Luminescent cyclometallated rhodium(iii) bis(pyridylbenzaldehyde) complexes with long-lived excited states

Abstract

A series of luminescent cyclometallated rhodium(III) diimine complexes containing two aldehyde functional groups [Rh(pba)₂(N–N)]Cl (Hpba = 4-(2-pyridyl)benzaldehyde; N–N = 2,2′-bipyridine, bpy (2); 4,4′-dimethyl-2,2′-bipyridine, 4,4′-Me₂bpy (3); 1,10-phenanthroline, phen (4); 3,4,7,8-tetramethyl-1,10-phenanthroline, 3,4,7,8-Me₄phen (5); 4,7-diphenyl-1,10-phenanthroline, 4,7-Ph₂phen (6)) have been synthesised, and their photophysical and electrochemical properties investigated. The X-ray crystal structures of complexes 3, 4 and the precursor complex, [Rh₂(pba)₄Cl₂] (1), have also been determined. Upon photoexcitation, complexes 2–6 display long-lived emission in solutions at 298 K and in low-temperature glass. Remarkably, the luminescence lifetimes of the complexes in solutions at 298 K are extraordinarily long (ca. 4.2–8.7 µs). To the best of our knowledge, there is no precedent for such long emission lifetimes observed in other related cyclometallated rhodium(III) systems. The solution emission spectra show structured bands with emission maxima at ca. 506 nm. The emission is tentatively assigned to an excited state of triplet intra-ligand ³IL (π → π*)(pba⁻) character, probably mixed with some triplet metal-to-ligand charge-transfer ³MLCT (d_π(Rh) → π*(pba⁻)) character. On the basis of the facile reaction between the aldehyde group and the primary amine group, to form a secondary amine after reductive amination, complexes 2–6 have been used to label the protein bovine serum albumin. The photophysical properties of the bioconjugates have also been investigated.