Small D-p-A-p-D organic dyes for near-infrared emitting OLEDs with excellent external quantum efficiency

Organic-light emitting diodes (OLEDs) with near-infrared (NIR) emission have an extremely broad range of attractive applications. Compounds containing donor (D) and acceptor (A) fragments are promising materials for use as active emitting layer in OLEDs due to their fast luminescence and because their luminescence properties can be fine-tuned at the synthesis stage. In this work, we synthesized and investigated two D-x-A-x-D NIR-emitting dyes based on a new pyridazine-thiadiazole electron acceptor with two different x linkers. The introduction of x linkers, together with the geometry modification, provides an additional redshift of the emission maximum from 700 to 850 nm. These dyes demonstrate moderate quantum yields up to 15% in chloroform solutions and to 4% in the solid state. Spectroscopic studies were supported by Time Dependent Density Functional Theory (TD-DFT) and multireference (XMCQDPT/CASSCF) calculations. A series of OLEDs with electroluminescence at 700 and 850 nm was fabricated. One of the devices exhibited maximum optical power 76 mu W/cm2 and maximum external quantum efficiency (EQE) 1.2%, which is close to the theoretical limit calculated for fluorescent NIR OLEDs based on low band gap small organic molecules.