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Three new pyridyl thienopyridines 5-pyridin-2-yl-thieno[3,2-b]pyridine (1), 6-pyridin-2-yl-thieno[2,3-b]pyridine (2) and 6-pyridin-2-yl-thieno[3,2-c]pyridine (3) have been synthesized. 1 was synthesized via a chalcone-like precursor derived from (2-formyl-thiophen-3-yl)-carbamic acid ethyl ester (9). Removal of the carbamate group with HBr led to an immediate ring-closure of the intermediate amine to give 1 in an overall yield of 32% after 5 steps. 2 was synthesized by a Beckmann rearrangement of E-1-(5-bromo-thiophen-2-yl)-ethanone oxime (16) to yield N-(5-bromo-thiophen-2-yl)-acetamide (18). Reaction of 18 under modified Vilsmeier conditions led to the ring-closed product, 2-bromo-6-chloro-thieno[2,3-b]pyridine (19). Debromination followed by a Negishi cross-coupling with 2-pyridyl zinc bromide gave 2 in an overall yield of 8%. 3 was synthesized via formylation of 2-thiophen-3-yl-[1,3]dioxolane (22). A Wittig reaction between 22 and triphenyl-pyridin-2-yl-phosphonium bromide (25) gave the alkene, 2-[2-(3-[1,3]dioxolan-2-yl-thiophen-2-yl)-vinyl]­pyridine (26) in moderate yield.   Deprotection and ring closure of the oxime derivative of the resulting aldehyde gave 3 in an overall yield of 7.5% for this six step reaction.

The luminescence lifetimes (in CH 3CN at room temperature) and electrochemical potentials (in CH 3CN) of a range of mono- and bis(bidentate) 2,2′-bipyridine-capped oligothiophene-bridged Ru II complexes based on the 6-(2-thienyl)-2,2′-bipyridine and 4-(2-thienyl)-2,2′-bipyridine motifs have been measured. The redox potentials occurred in a very narrow range and showed only small shifts from that of [Ru(bpy) 3] 2+, which indicates that the inductive effects of the substituents on the 2,2′-bipyridine ligands are very similar across this series. In the complexes that incorporated a bithiophene moiety the oxidation of the bithienyl group occurred at higher potentials than the metal-centered Ru III/II oxidation. No or very weak interaction between the metal cores in the dinuclear complexes was observed. It was found that the luminescence lifetimes of the complexes where the attachment point of the oligothiophene bridge is in the 4-position of a 2,2′-bipyridine ligand were extended compared to [Ru(bpy) 3] 2+, whereas the luminescence was very short-lived (<30 ns) or completely quenched in the complexes where the oligothiophene bridge was attached in the 6-position. The difference in lifetimes is probably due to steric interactions between the thienyl bridge and the auxiliary bipyridyl ligands, resulting in disturbances in coordination symmetry of the metal core.

The electrochemical and photophysical properties for a range of ruthenium(II) tris-2,2’-bipyridine complexes in which a thiophene substituent is attached to one of the bipyridine ligands via either a pendant or a fused mode have been determined. The fused mode of attachment eliminates torsional movement between the thiophene unit and the chelating bipyridine, thereby offering optimal overlap between the p systems of the chelating unit and the attached thiophene unit. The electrochemical properties of these complexes were found to be similar, however the luminescence lifetimes and intensities (in CH₃CN at room temperature) were found to be correlated to the mode of attachment. The longest luminescence lifetime was

observed for the complex [Ru(bpy)₂(4-thiophen-2-yl-[2,2']-bipyridine)]²⁺ (3000 ns), as compared to the prototypic [Ru(bpy)₃]²⁺ (1745 ns). This complex also had the highest quantum yield (0.045). In the four isomeric complexes where the thiophene ring was fused to the b or c face of the pyridine the lifetimes fell in the interval 275-1510 ns and the quantum yield ranged between 0.0047-0.014.

(© WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)