title: Triplet energies and excimer formation in meta- and para-linked carbazolebiphenyl matrix materials.
abstract: We present a spectroscopic investigation on the effect of changing the position where carbazole is attached to biphenyl in carbazolebiphenyl (CBP) on the triplet state energies and the propensity to excimer formation. For this, two CBP derivatives have been prepared with the carbazole moieties attached at the (para) 4- and 4(')-positions (pCBP) and at the (meta) 3- and 3(')-positions (mCBP) of the biphenyls. These compounds are compared to analogous mCDBP and pCDBP, i.e. two highly twisted carbazoledimethylbiphenyls, which have a high triplet energy at about 3.0 eV and tend to form triplet excimers in a neat film. This torsion in the structure is associated with localization of the excited state onto the carbazole moieties. We find that in mCBP and pCBP, excimer formation is prevented by localization of the triplet excited state onto the central moiety. As conjugation can continue from the central biphenyls into the nitrogen of the carbazole in the para-connected pCBP, emission involves mainly the benzidine. By contrast, the meta-linkage in mCBP limits conjugation to the central biphenyl. The associated shorter conjugation length is the reason for the higher triplet energy of 2.8 eV in mCBP compared with the 2.65 eV in pCBP.
authors.affiliation: Experimental Physics II, Department of Physics, University of Bayreuth, 95440 Bayreuth, Germany Bayreuth Institute of Macromolecular Research (BIMF), University of Bayreuth, 95440 Bayreuth, Germany.
authors.forename: Sergey A
authors.initials: SA
authors.lastname: Bagnich
authors.affiliation: Experimental Physics II, Department of Physics, University of Bayreuth, 95440 Bayreuth, Germany Bayreuth Institute of Macromolecular Research (BIMF), University of Bayreuth, 95440 Bayreuth, Germany.
authors.forename: Alexander
authors.initials: A
authors.lastname: Rudnick
authors.affiliation: Macromolecular Chemistry I, University of Bayreuth, 95440 Bayreuth, Germany.
authors.forename: Pamela
authors.initials: P
authors.lastname: Schroegel
authors.affiliation: Macromolecular Chemistry I, University of Bayreuth, 95440 Bayreuth, Germany Bayreuth Institute of Macromolecular Research (BIMF), University of Bayreuth, 95440 Bayreuth, Germany.
authors.forename: Peter
authors.initials: P
authors.lastname: Strohriegl
authors.affiliation: Experimental Physics II, Department of Physics, University of Bayreuth, 95440 Bayreuth, Germany Bayreuth Institute of Macromolecular Research (BIMF), University of Bayreuth, 95440 Bayreuth, Germany anna.koehler@uni-bayreuth.de.
authors.forename: Anna
authors.initials: A
authors.lastname: Köhler
keywords: blue electrophosphorescence; carbazole; excimer; matrix material; organic light-emitting diodes; phosphorescent