Photophysical and electrochemical properties of [Re(CO)₃Cl(NN)] (NN = dppp3, dppp2, dppp2Br) complexes functionalised with pendant pyridyl ligands

We report the synthesis, electrochemical, and photophysical properties of a series of rhenium complexes, [Re(CO)₃Cl(NN)] (NN = dppp2, dppp3, and dppp2Br; dppp2 = pyrido[2′,3′:5,6]pyrazino[2,3-f][1,10]phenanthroline, dppp3 = pyrido[3′,4′:5,6]pyrazino[2,3-f][1,10]phenanthroline, dppp2Br = 8-bromopyrido[2′,3′:5,6]pyrazino[2,3-f][1,10]phenanthroline), which have potential for use as CO₂ photoreduction catalysts. The complexes are related to [Re(CO)₃Cl(dppz)] (dppz = dipyrido[3,2-a:2′,3′-c]phenazine) but have an additional nitrogen heteroatom in the distal phenazine (phz) aromatic ring of the dppz ligand which is known to promote potential photoinduced CO₂ binding and reduction in related complexes. We use Fourier transform infrared (FTIR), UV/Visible, electron paramagnetic resonance (EPR) and time-resolved infrared (TRIR) spectroscopies to investigate how the location of the additional nitrogen atom, and how the nature of the substituents affects the photophysical properties. In addition, the one electron reduced catalysts are generated following bulk electrochemical reduction and characterised in CH₂Cl₂. TRIR experiments in d₆-DMSO, CH₃CN, CH₂Cl₂, THF and toluene reveal that excitation generates a ³MLCT(phz) (phz = phenazine) excited state in all compounds. In CH₃CN and d₆-DMSO (not for [Re(CO)₃Cl(dppp2)]), we observe the concomitant formation of an ³IL ππ* (IL = intra ligand) excited state. For [Re(CO)₃Cl(dppp2)] and [Re(CO)₃Cl(dppp3)] in toluene, a mixture of ³MLCT(phen) (phen = phenanthroline) and ³MLCT(phz) states appear to form together, and decay at the same rate. Inclusion of an nitrogen atom in the phz moiety of the dppz ligand lowers the ‘phz’ localised lowest unoccupied molecular orbital (LUMO) energy potentially favouring population of the ³MLCT(phz) state. This effect is more pronounced for [Re(CO)₃Cl(dppp3)] with a more positive reduction potential (E_½ = −1.16 vs Fc⁺/Fc in CH₂Cl₂) compared to [Re(CO)₃Cl(dppp2)] (E_½ = −1.20 V vs Fc⁺/Fc in CH₂Cl₂). For [Re(CO)₃Cl(dppp2Br)], lowering of the LUMO energy results in an even more positive potential (E_½ = −1.10 vs Fc⁺/Fc in CH₂Cl₂) and this can be seen to affect the photophysics. We find the excited state lifetime for each complex increases with decreasing dielectric constant, where their lifetime in toluene is greater than 100 times longer than that observed in d₆-DMSO.