Effect of twisted molecular geometry on the solid-state emissions of an anthracene fluorophore

Abstract

An anthracene-derived luminogen, A-4OH, obtained by reacting anthracene-9-aldehyde with 4-hydroxy benzoic acid hydrazide, produced two polymorphic phases and two solvates, each of which could be distinguished by the solid state emission. The luminogen A-4OH was constructed by attaching the anthracene motif to a 4-hydroxyphenyl group by means of the hydrazide group, which was expected to determine the molecular geometry in terms of the relative orientation of the anthracene motif and 4-hydroxyphenyl group. We isolated a green emissive phase, A-4OH-G, wherein the molecules adopted twisted conformations in the lattice, with the anthracene groups oriented ‘cisoid’ with respect to the 4-hydroxyphenyl group (C–CN–N torsion angles of 2.4° and 2.7°). Two symmetry-independent molecules were observed in the lattice of A-4OH-G, wherein the anthracene motifs were held in place by end-to-face interactions (angles between the aromatic planes ∼70 to 72°) and slipped face-to-face interactions. Subsequently, we isolated the weakly emissive A-4OH-O form, as a solvate from N,N-dimethylformamide (DMF), while the non-emissive A-4OH-NE form was obtained from methanol. Crystal analysis of the A-4OH-O and A-4OH-NE forms revealed distinct aromatic–π interactions between the anthracene motifs with face-to-face overlap. Furthermore, thermal stimulation of the A-4OH-NE form triggered the release of the solvated DMF molecules from the lattice affording a yellow emissive phase, A-4OH-Y, which could be characterised using powder X-ray diffraction and thermal methods.