Enhanced Emission and Analyte Sensing by Cinchonine Iridium(III) Cyclometalated Complexes Bearing Bent Diphosphine Chelators

Ir(III) complexes of cyclometalating ligands derived from the natural
product cinchonine and bent (4,6-bis(diphenylphosphino)phenoxazine
(Nixantphos), 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos))
and planar diphosphine ligands (1,2-bis(diphenylphosphino)benzene
(dppb)) exhibit good luminescence with quantum efficiencies higher
than those of their parent congeners. Steric hindrance by both the
bulky cinchonine-derived ligand and bent diphosphine could limit nonradiative
energy transfer. The cinchonine-derived and parent complexes cover
a broad emission range from 472 to 569 nm with quantum efficiencies
up to 0.38 and lifetimes from 0.01 to 0.46 μs in degassed CH2Cl2 solution at room temperature. DFT calculations
on selected examples are in good agreement with solid-state structures
determined crystallographically and accurately predict wavelengths
of emission by excited electron decay from a quinoline-centered orbital
to an Ir 5d–phenyl molecular orbital. The complex [(pcn)2Ir(Nixantphos)][PF6] (2; pcn = 2′-phenyl-9-O-benzyl-10,11-dihydrocinchonine-C2,N) exhibits the highest quantum yield and
could detect electron-deficient aromatic species at ppm levels.