Synthesis and near IR photoluminescence of Os(II) bis(2,2′-bipyridine) (3,8-diarylethynyl-1,10-phenanthroline) complexes: anomalous behavior in the 3,8-dinitrophenylethynyl-substituted homologue

A large bathochromic shift (⩾50 nm) and emission in the near infrared is observed by attaching arylethynyl groups at the 3,8-positions of the 1,10-phenanthroline ligand (phen) of [Os(bipy)2(phen)]2+ (where bipy = 2,2′-bipyridine). Thus [Os(bipy)2(3,8-di-4-methoxyphenylethynyl-1,10-phenathroline)]2+ emits at 795 nm, while [Os(bipy)2(3,8-diphenylethynyl-1,10-phenanthroline)]2+ emits at 815 nm. According to this trend it would have been expected that [Os(bipy)2(3,8-di-4-nitrophenylethynyl-1,10-phenathroline)]2+ emits farther in the near infrared. Nevertheless, this complex is not photoluminescent because of intramolecular electron transfer quenching of the MLCT excited state by the nitroaromatic group. These results set structural and redox potential standards in the design of near infrared emitters based on [Os(bipy)2(phen)]2+ type complexes.