Novel ESIPT-based luminophores – synthesis and characterization

Abstract

Within this thesis, novel, purely organic luminophores were synthesized and investigated towards their photophysical properties. The luminescence originates from an intramolecular proton transfer in the energetically excited state, which is referred to as “Excited-State Intramolecular Proton Transfer“ (ESIPT). The incorporation of the central phenolic hydroxy group was realized by the development of a mild late-stage hydroxylation method. The directed ortho C(sp2)–H magnesiation on 2-aryloxazolines and subsequent oxidation using molecular oxygen or air was highly efficient. In addition to the high regio- and chemoselectivity, an ample range of functional groups were tolerated under the mild reaction conditions. The efficiency of this method was highlighted by the use of sustainable oxygen sources as oxidants and the implementation at ambient temperature. The isolated phenols strongly differ in their emission properties depending on the substitution pattern and the size of the π-system. In order to give a general statement towards emission tendencies, a panoply of luminophores were investigated in both solution and the solid state. In particular, minimalistic benzene-based fluorophores bearing nitrile substituents showed extremely high emission efficiencies. Quantum yields of up to 87.3 % in the solid state and strong π-π-interactions were found. In addition, ESIPT-based luminophores bearing electron-donating or -withdrawing groups were investigated towards their emission potency. As a consequence, electron poor aromatic compounds exhibited a greater emission potential. Another priority was the modification of fluorenes with π-expanded proton acceptors. The tuning of the emission color was easily achieved through chalcogen atom insertion into the benzazolebased proton acceptors. Additionally, a mild and regioselective ortho-functionalization method of fluorenes was developed, which enabled the covalent linkage of these chromophores on superior structures.