Photophysics of a mono-nuclear tetrahedral silver(I)N4 core and its copper(I) analog

Abstract

A neutral ligand L is prepared by condensation of benzil dihydrazone and acetone in 1:2 mol ratio and used for the syntheses of [CuL2]ClO4, [CuL2]PF6, [AgL2]ClO4 and [AgL2]PF6. The X-ray crystal structures of L and the two perchlorate salts have been determined. The metal complexes are found to be mononuclear containing tetrahedral N4 coordination spheres for Cu(I) and Ag(I). Cyclic voltammetrically, the Cu(II/I) and Ag(II/I) potentials are 1.23 and 0.76 V vs NHE respectively in CH2Cl2 at a Pt electrode. The reason for the such a low Ag(II/I) potential is that the silver(I) complex is adsorbed on the electrode surface with a free energy of adsorption of ∼ −14.99 kcal mol−1. DFT calculations at the BP86/LanL2DZ level show that the HOMO’s in [CuL2]ClO4 and [AgL2]ClO4 are both metal based and the LUMO’s have no contribution from the metals. Both complexes show weak emissions from the MLCT states upon excitation at 270 nm in ethanol solution at room temperature. Changing the counterion to PF6− leads to higher quantum yields for these processes. It is consistent with the general observation that ClO4− being more coordinating than PF6−, it binds the metals in the MLCT state to form exciplexes leading to a lower ϕ or total quenching. At 77 K in ethanol glass, L, [CuL2]+ and [AgL2]+ show ligand centered (LC) emissions. Thus the MLCT and LC states are thermally equilibrated in the silver(I) and copper(I) complexes. In keeping with this, a mixture of the two processes, MLCT and LC emissions, is observed at room temperature for [AgL2]ClO4 and [CuL2]ClO4 when they are incorporated in a rigid polymethyl methacrylate matrix.