Enhanced deep-red emission in donor-acceptor molecular architecture: The role of ancillary acceptor of cyanophenyl

Organic solid-state luminescent materials with high-efficiency deep-red emission have attracted considerable interest in recent years. Constructing donor-acceptor (D-A) type molecules has been one of most commonly used strategies to achieve deep-red emission, but it is always difficult to achieve high photoluminescence (PL) quantum yield (η_PL) due to forbidden charge-transfer state. Herein, we report a new D-A type molecule 4-(7-(4-(diphenylamino)phenyl)-9-oxo-9H-fluoren-2-yl)benzonitrile (TPAFOCN), deriving from donor-acceptor-donor (D-A-D) type 2,7-bis(4-(diphenylamino)phenyl)-9H-fluoren-9-one (DTPA-FO) with a fluorescence maximum of 627 nm in solids. This molecular design enables a transformation of acceptor from fluorenone (FO) itself to 4-(9-oxo-9H-fluoren-2-yl) benzonitrile (FOCN). Compared with DTPA-FO, the introduction of cyanophenyl not only shifts the emission of TPA-FOCN to deep red with a fluorescence maximum of 668 nm in solids, but also maintains the high η_PL of 10%. Additionally, a solution-processed non-doped organic light-emitting diode (OLED) was fabricated with TPA-FOCN as emitter. TPA-FOCN device showed a maximum luminous efficiency of 0.13 cd/A and a maximum external quantum efficiency (EQE) of 0.22% with CIE coordinates of (0.64, 0.35). This work provides a valuable strategy for the rational design of high-efficiency deep-red emission materials using cyanophenyl as an ancillary acceptor.