversally thermalized partons

The idea that the parton system created in relativistic heavy-ion collisions (i) emerges in a state with transverse momenta close to thermodynamic equilibrium and (ii) its evolution at early times is dominated by the 2-dimensional (transverse) hydrodynamics of the ideal fluid is investigated. It is argued that this mechanism may help to solve the problem of early equilibration.     

Bis(merocyanine) Homo-Folda-Dimers: Evaluation of Electronic and Spectral Changes in Well-Defined Dye Aggregate Geometries

A series of three bis(merocyanine) dyes comprising chromophores of different conjugation lengths has been synthesized and the intramolecular aggregation process was investigated by UV/Vis absorption spectroscopy. The spectral changes observed upon variation of the solvent polarity reveal a folding process resulting in a cofacial π-stack of two chromophores with a decrease of the aggregation tendency with increasing chromophore length and solvent polarity. Solvent-dependent UV/Vis studies of the monomeric reference dyes show a significant increase of the polyene-like character for dyes with longer polymethine chains in nonpolar solvents, which is reversed upon aggregation due to the polarizability effect of the adjacent chromophore within the dye stack. The pronounced hypsochromic shift of the absorption band observed upon aggregation indicates strong coupling of the dyes’ transition dipole moments, which was confirmed by quantum-chemical analysis.     

The Rényi entropy H2 as a rigorous, measurable lower bound for the entropy of the interaction region in multi-particle production processes

A model-independent lower bound on the entropy S of the multi-particle system produced in high energy collisions, provided by the measurable Rényi entropy H₂, is shown to be very effective. Estimates show that the ratio H₂/S remains close to one half for all realistic values of the parameters.