Formation and decomposition of Poly(l-lactic acid) charge-transfer complex with iodine: A new molecular switch

Abstract

PLLA forms a charge-transfer (CT) complex with iodine in trichloromethane solution with Keq = 24.8 ± 1.8 M-1, much higher than the Keq = 0.75 M-1 measured with the model compound ethyl lactate-iodine in CHCl3. The higher equilibrium constant of the PLLA-I2 complex suggests a special polymer-iodine interaction which was confirmed by optical rotatory dispersion (ORD) measurements, taking advantage of the fact that PLLA is an optically active polymer. In fact, ORD measurement shows an induced Cotton effect at 500 nm giving further support to the model of iodine accommodated inside the helical coils of PLLA chains in solution. The PLLA-I2 complex was found very stable if stored in the dark and in the cold (i.e. +10 °C). The PLLA-I2 complex shows both the growth of the CT bands in the electronic spectrum and the induced Cotton effect in the ORD spectrum after prolonged storage. However, the PLLA-I2 complex is sensitive to visible light and it dissociates upon irradiation, but in the dark the complex is restored again, so that the complex could be defined as “photoreversible”. Furthermore, the PLLA-I2 complex is thermoreversible as it dissociates when heated at +50 °C but the complex is restored in full when cooled down to +10 °C in the dark. With these quite unique photoreversible and thermoreversible properties the PLLA-I2 complex behaves as a molecular switch i.e. a molecular system that can be reversibly interconverted between two states A and B by an external action or stimulus. Molecular switches are of key importance in nanotechnological applications.