Stable Singlet Biradicals of Rare-Earth-Fused Diporphyrin-Triple-Decker Complexes with Low Energy Gaps and Multi-Redox States

Dinuclear rare-earth (Tb^III, Y^III) triple-decker complexes with a fused diporphyrin (FP) and two tetraphenylporphyrin (TPP) ligands were synthesized in neutral, dianionic, and diprotonated forms. The neutral forms have large open shell biradical character, as determined from the temperature dependency of the magnetic susceptibility measurements and theoretical calculations. The coupling value (J=−1.4 kcal mol⁻¹, −487 cm⁻¹) of the radical centers in the neutral form of the Y^III complex indicates weak pairing interactions. Theoretical calculations on the neutral form reveal a significant biradical character (y=68 %). Furthermore, the Tb^III complex exhibits multi-redox states, having more than eight clear peaks in the voltammetry curves. The optical (3700 nm, 0.33 eV) and electrochemical measurements (3400 nm, 0.36 eV) indicate that the neutral form has very small HOMO–LUMO energy gap. Despite the large biradical character, the neutral complexes are thermally stable and do not decompose on heating at 120 °C. These complexes with unique characteristics are promising candidates for use in molecular electronics, optics, and spintronics.