Theoretical investigations of charge transport in organic materials are generally based on the "energy splitting in dimer" method and routinely assume that the transport parameters (site energies and transfer integrals) determined from monomer and dimer calculations can be reliably used to describe extended systems. Here, we demonstrate that this transferability can fail even in molecular crystals with weak van der Waals intermolecular interactions, due to the substantial (but often ignored) impact of polarization effects, particularly on the site energies. We show that the neglect of electronic polarization leads to qualitatively incorrect values and trends for the transfer integrals computed with the energy splitting method, even in simple prototypes such as ethylene or pentacene dimers. The polarization effect in these systems is largely electrostatic in nature and can change dramatically upon transition from a dimer to an extended system. For example, the difference in site energy for a prototypical "face-to-edge" one-dimensional stack of pentacene molecules is calculated to be 30% greater than that in the "face-to-edge" dimer, whereas the site energy difference in the pentacene crystal is vanishingly small. Importantly, when computed directly in the framework of localized monomer orbitals, the transfer integral values for dimer and extended systems are very similar.
We report on a joint theoretical and experimental investigation of the electronic structure of a series of bis(diphenylphosphine oxide) derivatives containing a central aromatic core with high triplet energy. Such molecules can serve as host material in the emissive layer of blue electro-phosphorescent organic devices. The aromatic cores considered in the theoretical study consist of biphenyl, fluorene, dibenzofuran, dibenzothiophene, dibenzothiophenesulfone, or carbazole, linked to the two phosphoryl groups in either para or meta positions. With respect to the isolated core molecules, it is found that addition of the diphenylphosphine oxide moieties has hardly any impact on the core geometry and only slightly reduces the energy of the lowest triplet state (by, at most, ∼0.2 eV). However, the diphenylphosphine oxide functionalities significantly impact the ionization potential and electron affinity values, in a way that is different for para and meta substitutions. Excellent comparison is obtained between the experimental UPS and IPES spectra of the para biphenyl and meta dibenzothiophene and dibenzothiophenesulfone compounds and the simulated spectra. In general, the phosphine oxide derivatives present triplet energies that are calculated to be at least 0.2 eV higher than those of currently widely used blue phosphorescent emitters.
We have investigated the electronic structure of triscarbazole derivatives used as host materials in blue phosphorescent organic light-emitting diodes. The results of density functional theory calculations show that, in the case of triscarbazole derivatives where the carbazole units are linked via C−C bonds, the frontier molecular orbital energies are modulated by strong molecular orbital interactions between the central and side carbazole units. On the other hand, in the case of triscarbazoles linked via C−N bonds, the combination of inductive effects and molecular orbital interactions tunes the frontier level energies and, interestingly, gives rise to an ambipolar character. In the C−N linked systems, the lowest triplet states are characterized mainly by an electronic transition localized within the central carbazole, while in the C−C linked compounds it is the longest oligo-para-phenyl segment to be found in the chemical structure that defines the lowest triplet transition. When the N− H group of the central carbazole unit is replaced by other groups [O, S, CH 2 , C(CH 3 ) 2 , C(CH 3 )(CF 3 ), and C(CF 3 ) 2 ], the HOMO/LUMO energies fluctuate substantially in the absence of the side carbazoles, but these variations are significantly reduced in their presence; also, the singlet−triplet energy differences decrease substantially when going from the isolated central unit to the triscarbazole-like derivatives.
Polymethacrylates, polystyrenes, and polynorbornenes bearing 2-(3-(carbazol-9-yl)phenyl)-5-phenyl-1,3,4-oxadiazole, 2-(4-(carbazol-9-yl)phenyl)-5-phenyl-1,3,4-oxadiazole, 2-(3,5-di(carbazol-9-yl)phenyl)-5-phenyl-1,3,4-oxadiazole) groups linked to the polymer backbone through the 3-position of their terminal phenyl groups have been synthesized for use as solution-processable ambipolar hosts for phosphorescent light-emitting diodes. The polymers exhibit good thermal stabilities, with no weight loss below 350 °C, and have glass-transition temperatures in the range 118–209 °C. Spectroscopic, electrochemical, and quantum-chemical data for small-molecule model compounds suggest that the side chain groups are suitable hosts for a range of phosphors, including the green-emitter fac-tris(2-phenylpyridinato-N,C 2′)iridium (Ir(ppy)3). Light-emitting diodes were fabricated, each with a solution-processed photo-cross-linked hole-transport layer, a solution-processed emissive layer composed of a carbazole-oxadiazole-functionalized polymethacrylate doped with Ir(ppy)3, and an evaporated electron-transport layer. The polymer with 2-(3,5-(dicarbazol-9-yl)phenyl)-5-phenyl-1,3,4-oxadiazole units in the side chain exhibited the lowest turn-on voltage (6.0 V), and the highest efficiency (external quantum efficiency of 10.0%, current efficiency of 34.1 cd/A).
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