X-ray Analysis of Excited-State Molecular Structure of [AuCl(PPh3)2]

When a chloroform solvate crystal of the three-coordinated Au(I) complex [AuCl(PPh3)2]·CHCl3 was irradiated with a mercury lamp at low temperatures, the crystal showed bright green emission from the excited molecules. From X-ray analysis, the unit-cell volume significantly decreased under photoirradiation. The three-dimensional intensity data were collected in the dark (light-off) and under photoirradiation (light-on) at 175, 156, 136, and 84 K. As the temperature decreased, the difference in unit-cell volume between the light-off and -on experiments increased. The molecular structures in the light-off and -on experiments revealed that the Au–P and Au–Cl bonds are significantly contracted by photoirradiation at every temperature. The changes in the other bond lengths were within the experimental errors. The shortening becomes greater as the temperature is lowered. The bond shortening, which causes the decrease in unit-cell volume, is in good agreement with the theoretical assumption that excitation from an anti-bonding orbital to a bonding orbital occurs in the Au(I) complex by photoirradiation.