Alkylated, naphthalimide-containing ionic compounds with rich thermotropic behaviour and nonlinear optical response

Abstract

Chemical functionalization of π-conjugated units plays a key role in fine tuning their supramolecular organizations and functions. Herein, five 1,8-naphthalimide derivatives were prepared where the naphthalimide moiety was attached to the imidazolium ring through a propyl linker. On the other side, the imidazolium ring was modified with either a branched aliphatic chain of 2-ethyl-hexyl (C₂C₆, 1), 2-hexyl-decyl (C₆C₁₀, 2) and 2-decyl-tetradecyl (C₁₀C₁₄, 3), or a linear aliphatic chain of octyl (C₈, 1′) and hexadecyl (C₁₆, 2′). Temperature-dependent structural evolution of these alkylated, naphthalimide-modified imidazolium bromides (abbreviated to a-NaphImiBrs hereafter) were investigated in detail by differential scanning calorimetry and small-angle X-ray scattering measurements as well as polarized optical microscopy observations. The compound with the shortest branched aliphatic chain (1) self-organized into a columnar oblique (Col_o) phase at room temperature, which changed to a columnar rectangular (Col_r) phase upon heating. In comparison, its counterpart with a linear aliphatic chain of the same carbon number (1′) formed a crystal at room temperature, which shifted to a Col_o phase at elevated temperature. The compound with a medium branched aliphatic chain (2) showed a columnar hexagonal (Col_h) organization at room temperature, which changed to a smectic (Sm) phase upon heating. Compounds with longer aliphatic chains (2′, 3), regardless of whether branched or linear, only exhibit the Sm phase. Films of a-NaphImiBrs formed at room temperature were subjected to evaluations for their nonlinear optical (NLO) responses where reverse saturated absorption was observed in all the cases. It was found that 2 showed the best NLO response with a third order nonlinear absorption coefficient of up to 0.49 cm W⁻¹.