Bonding and Electronic Properties of Linear Diethynyl Oligothienoacene-Bridged Diruthenium Complexes and Their Oxidized Forms.

A series of five diruthenium diethynyl complexes based on α,β-fused oligothienoacenes in the core of the bridging ligands [{Ru(dppe)Cp*}(μ-C≡C-L-C≡C)] [dppe = 1,2-bis(diphenylphosphino)ethane, Cp* = η-CMe; L = thieno[3,2-b]thiophene (4), thieno[2,3-b]thiophene (5), 3,4-dimethylthieno[2,3-b]thiophene (6), dithieno[3,2-b:2',3'-d]thiophene (7), and thieno[3,2-b]thieno[2',3':4,5]thieno[2,3-d]thiophene (8)] have been synthesized and fully characterized electrochemically and spectroscopically. Elongation of the redox noninnocent oligothienoacene bridge core causes a smaller potential difference between the initial two anodic steps, not seen for free dialkyl oligothienoacenes, and increased positive charge delocalization over the conjugated bridge backbone. The highest occupied molecular orbital of the parent complexes resides predominantly on the oligothienoacene core, with strong participation of the ethynyl linkers and slightly smaller contribution from the metallic termini. This bonding character makes the initial one-electron oxidation symmetrical, as revealed by combined voltammetric and spectroscopic (IR, UV-vis-near-IR, and electron paramagnetic resonance) methods as well as density functional theory (DFT) and time-dependent DFT calculations of truncated and selected nontruncated models of the studied series. The remarkable gradual appearance of two C≡C stretching absorptions in the IR spectra of the monocationic diethynyl complexes is ascribed to increasing vibronic coupling of the IR-forbidden ν(C≡C) mode of the oxidized -[C≡C-core-C≡C]- bridge with a low-lying π-π*(intrabridge)/metal-to-ligand charge-transfer electronic transition in the near-to-mid-IR spectral region.