We describe the phosphorescent characteristics of two platinum complexes containing an N∧C∧N-coordinating tridentate ligand: platinum(II) 3,5-di(2-pyridyl)toluene chloride [Pt(dpt)Cl] and a newly synthesized platinum(II) 3,5-di(2-pyridyl)toluene phenoxide [Pt(dpt)(oph)], together with the performance of organic light-emitting diodes (OLEDs) prepared with these complexes as phosphors. Films containing each one of the complexes in a 4,4’-N,N’-dicalbazolylbiphenyl (CBP) host showed highly efficient photoluminescence. The fabricated OLEDs exhibited high efficiencies; the maximum external quantum efficiency of the device with Pt(dpt)(oph) phosphor after correction of angular dependence of emission was found to be 16.5%. The luminance half decay time of the Pt(dpt)(oph) device under a constant-current operation was considerably longer than that of the Pt(dpt)Cl device.
The absorption and emission spectra of the Pt(II) complexes containing N wedge C wedge N-coordinating tridentate ligands, platinum(II) 1,3-di(2-pyridyl)benzene chloride [Pt(dpb)Cl] and platinum(II) 3,5-di(2-pyridyl)toluene chloride [Pt(dpt)Cl], together with their corresponding free ligands, 1,3-di(2-pyridyl)benzene (dpbH) and 3,5-di(2-pyridyl)toluene (dptH), have been analyzed by density functional theory (DFT) for the ground state and time-dependent DFT (TDDFT) for the excited states. T(1)(A(1)) and S(1)(B(2)) of the complexes (in C(2)(v) symmetry) were assigned on the basis of the calculated excitation energies as well as comparison of the experimental spectroscopic properties and the calculated states' characteristics. The calculated excitation energies for T(1) and S(1) of the complexes as well as those for T(1) of the free ligands were in good agreement with their observed values within 600 cm(-1). The d-pi* characters of the excited states were evaluated from the change in electron densities between the ground and excited states by Mulliken population analysis; values of 25% for T(1) and 32% for S(1) were obtained for both complexes. The calculated values of d-pi* character were found to be consistent with the reported emission lifetimes as well as the observed emission energy shifts from the corresponding free ligands. Most spectroscopic properties of the complexes and the free ligands, which include solvatochromic shift, Stokes shifts, methyl substitution shifts, and emission spectra profiles, were well explained from the calculation results.
ABSTRACT:We have conducted a study of composite polymer particles with a gradated resin composition by suspension polymerization, in which the resin composition gradually changes from the surface to the center of the particles. The binder resin of the polymer particles consists primarily of styrene, butyl acrylate, and methacrylic acid (MAA). Fourier transform infrared/photoacoustic spectroscopy analysis of the polymer particles by suspension polymerization has proved that MAA, having a higher polarity, concentrates near the surface of the particles, and this results in a formation similar to a core-shell structure. These composite polymer particles are excellent in blocking resistivity because the resin, containing a higher concentration of MAA, has a high glass-transition temperature. Composite polymer particles with a gradated MAA concentration could be used for toner applications in low-power hot-roll fusing electrophotography systems because the melting property of the core resin is controlled and allows fusing at lower temperatures.
We have developed a new class of highly-fluorescent blue emitter for organic light-emitting diodes (OLEDs) consisting of tetrasubstituted pyrenes. From the analysis of the excited state diagrams of pyrene and its derivatives by molecular orbital calculations, we found that the new tetra-substituted pyrenes are highly fluorescent. OLEDs fabricated using the synthesized tetrasubstituted pyrenes as emitters showed high efficiency and good color purity. IntroductionSince the report of the first multilayer organic light-emitting-diode (OLED) in 1987 [ 1 ], considerable efforts have been made to utilize the OLEDs to full-color displays. It should be mentioned that development of efficient and pure red, green, and blue (RGB) emitters is considered to be one of the most important technologies in achieving this target, since the characteristics of an OLED highly depend on the emitter. Especially, blue emitters are indispensable for all OLED displays with any of full-color panel constitution such as array of RGB emitting OLEDs, fluorescent color changing media with blue OLEDs, and color filters with white emitting OLEDs.Polycyclic Aromatic Hydrocarbons (PAHs) are well known as a major group of organic fluorescent materials. Anthracene [2,3], naphthacene [ 4 ], perylene [ 5 ] and their derivatives (such as rubrene [6]) have been widely used as emitters in OLEDs for their promising fluorescent properties. However, pyrene, a basic PAH, has not attracted much attention as an OLED emitter because of its moderate fluorescence quantum yield (q F = 0.32 [7]) even in dilute solutions, and strong tendency to form excimer that leads to decrease the fluorescence efficiency in condensed media [8].In this report we explain the applicability of tetra-substituted pyrenes as blue emitters for OLEDs. It should be noted that unsubstituted pyrene is not a strong fluorescent emitter whereas 1,3,6,8-tetraphenylpyrene (tppy) has been measured to be highly fluorescent (q F = 0.90) in pure blue spectral region [7]. In this study, we analyzed the excited state diagrams of these molecules using molecular orbital (MO) method in order to understand the reason behind this remarkable change in fluorescent properties due to the introduction of phenyl moieties to pyrene. From this analysis, we understood that the tetra-substituted pyrenes are highly fluorescent. Here, we describe the excited-states analysis and the fluorescent properties of the pyrenes and the OLED performance of the devices using these compounds as emitters. Computational chemical surveyThe lowest singlet state of PAHs may be classified according to its electronic character determined by the perimeter free electron orbital model [7], into either L a state that is represented by the highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) electronic transition, or L b state that is represented by a combination of the HOMO-1 to LUMO and HOMO to LUMO+1 transitions as given in Fig. 1. The L a state generally has strong oscillator strength (f) to the ground stat...
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