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

Ultraviolet-initiated photochemistry and electronic excited state rela xation of Rh(CO)(2)(acac) (acac = acetylacetonate), CpRh(CO)(2) (Cp* = eta(5)-C-5(CH3)(5)), CpIr(CO)(2), and Ir(CO)(2)(acac) in 295 K solu tions were studied by ultrafast transient infrared spectroscopy. The p rogress of these dynamics was monitored through changes in the CO-stre tch 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 specie s produces quite different results from that of Rh(CO)(2)(acac). In th ese compounds no monocarbonyl photoproducts are observed because the C O loss quantum yield is small (approximate to 1%). However, transient bleaches at the CO-stretch frequencies of each compound are found to r ecover with approximate to 40 ps lifetimes; this recovery is attribute d to rapid relaxation of electronic excited states. These results prov ide guidance for the interpretation of transient spectra of species su ch as Rh(CO)(2)(acac) which have a high quantum yield for CO loss.