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-d(5))]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 eta, defined as eta(hkl) = [I-on(hkl) - I-off(hkl)]/I-off(hkl), give evidence of a small (similar or equal to 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 eta. 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.