Waterborne Polyurethanes with Tunable Fluorescence and Room-Temperature Phosphorescence

Abstract

Single-component materials with both fluorescence and room-temperature phosphorescence (RTP) are useful for ratiometric sensing and imaging applications. On the basis of a general design principle, an amino-substituted benzophenone is covalently incorporated into waterborne polyurethanes (WPU) and results in fluorescence and RTP single-component dual-emissive materials (SDMs). At different aminobenzophenone concentrations, the statistical, thermal, and optical properties of these SDMs are characterized. Despite their similar thermal behaviors, the luminescence properties as a function of the chromophore concentration are quite different: increasing concentrations led to progressively narrowed singlet triplet energy gaps. The tunability of fluorescence and RTP via chromophore concentration is explained by a previously proposed model, polymerization-enhanced intersystem crossing (PEX). The proposal of PEX is based on Kasha's molecular exciton theory with a specific application in polymeric systems, where the polymerization of luminophores results in excitonic coupling and enhanced forward and reverse intersystem crossing. The mechanism of PEX is also examined by theoretical calculations for the WPU system. It is found that the presence of K1 aggregates indeed enhances the crossover from singlet excited states to triplet ones.