Rational molecular design of novel host material combing intra- and intermolecular charge transfers for efficient solution-processed organic light-emitting diodes

Abstract

Two novel D-σ-A molecules bearing 2,4,6-diphenyl-1,3,5-triazine (TRZ) units as electron-accepting segment and 1,3-di (9H-carbazol-9-yl)benzene (mCP) or 9'-(4-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenoxy)phenyl)-9′H-9,3':6′,9″-tercarbazole (TCz) as electron-donating moiety, named as 9,9'-(5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl) phenoxy) −1,3- phenylene) bis(9H-carbazole) (mCP-o-TRZ) and 9'-(4-(4-(4,6-diphenyl-1,3,5-triazin-2-yl) phenoxy)) phenyl)- 9′H-9,3':6′,9″-tercarbazole (TCz-o-TRZ), are designed and synthesized. The oxygen atom is applied as a bridge between the various donor and acceptor parts to realize the conjugation-forbidden linkage of the molecular backbone. The photophysical properties, solvatochromism, theoretical calculations, thermal properties and device performance are systematically investigated. As anticipated, the mCP-o-TRZ molecule was endowed with intramolecular charge transfer (intra-CT) and intermolecular charge transfer (inter-CT), while only intra-CT can be obtained in the TCz-o-TRZ material. The solution-processed OLEDs employed mCP-o-TRZ as host for the phosphorescence guest (tris(2-methylphenylpyridine) iridium (III)) (Ir (mppy)3) realized a maximum EQE of 9.0%, which is superior to that of TCz-o-TRZ. The results demonstrate that the development of new D-σ-A molecule equipped with dual CTs is a promising strategy for fabricating organic light-emitting diodes.