Thiazolo [5, 4-d] thiazoles with spirobifluorene moiety as the novel D-?-A type organic hosts: Design, synthesis, structure-properties relations and applications in electroluminescent devices

A series of thiazolo[5,4-d]thiazoles with bipolar character has been designed and synthesized. Compounds 4a-d and 5a,b consist of a central thiazolo[5,4-d]thiazole as the electron acceptor (A) and a terminal 9,9?-spirobifluorene fragment as the electron-donor (D). Both key structural features are connected through the thiophene ?-linker. The structure of all the compounds is fully ?-conjugated providing proximity of the frontier molecular orbitals (FMO). Accordingly, thiazolo[5,4-d]thiazole-based compounds 4a-d and 5a,b exhibit excellent electronic structures with energy gaps falling in the range of 2.5-3.0 eV. Appropriate thermal stability and valuable photophysical properties make these molecules valuable for applications in optoelectronic materials. Hence, electroluminescent devices with 4b and 5b as host materials showed yellowish-green and yellow-green emissions, which is quite unique for organic-small molecules demonstrating their potential in the development of novel, low-weight and fully organic dopants for electroluminescent (EL) devices.