Morphology dependent interaction between Co(II)-tetraphenylporphyrin and the MgO(100) surface.

Porphyrins are key elements in organic-inorganic hybrid systems for a wide range of applications. Understanding their interaction with the substrate gives a handle on structural and electronic device properties. Here we investigate a single transition-metal porphyrin, namely Co(ii)-tetraphenylporphyrin (CoTPP), on the MgO(100) surface and the effect of multilayer film formation within hybrid density-functional theory and many-body perturbation theory. We focus on the relevant adsorption sites, simulate their photoemission spectra as a key fingerprint and compare with experiments on MgO(100) films on Ag(100). While we find only weak interaction between the cobalt centre and terrace sites on the MgO(100) surface, a strong interaction manifests itself with the low-coordinated sites. This leads to distinct features in both the valence and core-level regions of the electronic structure, as observed in the ultraviolet and X-ray photoemission spectra, corroborated by simulated spectra and calculated cobalt core-level shifts. Our work thus demonstrates the relevance of morphology-related low-coordinated sites and their properties in the adsorption of CoTPP on the MgO(100) surface.

Effect of crystallization on the electronic and optical properties of archetypical porphyrins.

Thin porphyrin films as employed in modern optical devices or photovoltaic applications show deviating electronic and optical properties from the gasphase species. Any understanding of the physical origin may pave way to a specific engineering of these properties via ligand or substituent control. Here we investigate the impact of crystallization of prototypical porphyrins on the electronic levels and optical properties in the framework of density functional theory and many-body perturbation theory. Crystallization substantially shrinks the HOMO-LUMO gap based on polarization effects. We find a shift of the HOMO to higher energy is consistent with recent experiment of MgTPP multilayer film on Ag (100) [A. Classen et al., Phys. Rev. B, 2017, 95, 115414]. Calculated excitation spectra demonstrate a significant redshift of excitation bands except for the Q bands. These lowest excitation bands, in stark contrast to the strong HOMO-LUMO gap renormalization, remain essentially the same as in the gas phase. Our work underlines the possibility of band-gap engineering via ligand-controlled modification of the polarizability.

Dielectric and thermal effects on the optical properties of natural dyes: a case study on solvated cyanin.

The optical properties of the flavylium state of the cyanin dye are simulated numerically by combining Car-Parrinello molecular dynamics and linear-response time-dependent density functional theory calculations. The spectrum of the dye calculated in the gas phase is characterized by two peaks in the yellow and in the blue (green and violet), using a GGA-PBE (hybrid-B3LYP) DFT functional, which would bring about a greenish (bright orange) color incompatible with the dark purple hue observed in nature. Describing the effect of the water solvent through a polarizable continuum model does not modify qualitatively the resulting picture. An explicit simulation of both solvent and thermal effects using ab initio molecular dynamics results instead in a spectrum that is compatible with the observed coloration. This result is analyzed in terms of the spectroscopic effects of the molecular distortions induced by thermal fluctuations.

Functionality of C(4,4) carbon nanotube as molecular detector.

Alterations in the electronic transport properties of C(4,4) single walled carbon nanotube when an agent is introduced to the outer surface are investigated theoretically. Several chemical agents in this context are investigated. The calculations are performed in two steps: First an optimized geometry for the functionalized carbon nanotube is obtained using semi-empirical calculations at the PM3 level, and then the transport relations are obtained using non equilibrium green-function approach. Gaussian and Transiesta-C simulation packages are used in the calculations correspondingly. The "electrodes" are chosen to be ideal geometry of the particular carbon nanotube, eliminating current quantization effects due to contact region. By varying chemical potential in the electrode regions, an I-V curve is traced for each particular functionalisation. Conductance in carbon nanotubes show a strong dependence on the geometry and aromaticity, both are which altered when the suitable agent is introduced. This dependence results in rather dramatic response in the I-V trace, the current is reduced significantly, and quantization effects are observed, even for a single molecule. However due to chemically stable nature, not all agents form a chemical bond to the surface. Overall, the material is a promising candidate for detector equipment.

Thermal stability of benzorods: molecular-dynamics simulations.

Thermal stability of benzorods 2C6-20C6, which are obtained by stacking n (n=2-20) dehydrogenated benzene, have been investigated by molecular-dynamics simulations. It has been found that these structures assume a geometrical form depending on the number of dehydrogenated benzene layers, and they are stable under heat treatment up to elevated temperatures with a dependence on length.