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

The influence of π-conjugation on 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 Blatter radicals 1N^• and 3N^• were synthesized via Pd(0)-catalyzed C─C coupling, while 2N^• was synthesized via Pd(II)/Cu(I)-catalyzed C─C coupling, starting from 7-bromo-Blatter radical (4N^•), incorporating phenanthrene, naphthyl, and ethynylphenanthrene substituents at C7-position, respectively. The radicals 1N^• - 3N^• exhibited bathochromically shifted UV-vis absorption and optical bandgaps (ca. 2.13 eV) relatively lower than 4N^• (2.30 eV) due to enhanced π-delocalization. Upon one-electron oxidation (nN^• → nN⁺, n = 1 - 4), additional red-shifted absorption bands were observed and corroborated by time-dependent density functional theory (TD-DFT) calculations. Intermolecular π-π stacking interactions between 1,2,4-benzotriazinyl units facilitated intermolecular spin-spin interactions, strong antiferromagnetic interactions with 2J/k_B values of -35.22 and -23.73 K, for 1N^• and 4N^•, respectively. Notably, the extended π-conjugation and structurally favorable packing in 1N^• - 3N^• led to reduced bandgaps and an order-of-magnitude increase in electronic conductivity (ca. 7.2 × 10^-6 S m^-1) compared to 4N^• (2.85 × 10^-7 S m^-1).