Inhibition of the fac-(RCO₂)Re^I(CO₃)(bis-azine) Photodecarboxylation of the Carboxylate Ligand, RCO₂^-, When R Is a Strongly Electron Donating Group:  Thermal and Photochemical Properties of Complexes Where R = Ferrocene, 4-(Dimethylamino)benzyl

The photophysical and phtochemical properties of fac-(RCO₂)Re^I(CO)₃L (R = ferrocene, L = phen, 2,2‘-bpy; R = 4-(dimethylamino)benzyl (4-DMAB), L = phen) were investigated with continuous and flash photolyses. Their properties were compared with those of fac-(CH₃CO₂)Re^I(CO)₃L (L = phen, 2,2‘-bpy) complexes. On a femtosecond to 10 ps time scale, the transient spectra of the singlet Re(I) to L charge-transfer excited states, ¹MLCT_Re→L, of fac-(RCO₂)Re^I(CO)₃L (R = ferrocene, 4-DMAB) transform into the spectra of two triplet excited states, the ³MLCT_Re→L excited state and a Re(I) to RCO₂^- charge-transfer excited state, ³MLCT_Re→RCO2. The ³MLCT_Re→RCO2 and ³MLCT_Re→L excited states have lifetimes that span 10² ns to several microseconds. The photodecarboxylation of fac-(RCO₂)Re^I(CO)₃L, initiated in ligand-to-ligand charge-transfer excited states, ³LLCT_Re→L, is markedly affected by the positions of the ³MLCT_Re→RCO2 and ³MLCT_Re→L excited states. In addition, the low energy of ³LLCT_RCO2→L in the fac-(RCO₂)Re^I(CO)₃L (L = phen, 2,2‘-bpy) complexes, relative to those of fac-(CH₃CO₂)Re^I(CO)₃L (L = phen, 2,2‘-bpy) decreases the efficiency of the photodecarboxylation reaction.