Enhanced upconversion emission in air using novel stretched poly(vinyl alcohol) thin films

Abstract

Dense chromophore systems have been proposed as an alternative approach to creating solid-state triplet–triplet annihilation upconversion (TTA-UC) materials. In order to extend this system to polymer-based TTA-UC materials, a new method for the in situ preparation of dense chromophore systems in poly(vinyl alcohol) (PVA) was developed using a mechanical stretching technique. The chromophores used in this study were newly synthesised to dissolve homogeneously in a PVA aqueous solution. These chromophores exhibited a blue upconverted emission in MeOH solution upon excitation with a 532 nm green laser. Remarkably, a clear upconversion emission was observed from the chromophore-blended PVA films even under ambient conditions after the stretching process. In addition, a correlation between the elongation length of the films and the ratio of UC emission/phosphorescence was clearly observed. The stretching process made the chromophores more condensed in the oriented and cross-linked polymer chains, thereby creating favourable conditions for triplet–triplet annihilation.