Structures, Electronic States, Photoluminescence, and Carrier Transport Properties of 1,1-Disubstituted 2,3,4,5-Tetraphenylsiloles

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 unique
EL properties. Therefore, we investigated the structures, photoluminescence (PL), and charge carrier
transport properties of 1,1-disubstituted 2,3,4,5-tetraphenylsiloles as well as the effect of substituents on
these 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 steric
hindrance and have predominant influence on the twisted degree of phenyl groups at ring carbons. This
nonplanar structure reduces the intermolecular interaction and the likelihood of excimer formation, and
increases 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 siloles
were 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 two
phenyl groups at 2,5-positions in all cases, while the LUMOs have a significant orbital density at two exocyclic
Si−C bonds. The extremely theoretical studies of luminescent properties were carried out. We calculated
the nonradiative decay rate of the first excited state as well as the radiative one. It is found that the faint
emission 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-disubstituted
2,3,4,5-tetraphenylsiloles in the thin films exhibit high fluorescence quantum yields. The charge carrier
mobilities of the MPPS and HPS films were measured using a transient EL technique. We obtained a
mobility of 2.1 × 10-6 cm2/V·s in the MPPS film at an electric field of 1.2 × 106 V/cm. This mobility is
comparable to that of Alq3, which is one of the most extensively used electron transport materials in organic
light-emitting diodes (LEDs), at the same electric field. The electron mobility of the HPS film is about ∼1.5
times higher than that of the MPPS film. To the best of our knowledge, this kind of material is one of the
most excellent emissive materials that possess both high charge carrier mobility and high PL efficiency in
the solid states simultaneously. The excellent EL performances of MPPS and HPS are presumably ascribed
to these characteristics.