Inhibition of the <i>fac</i>-(RCO<sub>2</sub>)Re<sup>I</sup>(CO<sub>3</sub>)(bis-azine) Photodecarboxylation of the Carboxylate Ligand, RCO<sub>2</sub><sup>-</sup>, 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 <i>fac</i>-(RCO<sub>2</sub>)Re<sup>I</sup>(CO)<sub>3</sub>L (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 <i>fac</i>-(CH<sub>3</sub>CO<sub>2</sub>)Re<sup>I</sup>(CO)<sub>3</sub>L (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, <sup>1</sup>MLCT<sub>Re</sub><sub>→</sub><sub>L</sub>, of <i>fac</i>-(RCO<sub>2</sub>)Re<sup>I</sup>(CO)<sub>3</sub>L (R = ferrocene, 4-DMAB) transform into the spectra of two triplet excited states, the <sup>3</sup>MLCT<sub>Re</sub><sub>→</sub><sub>L</sub> excited state and a Re(I) to RCO<sub>2</sub><sup>-</sup> charge-transfer excited state, <sup>3</sup>MLCT<sub>Re</sub><sub>→</sub><sub>RCO</sub><sub><sub>2</sub></sub>. The <sup>3</sup>MLCT<sub>Re</sub><sub>→</sub><sub>RCO</sub><sub><sub>2</sub></sub> and <sup>3</sup>MLCT<sub>Re</sub><sub>→</sub><sub>L</sub> excited states have lifetimes that span 10<sup>2</sup> ns to several microseconds. The photodecarboxylation of <i>fac</i>-(RCO<sub>2</sub>)Re<sup>I</sup>(CO)<sub>3</sub>L, initiated in ligand-to-ligand charge-transfer excited states, <sup>3</sup>LLCT<sub>Re</sub><sub>→</sub><sub>L</sub>, is markedly affected by the positions of the <sup>3</sup>MLCT<sub>Re</sub><sub>→</sub><sub>RCO</sub><sub><sub>2</sub></sub> and <sup>3</sup>MLCT<sub>Re</sub><sub>→</sub><sub>L</sub> excited states. In addition, the low energy of <sup>3</sup>LLCT<sub>RCO</sub><sub><sub>2</sub></sub><sub>→</sub><sub>L</sub> in the <i>fac</i>-(RCO<sub>2</sub>)Re<sup>I</sup>(CO)<sub>3</sub>L (L = phen, 2,2‘-bpy) complexes, relative to those of <i>fac</i>-(CH<sub>3</sub>CO<sub>2</sub>)Re<sup>I</sup>(CO)<sub>3</sub>L (L = phen, 2,2‘-bpy) decreases the efficiency of the photodecarboxylation reaction.