Browsing by Author "Monkman, Andrew P."

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Effects of Asymmetric Acceptor and Donor Positioning in Deep Blue Pyridyl-Sulfonyl Based TADF Emitters
(Elsevier Sci Ltd, 2021) Haykir, Gulcin; Aydemir, Murat; Danos, Andrew; Gumus, Selcuk; Hizal, Gurkan; Monkman, Andrew P.; Turksoy, Figen
In this work, we report synthesis and photophysical properties of deep-blue emitting donor-acceptor (D-A) and donor-acceptor-donor (D-A-D) thermally activated delayed fluorescence (TADF) molecules, where the molecules designed using carbazole as a donor (D) and a pyridyl (a)-sulfonyl (A) based bifunctional group as an acceptor. The work reveals how structural changes favor reverse intersystem crossing (rISC) by forming emissive charge transfer (CT) state, which is thoroughly investigated in different donor and asymmetric acceptor positions. Three comparison sets of regioisomers are investigated. 2,5-substituted pyridine derivatives in Set-1, are D-Aa, D-aA and D-Aa-D structures with asymmetric acceptor systems, revealing that the donor nearer to the pyridine group substantially controls the TADF properties In Set-2, modified the D-Aa-D structures reveal how ortho and meta positioned a relative to A (keeping the carbazole at meta to the A) affects the emission properties, deactivating TADF and promotion triplet-triplet annihilation. In the final set, 2,4-substituted pyridyl-sulfonyl derivatives show that the positioning of the donor far from the pyridine group has minimal influence. This final set of molecules show superior optical and physical properties though, indicating the importance of correct positioning between D, a, and A.
Photophysics of an Asymmetric Donor-Acceptor-Donor' TADF Molecule and Reinterpretation of Aggregation-Induced TADF Emission in These Materials
(Amer Chemical Soc, 2017) Aydemir, Murat; Xu, Shidang; Chen, Chengjian; Bryce, Martin R.; Chi, Zhenguo; Monkman, Andrew P.
We report an in-depth photophysical investigation of an asymmetric donor-acceptor-donor' (D-A-D') thermally activated delayed fluorescence (TADF) molecule (4- (9H-carbazol-9-yl)phenyl) (4- (10 H-phenothiazin-10-yl)-phenyl)sulfone and compare its photophysical properties to the parent symmetric D-A-D and D'-A-D' molecules. These D-A-D type small molecules all show strong TADF. The work reveals how the relative orientations of D-A (D'-A) moieties favor reverse intersystem crossing (rISC) by forming stable charge transfer (CT) states. The key requirement for the efficient TADF emitters is to achieve a very small CT-local triplet state energy splitting, which is shown to be complex in the asymmetric molecule. Throughout the investigations, we show that in the asymmetric D-A-D' system, even though E-CT (D-A) > E-CT (D'-A), no evidence of energy transfer from D-A to A-D' is observed, nor from excited D to D'. This is ascribed to the near orthogonality of the D and D' units and the very strong decoupling of the electrons on the D and A in the CT state. In addition, the possibility of aggregation-induced TADF (AI-TADF) is examined and shown to be a manifestation of solvatochromism in these particular molecules.
Synthesis of Biscyclometalated Iridium(III) Acetylacetonate Complexes Via a 15 Min Bridge-Splitting Reaction, Their Characterisations and Photophysical Properties
(Elsevier Science Sa, 2017) Altinolcek, Nuray; Aydemir, Murat; Tavasli, Mustafa; Dos Santos, Paloma L.; Monkman, Andrew P.
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. (C) 2017 Elsevier B.V. All rights reserved.