Acene Size-Dependent Transition of The Radical Centers From the Metal to The Acene Parts In Monocationic Dinuclear (Diethynylacene)diyl Complexes

Controlling radical localization/delocalization is important for functional materials. The present paper describes synthesis and results of electrochemical, spectroscopic, and theoretical studies of diruthenium (p-diethynylacene)diyl complexes, Me₃Si-(C≡C)₂-Ru(dppe)₂-C≡C?Ar-C≡C?Ru(dppe)₂-(C≡C)₂-SiMe₃ ( 1–6 ) (dppe: 1,2-bis(diphenylphosphino)ethane), and their monocationic radical species ( 1 ]⁺– 6 ]⁺). The HOMO-LUMO energy gaps can be finely tuned by the acene rings in the bridging ligands installed, as indicated by the absorption maxima of the electronic spectra of 1–6 ranging from the UV region even to the NIR region. The cationic species 1 ]⁺– 6 ]⁺ show two characteristic NIR bands, which are ascribed to the charge resonance (CR) and π-π* transition bands, as revealed by spectroelectrochemistry. Expansion of the acene rings in 1 ]⁺– 6 ]⁺ causes (1) blue shifts of the CR bands and red shifts of the π-π* transition bands and (2) charge localization on the acene parts as evidenced by the ESR, DFT and TD-DFT analyses. Notably, the monocationic complexes of the larger acene derivatives are characterized as the non-classical acene-localized radicals.