Rational color tuning and luminescent properties of functionalized boron-containing 2-pyridyl pyrrolide complexes

Three systematically functionalized pyrrolide ligands were prepared via the coupling of methyl vinyl ketone and the respective carbaldehyde reagents, followed by treatment of the pre-formed dicarbonyl compounds with (NH₄)₂CO₃ in order to generate the required pyrrole fragment. These ligands readily reacted with the boron reagent BPh₃ to afford the complexes [(pyro)BPh₂] (2a), [(noro)BPh₂] (2b), and [(xaro)BPh₂] (2e), where (pyro)H, (noro)H, and (xaro)H represents the 2-pyridyl, 2-quinolinyl, and 2-quinoxalinyl pyrrole groups, respectively. Complexes 2a-2c give stable solutions in air, and show strong photoluminescence with emission peak maxima located at 490 nm, 510 nm, and 575 nm, respectively. Calculations based upon time-dependent density function theory (TDDFT) show that the S₁ state in these complexes is attributed to an allowed (π-symmetry) → π* (π-symmetry) transition located at the chelating pyrrolide moieties. Electroluminescence (EL) devices based on 2c were fabricated. The EL emission from 2c as the host-emitter, with the emission peak maximum shifted to 580 nm, was observed when BCP was used as the hole blocking material. This device produces saturated red-orange light-emission at an onset voltage of 8 V and a maximum brightness of 5000 cd m^-2 at a driving voltage of 15 V; the external quantum yield is estimated to be 0.5 %.