Excited-state structure by time-resolved X-ray diffraction

X-ray crystallography has traditionally been limited to the study of the ground-state structure of molecules and solids. Recent technical advances are removing this limitation as demonstrated here by a time-resolved stroboscopic study of the photo-induced 50 µs lifetime excited triplet state of the [Pt₂(pop)₄]⁴⁻ ion [pop = pyrophosphate, (H₂P₂O₅)²⁻], performed at helium temperatures with synchrotron radiation. The shortening of the Pt—Pt bond by 0.28 (9) Å upon excitation is compatible with the proposed mechanism involving promotion of a Pt—Pt antibonding dσ* electron to a weakly bonding p orbital. The contraction is accompanied by a 3° molecular rotation. The time-resolved diffraction technique described here is applicable to reversible light-driven processes in the crystalline solid state.