Abstract
A series of four heterocyclic dimers has been synthesized, with twisted geometries imposed across the central linking bond by ortho‐alkoxy chains. These include two isomeric bicarbazoles, a bis(dibenzothiophene‐S,S‐dioxide) and a bis(thioxanthene‐S,S‐dioxide). Spectroscopic and electrochemical methods, supported by density functional theory, have given detailed insights into how para‐ vs. meta‐ vs. broken conjugation, and electron‐rich vs. electron‐poor heterocycles impact the HOMO–LUMO gap and singlet and triplet energies. Crucially for applications as OLED hosts, the triplet energy (E T) of these molecules was found to vary significantly between dilute polymer films and neat films, related to conformational demands of the molecules in the solid state. One of the bicarbazole species shows a variation in E T of 0.24 eV in the different media—sufficiently large to “make‐or‐break” an OLED device—with similar discrepancies found between neat films and frozen solution measurements of other previously reported OLED hosts. From consolidated optical and optoelectronic investigations of different host/dopant combinations, we identify that only the lower E T values measured in neat films give a reliable indicator of host/guest compatibility. This work also provides new molecular design rules for obtaining very high E T materials and controlling their HOMO and LUMO energies.
Highlights
Carbazole is an important heterocyclic motif in organic electronics
A new series of high ET molecules has been synthesized displaying large dihedral angles enforced between two heterocycles, with the meta-conjugated carbazole dimer 33Cz displaying the highest ET of 3.07 eV in dilute polymer film
For the linearly conjugated 22Cz and 33DBS with para-quaterphenyl backbones the HOMO–LUMO gap is controlled by the long conjugation length of the molecule, but the relative energies of the HOMO and LUMO themselves are controlled by the nature of the heteroatom in the cycle
Summary
Versatility permitting systematic variations through substitution, interconversion and coupling reactions, allowing optimisation of the optoelectronic properties of carbazole derivatives for many applications.[1,2,3,4,5,6,7,8,9,10,11] Among the most important of these applications are their use as hole transport materials (HTMs),[12] or as host materials in the emissive layer (EML) of organic light emitting diodes (OLEDs).[13,14,15,16,17] In OLEDs the host material is doped at low levels with an emissive molecule such as an organometallic phosphor (PhOLED) or a thermally activated delayed fluorescent (TADF) molecule. The positioning of the p-bridge and the nature of its substituents had important consequences on the optoelectronic properties and led to host materials such as 1,4-bis(9-phenyl-2-carbazolyl)-2,5-dimethylbenzene 1 (Scheme 1) which outperformed the archetypical host material 4,4’-bis(9-carbazolyl)-1,1’-biphenyl (CBP) in OLED devices using the organometallic iridium phosphor FIrpic Despite this and other advances reported in recent years, carbazole-containing EML hosts generally possess ET too low for deep-blue TADF emitters (ET < 3 eV).[25] we were motivated by the fact that ET 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 p-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.
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