Solution- and Solid-State Luminescent Borate Complexes Based on a Substituted π-Conjugated 2-(6′-Hydroxy-5′-benzofuryl) Scaffold

This paper describes the synthesis of pi-conjugated fluorophores based either on an anil or a benzoxazole scaffold incorporating a rigid 2-(6'-hydroxy-5'-benzofuryl) fragment. Their subsequent coordination to a BF2 motif led to highly luminescent B-III complexes called boranils or HBBO borate complexes, respectively. All the new compounds were characterized by NMR spectroscopy, mass spectrometry, and elemental analysis. The study of their optical properties in solution revealed two distinct photophysical behaviors depending on the substitution. Complexes bearing a strong electron-donating p-nBu(2)NC(6)H(4) group displayed a marked internal charge transfer (ICT) leading to a pronounced solvatochromism with lambda(em) ranging from 507 to 663 nm and quantum yields of up to 87%. Alternatively, borate complexes functionalized with p-tBuC(6)H(4) or p-OMeC6H4 displayed fluorescence in the visible range that is not influenced by the nature of the solvent. Although the emission of the boranils was quenched in the solid state, HBBO borate complexes displayed intense fluorescence from 489 to 567 nm and quantum yields of up to 23%. Finally, the excited states of a selection of borate complexes were modeled by using time-dependent DFT, which allowed calculation of the dipole moments of the dyes in their excited states.