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

484-492Using 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 (1a) and [Cu2L2(1,2-bis(4-pyridyl)ethane)](ClO4)2 (1b) are synthesised. The two metal ions in 1a are bridged by 4,4'-bipyridine and those in 1b by 1,2 -bis(4-pyridyl)ethane. From the X-ray crystal structure of 1a, 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 1a lie in the range of 1.998(5)-2.229(6)Å. Electrochemical studies in dichloromethane (DCM) show that the CUI2N8 moieties in 1a and 1b are composed of two essentially non-interacting CuIN4 cores with CuII/I potential of 0.44 V vs. SCE. While 1a displays metal induced quenching of the inherent emission of 4,4'-bipyridine in DCM solution, 1b 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 1b is from Cu(I) → bridging-ligand charge transfer excited state and the lower energy one in 1b from Cu(I) → L charge transfer excited state