Synthesis and photophysical and supramolecular study of π-conjugated (diethylene glycol methyl ether) benzoateethynylene oligomers and polymers

Abstract

We report on the synthesis of a trimer, pentamer, heptamer and a polymer of the (benzoate)ethynylene type bearing highly polar diethylene glycol methyl ethers as side chains. All of the materials were characterized by ¹H, DEPT-135, APT, ¹³C NMR, UV-Vis, static and dynamic fluorescence spectroscopy. The X-ray scattering patterns of the pentamer and heptamer revealed a Smectic C lamellar order of molecules, while that of the polymer was of the Nematic type; however, any known texture reported in the literature was observed by POM. The general term to classify the mesomorphic behaviour of these materials is Sanidic liquid crystals (LCs), which is consistent with their molecular shape assembly resembling bricks or board-like structures. The distance within the lamellae of 2.1, 2.3 and 2.0 nm for the pentamer, heptamer and polymer, respectively, is almost twice the distance of a single molecule in its more extended conformation of 1.48 nm. Therefore, the model of organization is consistent with a supramolecular assembly in bilayers, as was found in other (alkyl)benzoateethynylenes, where the side chains are oriented in the opposite way, in a back-to-back “comb-like” style. The strong π–π interaction that governs these materials was evidenced by the repeated distance of 0.35 nm found by both X-ray scattering and HRTEM microscopy, which corresponds to the distance between conjugated chains. Photophysical properties are discussed as a function of the conjugation length and of the iodine or hydrogen terminal groups and are supported by theoretical calculations.