When Light Challenges Heat: Mechanistic Insights Into a Reaction Competing With Cadogan Cyclisation in...

Visible-light-driven transformations have emerged as powerful and sustainable tools in modern organic synthesis. However, the intrinsic photochemical reactivity of polycyclic aromatic hydrocarbons (PAHs) remains underexplored. Among π-conjugated chromophores, perylenediimides (PDIs) combine exceptional photostability, strong visible-light absorption, and rich redox properties, yet their light-induced chemical transformations are still poorly understood. Herein, we report an unprecedented divergence between thermal and photochemical reactivity in the reaction of bay-nitrated PDIs (PDI-NO₂) with triphenylphosphine. While thermal activation promotes a classical Cadogan-type reductive cyclization to afford N-annulated PDI carbazole, visible-light irradiation redirects the reaction toward a previously unobserved pathway, yielding a bay-functionalized 1-(iminophosphorane)-12-hydroxy PDI derivative in excellent yield. Experimental studies reveal a strong wavelength dependence, with blue light dominating the photochemical transformation. Notably, the initial nitro-to-nitroso conversion is not phosphine-mediated but arises from the strong reducing power of photoexcited PDI-NO₂. Combined experimental and theoretical investigations demonstrate that light irradiation reshapes the reaction landscape by enabling access to charge-transfer and π-π* excited states, involving the population of an asymmetric unoccupied orbital localized on the nitroso moiety, thereby unlocking a phosphine-addition pathway inaccessible under thermal conditions. These findings establish orbital-selective excitation as a general design principle for exploiting visible light to control reaction pathways in π-conjugated chromophores.