Theoretical studies on the absorption spectra of cis-[Ru(4,4′-COO-2,2′-bpy)2(X)2]4−, (X = NCS, Cl) and panchromatic trans-terpyridyl Ru complexes including strong spin–orbit coupling

Abstract

In this paper, we theoretically and experimentally investigate the photophysical and chemical characteristics and absorption spectra of various ruthenium complexes in solution used as efficient dye-sensitized solar cells. The target molecules are two well-known complexes, cis-[Ru(4,4′-COO-2,2′-bpy)₂(X)₂]⁴⁻, (X = NCS, Cl) and trans-terpyridyl Ru. The experimental absorption spectra of these molecules, which show strong spin–orbit (SO) coupling, are simulated using first-order perturbation theory based on time-dependent density functional theory and quantum chemistry calculations. It turns out that the theory can simulate the experimental data very well, which indicates that SO coupling is very important and the mixing between singlet and triplet states is strong in these molecules because of the large SO coupling constant of the Ru atom. The exact absorption spectra can only be reproduced by including the perturbation by SO coupling. The physical and chemical differences between cis-[Ru(4,4′-COO-2,2′-bpy)₂(X)₂]⁴⁻, (X = NCS, Cl) and trans-terpyridyl Ru complexes are elucidated by natural bond orbital and natural transition orbital analyses. From these analyses, we have found that the two kinds of Ru complexes are quite different in terms of photoexcitation response and chemical bonding between the central Ru atom and the surrounding ligands.