Enhanced emission and analyte sensing by cinchonine iridium(III) cyclometalated complexes bearing bent diphosphine chelators

Abstract

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. © 2013 American Chemical Society.