Mechanochromic Properties of Boron-Difluoride Complexes Bearing π-Expanded Pyridine Ligands: Effects of π-Conjugated Skeletons and Halogen Atoms

Halogenated and non-halogenated boron complexes bearing pyrrole and π-expanded pyridine structures, such as quinoline (for 1 a-H, 1 a-Cl, 1 a-Br, and 1 a-I), isoquinoline (for 1 b-Br), and phenanthridine (for 1 c-Br), were prepared to investigate their optical properties. Fluorescence studies of the different solid states revealed that the boron complexes 1 a-H, 1 a-Cl, and 1 a-Br exhibit the mechanochromic properties. The present paper also examined the optical properties of other analogs 1 a-I, 1 b-Br, and 1 c-Br both in-solution and solid state. Moreover, the interrelationship between their emission character, molecule structures, and solid state were studied by X-ray crystallographic analysis and X-ray diffraction investigation.