New [1]benzothieno[3,2-b]benzothiophene-tetraoxide-based TADF emitters with a D–A–D structure for OLED applications

Abstract

Luminescent organic molecules showing thermally activated delayed fluorescence (TADF) are appealing materials for high-efficiency OLEDs. Here, we report a new class of organic luminescent materials with TADF properties, and a D–A–D electronic structure based on [1]benzothieno[3,2-b]benzothiophene-tetraoxide BTBTOx₄ as the acceptor unit A. Three donor units D were selected and coupled with BTBTOx₄, using a straightforward synthetic protocol based on microwave-assisted Buchwald–Hartwig cross-coupling, to yield three organic luminescent molecules labelled PTz₂-BTBTOx₄, MPA₂-BTBTOx₄ and POCz₂-BTBTOx₄. Chemico-physical and structural properties were investigated by cyclic voltammetry, electrical measurements, crystallographic analysis, theoretical study and photophysical characterization. All three emitters showed high electrochemical stability with reversible oxidation waves. MPA₂-BTBTOx₄ was selected as the reference molecule for X-ray analysis, which revealed torsion angles of −59° and 86° between the donor (MPA) and acceptor (BTBTOx₄) units supporting their appropriate structural configuration to have TADF properties. Photophysical studies highlighted a noteworthy increase in PL efficiency upon deoxygenation for all three compounds. The oxygen-induced quenching of delayed fluorescence and time-resolved photoluminescence studies supported the presence of the TADF properties, further corroborated for PTz₂-BTBTOx₄ and MPA₂-BTBTOx₄ by DFT studies. Preliminary steady-state photophysical studies were also carried out on neat films of all three emitters, revealing a pronounced self-quenching of photoluminescence for PTz₂-BTBTOx₄ and MPA₂-BTBTOx₄ and a minimal self-quenching for POCz₂-BTBTOx₄, which maintains a high Φ_PL (22%) comparable to that in Zeonex and half of that in PMMA. As a proof of concept, the three emitting molecules were tested as neat films in simple-structure OLED devices. In accordance with the photoluminescence data, POCz₂-BTBTOx₄, thanks to its sterically bulky structure, retains a good emission capacity even in a neat film and was also selected as an active matrix to build OLED devices by using two different deposition techniques: inkjet-printing and spin coating.