Capturing Conformation-Dependent Photoreactivity of Crystalline 3-Azido-1,3-diphenylisobutyrophenone

Solid-state photolysis of 3-azido-1,3-diphenyl-isobutyrophenone 1 results in selective formation of isobutyrophenone 2 and benzonitrile 3. X-ray structure analysis of azide 1 demonstrates that the conformer (1Cr) adopted for packing in the crystal lattice has the carbonyl moiety perpendicular to the α-phenyl group (Ph-C=O torsional angle of 85°). Laser flash photolysis of azide 1 nanocrystals allows direct detection of the lowest excited triplet ketone (T_K) of 1Cr (λ_max≈475 nm). Thus, in crystals, 1 is proposed to react via the T_K state to cleave the Cβ−Cγ bond. Time-dependent density functional theory (TD-DFT) calculations verify that the energy of the T_K of 1Cr is 79 kcal mol⁻¹ above the ground state (S₀), and thus T_K has sufficient energy for cleavage of the Cβ−Cγ bond to form products 2 and 3, further supporting this solid-state reaction mechanism.