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  • Title: Interplay of structural dynamics and electronic effects in an engineered assembly of pentacene in a metal-organic framework.
    Author: Haldar R, Kozlowska M, Ganschow M, Ghosh S, Jakoby M, Chen H, Ghalami F, Xie W, Heidrich S, Tsutsui Y, Freudenberg J, Seki S, Howard IA, Richards BS, Bunz UHF, Elstner M, Wenzel W, Wöll C.
    Journal: Chem Sci; 2021 Feb 08; 12(12):4477-4483. PubMed ID: 34168750.
    Abstract:
    Charge carrier mobility is an important figure of merit to evaluate organic semiconductor (OSC) materials. In aggregated OSCs, this quantity is determined by inter-chromophoric electronic and vibrational coupling. These key parameters sensitively depend on structural properties, including the density of defects. We have employed a new type of crystalline assembly strategy to engineer the arrangement of the OSC pentacene in a structure not realized as crystals to date. Our approach is based on metal-organic frameworks (MOFs), in which suitably substituted pentacenes act as ditopic linkers and assemble into highly ordered π-stacks with long-range order. Layer-by-layer fabrication of the MOF yields arrays of electronically coupled pentacene chains, running parallel to the substrate surface. Detailed photophysical studies reveal strong, anisotropic inter-pentacene electronic coupling, leading to efficient charge delocalization. Despite a high degree of structural order and pronounced dispersion of the 1D-bands for the static arrangement, our experimental results demonstrate hopping-like charge transport with an activation energy of 64 meV dominating the band transport over a wide range of temperatures. A thorough combined quantum mechanical and molecular dynamics investigation identifies frustrated localized rotations of the pentacene cores as the reason for the breakdown of band transport and paves the way for a crystal engineering strategy of molecular OSCs that independently varies the arrangement of the molecular cores and their vibrational degrees of freedom.
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