Phonons crucially impact a variety of properties of organic semiconductor materials. For instance, charge-and heat transport depend on low-frequency phonons, while for other properties, such as the free energy, especially high-frequency phonons count. For all these quantities one needs to know the entire phonon band structure, whose simulation becomes exceedingly expensive for more complex systems when using methods like dispersion-corrected density functional theory (DFT). Therefore, in the present contribution we evaluate the performance of more approximate methodologies, including density functional tight binding (DFTB) and a pool of force fields (FF) of varying complexity and sophistication. Beyond merely comparing phonon band structures, we also critically evaluate to what extent derived quantities, like temperature-dependent heat capacities, mean squared thermal displacements and temperaturedependent free energies are impacted by shortcomings in the description of the phonon bands. As a benchmark system, we choose (deuterated) naphthalene, as the only organic semiconductor material for which to date experimental phonon band structures are available in the literature.Overall, the best performance amongst the approximate methodologies is observed for a systemspecifically parametrized second-generation force field. Interestingly, in the low-frequency regime also force fields with a rather simplistic model for the bonding interactions (like the General Amber Force Field) perform rather well. As far as the tested DFTB parametrization is concerned, we obtain a significant underestimation of the unit cell volume resulting in a pronounced overestimation of the phonon energies in the low frequency region. This cannot be mended by relying on the DFT-calculated unit cell, since with this unit cell the DFTB phonon frequencies significantly underestimate the experiments.3

Metal-organic frameworks (MOFs) are crystalline materials consisting of metal centers and organic linkers forming open and porous structures. They have been extensively studied due to various possible applications exploiting their large amount of internal surface area. Phonon properties of MOFs are, however, still largely unexplored, despite their relevance for thermal and electrical conductivities, thermal expansion, and mechanical properties. Here, we use quantum-mechanical simulations to provide an in-depth analysis of the phonon properties of isoreticular MOFs. We consider phonon band structures, spatial confinements of modes, projected densities of states, and group velocity distributions. Additionally, the character of selected modes is discussed based on real-space displacements and we address, how phonon properties of MOFs change, when their constituents are altered, e.g., in terms of mass and spatial extent, bonding structure etc. We find that more complex linkers shift the spectral weight of the phonon density of states towards higher frequencies, while increasing the mass of the metal atoms in the nodes has the opposite effect. As a consequence of the high porosity of MOFs, we observe a particularly pronounced polarization dependence of the dispersion of acoustic phonons with rather high group velocities for longitudinal acoustic modes (around 6000 ms -1 in the long wavelength limit). Interestingly, also for several optical phonon modes group velocities amounting to several thousand ms -1 are obtained. For heterogeneous systems like MOFs correlating group velocities and the displacement of modes is particularly relevant. Here we find that high group velocities are generally associated with delocalized vibrations, while the inverse correlation does not necessarily hold. These results provide the foundations for an in-depth understanding of the vibrational properties of MOF, and, therefore, pave the way for a future rational design of systems with welldefined phonon properties.

Covalent organic frameworks (COFs) have attracted significant attention due to their chemical versatility combined with a significant number of potential applications. Of particular interest are two-dimensional COFs, where the organic...

Controlling the transport of thermal energy is key to most applications of metal-organic frameworks (MOFs). Analyzing the evolution of the effective local temperature, the interfaces between the metal nodes and the organic linkers are identified as the primary bottlenecks for heat conduction. Consequently, changing the bonding strength at that node-linker interface and the mass of the metal atoms can be exploited to tune the thermal conductivity. This insight is generated employing molecular dynamics simulations in conjunction with advanced, ab initio parameterized force fields. The focus of the present study is on MOF-5 as a prototypical example of an isoreticular MOF. However, the key findings prevail for different node structures and node-linker bonding chemistries. The presented results lay the foundation for developing detailed structure-to-property relationships for thermal transport in MOFs with the goal of devising strategies for the application-specific optimization of heat conduction. The structure of a metal-organic framework (MOF) is characterized by an open framework of metal ions interconnected by organic linkers. Its porous structure is particularly interesting for various applications including catalysis [1] and the capture,

Processing oriented metal–organic frameworks (MOFs) as thin films is a key challenge for their application to device fabrication. However, typical fabrication methods cannot generate precisely oriented crystals on commercially relevant scales (i.e., cm2). This limits access to applications that require anisotropic functional properties (e.g., separation, optics, and electronics). Currently, highly oriented copper‐based MOFs are synthesized via the addition of the organic MOF component to an ethanolic solution of manually aligned Cu(OH)2 nanobelt films. In this work, the optimization of a semi‐automatic method for the fabrication of precisely oriented MOF films that affords a 100% yield of high quality ceramic films at the centimeter scale is reported. This improved fabrication protocol will facilitate the progress of heteroepitaxially grown MOFs for molecular separators and micro‐opto‐electronic devices.