TD-DFT investigation of triple-stranded helicates with bis(benzene- o-dithiolato) ligands

Abstract

The electronic spectra of triple-stranded complexes of Ti(IV) with bis(benzene- o-dithiolato) ligands (H 4-L 1 = 1,2-bis(2,3-dimercaptobenzamido)ethane; H 4-L 2 = 1,2-bis(2,3-dimercaptobenzamido)benzene) were investigated at the TD-DFT level of theory employing B3LYP/LANL2DZ. The influence of the nature (aliphatic or aromatic) of the spacer bridging both {Ti(S 2C 6H 3) 3} units on the absorptive features of the dinuclear complexes was also studied. B3LYP/LANL2DZ leads to spectra accounting for four absorption bands. At the lowest-energy region, the most important transitions are due to ligand-to-metal charge transfer (LMCT), in which out-of-plane ligand-centered orbitals and titanium-based MOs are involved. In going to the blue-region, a third band was detected with excitations showing an important contribution from ligand-to-ligand charge transfer (LLCT) and indeed, a combined LMCT + LLCT character has been considered. This observation seems to arise from a decrease in the metallic character of the LUMO-derivatives involved in the excitations. The origin of the absorption band at the highest-energy part of the spectrum is assigned to a LLCT. The influence of the nature of the spacer on the molar absorption coefficients ( ɛ) for this band has been clearly observed. The complex bearing aliphatic spacer shows ɛ of about 5 E +4 M −1 cm −1, while the one containing an aromatic spacer present a value of ɛ of about 2 E +5 M −1 cm −1.