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)(3)Br]

Abstract

The spectroscopic, electrochemical and photophysical properties of the new complex [P,N-{(C6H5)(2)(C5H4N)NHP)Re(CO)(3)Br] (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 pi(py) -> pi*(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 eully 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 pi(py) -> pi*(py) and pi(py) -> pi*(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 (MLCT)-M-3 to the ground state in the RePNN complex compared to those of 2-pyridylphosphine, [P,N-{(C6H5)(2)(C5H4N)PRe(CO)(3)Br] (RePN).