Browsing by Author "Monkman, Andrew P."

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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%.
High efficiency OLEDs based on anthracene derivatives: The impact of electron donating and withdrawing group on the performance of OLED
(Elsevier, 2016-03) Aydemir, Murat; Haykır, Gülçin; Battal, Ahmet; Jankus, Vygintas; Sugunan, Sunish K.; Dias, Fernando B.; Al-Attar, Hameed; Türksoy, Figen; Monkman, Andrew P.; Tavaslı, Mustafa; Uludağ Üniversitesi/Fen Edebiyat Fakültesi/Kimya Bölümü.; 0000-0002-9466-1111; AAB-1630-2020; 6506308760
New well-defined bulky anthracene derivatives with side groups having electron donating or withdrawing properties 8a-d were synthesized. The compounds contain substituted anthracene as the central core attaching 2-(4-(2-pyridinyl)-phenyl)vinyl and 4-R-phenyl [R:H (a), OMe (b) and CF3 (c), N(Ph)(2) (d)] groups at 9- and 10-positions. The impact of electron donating, withdrawing and neutral groups and their influence on the molecules photophysical, charge transfer (CT), triplet transfer (TT) and triplet-triplet annihilation (TTA) properties has been investigated. Based on the photophysical studies the most promising molecule (8d) has been selected and high efficiency fluorescent OLEDs with EQE at very low current efficiency reaching 7% were obtained. The value at low current density implies a Triplet Fusion (TF) contribution of 45%, very close to the maximum theoretical value of 50% when only the singlet decay channel is open to TTA, however we believe that in this case both TTA and TADF contribute to the triplet harvesting to yield high EQE values, and this mixed triplet harvesting arises through the heterogeneity of the films. At high current density a brightness of 20000 cd/m(2) was achieved and it is assigned partially to the material crystallisation.
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.
Synthesis of biscyclometalated iridium(III) acetylacetonate complexes via a 15 min bridge-splitting reaction, their characterisations and photophysical properties
(Elsevier, 2017-09-19) Aydemir, Murat; Dos Santos, Paloma L.; Monkman, Andrew P.; Altınölçek, Nuray; Tavaslı, Mustafa; Uludağ Üniversitesi/Fen-Edebiyat Fakültesi/Kimya Bölümü.; 0000-0002-9466-1111; AAB-1630-2020; 56955836600; 6506308760
Chloro-bridged diiridium(III) complexes (4a and 4b) were subjected to a bridge-splitting reaction with acetylacetone and sodium carbonate in 2-ethoxyethanol. The reaction was complete within 15 min and two novel biscyclometalated iridium(III) acetylacetonate complexes (5a and 5b) were successfully obtained in 41% and 63%, respectively. Complexes (5a and 5b) were fully characterised by H-1, C-13 NMR, FT-IR and elemental analysis. UV-Vis spectra of complexes (5a and 5b) showed two absorption bands: a strong one appears below 400 nm due to ligand-centered (LC, pi-pi*) transitions and the weak one arises between 400 and 600 nm due to metal-to-ligand charge-transfer (MLCT, d-pi*) transitions. The emission of the complexes (5a and 5b) arises in the red region of the spectrum, emitting ca. 675 nm for 5a and ca. 625 nm for 5b. A short emission lifetime (62 ns) for complex 5a indicates emission originates from (LC)-L-3 transitions. However longer emission lifetime (657 ns) for complex 5b confirms that the emission totally results from (MLCT)-M-3 transitions.
Synthesis, thermal and optical properties of a novel bis(2-(4-dicyanovinylphenyl) pyridinato-N, C2) iridium(acetylacetonate)
(Elsevier, 2015-10-09) Aydemir, Murat; Abay, Bahattin; Monkman, Andrew P.; Altınölçek, Nuray; Tavaslı, Mustafa; Uludağ Üniversitesi/Fen-Edebiyat Fakültesi/Kimya Bölümü.; 0000-0002-9466-1111; AAB-1630-2020; 56955836600; 6506308760
Bis(2-(4-formylphenyl) pyridinato-N, C2) iridium(acetylacetonate) (5) was subjected to a Knoevenagel reaction with malononitrile in dichloromethane in the presence of catalytic amount of pyrrolidine. This gave successfully a novel bis(2-(4-dicyanovinylphenyl) pyridinato-N, C2) iridium(acetylacetonate) (6) in good yield (83%). Complex 6 was fully chracterised by H-1, C-13 NMR, FT-IR, elemental analysis. Complex 6 showed a strong ligand centred (LC) absorption band at 355 nm (epsilon 56490 L mol (1) cm (1)) and weak metal-to-ligand charge transfer (MLCT) bands at 445 (epsilon 4154 L mol (1) cm (1)) and 535 nm (epsilon 2606 L mol (1) cm (1)). Complex 6 loses its acac moiety between 248 and 336 Alpha degrees C (under N-2 atmosphere) and emits a red light at 695 nm with a large Stokes shift (160 nm).