Structure and Charge Transport Properties of Cycloparaphenylene Monolayers on Graphite

Abstract

The synthesis of CPPs has been experimentally challenging due to the strain energy needed to close the ring. [7] Vögtle and co-workers [8,9] attempted to synthesize CPPs from macrocycles formed by arene and cyclohexane units to therefore reduce the strain energy of the system. However, the first synthesis of CPPs was afforded only by Jasti et al. [10] in 2008, followed by the first selective synthesis of CPPs by Itami and co-workers [11,12] using a modular approach to build [n]CPPs with n ⩾ 12. Other successfully strategies (Yamago et al. [13] ) use a square-shaped tetra platinum biphenyl complex as an intermediate to selectively synthesize the [8]CPP molecule. In the above mentioned works, it was possible to obtain CPPs in the milligram-scale, with Jasti and co-workers [6,14] producing the first gram-scale synthesis. The smallest CPP synthesized up to date is [5]CPP, [15] which shows the great success achieved so far in overcoming the strain energy.Concerning the supramolecular packing, the solid-state (3D) structure of CPPs has been characterized in several works by means of X-ray diffraction. [16,17] The molecules self-assemble in a herringbone (HB) pattern independently of their size, as it often happens for other π-conjugated systems. An exception is the [6]CPP case, for which three different polymorphs have been found up to date. The first solid-state characterization of [6]CPP revealed a tubular-like structure, [14] but a recent study revealed that the crystallographic structure might depend on the crystallization conditions, also finding a herringbone configuration a few kcal mol −1 more stable than the tubular one. [18] More recently, another polymorph of [6]CPP has been found when the crystallization is carried out by sublimation at 220 °C. In these conditions, no solvent molecules are occluded inside the [6]CPP cavity, and thus, a concave-convex structure appears (i.e., molecules tightly packed in a T-like shape minimizing the internal space inside the cavities). [19] These findings clearly reveal the subtle yet dominant effect of the weak intermolecular interactions driving the supra molecular self-assembly of these systems. Motivated by this, some of us investigated in a previous theoretical work the adsorption energies of [8]CPP on small carbon nanoflakes (circumcirmcumcoronene) and how these through-space weak forces can serve to immobilize the systems in energetically favored configuration. [20] In the present study, we wish to go a step further and employ a tailored force field and molecular dynamics (MD) simulations to predict the supramolecular structure of thin films of CPPs on graphite surfaces. The systematic exploration of the adsorption The nanoscale organization of cycloparaphenylene molecules when physisorbed on a graphite surface is theoretically investigated by means of atomistic molecular dynamics simulations employing a tailored and benchmarked force field. The landing of a single molecule is first considered, to progressively deposit more molecules to finally reach the full coverage...

Theoretical Insights for Materials Properties of Cyclic Organic Nanorings

“…[ 228 ] The calculation of the electronic coupling values for the monolayers formed upon physisorption to a graphite surface yielded to hole (electron) mobility values of 5.6 (1.7) and 0.1 (0.3) for the hexagonal and concave‐convex configurations described before, respectively. [ 132 ] These values, slightly higher than those found for bulk hole mobilities, [ 66 ] might indicate a stronger disorder of real samples with respect to the monolayer investigated as a model case. For the field of organic and molecular electronics, note that: i) other transport regimes (e.g., metal‐doped 6CPP coupled with Au electrodes exhibiting negative differential resistence [ 229,230 ] ) have also been explored; ii) CPPs have also been proposed as additives to improve the electrical and mechanical (stretchability) properties of polymer‐based semiconductors.…”

“…Those results have also allowed to explore the consequences for the controlled growth of armchair or zigzag CNTs of various diameters and lengths, and more specifically how this could benefit from the use of nCPPs and nCCs molecular templates, respectively, of varying size. Interestingly, the chemical nature of individual CPPs allows a moderate radicaloid character of armchair CNTs of increasing size built from them, [ 132 ] which translates in practice toward the successful synthesis of extended CPP rings [ 154–156 ] or even armchair CNTs with the same diameter than a 12CPP system used as template. [ 70 ] Several synthetic routes have also been theoretically explored to better understand the complex mechanism and activation barrier heights for this growth; for example through a radical‐mediated mechanism involving CPP and C 2 H 2 species as reactants, [ 91 ] in line with previous experimental cycloaddition (diene – dienophile) studies.…”

“…Note that the roughness of graphite is very low, and thus this behaviour could be different in other surfaces (e.g., silica). If more molecules are progressively deposited on the surface to reach a full coverage, two stable configurations are found [ 132 ] as shown in Figure 5: i) a hexagonal structure arising from horizontally physisorbed molecules, consequence (but not only) of the molecule‐substrate interactions; and ii) a rectangular structure of vertical, concave–convex arranged molecules, consequence (but not only) of the molecule–substrate interactions.…”

“…These adsorption energy profiles were further used to calibrate a force‐field expression [ 132 ] ; that is, optimizing the Lennard–Jones parameters to fit the curves previously calculated at the HF‐3c level. Partial atomic charges of the adsorbate were calculated by fitting to the electrostatic potential, and then used to feed the force‐field.…”

“…">5.The process of physisorption and molecular friction of these systems on, for example, graphite surfaces, [ 131 ] which could modulate a highly directional growth of solid‐state samples, as well as the emerging semiconducting properties of the deposited monolayer upon diffusion of the charge carriers injected from the surface. [ 132 ]…”