Third-order pump-probe spectroscopy applied to molecular dimers: characterization of relaxation dynamics and exciton-exciton annihilation.

We investigate theoretically the excitonic dynamics in molecular dimers which is monitored by two time-delayed femtosecond laser pulses. A two-photon absorption leads to a wave packet dynamics in the manifold of second excited states. This opens up the channel for exciton-exciton annihilation (EEA) which involves non-radiative electronic transitions.

Dynamic diffusion and precipitation processes across calcium silicate membranes.

Hypothesis: Chemical gardens are tubular inorganic structures exhibiting complex morphologies and interesting dynamic properties upon ageing, with coupled diffusion and precipitation processes keeping the systems out of equilibrium for extended periods of time. Calcium-based silica gardens should comprise membranes that mimic the microstructures occurring in ordinary Portland cement and/or silicate gel layers observed around highly reactive siliceous aggregates in concrete.

Experiments: Single macroscopic silica garden tubes were prepared using pellets of calcium chloride and sodium silicate solution.

Optically Induced Electron Transfer in Mixed-Valence States: A Model Study on Electronic Transitions, Relaxation Dynamics, and Transient Absorption Spectroscopy.

Quantum dynamical model calculations are performed on the optically induced electron transfer in a mixed-valence system interacting with different solvents. The simultaneously occurring processes of population transfer between electronic states and relaxation are studied in detail. Transient absorption traces, as recently recorded in our laboratory, are simulated, and the features of the spectra are related to the dynamics.

Release of Isonitrile- and NHC-Stabilized Borylenes from Group VI Terminal Borylene Complexes.

A family of doubly isonitrile-stabilized terphenyl borylenes could be obtained by addition of three equivalents of isonitrile to the corresponding Cr and W terminal terphenyl-borylene complexes. The mechanism of isonitrile- and carbon-monoxide-induced borylene liberation was investigated computationally and found to be significantly exergonic in both cases. Furthermore, addition of a small N-heterocyclic carbene (NHC) to a terminal Cr borylene complex results in release of an NHC-stabilized borylene carbonyl species, whereas the analogous reaction with bulkier phosphines results in metal-centered substitution.

Precipitation and Crystallization Kinetics in Silica Gardens.

Silica gardens are extraordinary plant-like structures resulting from the complex interplay of relatively simple inorganic components. Recent work has highlighted that macroscopic self-assembly is accompanied by the spontaneous formation of considerable chemical gradients, which induce a cascade of coupled dissolution, diffusion, and precipitation processes occurring over timescales as long as several days. In the present study, this dynamic behavior was investigated for silica gardens based on iron and cobalt chloride by means of two synchrotron-based techniques, which allow the determination of concentration profiles and time-resolved monitoring of diffraction patterns, thus giving direct insight into the progress of dissolution and crystallization phenomena in the system.

Experimental techniques for the growth and characterization of silica biomorphs and silica gardens.

Silica biomorphs and silica gardens are canonical examples of precipitation phenomena yielding self-assembled nanocrystalline composite materials with outstanding properties in terms of morphology and texture. Both types of structures form spontaneously in alkaline environments and rely on simple, and essentially similar, chemistry. However, the underlying growth processes are very sensitive to a range of experimental parameters, distinct preparation procedures, and external conditions.

The effect of silica on polymorphic precipitation of calcium carbonate: an on-line energy-dispersive X-ray diffraction (EDXRD) study.

Calcium carbonate is the most abundant biomineral and a compound of great industrial importance. Its precipitation from solution has been studied extensively and was often shown to proceed via distinct intermediate phases, which undergo sequential transformations before eventually yielding the stable crystalline polymorph, calcite. In the present work, we have investigated the crystallisation of calcium carbonate in a time-resolved and non-invasive manner by means of energy-dispersive X-ray diffraction (EDXRD) using synchrotron radiation.

Growth behavior and kinetics of self-assembled silica-carbonate biomorphs.

Upon slow crystallization from silica-containing solutions or gels at elevated pH, alkaline-earth carbonates spontaneously self-assemble into remarkable nanocrystalline ultrastructures. These so-called silica biomorphs exhibit curved morphologies beyond crystallographic symmetry and ordered textures reminiscent of the hierarchical design found in many biominerals. The formation of these fascinating materials is thought to be driven by a dynamic coupling of the components' speciations in solution, which causes concerted autocatalytic mineralization of silica-stabilized nanocrystals over hours.

Stabilization of amorphous calcium carbonate in inorganic silica-rich environments.

In biomineralization, living organisms carefully control the crystallization of calcium carbonate to create functional materials and thereby often take advantage of polymorphism by stabilizing a specific phase that is most suitable for a given demand. In particular, the lifetime of usually transient amorphous calcium carbonate (ACC) seems to be thoroughly regulated by the organic matrix, so as to use it either as an intermediate storage depot or directly as a structural element in a permanently stable state. In the present study, we show that the temporal stability of ACC can be influenced in a deliberate manner also in much simpler purely abiotic systems.