Geometric control of multi-resonance backbone DABNA for narrowband deep-blue electroluminescence

Molecular geometry is well known as a determining factor for the photophysical properties of conventional organic emitters, but its potential influences on multi-resonance (MR) emitters have yet to be investigated. Herein, taking the famous MR backbone DABNA as the prototype, we demonstrated that the photophysical properties can be significantly adjusted by modulating the conjugated geometry. Unlike the common DABNA series (<i>e.g.</i>, DABNA-1) which exhibit slightly distorted geometries due to the steric hindrance in the bay area, our newly designed DABNA derivative, c-DABNA, demonstrates a nearly ideal planar conjugated backbone, due to insertion of a sp³-carbon atom in the bay. Importantly, such a geometrical change promotes the lowest unoccupied molecular orbital delocalization and leads to a significantly enhanced optical bandgap without affecting the MR properties. Therefore, c-DABNA in dilute toluene exhibits a significant emission blue shift of ∼30 nm compared to DABNA-1 and reaches the deep-blue region at 431 nm, while retaining a narrow full width at half maximum of 23 nm. The corresponding organic light-emitting diodes achieve deep-blue emission with a Commission Internationale de l'Éclairage <i>y</i>-coordinate down to 0.033 and improved external quantum efficiency. Our work thus opens a new avenue for modulating the photophysical properties of the MR backbone.