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.

以天然产物辛可宁（cinchonine）衍生的环金属化配体（cyclometalating ligand），分别与弯折型双膦配体[4,6-双(二苯基膦基)吩噁嗪（Nixantphos）、4,5-双(二苯基膦基)-9,9-二甲基呫吨（Xantphos）]及平面型双膦配体1,2-双(二苯基膦基)苯（dppb）配位得到的铱(III)（Ir(III)）配合物，展现出优异的发光性能，其量子效率高于对应母体配合物。庞大的辛可宁衍生配体与弯折型双膦配体所带来的空间位阻，可有效限制非辐射能量转移过程。该系列辛可宁衍生及母体配合物在室温脱气二氯甲烷（CH₂Cl₂）溶液中，展现出472至569 nm的宽发射范围，量子效率最高可达0.38，荧光寿命介于0.01至0.46 μs之间。对部分代表性配合物开展的密度泛函理论（DFT, Density Functional Theory）计算结果，与晶体学测定的固态结构高度吻合，且可精准预测激发态下电子从喹啉中心轨道跃迁至铱5d-苯基分子轨道所对应的发射波长。配合物[(pcn)₂Ir(Nixantphos)][PF₆]（2；pcn = 2′-苯基-9-O-苄基-10,11-二氢辛可宁-C2,N）拥有最高的量子产率，可实现ppm级别缺电子芳香类物种的检测。