Fluorescent columnar bis(boron difluoride) complexes derived from tetraketonates

Three new series of bis-(boron difluoride) complexes 1a-c derived from substituted tetraketonates 2a-c are reported, and their mesomorphic and optical properties have been investigated. Two single crystals of mesogenic ligand 2a (n = 6) and nonmesogenic diboron complex 1a (n = 14) were obtained, and their single crystal and molecular structures were resolved. Results show that all 2a ligands (n = 6, 8, 10, 12, 14) formed nematic and/or smectic C phases; however, all 2b-c compounds were not mesogenic. In contrast, bis(BF₂) complexes 1a-b were nonmesogenic, and 1c (n = 8, 10, 12) exhibited enantiotropic columnar mesophases. N_cell = 2.70-2.86 within a column slice of 9.0 Å thick was obtained for compound 1c, indicating that a single disc-shaped molecule was stacked within columns in Col_h phases. Bis(BF₂) complexes 1a-c (n = 8) possess a pronounced yellow-red emission in solution at room temperature. The luminescent emission showed a strong dependence on structural moiety incorporated, with λ_max = 538 (1a) < 641 (1b) < 708 nm (1c). To the best of our knowledge, this is the first example of columnar bis(BF₂) complexes.