Li ⁺- and Eu ³⁺-doped poly(ε-caprolactone)/ siloxane biohybrid electrolytes for electrochromic devices

The sol-gel process has been successfully combined with the "mixed cation" effect to produce novel luminescent and ion conducting biohybrids composed of a diurethane cross-linked poly(ε-caprolactone) (PCL530)/siloxane hybrid network (PCL stands for the poly(ε-caprolactone) biopolymer and 530 is the average molecular weight in gmol ^-1) doped with a wide range of concentrations of lithium and europium triflates (LiCF ₃SO ₃ and Eu(CF ₃SO ₃) ₃, respectively) (molar ratio of ca. 50:50). The hybrid samples are all semicrystalline: whereas at n = 52.6 and 27.0 (n, composition, corresponds to the number of (C(=O)(CH ₂) ₅O) repeat units of PCL(530) per mixture of Li ⁺ and Eu ³⁺ ions) a minor proportion of crystalline PCL(530) chains is present, at n = 6.1, a new crystalline phase emerges. The latter electrolyte is thermally stable up to 220 °C and exhibits the highest conductivity over the entire range of temperatures studied (3.7 × 10 ^-7 and 1.71 × 10 ^-4 S cm ^-1 at 20 and 102 °C, respectively). According to infrared spectroscopic data, major modifications occur in terms of hydrogen bonding interactions at this composition. The electrochemical stability domain of the biohybrid sample with n = 27 spans more than 7 V versus Li/Li ⁺. This sample is a room temperature white light emitter. Its emission color can be easily tuned across the Commission Internationale dÉclairage (CIE) chromaticity diagram upon simply changing the excitation wavelength. Preliminary tests performed with a prototype electrochromic device (ECD) comprising the sample with n = 6.1 as electrolyte and WO ₃ as cathodically coloring layer are extremely encouraging. The device exhibits switching time around 50 s, an optical density change of 0.15, good open circuit memory under atmospheric conditions (ca. 1 month) and high coloration efficiency (577 cm ² C ^-1 in the second cycle).