| 1. |
- Yu, G., et al.
(författare)
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Structures, electronic states, photoluminescence, and carrier transport properties of 1,1-disubstituted 2,3,4,5-tetraphenylsiloles
- 2005
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 127:17, s. 6335-6346
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Forskningsöversikt (refereegranskat)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 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 HIPS 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 x 10(-6) cm(2)/V(.)s in the MPPS film at an electric field of 1.2 x 10(6) V/cm. This mobility is comparable to that of Alq(3), 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 similar to 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.
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| 2. |
- Zhao, Y. S., et al.
(författare)
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Single crystalline submicrotubes from small organic molecules
- 2005
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 17:25, s. 6430-6435
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Tidskriftsartikel (refereegranskat)abstract
- The single crystalline submicrotubes of a small organic functional molecule, 2,4,5-triphenylimidazole (TPI), were successfully prepared with a facile method. A series of characterizations indicated that the tubes were obtained from the rolling followed by seaming of a preorganized two-dimensional sheet-like structure, whose formation was due to the efficient cooperation of several molecular recognition elements. The length and diameter of the TPI tubes can be readily controlled by adjusting the experimental conditions. The as-prepared submicrotubes have intensive luminescence and size-dependent optical properties, which allows them to find potential applications in novel optical and optoelectronic devices together with their single crystalline structure and good stability. The strategy described here should give a useful enlightenment for the design and fabrication of tubular structures from small organic molecules.
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| 5. |
- Yin, S., et al.
(författare)
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Aggregation-enhanced luminescence and vibronic coupling of silole molecules from first principles
- 2006
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 73:20, s. 205409-
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Tidskriftsartikel (refereegranskat)abstract
- Aggregate formation in molecular solids usually quenches the luminescence, a piece of bad news for molecular electronic devices. However, siloles present extremely high luminescent efficiency in solid state as well as in aggregation, but have almost no luminescence in solution. By employing a first-principles calculation to study excited states and vibronic couplings, we find that it is the low-frequency twisting motions of side rings which enhance the nonradiative decay. These motions can be suppressed either by solid-state packing, by aggregation formation in polar solvents, or by increasing the solvent viscosity; thereby, the radiative decay becomes dominant, resulting in peculiar aggregate-induced emission phenomena in siloles.
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