Hybridized local and charge-transfer excitation in 2,5-substituted D–A type siloles for efficient OLEDs†

Abstract

Hybridized local and charge-transfer (HLCT) excitation in molecules can utilize triplet excitons efficiently via the high-energy reverse intersystem crossing (hRISC) process. The molecular design to realize HLCT is usually based on the structural arrangement of the constituent electron donor (D) and acceptor (A) units to form D–A or D–π–A structures. A motif that enables local excitation, good carrier transport and efficient emission – silole, specifically its 2,5-substituted derivatives – can be structurally modified to tune the optoelectronic properties. Here, we designed two derivatives, Silole-2,5DPA-TRZ and Silole-2,5Cz-TRZ, in which the electron acceptor triazine (TRZ) and the electron donor diphenylamine (DPA) or carbazole (Cz) modified at the 2,5-positions of a silole core form silole-D–A structures. This structural design, for the first time, allowed us to realize the HLCT character in 2,5-substituted silole derivatives. Experimental and theoretical studies revealed that Silole-2,5DPA-TRZ exhibits more balanced contributions of local excitation and charge transfer components to the HLCT state compared to the stronger charge transfer state in Silole-2,5Cz-TRZ. The organic light-emitting diode (OLED) based on the Silole-2,5DPA-TRZ emitter displays a high maximum external quantum efficiency (EQE_max) of 6.13% with a maximum emission peak wavelength at 532 nm, which is the hitherto highest reported EQE_max for OLEDs based on 2,5-substituted silole derivatives.