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ABSTRACT The pursuit of sustainable organic synthesis has renewed interest in photochemistry, as sunlight‐driven reactions provide eco‐friendly alternative methods. Although the relationships among structure, properties, and reactivity are well established for ground‐state molecules, the understanding of excited states and reactive intermediates, such as triplet and singlet arylnitrenes, remains limited. Herein, we investigated the properties of triplet and singlet 4‐nitrenopyridine‐1‐pyridine oxide (1N), 3‐nitrenopyridine‐1‐pyridine oxide (2N), and phenylnitrene (PhN) using density functional theory (DFT), complete active space self‐consistent field (CASSCF(10,9)), and complete active space second‐order perturbation theory (CASPT2(10,9)) calculations. Bond length analysis demonstrated that³1Nand¹1N, as well as¹2Nand¹PhN, exhibit significant imine biradical character, whereas the structures of³2Nand³PhNare better described as benzene‐like. Nucleus‐independent chemical shift (NICS(0), NICS(1.7)_ZZ) and anisotropy of induced current density (ACID) calculations were performed to compare the induced magnetic currents in these molecules. These analyses demonstrated that³1Nis weakly aromatic, whereas³2Nand³PhNare best described as having Baird aromaticity. In contrast, singlet nitrenes¹1N,¹2N, and¹PhNare nonaromatic. In addition, irradiation of1in argon matrices verified that³1Nreacts photochemically to form corresponding ketenimine1K. Finally, the absorption difference spectrum of³1Nin a frozen 2‐methyltetrahydrofuran (mTHF) matrix exhibited resolved vibrational structure, suggesting the vibrational coupling to another electronic state. These insights into the structure and aromaticity of heterocyclic nitrenes could provide new avenues for modulating the reactivity of triplet ground state and triplet excited molecules. 

Organic azides are valuable precursors in synthetic chemistry, particularly for nitrogen-based functionalization through photochemical activation. In this study, the photoreactivities of 4-azido-1-phenylbutan-1-one (1a) and 4-azido-(4-methoxy)phenylbutan-1-one (1b) were investigated using visible-light photocatalysts [Ir(dF(CF3)ppy)2(dtbpy)]PF6 and [Ru(bpy)3]Cl2 to elucidate the mechanistic differences between triplet energy transfer and photoreductive electron transfer pathways. Direct irradiation of 1a in methanol favors the formation of a biradical species via intramolecular H atom abstraction to generate its lowest triplet ketone (T1K) with an (n,π*) configuration, which selectively yields 2-phenyl-1-pyrroline derivative 2a. However, 1b reacts through its less reactive T1K, which has a (π,π*) configuration, to form 2-phenyl-1H-pyrrole as the major product. When sensitized by [Ir(dF(CF3)ppy)2(dtbpy)]PF6, selective excitation of the triplet azido moiety (TA) of both 1a and 1b yields the corresponding pyrroline (2a and 2b) via triplet alkylnitrene (31aN and31bN) formation. In contrast, photoactivation of [Ru(bpy)3]Cl2 in the presence of diisopropylethylamine (DIPEA) results in photoreductive electron transfer, forming azido radical anion intermediates, which cyclize to also yield 2a and 2b. Product studies, cyclic voltammetry, laser flash photolysis, and DFT calculations supported these mechanistic assignments. This work demonstrates complementary approaches to control alkyl azide photoreactivity and unlock new strategies for visible-light-induced nitrogen incorporation.