Interplay of structural dynamics and electronic effects in an engineered assembly of pentacene in a metal-organic framework

• Published in: Chemical science (Cambridge) 2021-02, Vol.12 (12), p.4477-4483
• Main Authors: Haldar, Ritesh, Kozlowska, Mariana, Ganschow, Michael, Ghosh, Samrat, Jakoby, Marius, Chen, Hongye, Ghalami, Farhad, Xie, Weiwei, Heidrich, Shahriar, Tsutsui, Yusuke, Freudenberg, Jan, Seki, Shu, Howard, Ian A, Richards, Bryce S, Bunz, Uwe H. F, Elstner, Marcus, Wenzel, Wolfgang, Wöll, Christof
• Format: Article
• Language: English
• Subjects: Assembly; Banded structure; Carrier mobility; Charge transport; Chemistry; Coupling (molecular); Crystal defects; Crystal structure; Crystals; Current carriers; Dynamic structural analysis; Figure of merit; Long range order; Metal-organic frameworks; Molecular dynamics; Parameter sensitivity; Quantum mechanics; Substrates
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/d0sc07073d

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. Pentacene assembled into 1D arrays using a metal-organic framework (MOF) approach. This cofacial packing motif, which is not present in pentacene bulk, shows an interesting interplay of band-like and hopping-type transport.