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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 7, Number 2
February 20, 1986 

Anomalous Luminescence and Emission Quenching Behaviors of Tris(2,2'-bipyridine)Ruthenium(Ⅱ) in Poly(methacrylic acid) Solutions
Joon Woo Park*, Young Hee Paik
The luminescence spectra of Ru(bpy)32+ in poly(methacrylic acid) (PMA) solutions varied sensitively with pH. At pH < 5.5, the emission intensity increased with pH up to 4 times, while it decreased with pH beyond the pH. The enhanced emission intensity was accompanied by blue-shift of the emission maxima as much as 15 nm. The enhancement of emission intensity was attributed to the restricted rotational mobility of ligand of the cation bound to densely coiled PMA molecules at pH < 5.5. The sharp decrease in emission intensity with increasing pH near pH 5.6 was accounted for conformational transition of the polymer to more extended structure, which was also revealed in viscosity measurement. The enhancement of emission intensity became higher as NaCl concentration of the solution increased. The binding constant of Ru(bpy)32+ with two carboxylate groups of PMA was calculated as 2 × 105 M-1 in 0.1 M NaCl at pH 5.2. The pH dependence of luminescence quenching rate of Ru(bpy)32+ by Cu++ also showed maximum near pH 5, and the rate was more than 103 times higher than that in water, whereas the maximum enhancement of quenching rate (about 20 times) in poly(acrylic acid) (PAA) solution occurred at pH 4.5. On the other hand, the pH dependence for neutral water soluble nitrobenzene (NB) exhibited opposite trend to that of Cu++. The quenching constant vs pH curve for MV++ was composite of those for Cu++ and NB. The anomalous high quenching rate for Cu++ in PMA solution at pH < 5.5 was attributed to the binding of Ru(bpy)32+ and Cu++ to the same region of PMA, when it conforms densely coiled structure in the pH range. The observation of mininium quenching rate for NB near pH 5.5 indicated that the Ru(bpy)32+ bound to the densely coiled PMA is not accessible by NB, which is in bulk water phase. The composite nature of the pH dependence of quenching rate for MV++ in PMA solution was attributed to the smaller binding affinity of the cation to PMA, compared to that of Cu++. The sharp, cooperative conformational transition with pH observed in PMA was not revealed in PAA. But, the pH dependence of quenching rates in this polymer reflected increased charge density and, thus, binding of cations to the polymer, and expansion of the polymer chain with pH.
137 - 142
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