Aiming at a better understanding of the molecular details in light absorption during photosynthesis, spatial and temporal correlation functions as well as spectral densities have been determined. At the focus of the present study are the light-harvesting II complexes of the purple bacterium Rhodospirillum molischianum. The calculations are based on a time-dependent combination of molecular dynamics simulations and quantum chemistry methods. Using a 12 ps long trajectory, different quantum chemical methods have been compared to each other. Furthermore, several approaches to determine the couplings between the individual chromophores have been tested. Correlations between energy gap fluctuations of different individual pigments are analyzed but found to be negligible. From the energy gap fluctuations, spectral densities are extracted which serve as input for calculations of optical properties and exciton dynamics. To this end, the spectral densities are tested by determining the linear absorption of the complete two-ring system. One important difference from earlier studies is given by the severely extended length of the trajectory along which the quantum chemical calculations have been performed. Due to this extension, more accurate and reliable data have been obtained in the low frequency regime which is important in the dynamics of electronic relaxation.

Time-dependent atomistic view on the electronic relaxation in light-harvesting system II.

Molecular self-assembly constitutes a versatile strategy for creating functional structures on surfaces. Tuning the subtle balance between intermolecular and molecule-surface interactions allows structure formation to be tailored at the single-molecule level. While metal surfaces usually exhibit interaction strengths in an energy range that favors molecular self-assembly, dielectric surfaces having low surface energies often lack sufficient interactions with adsorbed molecules. As a consequence, application-relevant, bulk insulating materials pose significant challenges when considering them as supporting substrates for molecular self-assembly. Here, the current status of molecular self-assembly on surfaces of wide-bandgap dielectric crystals, investigated under ultrahigh vacuum conditions at room temperature, is reviewed. To address the major issues currently limiting the applicability of molecular self-assembly principles in the case of dielectric surfaces, a systematic discussion of general strategies is provided for anchoring organic molecules to bulk insulating materials.

https://pub.uni-bielefeld.de/download/2913801/2930780/AdvMater_25_2013_Rahe.pdf

Tuning Molecular Self-Assembly on Bulk Insulator Surfaces by Anchoring of the Organic Building Blocks

Structure of Epitaxial Layers of KCl on Ag(100)

Surface structures on wide band gap insulators and their use as templates for the growth of adsorbates are reviewed. Surface structures include evaporation structures, vicinal surfaces, facetted surfaces, epitaxial structures, or structures transferred to or induced by the growth of thin films. Most structures have been realized so far on Al2O3 and on alkali halide crystals. The guided growth of adsorbates is discussed, considering the examples of metallic clusters or wires and ordered films of organic molecules.

Structured surfaces of wide band gap insulators as templates for overgrowth of adsorbates.

Thin films of the model molecule perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) were vacuum deposited on epitaxial NaCl films on Ag(100) and investigated by spot-profile-analysis low-energy electron diffraction (SPA-LEED). At growth temperatures between 193 K and room temperature (RT), the first layer (monolayer) exhibits a commensurate $(3\sqrt{2}\ifmmode\times\else\texttimes\fi{}3\sqrt{2})R45\ifmmode^\circ\else\textdegree\fi{}$ superstructure with a quadratic unit cell, which is identical to that found for PTCDA growth on bulk NaCl crystals [Burke et al., Phys. Rev. Lett. 100, 186104 (2008)]. At RT, dewetting occurs and complicates the formation of multilayers. However, at low surface temperatures (223 K), closed PTCDA multilayers grow. The corresponding unit cell $[19.69(4)\text{ }\text{\AA{}}\ifmmode\times\else\texttimes\fi{}12.23(3)\text{ }\text{\AA{}}]$ is rectangular and similar to the herringbone structure of the (102) bulk plane of PTCDA. This growth form is metastable and the surface dewets upon annealing at 300 K. Submonolayers deposited at low temperatures (20--100 K) are disordered; upon annealing, they transform into the $(3\sqrt{2}\ifmmode\times\else\texttimes\fi{}3\sqrt{2})R45\ifmmode^\circ\else\textdegree\fi{}$ structure via a third metastable phase with a herringbone-type rectangular unit cell.

/pdf/stable-and-metastable-phases-of-ptcda-on-epitaxial-nacl-5b8wffg83n.pdf

Stable and metastable phases of PTCDA on epitaxial NaCl films on Ag(100)

Ordered growth of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) on Ag(111) partially covered by one or two monolayers of KBr was investigated by non-contact AFM with molecular resolution. Different adsorption patterns are found on the pure substrate, the one covered by a single monolayer, and the one covered by two monolayers KBr. Simulations with an extended Ising-type model reproduce these experimental patterns very well. The adsorbate-adsorbate and the adsorbate-substrate interaction parameters obtained from the simulation are discussed with respect to the interactions at the Ag(111)|KBr interface. As a result, alkali halide covered metals can be used for tuning the interactions and designing adsorption systems, which opens up new possibilities in the control of self-assembled nanostructures.

https://iopscience.iop.org/article/10.1088/0957-4484/17/6/006/pdf

Adsorption of PTCDA on a partially KBr covered Ag(111) substrate

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Adsorption of PTCDA on a partially KBr covered Ag(111) substrate