π-Conjugated Blatter Radicals: Molecular Structure-Driven Modulation of Optoelectronic Properties and Electrical Conductivity

The influence of π-conjugation on the optoelectronic properties, particularly electronic conductivity of air-stable 1,2,4-benzotriazinyl radicals (known as Blatter radicals), remains largely unexplored, yet is critical for advancing organic electronics. Herein, we elucidate the modulation of optoelectronic properties and electronic conductivities of Blatter radicals functionalized with varied conjugating units in two-terminal solid-state devices. The 7-bromo-Blatter radical (4N•) was stable under Pd(0)-catalyzed C-C coupling conditions and yielded 1N•, 2N•, and 3N•, incorporating phenanthrene, ethynylphenanthrene, and naphthyl substituents at the C7-position, respectively. The radicals 1N• to 3N• exhibited bathochromically shifted absorption properties, reduced optical band gaps (ca. 2.13 eV) relative to 4N• (2.30 eV) due to enhanced π-delocalization. Upon one-electron oxidation (nN• → nN+), additional red-shifted absorption bands were observed, corroborated by time-dependent density functional theory (TD-DFT) calculations. Intermolecular π-π stacking interactions between 1,2,4-benzotriazine units facilitated intermolecular spin-spin interactions and strong antiferromagnetic interactions with 2J/kB values of −35.22 K and −23.73 K, for 1N• and 4N•, respectively. Notably, the extended π-conjugation in 1N• and 2N• led to reduced band gaps and an order-of-magnitude increase in electronic conductivity (7.5×10−6 S m−1) compared to 4N• (1×10−7 S m−1). The stable radicals can be extended for low-bias operational optoelectronic applications.