Enabling red thermally activated delayed fluorescence by increasing the push–pull strength in naphthalimide-phenothiazine derivatives

In this study, we report the synthesis of a series of six new push-pull compounds, bearing naphthalimide as the electron acceptor and phenothiazine or phenothiazine dioxide as the electron donor, arranged in dipolar or octupolar molecular structures and showing either a single bond or a phenyl spacer as the linker between units. This synthetic effort allowed a detailed investigation of the structure-property relationships and of the key factors enabling thermally activated delayed fluorescence (TADF) in these fluorophores, performed through a joint advanced time-resolved spectroscopic and TD-DFT computational approach. Our ultrafast spectroscopic results showed larger push-pull degree for the phenothiazine compared to the phenothiazine dioxide derivatives. This resulted in negligible energy gap between the intramolecular charge transfer S-1 and the naphthalimide localized T-1 excited states, and thus lead to effective reverse intersystem crossing and TADF only in the case of the oxygen-free compounds. The increased conjugation in the octupolar relative to the dipolar systems as well as in the phenyl-bridged structures allowed higher fluorescence efficiencies to be achieved. The red TADF of these luminophores, less common and more difficult to achieve than blue and green TADF, was found to be enhanced in their aggregates produced both in water dispersions and in thin films. The observation of intense red TADF in the solid state makes these new all-organic materials highly promising for applications in third generation OLED devices.