Carboranes tuning the phosphorescence of iridium tetrazolate complexes.

Abstract

New iridium tetrazolate complexes containing o-, m-, or p-carboranyl substitution in different positions of a phenylpyridine ligand have been prepared. The carborane isomers and the effect of their substitution position in the tuning of optical properties have been examined. The neutral complexes with the carboranyl substituent on the phenyl ring in meta position relative to the metal exhibit redshifted emission bands in contrast to blueshifts for those with carboranyl in para position. All cationic complexes display evidently blueshifted dual-peak emission compared with the carborane-free complex (c-TZ) with a broad single-peak emission. Introduction of carborane leads to a blueshift over 70 nm relative to c-TZ. Carboranes also significantly improve phosphorescence efficiency (ΦP ) and lifetime (τ), that is, ΦP =0.64 versus 0.21 (c-TZ) and τ=880 ns versus 241 ns (c-TZ). The unique hydrophilic nido-carborane-based Ir(III) complex nido-o-1 shows the largest phosphorescence efficiency (abs ΦP =0.57) among known water-soluble iridium complexes, long emission lifetime (τ=4.38 μs), as well as varying emission efficiency and lifetime with O2 content in aqueous solution. Therefore, nido-o-1 has been used as an excellent oxygen-sensitive phosphor for intracellular O2 sensing and hypoxia imaging.