Tuning the photophysical properties of a (P,N)Re^I complex by adding a -NH- fragment into a P,N-bidentate ligand: The case of [P,N-{(C₆H₅)₂(C₅H₄N)NHPRe(CO)₃Br]

The spectroscopic, electrochemical and photophysical properties of the new complex [P,N-{(C₆H₅)₂(C₅H₄N)NHP}Re(CO)₃Br] (RePNN) are reported. The UV-Vis spectrum in dichloromethane (DCM) shows an absorption maximum centered at 300 nm and a shoulder at 350 nm. These absorption bands have been characterized, having a π_py → π∗_py and a MLCT character. Excitation at the first wavelength (higher energy) leads to an emission band centered at 365 nm. Cyclic voltammetry shows a fully irreversible oxidation wave around +1.45 V, assigned to Re^I/Re^II couple, whereas two irreversible reduction signals centered at -0.75 and -1.20 V are assigned to ligand reduction processes. The photophysical results show that emission from the RePNN complex comes exclusively from the ligand. The MLCT excited state shows to be non-emissive by coupling to vibrational modes leading to non-radiative decay path to ground state. The luminescent decay fits to a bi-exponential function. DFT calculations suggest that intra-ligand π_py → π∗_py and π_py → π∗_py excited states may account for the existence of these two decay lifetimes. The additional -NH- fragment present in the 2-(diphenylphosphinoamino)pyridine ligand compared to 2-pyridylphosphine, makes it more flexible and in consequence, enhances the non-radiative decay rate constant from the ³MLCT to the ground state in the RePNN complex compared to those of 2-pyridylphosphine, [P,N-{(C₆H₅)₂(C₅H₄N)PRe(CO)₃Br] (RePN).