Conformationally-restricted bicarbazoles with phenylene bridges displaying deep-blue emission and high triplet energies: systematic structure–property relationships

Abstract

The synthesis is reported of twelve new symmetrical carbazole dimers in which the carbazole units are linked via 1,4-phenylene spacers. There are two distinct series of compounds based on the position on the carbazole ring where the phenylene spacer is attached: this is either at carbazole C(3) (series 1a-1f) or at C(2) (series 2a-2f). The central phenylene ring is substituted with either two methyl, two methoxy or two cyano substituents which impart an intramolecular torsional angle between the phenylene and carbazole rings, thereby limiting the extent of π-conjugation between the carbazole units, and raising the triplet energies of the molecules to E 2.6-3.0 eV, as determined from their phosphorescence spectra at 80 K. Structure-property relationships were studied by UV-vis and fluorescence spectroscopy, cyclic voltammetry and theoretical calculations. A notable observation is that substitution at the 2-position of carbazole (linear conjugation) exerts control over the position of the HOMO, while substitution at the 3-position of carbazole (meta conjugation) allows greater control over the LUMO. X-ray crystal structures are reported for two of the bicarbazoles. Compound 2d is shown to be a suitable host for the sky-blue emitter FIrpic in PhOLEDs, with improved device performance compared to CBP as host.

Conformational Dependence of Triplet Energies in Rotationally Hindered N‐ and S‐Heterocyclic Dimers: New Design and Measurement Rules for High Triplet Energy OLED Host Materials

“…The results are presented in Figure 3 and Table 2. Carbazole dimers are known to be readily oxidized at low potentials [7, 17, 24] . Consistent with this, both 22Cz and 33Cz displayed two sequential and reversible single‐electron oxidations corresponding to radical cation formation on each of the carbazole rings.…”

“…33Cz is unique in this present series, with the meta ‐conjugated dimer displaying some degeneracy between the LUMO and LUMO+1. Interestingly, our previous observation of significant degeneracy in the HOMO manifold of para ‐conjugated 1,4‐phenylene bridged bicarbazoles, and in the LUMO manifold of meta ‐conjugated 1,4‐phenylene π‐bridged bicarbazoles, [24] is not reproduced as a general trend in these non‐π‐bridged dimers.…”

“…However, we were motivated by the fact that E T increases for the non‐planar host materials 4,4’‐bis(9‐carbazolyl)‐2,2’‐dimethylbiphenyl CDBP and 1,3‐bis(9‐carbazolyl)benzene mCP as conjugation length is decreased compared to planar CBP (structures of these and other host materials mentioned throughout this work are shown in Figure S1) [26, 27] . Applying this approach to the previously reported bicarbazole series, [24] removing the central 1,4‐phenylene π‐bridge and instead placing the alkoxy substituents directly onto adjacent carbazole rings of a simple carbazole‐carbazole dimer was anticipated to impose a larger dihedral angle between the carbazoles through steric clash. This modification would cause the conjugation length of the molecule to decrease further, leading to a desirable blue‐shift in optical properties and increased E T .…”

“…Toward establishing rules for maximizing E T and developing new host materials appropriate for high E T , deep‐blue emitters, we previously reported a large family of conformationally‐restricted π‐bridged bicarbazoles. In each, the two carbazole moieties were separated by 1,4‐phenylene π‐bridges and bound either meta ‐ or para ‐ to the nitrogen atom of the carbazole [24] . Sterically demanding substituents were attached in the 2‐ and 5‐positions of the phenylene bridge which imposed a twist in the molecules.…”

The impact of phenyl–phenyl linkage on the thermodynamic, optical and morphological behavior of carbazol derivatives

“…According to recent reports, the short conjugation of carbazole derivatives leading to a wide band gap is a key factor for the high triplet energy presented by this class of semiconductors. 24,[60][61][62][63] Optical band gaps (E g ) of 3.57, 3.51 and 3.59 eV were estimated from the absorption edge wavelengths (l abs.edge ) of the UV-vis spectra for mCP, CBP and TCB, respectively (E g z 1240/l abs.edge ). These values are in nice agreement with literature reports presented by other authors for samples studied in DCM solution and/or in the thin-lm state.…”