Electron transfer quenching of tris(2,2'-bipyridine) ruthenium(II) complex derivatives covalently linked to poly (N-isopropylacrylamide) in aqueous solutions

Tris (2,2'-bipyridine) ruthenium(II) complex, ionic probe, was incorporated into poly(N-isopropylacrylamide) (PNIPA), which is known to be a thermoresponsive polymer, by a copolymerization method. Electron transfer quenching of the complex probe by methyl viologen was investigated as a function of temperature. The electron transfer quenching rate constant jk,) in a globular state (higher temperature than the LCST (31 degrees C)) is 4-5 times as large as that in a coil state (lower temperature) from the Stern-Volmer analysis. The result is quite different from the quenching of pyrene probe incorporated into PNIPA in the previous study. This is because hydrophilic ruthenium probe is located at the interface of polymer globular matrix even in a globular state, whereas pyrene probe was embedded into the hydrophobic matrix. The quenching behavior is discussed by a difference in molecular environment of the probes in phase transition of PNIPA in the aqueous solution.