Variation from closed-shell to open shell electronic structures in oligothiophene bis(dioxolene) complexes

A series of oligothiophene bis(dioxolene) complexes, SQ-Th-SQ (SQ = S = ½Tp^Cum,MeZn^II(3-tert-butyl-orthosemiquinonate); Tp^Cum,Me = tris(5-cumenyl-3-methylpyrazolyl)borate anion) have been synthesized, structurally characterized, and studied as a function of the number of thiophene bridging units, n (n = 0-3) using a combination of variable-temperature (VT) electronic absorption and EPR spectroscopies, and VT magnetic susceptibility measurements. The thiophene bridge bond lengths determined by X-ray crystallography display dramatic differences across the SQ-Th-SQ series. Bridge bond deviation values (Σ|Δ_i|) display a progressive change in the nature of the bridge fragment bonding as the number of thiophene groups increases, with quinoidal bridge character for n = 1 (SQ-Th-SQ) and biradical character with "aromatic" bridge bond lengths for n = 3 (SQ-Th₃-SQ). Remarkably, for n = 2 (SQ-Th₂-SQ) the nature of the bridge fragment is intermediate between quinoid and biradical aromatic, which we describe as having open-shell character as opposed to biradicaloid since the open-shell biradical configuration does not have the correct symmetry to mix with the quinoidal ground-state configuration. This bridge bonding character is reflected in the energies of the lowest lying open-shell states for these three molecules. The SQ-Th-SQ molecule is diamagnetic at all temperatures studied, and we provide evidence for SQ-SQ antiferromagnetic exchange coupling and population of triplet states in SQ-Th₂-SQ and SQ-Th₃-SQ, with _SQ-SQ(ave) = -279 cm^-1 (VT EPR/electronic absorption/magnetic susceptibility) and _SQ-SQ = -117 cm^-1 (VT EPR/electronic absorption/magnetic susceptibility), respectively. The results have been interpreted in the context of state configurational mixing within a simplified 4-electron, 3-orbital model that explicitly contains contributions of a bridge fragment. Variable-temperature spectroscopic- and magnetic susceptibility data are consistent with two low-lying open-shell states for SQ-Th₃-SQ, but three low-lying states (one closed-shell and two open-shell) for SQ-Th₂-SQ. This model provides a simple symmetry-based framework to understand the continuum of electronic and geometric structures of this class of molecules as a function of the number of thiophene units in the bridge.