Photophysical properties of [Cu(binap) ₂] ⁺ and [Pd(binap) ₂] complexes: A theoretical study

A theoretical study of the electronic and molecular structure of [Cu(binap) ₂] ⁺ (1) and [Pd(binap) ₂] (2) complexes (binap = 2,20-bis(diphenylphosphino)-1,10-binaphthyl) was performed. We examined the ground states and the lowest excited states (S ₁ and T ₁) in order to get insights about the luminescence processes of these systems. Geometry optimizations were performed at density functional theory DFT (PW91 functional) employing Slater type orbitals (STO) including two polarization functions (TZ2P) and scalar relativistic corrections via the ZORA Hamiltonian. The UV-Vis absorption spectra were simulated employing time-dependent density functional theory (TD-DFT) and solvent effects were estimated using the COSMO model. The calculated excitation energies are in good agreement with experimental data, which is reflected by the calculated UV-Vis spectra. The results suggest that the lowest-energy excited state in each compound corresponds to a ML-LCT (metal-ligand to ligand charge transfer) state. We constructed Jablonsky diagrams for the luminescence phenom. In (1), this process consists of flourescence originated from the S ₁ state and the phosphorescence from the T ₁ state. In (2) the emission energy only results in phosphorescence. We found mainly differences in the geometrical parameters that envolve the Pd metal. The structural changes observed in the calculated geometries of the first excited states respect to those of the ground state have significative relevance in the values of the calculated Stokes shifts showed by the complexes in their UV-Vis/emission spectra.