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 (S 0 ), 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.