Propeller-Shaped Blatter-Based Triradicals: Distortion-Free Triangular Spin System and Spin-State-Dependent Photophysical Properties.

Herein, we report the synthesis and properties of triptycene-based C_3v- and C_s-symmetric stable triradicals. SQUID magnetometry showed the propeller-shaped triradicals were both an antiferromagnetic equilateral triangle spin system with small spin-spin interactions J/k_B~-120 K and -106 K, leading to ca. 4/6 coexistence of the doublet/quartet states in thermal equilibrium at room temperature. The triradicals exhibited characteristic NIR absorption bands reaching 1000 nm, which was solely ascribed to the triradical in the doublet state by variable-temperature spectroscopic studies and quantum chemical calculations. Taking advantage of the spin-specific absorption feature, we found that the triradicals in the low-spin state undergo symmetry-breaking charge transfer, while those in the high-spin state fall into a monoradical-like excited state.