This article reports a detailed mechanistic and kinetic study of an unusual photoreaction leading to the (diazonia)tetrabenzonaphthacene skeleton. The photo-triggered double intramolecular nucleophilic aromatic substitution (SNAr*) has been investigated by varying the leaving groups. Photoreaction quantum yields have been determined and mechanistic insights have been supported by theoretical calculations using DFT and TD-DFT methods. Additionally, we show that this light-triggered formed diazonia constitutes a potent photosentitizer with a singlet oxygen generation quantum yield of 0.55, both in organic solvents and in water, which is an extremely relevant value in view of PDT applications or use as an oxidation photocatalyst in aqueous media. Once again, the experimental observations were supported by TD-DFT calculations showing a large density of triplet states below the S1 excited state along with large spin-orbit couplings. The reaction is not restricted to solutions but can also occur in solid PDMS matrices thus allowing for photochemical encoding of information that will progressively vanish upon prolonged UV-exposure.

Controlling the formation of photoexcited triplet states is critical for many (photo)chemical and physical applications. Here, we demonstrate that a permanent out-of-plane distortion of the benzothioxanthene imide (BTI) dye promotes intersystem crossing by increasing spin-orbit coupling. This manipulation was achieved through a subtle chemical modification, specifically the bay-area methylation. Consequently, this simple yet efficient approach expands the catalog of known molecular engineering strategies for synthesizing heavy atom-free, dual redox-active, yet still emissive and synthetically accessible photosensitizers.