Reversible Molecular Switching of Ruthenium Bis(bipyridyl) Groups Bonded to Oligothiophenes: Effect on Electrochemical and Spectroscopic Properties

We report the preparation of complexes in which ruthenium(II) bis(bipyridyl) groups are coordinated
to oligothiophenes via a diphenylphosphine linker and a thienyl sulfur (P,S bonding) to give [Ru(bpy)2PT3-P,S](PF6)2 (bpy = 2,2‘-bipyridyl, PT3 = 3‘-(diphenylphosphino)-2,2‘:5‘,2‘ ‘-terthiophene), [Ru(bpy)2PMeT3-P,S](PF6)2 (PMeT3 = 3‘-(diphenylphosphino)-5-methyl-2,2‘:5‘,2‘ ‘-terthiophene), [Ru(bpy)2PMe2T3-P,S](PF6)2
(PMe2T3 = 5,5‘ ‘-dimethyl-3‘-(diphenylphosphino)-2,2‘:5‘,2‘ ‘-terthiophene), and [Ru(bpy)2PDo2T5-P,S](PF6)2
(PDo2T5 = 3,3‘ ‘‘ ‘-didodecyl-3‘ ‘-diphenylphosphino-2,2‘:5‘,2‘ ‘:5‘ ‘,2‘ ‘‘:5‘ ‘‘,2‘ ‘‘ ‘-pentathiophene). These complexes react with base, resulting in the complexes [Ru(bpy)2PT3-P,C]PF6, [Ru(bpy)2PMeT3-P,C]PF6,
[Ru(bpy)2PMe2T3-P,C]PF6, and [Ru(bpy)2PDo2T5-P,C]PF6, where the thienyl carbon is bonded to ruthenium
(P,C bonding). The P,C complexes revert back to the P,S bonding mode by reaction with acid; therefore,
metal−thienyl bonding is reversibly switchable. The effect of interaction of the metal groups in the different
bonding modes with the thienyl backbone is reflected by changes in alignment of the thienyl rings in the
solid-state structures of the complexes, the redox potentials, and the π → π* transitions in solution. Methyl
substituents attached to the terthiophene groups allow observation of the effect of these substituents on
the conformational and electronic properties and aid in assignments of the electrochemical data. The PTn
ligands bound in P,S and P,C bonding modes also alter the electrochemical and spectroscopic properties
of the ruthenium bis(bipyridyl) group. Both bonding modes result in quenching of the oligothiophene
luminescence. Weak, short-lived Ru → bipyridyl MLCT-based luminescence is observed for [Ru(bpy)2PDo2T5-P,S](PF6)2, [Ru(bpy)2PT3-P,C]PF6, [Ru(bpy)2PMeT3-P,C]PF6, and [Ru(bpy)2PMe2T3-P,C]PF6, and
no emission is observed for the alternate bonding mode of each complex.