Browsing by Author "Fox, Mark A."

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    Colour tuning from green to red by substituent effects in phosphorescent tris-cyclometalated iridium(III) complexes of carbazole-based ligands: Synthetic, photophysical, computational and high efficiency OLED studies
    (Royal Soc Chemistry, 2012) Moore, Tom N.; Zheng, Yonghao; Bryce, Martin R.; Fox, Mark A.; Griffiths, Gareth C.; Jankus, Vygintas; Al-Attar, Hameed A.; Monkman, Andrew P.; Tavaslı, Mustafa; Uludağ Üniversitesi/Fen-Edebiyat Fakültesi/Kimya Bölümü.; 0000-0002-9466-1111; AAB-1630-2020; 6506308760
    Two series of fac-tris-cyclometalated iridium(III) complexes, series 1 from the 2-(carbazol-3'-yl)-pyridine ligands, and series 2 from the isomeric 2-(carbazol-2'-yl)-pyridine ligands, have been characterised. The photoluminescence and electroluminescence from series 2 complexes are red shifted compared to series 1 complexes, due to the increased electron donating ability of the carbazole unit in series 2. The attachment of trifluoromethyl and methoxy substituents to the pyridyl ring in these complexes results in colour tuning of phosphorescence energy maxima over the range 494-637 nm (green to red). These complexes possess predominantly (MLCT)-M-3 (metal-to-ligand-charge transfer) excited states. DFT/TD-DFT computations correctly predict the phosphorescence emission maxima and show that the HOMOs in these complexes contain mixed iridium and carbazolyl character. The carbazolyl ligand contributions to the excited states increase in series 2 compared to series 1. Complexes of series 1 exhibit high phosphorescence quantum yields whereas complexes of series 2 show lower quantum yields. Solution processed organic light emitting devices (OLEDs) with series 1 complexes using the high triplet poly(9-vinylcarbazole) (PVK) as the host polymer exhibit very high performances of up to 40 cd A(-1) and external quantum efficiency of 12%. For series 2 the highest current efficiency is 10.3 cd A(-1) and external quantum efficiency of 5.6%.
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    Highly efficient, solution-processed, single-layer, electrophosphorescent diodes and the effect of molecular dipole moment
    (Wiley, 2011-06-21) Al-Attar, Hameed A.; Griffiths, Gareth C.; Moore, Tom N.; Fox, Mark A.; Bryce, Martin R.; Monkman, Andrew P.; Tavaşlı, Mustafa; Uludağ Üniversitesi/Fen-Edebiyat Fakültesi/Kimya Bölümü.; 0000-0002-9466-1111; AAB-1630-2020; 6506308760
    A new family of highly soluble electrophosphorescent dopants based on a series of tris-cyclometalated iridium(III) complexes (1-4) of 2-(carbazol-3-yl)-4/5-R-pyridine ligands with varying molecular dipole strengths have been synthesized. Highly efficient, solution-processed, single-layer, electrophosphorescent diodes utilizing these complexes have been prepared and characterized. The high triplet energy poly(9-vinylcarbazole) PVK is used as a host polymer doped with 2-(4-biphenylyl)-5-(4-tert-butyl-phenyl)-1,3,4-oxadiazole (PBD) for electron transport. Devices with a current efficiency of 40 cd A(-1) corresponding to an EQE of 12% can thus be achieved. The effect of the type and position of the substituent (electron-withdrawing group (CF3) and electron-donating group (OMe)) on the molecular dipole moment of the complexes has been investigated. A correlation between the absorption strength of the singlet metal-to-ligand charge-transfer ((MLCT)-M-1) transition and the luminance spectral red shift as a function of solvent polarity is observed. The strength of the transition dipole moments for complexes 1-4 has also been obtained from TD-DFT computations, and is found to be consistent with the observed molecular dipole moments of these complexes. The relatively long lifetime of the excitons of the phosphorescence (microseconds) compared to the charge-carrier scattering time (less than nanoseconds), allows the transition dipole moment to be considered as a "quasi permanent dipole". Therefore, the carrier mobility is sufficiently affected by the long-lived transition dipole moments of the phosphorescent molecules, which are randomly oriented in the medium. The dopant dipoles cause positional and energetic disorder because of the locally modified polarization energy. Furthermore, the electron-withdrawing group CF3 induces strong carrier dispersion that enhances the electron mobility. Therefore, the strong transition dipole moment in complexes 3 and 4 perturbs both electron and hole mobilities, yielding a reduction in exciton formation and an increase in the device dark current, thereby decreasing the device efficiency.