Computational design of low singlet-triplet gap all-organic molecules for OLED application

► All-organic compounds with low singlet-triplet gap are highly attractive for OLEDs. ► Our computational study reveals the parameters controlling the singlet-triplet gap. ► The singlet-triplet gap in donor-acceptor compounds is mainly driven by the orbital offset. ► The nature of the bridging units is an important cofactor for low ST gaps. ► Consequences for organic LEDs and solar cells are discussed.