Irina V. Novozhilova

Capturing and Analyzing the Excited-State Structure of a Cu(I) Phenanthroline Complex by Time-Resolved Diffraction and Theoretical Calculations

Time-resolved crystallography and density functional theory calculations are used to analyze the geometric and electronic changes that occur upon photoexcitation of [Cu(I)(dmp)(dppe)](+) in crystalline [Cu(I)(dmp)(dppe)][PF(6)] [dmp = 2,9-dimethyl-1,10-phenanthroline; dppe = 1,2-bis(diphenylphosphino)ethane]. In the pump-probe experiment, laser and X-ray pulses are synchronized to capture an image of the instantaneous molecular distortions in the transient triplet state. Parallel theoretical calculations, with the phenyl groups replaced by methyl groups, yield information on the distortion of the isolated cation and the change in electron density upon excitation. The experimental distortions are significantly less than the calculated values and are different for the two independent molecules in the asymmetric unit; these findings are attributed to the constraining influence of the crystal matrix. The calculations indicate that the electron transfer upon excitation is mostly from the dmpe ligand to the dmp ligand, while the Cu atomic charge changes by only approximately +0.1e, although the charge distribution on Cu is significantly affected. As found for homoleptic [Cu(I)(dmp)(2)](+), the change in the population of the Cu atom is close to the calculated difference between the corresponding Cu(II) and Cu(I) complexes. Charge density difference maps confirm these conclusions and show a large rearrangement of the electron density on the Cu atom upon excitation.

Photo-Induced Linkage Isomerism of Transition Metal Nitrosyl and Dinitrogen Complexes Studied by Photocrystallographic Techniques

New metastable species are formed by low-temperature irradiation of crystals of transition metal nitrosyl complexes. In the work described, photocrystallographic techniques, in which crystals are exposed in situ, are combined with differential scanning calorimetry measurements, IR spectroscopy and theoretical calculations. The new species are identified as isonitrosyl and h 2 -NO linkage isomers, which revert to the ground state on subsequent warming. The stability of the linkage isomers increases with increasing p-withdrawing ability of the ligand trans to NO, indicating the crucial role of electronic factors in determining relative stability of the isomers of different complexes. Isomers with decay temperatures up to ,08C have been prepared. Solid state effects are evident from comparison of the behavior of different salts of the same ion, of sodiumnitroprusside in the neat crystal and in a co-crystal with 18-crown-6 ether, and from the observation of different photochemical behavior when (Ru(NO)(NH3)5)(NO3)3 is absorbed in a sol-gel glass. The two independent molecules in the asymmetric unit of (Ni(NO)(h 5 -Cp p )) show different orientations of the h 2 -NO ligand after irradiation, the difference being related to the shape of the reaction cavities at the two sites. A previously unknown side-on binding mode, similar to that occurring for NO, has been discovered for the dinitrogen ligand in (Os(NH3)5(N2))(PF6)2, indicating the potential of the techniques used in the search for new species and novel binding modes of transition metal atoms. q 2000 Elsevier Science Ltd. All rights reserved.

Time-resolved synchrotron diffraction and theoretical studies of very short-lived photo-induced molecular species

Definitive experimental results on the geometry of fleeting species are at the time of writing still limited to monochromatic data collection, but methods for modifications of the polychromatic Laue data to increase their accuracy and their suitability for pump-probe experiments have been implemented and are reviewed. In the monochromatic experiments summarized, excited-state conversion percentages are small when neat crystals are used, but are higher when photoactive species are embedded in an inert framework in supramolecular crystals. With polychromatic techniques and increasing source brightness, smaller samples down to tenths of a micrometre or less can be used, increasing homogeneity of exposure and the fractional population of the excited species. Experiments described include a series of transition metal complexes and a fully organic example involving excimer formation. In the final section, experimental findings are compared with those from theoretical calculations on the isolated species. Qualitative agreement is generally obtained, but the theoretical results are strongly dependent on the details of the calculation, indicating the need for further systematic analysis.

A very large Rh-Rh bond shortening on excitation of the [Rh 2(1,8-diisocyano-p-menthane)4]2+ ion by time-resolved synchrotron X-ray diffraction

A very large Rh-Rh contraction of approximately 0.85 A occurs on excitation of the [Rh(2)(1,8-diisocyano-p-menthane)(4)](2+) ion to its triplet state.