Monolayers of twisted binaphthyls for aromatic crystallization at low nucleation densities and high growth rates

Monolayers of 1,1'-bi-2-naphthol (BN) derivatives, of which the two naphthalene rings are twisted along the carbon(1)-carbon(1') single bond, were studied for their conformational effect on the growth of pentacene crystals on their monolayer surface. BN monolayers with H and Br at 6,6'-positions (H-BN and Br-BN) were prepared by immersion-coating in toluene solution of the corresponding BNSiCl2. Pentacene was thermally evaporated onto the H-BN and Br-BN monolayers, silica, octadecylsilyl (ODTS) SAM, and a micropattern of H-BN and ODTS SAM. Pentacene crystals were also grown on the SAMs of 1-naphthylsilyl(NPh), phenylsilyl(Ph), and diphenylsilyl (DPh) groups, which are aromatic and have contact angle values similar to those of the the BN monolayers. AFM images of the crystals at the early stage of growth indicated that the BN monolayers suppressed the nucleation while facilitating the growth of nuclei to larger crystals. The low nucleation density and high growth rate are accounted for by the amorphous nature of the twisted BN monolayer surface where the intermolecular interaction between neighboring adsorbates is likely to be suppressed. The results offer new insights into designing surfaces for controlling the crystallization kinetics of organic materials.