Fluorescence behavior of water-soluble copolymers with pendant (4-carboxylatophenoxy)cyclotriphosphazene/europium ion complexes

The water-soluble copolymers composed of styrene derivative with (4-carboxylatophenoxy)cyclotriphosphazene (CCP) and N-isopropylacrylamide (NIPAM) have been prepared as host polymers of Eu3+ ion. When CCP units were excited at 260 nm, the characteristic fluorescence of Eu3+ was remarkably enhanced compared to that of the excitation of Eu3+ at 396 nm, suggesting occurrence of efficient energy transfer from CCP units to Eu3+ ion. The content of CCP units in the polymer chains significantly affected the fluorescence behaviors; for the copolymer with 1.7 mol% CCP unit (6a) the intensity increased with increasing of Eu3+ ion and reached a limiting value at [Eu3+]/[CCP unit]=1.0, whereas the fluorescence intensity of the copolymer with 12.4 mol% CCP unit (6d) reached a maximum at [Eu3+]/[CCP unit]=0.25, decreased by the further addition of Eu3+, and then has a constant value. In the 6d–Eu3+ complexes, the number of water molecules coordinated with Eu3+ ions, which act as a quencher, varies from 2.8 at [Eu3+]/[CCP unit]=0.25 to 4.1 at [Eu3+]/[CCP unit]=1.5. The reduced viscosity of 6d decreased with increasing the concentration of Eu3+ ions, indicating the contraction of polymer chains. The environment of Eu3+ bound to the copolymer is not affected by the coil–globule transition of the copolymer chain, although the 6a–Eu3+ system showed a lower critical solution temperature at 38 °C. The fluorescence behaviors of Eu3+ trapped by new host polymers based on cyclotriphosphazene are described.