Self-Assembly of Fluorescent Amphipathic Borondipyrromethene Scaffoldings in Mesophases and Organogels

Abstract

Mesogenic scaffoldings are constructed from highly luminescent 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (Bodipy) linked via an amide or an ethynyl spacer to a functional amphipathic platform. A single-crystal X-ray structure of a parent methoxy compound revealed that the Bodipy fragment is almost perpendicular to the aromatic core of the functional platform and that hydrogen-bonded dimers form one-dimensional crystallographic chains through the stacking of two Bodipy residues. The functional molecules self-assemble into thermotropic and fluorescent columnar mesophases with a hexagonal symmetry as deduced from small-angle X-ray scattering. An unprecedented texture of the Bodipy mesophase was observed by fluorescence microscopy and correlates the classical texture observed by polarized optical microscopy. Molecular modeling and FT-IR strongly suggest that the driving forces for the emergence of the mesophase is the formation of a tight hydrogen-bonded network, combined with stacking of the Bodipy fragments. Besides, the tris-amide (n = 16), Bodipy compound proved to be an excellent gelator of nonane in particular. The resulting robust gels are highly luminescent, and transmission electron microscopy revealed that the gels are mastered through the formation of an interconnected network of fibrils. Temperature-dependent FT-IR spectroscopy shows that the amide functions are engaged in hydrogen bonds that are not disrupted above the sol−gel transition. Temperature-dependent 1H, 11B NMR experiments and fluorescence spectroscopy results are in line with a gelification process driven by the aggregation of the Bodipy residues into large objects by formation of J-aggregates.