Intramolecular Coumarin-Dimer Containing Polyurethanes: Optical Tuning via Single- and Two-Photon Absorption Processes

Abstract

The synthesis of an isomer-pure intramolecular coumarin dimer (ICD) in virtually unlimited amounts using a photo flow reactor is described. Based on this compound, polyurethanes with different ratios of hexamethylene diisocyanate and polytetrahydrofuran were synthesized and characterized. The reversibility of the photoinduced cycloaddition and cycloreversion reactions, one of the critical features, was compared for polymers prepared from closed ICDs and open ICDs. It was found that closed ICDs lead to a pre-orientation of the photoactive groups in the polymers, which results in a reduced loss in photoswitchable absorption. By varying the ICD content of the polymers, refractive indices ranging from 1.47 to 1.59 were achieved. With illumination, refractive index changes of Δn ≥ 0.02, as well as fluorescence changes, were achieved. Photoreactions induced by both single-photon absorption (SPA) and two-photon absorption (TPA) were studied. For the TPA-induced reaction, an autocatalytically sensitized mechanism was found. These novel polymers based on an intramolecular dimeric coumarin compound have great potential as optical materials with light-switchable properties, in particular, by TPA processes.