Large area, soft crystalline thin films of N,N′,N′′-trialkyltriazatriangulenium salts with homeotropic alignment of the discotic cores in a lamellar lattice

Abstract

N,N′,N′′-Trialkyltriazatriangulenium (R-TATA⁺) tetrafluoroborate (BF₄⁻) salts form highly ordered thin films directly when spin-cast onto rotating substrates. The homogeneous and crystalline thin films show macroscopic order over centimetres. The crystal structures of the R-TATA·BF₄ salts are investigated and compared for the propyl, 3-methylpentyl and octyl derivatives. In all cases the molecules pack in hexagonally ordered bilayers, with segregation of the ionic groups and the alkyl chains. This is a rare packing motif with the rigid discotic TATA⁺ cores organized co-planarly in sheets separated by perpendicularly oriented alkyl chains, somewhat similar to a crystallized lamellar phase of phospholipid bilayers and the lamellar smectic mesophase of liquid crystals. The structure of the thin films is investigated by optical spectroscopy, X-ray reflectometry and grazing incidence X-ray diffraction. All techniques confirm that the 15–30 nm thick films maintain the lamellar structure of the bulk crystals, are flat on macroscopic length scales, and are completely ordered relative to the substrate. The triazatriangulenium molecules are electroactive dyes, and in the highly anisotropic polycrystalline thin film structure they show intense fluorescence and efficient exciton transport.