Columnar liquid-crystalline J-aggregates based on N-core-substituted naphthalene diimides

Abstract

We report two distinct approaches to prepare unprecedented liquid-crystalline (LC) J-aggregates based on bis-core-dendronized naphthalene diimides (NDIs). The NDI 1, with free imidic NH groups, spontanously self-assembles via hydrogen bonds into a columnar LC structure where the chromophores organize with the cores parallel to the columnar axis. Quantum mechanics calculations (GFN2-xTB method) and middle angle X-ray scattering confirmed the uncommon columnar architecture for the LC 1 which enable strong J-type exciton coupling between NDI cores. On the other hand, the NDI 2, with N-butyl groups at the imide positions, spontaneously self-assembles by cofacial π-π stacking into columnar H-aggregates; however mechanical shearing of the samples induces the reversible formation of weakly coupled J-aggregates. Surprisingly, this mechanochromic behavior is induced by a molecular reorganization of the NDI cores, which is accompanied by a fluorescence turn on. Thus, in this work we successfully applied two approaches to prepare for the first time LC NDI J-aggregates, which are highly appreciated materials for photonic applications.