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Solid-State Order and Charge Mobility in [5]- to [12]Cycloparaphenylenes

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journal contribution
posted on 13.12.2018, 00:00 by Janice B. Lin, Evan R. Darzi, Ramesh Jasti, Ilhan Yavuz, K. N. Houk
We report a computational study of mesoscale morphology and charge-transport properties of radially π-conjugated cyclo­para­phenylenes ([n]­CPPs) of various ring sizes (n = 5–12, where n is the number of repeating phenyl units). These molecules are considered structural constituents of fullerenes and carbon nanotubes. [n]­CPP molecules are nested in a unique fashion in the solid state. Molecular dynamics simulations show that while intra­molecular structural stability (order) increases with system size, inter­molecular structural stability decreases. Density functional calculations reveal that reorganization energy, an important parameter in charge transfer, decreases as n is increased. Intermolecular charge-transfer electronic couplings in the solid state are relatively weak (due to curved π-conjugation and loose inter­molecular contacts) and are on the same order of magnitude (∼10 meV) for each system. Intrinsic charge-carrier mobilities were simulated from kinetic Monte Carlo simulations; hole mobilities increased with system size and scaled as ∼n4. We predict that disordered [n]­CPPs exhibit hole mobilities as high as 2 cm2/(V·s). Our computations show a strong correlation between reorganization energy and hole mobility (μ ∼ λ–4). Quantum mechanical calculations were performed on cofacially stacked molecular pairs for varying phenyl units and reveal that orbital delocalization is responsible for both decreasing reorganization energies and electronic couplings as n is increased.