Abstract
This paper presents a detailed spectroscopic investigation of luminescence properties of 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) and N,N,N′,N′-tetraphenylbenzidine (TAD) in solutions and neat films. These compounds are compared to their derivatives CDBP and TDAD that contain methyl groups in the 2 and 2′ position of the biphenyl core. We find that whereas steric twisting in CDBP and TDAD leads to a high triplet energy of about 3.0 and 3.1 eV, respectively, these compounds also tend to form triplet excimers in a neat film, in contrast to CBP and TAD. By comparison with N-phenylcarbazole (NPC) and triphenylamine (TPA), on which these compounds are based, as well as with the rigid spiro analogues to CBP and TAD we show that the reduced excimer formation in CBP and TAD can be attributed to a localization of the excitation onto the central biphenyl part of the molecule.
Highlights
The fabrication of organic light-emitting diodes (OLEDs) based on phosphorescence requires host materials with a triplet T1 energy that is higher than that of the emitting phosphorescent dye
Based on phosphorescence measurements supplemented by density-functional theory (DFT) calculations we find that an increase in planarity of the central biphenyl is associated with localization of the excitation there that efficiently prevents the formation of excimers
The spectroscopic investigations of CBP/CDBP and TAD/ TDAD have the aim to clarify the stronger propensity to excimer formation that has been observed in CDBP and TDAD compared to CBP and TAD
Summary
The fabrication of organic light-emitting diodes (OLEDs) based on phosphorescence requires host materials with a triplet T1 energy that is higher than that of the emitting phosphorescent dye. To obtain a more thorough understanding of the electronic properties of carbazole- and triphenylamine-based host materials, we have investigated two systematic series (Figure 1) where the degree of coupling between two triphenylamine or carbazole chromophores is modified gradually To this end, we have complemented CBP and TAD with a spiro-substituted derivative where the central two phenyl rings of the CBP are forced into planarity, and with a CH3-substituted derivative that increases the twist between the two central phenyl rings and so disrupts conjugation. Based on phosphorescence measurements supplemented by density-functional theory (DFT) calculations we find that an increase in planarity of the central biphenyl is associated with localization of the excitation there that efficiently prevents the formation of excimers This is in contrast to the derivatives with CH3 substitution on the 2- and 2′-position of the biphenyl unit, Received: December 22, 2014 Published: January 9, 2015. In contrast to the common view,[5] for the host materials based on N-phenylcarbazole and triphenylamine, introducing steric twisting increases the propensity to excimer formation
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