On the Analysis of Reversible Light-Induced Changes in Molecular Crystals

New photocrystallographic and cryogenic techniques are tested in a 40 K study of the 2.44 ms lifetime excited state of [RhBr₂(pyridine-d₅)]Br. A modulation method is used in which a laser beam is interrupted by a mechanical chopper with a preselected frequency and the diffracted X-ray beam gated in synchronization with the chopper phase. Wilson-type plots of the response ratio η, defined as η_hkl = [I_on(hkl)−Ioff(hkl)]/I_off(hkl), give evidence of a small (≃2 K) temperature increase upon laser irradiation of the crystal at 40 K. The temperature change decreases when the chopper frequency is increased from 20 to 40 to 80 Hz, pointing to a gradual temperature increase during the laser-on period. A least-squares method for the refinement of excited-state parameters, with the experimental response ratios as observations, is described and applied using 46 experimental values of 17. Although there is an indication that the Rh–Br bonds are shortened on excitation, the current data-set is too limited to allow unequivocal determination of the geometry changes upon excitation; the study presented is therefore primarily a description of the development and testing of new techniques.