Photoactivation Properties of Self-N-Doped Organic Semiconductors—Concentration-Dependent Radical and Biradical Formation

Perylene diimides (PDIs) have garnered attention as organic photocatalysts in recent years for their ability to drive challenging synthetic transformations, such as aryl halide reduction and iodoperfluroalkylation. During the photocatalytic cycle, photoirradiated PDI is reduced with a sacrificial extrinsic reductant in a diffusion-mediated process to form the radical anion <b>R^●-</b>, which is further excited to the doublet excited state ²[<b>R^●-</b>]*; a strong reducing agent. Previous work in this area employs pendant groups attached at the imide positions of PDI that do not participate in these reactions and are only added to impart solubility. In this work, we employ electron-rich pendant groups capable of self-n-doping the PDI core when irradiated with 405 nm light. We observe radical anion formation is favored at low concentrations where chain rotation is uninhibited, while biradical formation is favored at high concentrations, which we attribute to charge stabilization between interacting chromophores. Cyclic voltammetric measurements support this view by demonstrating steric encumbrance increases the Lewis basicity of anions by disrupting counterion interactions but inhibits biradical formation by disrupting aggregation. Finally, femtosecond transient absorption reveals low wavelength excitation (405 nm) preferentially favors the formation of ²[<b>R^●-</b>]*. In contrast, higher wavelength excitation (520 nm) favors the formation of the singlet excited state ¹[<b>N</b>]* in hydroxylated samples. These findings highlight the importance of dopant architecture, counterion selection, excitation wavelength, and concentration on electronic doping, which has substantial implications for future photocatalytic applications. We anticipate these findings will enable more efficient catalytic systems based on PDIs.