Fluorescent Molecular Rotors of Organoboron Compounds from Schiff Bases: Synthesis, Viscosity, Reversible Thermochromism, Cytotoxicity, and Bioimaging Cells

We report the design, synthesis,
and characterization of two new
fluorescent molecular rotors of boron derived from Schiff bases: (2,4,8,10-tetra-tert-butyl-6-phenyldibenzo­[d,h]­[1,3,6,2]­dioxazaboronine
(3) and 1,4-bis­(2,4,8,10-tetra-tert-butyldibenzo­[d,h]­[1,3,6,2]­dioxazaboronin-6-yl)­benzene (4), as well as the free ligand 2-[[(3,5-di-tert-butyl-2-hydroxyphenyl)­imino]­methyl]-4,6-di-tert-butylphenol 1. All compounds were fully
characterized by NMR (1H, 11B, and 13C), IR, UV/vis, fluorescence spectroscopy, and high-resolution mass
spectrometry. The crystal structures of 3 and 4 showed tetracoordinated boron atoms with semiplanar skeleton ligands.
The free rotation of the fluorescent molecular rotor, only observed
in the binuclear compound, was decreased with increasing viscosity,
while the quantum yield was increased. Interestingly, the property
of reversible thermochromism was found in organoboron 4 in the solid state. DFT calculations to determine the both complexes
have free rotation around the CPh–B1 bond. The boron compounds 3 and 4 have shown low cytotoxicity activity
in cell line A-431 and low green staining in cells.