| Abstract
| - The excellent electroluminescent (EL) properties of 1,1-disubstituted 2,3,4,5-tetraphenylsiloles,1-methyl-1,2,3,4,5-pentaphenylsilole (MPPS), and 1,1,2,3,4,5-hexaphenylsilole (HPS) have been found.Despite some studies devoted to these materials, very little is known about the real origin of their uniqueEL properties. Therefore, we investigated the structures, photoluminescence (PL), and charge carriertransport properties of 1,1-disubstituted 2,3,4,5-tetraphenylsiloles as well as the effect of substituents onthese characteristics. The single crystals of the three siloles involving 1,1-dimethyl-2,3,4,5-tetraphenylsilole(DMTPS), MPPS, and HPS were grown and their crystal structures were determined by X-ray diffraction.Three siloles have nonplanar molecular structures. The substituents at 1,1-positions enhance the sterichindrance and have predominant influence on the twisted degree of phenyl groups at ring carbons. Thisnonplanar structure reduces the intermolecular interaction and the likelihood of excimer formation, andincreases PL efficiency in the solid state. The silole films show high fluorescence quantum yields (75−85%), whereas their dilute solutions exhibit a faint emission. The electronic structures of the three siloleswere investigated using quantum chemical calculations. The highest occupied molecular orbitals (HOMOs)and the lowest unoccupied molecular orbitals (LUMOs) are mainly localized on the silole ring and twophenyl groups at 2,5-positions in all cases, while the LUMOs have a significant orbital density at two exocyclicSi−C bonds. The extremely theoretical studies of luminescent properties were carried out. We calculatedthe nonradiative decay rate of the first excited state as well as the radiative one. It is found that the faintemission of DMTPS in solutions mainly results from the huge nonradiative decay rate. In solid states,molecular packing can remarkably restrict the intramolecular rotation of the peripheral side phenyl ring,which has a large contribution to the nonradiative transition process. This explains why the 1,1-disubstituted2,3,4,5-tetraphenylsiloles in the thin films exhibit high fluorescence quantum yields. The charge carriermobilities of the MPPS and HPS films were measured using a transient EL technique. We obtained amobility of 2.1 × 10-6 cm2/V·s in the MPPS film at an electric field of 1.2 × 106 V/cm. This mobility iscomparable to that of Alq3, which is one of the most extensively used electron transport materials in organiclight-emitting diodes (LEDs), at the same electric field. The electron mobility of the HPS film is about ∼1.5times higher than that of the MPPS film. To the best of our knowledge, this kind of material is one of themost excellent emissive materials that possess both high charge carrier mobility and high PL efficiency inthe solid states simultaneously. The excellent EL performances of MPPS and HPS are presumably ascribedto these characteristics.
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