Electronic properties of PVP-ionic liquid composite: spectroscopic and DFT-based thermochemical studies on the effect of anions

摘要

Polymer ionic liquid composites are presently an active area of research because of the abounding benefits bestowed by the ionic liquid component to the polymer. They are increasingly investigated as smart materials for multifunctional applications. In this study, the effect of anions, such as Br- and BF4-, having a common N-butylpyridinium (bpy(+)) cation on the electronic and DFT-based thermochemical properties of polyvinyl pyrrolidone (PVP) polymer was comparatively studied. The PVP-bpy[Br] composites subjected to different thermal conditions were studied using fluorescence spectroscopy, UV-visible spectroscopy, FTIR and Raman spectroscopy. The fluorescence spectroscopic studies showed that bpy[Br] could create efficient electronic trapped states which decreases the electron-hole recombination rate in PVP and partially quenching the PVP fluorescence emission intensity. The PVP fluorescence arising from the C = O electronic transitions is temporarily scavenged by the aromatic pyridinium ring centers. The UV-visible spectroscopic study shows that the optical band-gap of pure PVP is decreased on thermal polymerization in presence of bpy[Br] ionic liquid while it is increased with bpy[BF4-]. The thermochemical calculations indicated that the binding process of bpy[Br] with PVP is spontaneous only at high temperatures and non-spontaneous at low temperatures. The composition analysis using 2D confocal Raman image showed that the use of Br- anions instead of BF4- will give more effective and uniform thermal polymerization of PVP matrix in addition to decreasing its band-gap and making it more conductive. The study indicated that bpy [Br] was a better dopant for PVP than bpy[BF4] and also improved the electronic properties of PVP-based polymer-ionic liquid composites.