Mixed ligand binuclear copper(I) complexes containing CU^I₂N₈ chromophores. Observation of emissions from MLCT excited states involving two different ligands

Abstract

Using the 1:2 condensate (L) of diethylenetriamine and benzaldehyde as the main ligand, binuclear copper(I) complexes [Cu₂L₂(4,4'-bipyridine)](ClO₄)₂.0.5H₂O (<b>1a</b>) and [Cu₂L₂(1,2-bis(4-pyridyl)ethane)](ClO₄)₂ (<b>1b</b>) are synthesised. The two metal ions in <b>1a</b> are bridged by 4,4'-bipyridine and those in <b>1b</b> by 1,2 -bis(4-pyridyl)ethane. From the X-ray crystal structure of <b>1a</b>, each metal ion is found to be bound to three N atoms of L and one of the two N atoms of the bridging ligand in a distorted tetrahedral fashion. The Cu(I)-N bond lengths in <b>1a</b> lie in the range of 1.998(5)-2.229(6)Å. Electrochemical studies in dichloromethane (DCM) show that the CU^I₂N₈ moieties in <b>1a</b> and <b>1b</b> are composed of two essentially non-interacting

Cu^IN₄cores with Cu^II/I<i> </i>potential of 0.44 V <i>vs. </i>SCE. While <b>1a</b> displays metal induced quenching of the inherent emission of 4,4'-bipyridine in DCM solution, <b>1b</b> exhibits two weak emission bands in DCM solution at 425 and 477 nm (total quantum yield = 3.59 × 10.5) originating from MLCT excited states. With the help of Extended Huckel calculations it

is established that the higher energy emission in <b>1b</b> is from Cu(I) → bridging-ligand charge transfer excited state and the lower energy one in <b>1b</b> from Cu(I) → L charge transfer excited state.