PHOTOCHEMISTRY OF RH(CO)(2)(ACETYLACETONATE) AND RELATED METAL DICARBONYLS STUDIED BY ULTRAFAST INFRARED-SPECTROSCOPY

Ultraviolet-initiated photochemistry and electronic excited state relaxation of Rh(CO)(2)(acac) (acac = acetylacetonate), Cp*Rh(CO)(2) (Cp* = eta(5)-C-5(CH3)(5)), Cp*Ir(CO)(2), and Ir(CO)(2)(acac) in 295 K solutions were studied by ultrafast transient infrared spectroscopy. The progress of these dynamics was monitored through changes in the CO-stretch region (near 2000 cm(-1)). Solvent-dependent photochemistry of Rh(CO)(2)(acac) is reported which supports the interpretation of previous metal carbonyl photolysis experiments. Excitation of the other species produces quite different results from that of Rh(CO)(2)(acac). In these compounds no monocarbonyl photoproducts are observed because the CO loss quantum yield is small (approximate to 1%). However, transient bleaches at the CO-stretch frequencies of each compound are found to recover with approximate to 40 ps lifetimes; this recovery is attributed to rapid relaxation of electronic excited states. These results provide guidance for the interpretation of transient spectra of species such as Rh(CO)(2)(acac) which have a high quantum yield for CO loss.