HOMOPLETIC AND HETEROPLETIC RuII COMPLEXES WITH EXTENDED PHENANTHROLINE-BASED LIGANDS

Four RuII complexes of general formula [Ru(PT)3]2+, [Ru(NT)3]2+, [Ru(PT)(bpy)2]2+, and [Ru(NT)(bpy)2]2+, where PT = 10,13-bis((triisopropylsilyl)ethynyl)dipyrido[3,2-a:2',3'-c]phenazine, NT = 10,15-bis((triisopropylsilyl)ethynyl)benzo[i]dipyrido[3,2-a:2',3'-c]phenazine and bpy = 2,2'-bipyridine have been synthesized. Our goal is to illustrate their electrochemical and photophysical properties with the aid of DFT and TD-DFT theoretical methods, in order to evaluate their potential for applications on solar energy conversion [1]. Additionally, we want to get exhaustive photophysical data for RuII homoleptic complexes with extended ligands, which have been scarcely investigated [2]. The electrochemical data for the phenanthroline ligands PT and NT show two successive one-electron reduction processes, while in the four complexes, also according to DFT calculations, the two first reduction processes are located on the extended phenanthrolines. Both ligands display green-yellow fluorescence at 298 and 77 K, whereas a strong and long-lived phosphorescence is detected only in the case of the PT ligand. In oxygen-free THF solution at 298 K, [Ru(PT)3]2+ and [Ru(PT)(bpy)2]2+ exhibit a very weak emission band, which is unequivocally attributed to emission from the lowest triplet level centered on the PT ligand since the emission band of the complexes is virtually identical to the phosphorescence of the free ligand. The two complexes of the more extended chelator, [Ru(NT)3]2+ and [Ru(NT)(bpy)2]2+, do not emit under any conditions; only a weak fluorescence signal attributable to a slight amount of free ligand in solution is observed. Photophysical data are fully rationalized with DFT methods, which always predict that the lowest triplet excited state is centered on the extended phenanthroline ligand in all of the complexes. These RuII compounds are long wavelength light absorbers, which are potentially useful for the harvesting of solar energy. [1] Monti F., Hahn U., Pavoni E., Delavaux-Nicot B., Nierengarten J.-F., Armaroli N., Polyehdron, 2014, doi:10.1016/j.poly.2014.05.030, Special Issue on Molecular Materials for Solar Energy Conversion. [2] Puntoriero F., Nastasi F., Galletta M., Campagna S., Photophysics and Photochemistry of Non-Carbonyl-Containing Coordination and Organometallic Compounds, in: Eds. J. Reedijk, K. Poeppelmeier Comprehensive Inorganic Chemistry II (2nd Edition), Elsevier, 2013, pp. 255-337.