Steric and Distance Effect on Electron Transfer in Dibenzo-Fused BODIPY-Based Photoredox Catalysts

π-Extended BODIPY compounds are a compelling class of fluorophores known for their red or near-infrared (NIR) emission and high quantum yields, which are crucial for applications in materials science, solar cells, and biomedical imaging. Our recent study shows that we can use a dibenzo-fused BODIPY as a singlet-driven red photoredox catalyst by installing a simple electron donor group. Despite their potential in these applications, knowledge of electron transfer reactions involving dibenzo-fused BODIPY is still scarce. This paper presents the synthesis and systematic photophysical investigations of donor-acceptor (D-A) and donor-bridge-acceptor (D-bridge-A) series of dibenzo-fused BODIPY with N,N'-diethylaniline fragments serving as an electron donor. We examined the effects of methyl substituents and bridge length on the rates of photoinduced electron transfer (PeT). Through steady-state and time-resolved optical spectroscopy, electrochemistry, and density functional theory calculations, we elucidated how these simple structural modifications controlled the PeT rates and examined their impacts on catalytic activities in atom transfer radical addition (ATRA) reactions. Our results support previous studies on the (D-A) design of red heavy atom-free photocatalysts.