The molecular structure and dynamics of the photoexcited metal-to-ligand-charge-transfer (MLCT) state of [Cu(I)(dmp)(2)](+), where dmp is 2,9-dimethyl-1,10-phenanthroline, in acetonitrile have been investigated by time-domain pump-probe X-ray absorption spectroscopy, femtosecond optical transient spectroscopy, and density functional theory (DFT). The time resolution for the excited state structural determination was 100 ps, provided by single X-ray pulses from a third generation synchrotron source. The copper ion in the thermally equilibrated MLCT state has the same oxidation state as the corresponding copper(II) complex in the ground state and was found to be penta-coordinate with an average nearest neighbor Cu-N distance 0.04 A shorter than that of the ground state [Cu(I)(dmp)(2)](+). The results confirm the previously proposed "exciplex" structure of the MLCT state in Lewis basic solvents. The evolution from the photoexcited Franck-Condon MLCT state to the thermally equilibrated MLCT state was followed by femtosecond optical transient spectroscopy, revealing three time constants of 500-700 fs, 10-20 ps, and 1.6-1.7 ns, likely related to the kinetics for the formation of the triplet MLCT state, structural relaxation, and the MLCT excited-state decay to the ground state, respectively. DFT calculations are used to interpret the spectral shift on structural relaxation and to predict the geometries of the ground state, the tetracoordinate excited state, and the exciplex. The DFT calculations also indicate that the amount of charge transferred from copper to the dmp ligand upon photoexcitation is similar to the charge difference at the copper center between the ground-state copper(I) and copper(II) complexes.

I. Introduction 883 II. Photocrystallography 883 A. Crystallography of Light-Induced Species 883 B. Supporting Techniques 863 III. Metastable Isomers of Transition-Metal Nitrosyl Complexes 863 A. The First Discoveries 863 B. The Nature of the Metastable States of Sodium Nitroprusside 864 1. Sodium Nitroprusside (SNP): Experimental Methods 864 2. Orbital Ordering of the SNP Ground-State Species 866 3. Theoretical Calculation of the Metastable and Excited States and the Mechanism of Photoinduced Interconversion 867 C. Other Small Molecule Complexes 868 1. [NiNO(η 5 -Cp)] 868 2. Ruthenium and Osmium Complexes and the Dependence of the Decay Temperature on Chemical Substitution and Solid-State Environment 869 3. Solid-State Effects 870 4. Theoretical Calculations on Ruthenium Complexes 871 5. Calculated Hyperfine Splittings and Comparison with Results from Mo ¨ssbauer Spectroscopy 874 IV. Heme Systems 875 A. Introduction 875 B. Experimental Evidence for Linkage Isomers of NO Porphyrins and Theoretical Confirmation 875 C. Further Theoretical Studies on NO-Porphyrins 875 V. Linkage Isomerism of Other Di-and Triatomic Ligand Transition-Metal Complexes 876 A. Dinitrogen 876 B. NO 2 878 C. Sulfur-Containing Ligands 879 VI. Concluding Remarks 880 VII. Acknowledgments 880 VIII. Abbreviations 881 IX. Note Added after ASAP Posting 881 X. References 881