Luminescent tricarbonyl pyridazine-rhenium complexes containing bridging -or/-sr ligands

There is still a high interest towards the photochemical and photophysical properties of luminescent rhenium(I) diimine tricarbonyl complexes, due to their broad range of applications [1-2]. Recently we reported the synthesis of a series of neutral dinuclear Re(I) complexes suitable for phosphorescent light emitting devices (OLEDs). These compounds, of general formula [Re2(CO)6(-1,2-diazine)(-X)2], where X could be halogen or hydride, exhibit emission in solution from triplet metal-to-ligand charge-transfer (3MLCT) excited states [3-4]. Wavelengths, lifetimes and quantum yields of the emission can be modulated by varying both the diazine substituents and/or the ancillary ligands. Photoluminescent quantum yield (PLQY) up to 0.53 have been measured for di-chloro complexes containing pyridazine (pydz) bearing alkyl substituents in  positions [5]. The aim of the work is to understand how the photophysical properties of the complexes can be tuned by modifying the ancillary ligands, estimating the possibility to functionalize these complexes without involving the diazine emitting centre. The synthesis of a new series of dinuclear complexes with general formula [Re2(CO)6(μ-pydz)(μ-ER)2], (E = S, O) – containing two (thio)phenol-type (or a alcoxo-type) unit as bridging ligands – has therefore been developed. The complexes have been characterized from the spectroscopical, photophysical, electrochemical and structural point of view. The emisssion - in deareated toluene – is localized in the red region of the visible spectrum (608 < em < 708 nm). The progressive bathochromic shift of both absorption and emission maxima follows the same trend of the oxidation potentials and agrees with the electron-withdrawing character of the alcoxo or phenoxo ligands, with respect to the halogen one. The PLQYs are in the range 0.1-5.5%, and, in agreement with the energy gap law, the highest value, comparable to that of the analogous dichloro derivative, is observed for compound 1, showing the emission at the highest energy. [1] Lundin, N. J.; Blackman, A. G.; Gordon, K. C.; Officer, D. L. Angew. Chem., Int. Ed. 2006, 45, 2582. [2] Mak, C. S. K.; Leung, Q. Y.; Li, C. H.; Chan, W. K. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 2311. [3] Panigati, M.; Donghi, D.; D'Alfonso, G.; Mercandelli, P.; Sironi, A.; D'Alfonso, L. Inorg. Chem. 2006, 45, 10909. [4] Donghi, D.; D’Alfonso, G.; Mauro, M.; Panigati, M.; Mercandelli, P.; Sironi, A.; Mussini, P.; D’Alfonso, L. Inorg. Chem. 2008, 47, 4243. [5] Mauro, M.; Quartapelle Procopio, E.; Sun, Y.; Chien, C-H.; Donghi, D.; Panigati, M.; Mercandelli, P.; Mussini, P.; D’Alfonso, G.; De Cola, L. Adv. Funct. Mater. 2009, 19, 2607.