Synthesis, photophysical properties and their potential application in thermally activated delayed fluorescence (TADF) emitters" presents the results obtained from the research work carried out on the synthesis, characterization and exploration of the chemistry of nido-carborane donors and organoboron acceptor -based thermally activated delayed fluorescence (TADF) compounds. The first part includes the general introduction of the research work presented in the thesis. The second part describes the synthesis and photophysical studies of nido-carborane-attached triarylboranes and their applications in TADF.
In this work, a methyl group was inserted into the 4-position of the phenylene ring in the PhBMes2 acceptor moiety and that the introduction of a steric group into the phenylene linker of the nido-carborane-triarylborane D-A dyads (nido-1 −4) greatly increases their TADF properties both in solution and in the solid state. In the last chapter, a series of systems based on o-nido-carborane and dibenzo oxaborine donors were successfully designed and synthesized. Insertion of the new dibenzo oxaborinine acceptor moiety in the ortho position of the phenylene linker of the nido donor system leads to the formation of the TADF system.
The torsion angles ( = CCb−CCb−CPh−CPh) and frontier molecular orbitals, HOMOs and LUMOs, of nido-1−4 (isovalence = 0.02) in their ground state (S0) geometries from DFT calculations.
List of Tables
Chapter 1. General Introduction
- Carborane
- Nido-carboranes
- Thermally Activated Delayed Fluorescence (TADF)
- References
Carborane structures are related to each other as the closo species adopt a closed polyhedral structure with all faces triangular, while those of the nido, arachno and hypho compounds can be viewed by successive removal of one, two and three vertices respectively of the close ones. The C-C bond in o-carborane derivatives has unusual properties, the bond distance varies depending on three main factors: (i) steric repulsion between one or two substituents, (ii) an electronic effect due to electronegativity differences between the hydrogen atom and the substituent group , and (iii) stereoelectronic effects due to charge transfer from the lone electron pair or occupied orbital of a substituent to the antibonding orbital of the Ccarb-Ccarb bond (in the range of 1.62–2.15 Ao).12 -14Although under highly acidic and oxidizing conditions, the carboranes can be prone to degradation in a nucleophilic environment due to its unusual structure.15 Removal of a boron atom from a closo-carborane (C2B10H12) using a strong nucleophile to form a nido-carborane anion C2B9H12 - has been known for more than three decades16. Most studies have focused primarily on nido-carbane capabilities as an axillary ligand.
The other part of the reported works mainly investigates the influence of the nido-carboranyl unit as a ligand for transition metal-based complexes, especially Iridium-based phosphorescent emitters (Figure 1.6. In p-type delayed fluorescence, RISC can occur by triplet- triplet annihilation (Figure 1.6). TTA)29, whether a higher triplet Tn state to the S1 state transition is possible depends on the circumstances of the molecules.30,31 In p-type delayed fluorescence these processes require the reverse ISC not thermal energy to initiate the process.32 ,33 Therefore, our focus will be exclusively on endothermic RISC, which we refer to in this thesis as 'RISC' Organic light-emitting diodes (OLEDs) are one of the most important devices that we correlate with TADF must discuss.
Enhancement of thermally activated delayed fluorescence of nido-carborane-attached triarylboranes by steric modification of the phenylene.
- Introduction
- Experimental Section
- Chemical and Instrumentation
- General synthesis of nido-carborane-appended triarylboranes, nido-1−4
- Results and Discussion
- Synthesis and Characterization
- Photophysical Properties
- Electrochemical Properties
- Theoretical Calculations
- Conclusion
- Reference
These findings indicate that the coiled structure is not sufficiently retained in solution due to the free rotation of the nido-cage. To confirm this hypothesis, we attached a methyl group to the 4-position of the phenylene ring bearing a nido-carborane, i.e. to the ortho position relative to the cage, in the acceptor part of PhBMes2 (nido-1-4 in Figure 0.3). Workup and purification of the crude product by silica gel column chromatography using hexane as eluent gave 2a as a white powder (0.61 g, 24%).
Workup and purification of the crude product by column chromatography afforded a white powder of closo-2 (0.18 g, 61%). To obtain sterically restructured nido-carboranes, a methyl group was introduced into position 4 of the phenylene ring in the acceptor part of PhBMes2. 1H NMR resonances (δ −2.2 to −2.7 ppm) attributable to the B−H−B bridging hydrogen of nidocarborane, 11B NMR signals appearing at δ ca.
As far as single crystals are concerned, the tetrabutylammonium salt of the nido-compound cannot be successfully obtained. However, crystals of the tetramethylammonium (Me4N) salt of nido-1 could be obtained for X-ray diffraction study (Figure 2.12). In order to get a good insight into the photophysical properties of nido-compounds, we investigated with UV/Vis absorption and photoluminescence (PL) spectroscopy (Figure and Table 2).
In particular, the emission profiles remain similar with varying doping concentration of nido compounds from 5 wt% to 20 wt%, probably due to the sterically bulky nidocarborane and BMes2 groups. This again indicates that the nature of the 8-R group on the nido-carborane does not significantly affect the TADF properties of nido-1-4. Due to the presence of the same boryl moiety, nido-1−3 shows very similar LUMO levels.
This could be attributed to the additional electron-donating 2-Me group on the phenylene ring of the LUMO-dominant triarylboryl group. In this present work, we have successfully introduced a methyl group in the 4-position of the phenylene ring in the PhBMes2 acceptor moiety, and that the introduction of a steric group in the phenylene linker of nido-carborane-triarylborane D-A dyads (nido-1−4 ) greatly improves their TADF properties. Regardless of the type of 8-R group (R = H, Me, iPr) on the nido-carborane, all compounds exhibited very small.
Lee, The effect of the number of o-carboranyl ligands on the photophysical and electroluminescent properties of iridium(iii) cyclometalates, J.
Synthesis and Photophysical Properties of p-Nido-Carborane- Triarylborane Conjugates with a Methyl‐Substituted Phenylene Linker Triarylborane Conjugates with a Methyl‐Substituted Phenylene Linker
- Introduction
- Experimental Section
- Chemical and Instrumentation
- Photophysical Measurements
- Cyclic Voltammetry
- Results and Discussion
- Synthesis and Characterization
- Photophysical Properties
- Electrochemical Properties
- Conclusion
- References
Thermally activated delayed fluorescence (TADF) OLED emitter materials can promise efficient performance with long life without heavy metals. To achieve this, we introduced a methyl group to the ortho position of the phenylene ring bearing the nido-carborane. Removal of the solvent under reduced pressure afforded a sticky residue which was subjected to column chromatography on silica gel using hexane to give ((4-(dimesitylboryl)-2-methylphenyl)ethynyl)trimethylsilane as a white powder (0.68 g, 59 % ).
Workup and purification of the crude product by silica gel column chromatography using hexane as eluent gave 2a as a white powder (1 g, 29.9%). Workup and purification of the crude product by column chromatography afforded a white powder of closo-2 (0.22 g, 61%). The lack of long-lived components and the strong decrease in CT emission intensity in aerated solutions confirm that the long-lived emission is delayed fluorescence originating from the T1 → S1 RISC process, as expected (Figure 2).
All three nido compounds show varying weak delayed fluorescence with emission lifetimes (d s for 10 wt% doped PMMA) which are significantly smaller compared to the meta analogue. Towards high-efficiency pure blue and stable thermally activated delayed fluorescent organic light-emitting diodes. Design of efficient thermally activated delayed fluorescence materials for pure blue organic light-emitting diodes.
Anthraquinone-based intramolecular charge-transfer compounds: computational molecular design, thermally activated delayed fluorescence, and highly efficient red electroluminescence. Triplet harvesting with 100% efficiency via thermally activated delayed fluorescence in charge transfer OLED emitters. Highly efficient organic light-emitting diode based on a hidden thermally activated delayed fluorescence channel in a hepacin derivative.
Ultrapure blue thermally activated delayed fluorescence molecules: efficient HOMO- LUMO separation through the multiple resonance effect. Efficient Donor-Acceptor-Donor Borylated Compounds with Extremely Small ΔEST for Thermally Activated Delayed Fluorescence OLEDs. Highly efficient white light-emitting diodes with a two-component emitting layer based on blue and yellow thermally activated delayed fluorescence emitters.
Dimesitylarylborane-based luminescent emitters exhibiting highly efficient thermally delayed fluorescence for organic light-emitting diodes.
Synthesis and Thermally Activated Delayed Fluorescence of o-Nido Carborane-Appended Dibenzo Oxaborinine
- Introduction
- Experimental Section
- Chemical and Instrumentation
- Photophysical Measurements
- Cyclic Voltammetry
- Results and Discussion
- Synthesis and Characterization
- Photophysical Properties
- Electrochemical Properties
To overcome the nature of the non-TADF solution state and improve the TADF property, we change the steric bulk of the phenylene ring bearing the nido-carborane. Removal of the solvent under reduced pressure gave a sticky residue which was subjected to silica gel column chromatography using hexane to give 2-(10H-dibenzo[1,4]oxaborinin-10-yl)-3-methylphenyl)ethynyl)trimethylsilane as a white powder (0.68 g, 59%). Electrochemical oxidation (Eonset) and reduction (E1/2) were used to determine the HOMO and LUMO energy levels, respectively.
To achieve this, we introduce a dibenzo-oxaborinin acceptor at the ortho position of the phenyl ring of the nido-carborane donor system. The target nido compounds, nido-o1−o3, were prepared by deboronation of the closo-carborane cage of the corresponding closo-1−3 with tetrabutylammonium fluoride (TBAF) in refluxing THF. (15,18) Depending on the 8-R- group a slightly different synthetic method was chosen for the synthesis of Closo-1−3 compounds. All closo and nido compounds were characterized by multinuclear NMR spectroscopy. For nido-o1-o3 gives 13C and 1H peaks respectively along with broad.
To investigate the influence of the position of the acceptor as well as the influence of the acceptor different from the previous analogue in the nido-carborane-based systems, UV/Vis absorption and photoluminescence (PL) spectroscopy studies were performed (Figure 4.6 and Table 4.1). . Next, the PL transient decay to CT emission was investigated in both the solution and film states to elucidate the nature of the excited states of the compounds. Indeed, the PL temporal decays of all three nido-o1-o3 show delayed components in the microsecond range (d = 6.1 and 6.8 s), with a larger fraction of the delayed component than that of the quickly (Table 4.1 and Figure 4.8).
This is probably due to the steric bulk of the nido-carborane and dibenoxaborinine groups. This property is maintained until the water fraction of the solution is increased to about 80%, for which nido-o2 is still dissolved. In 99% water, the emission intensity reached a PLQY of 27%, which is slightly lower than that of the doped PMMA films but higher than that of a pure powder of nido-o2 (ΦPL = 18% in powder).
Furthermore, the transient PL decays of aggregates show apparent delayed components (d s), confirming the TADF properties of AIE. PL spectra (left) and transient PL decay curves (right) of the PMMA films doped with nido-o1−o3 at doping concentrations of 5–10 wt%.
Acknowledgement
List of Publications
Curriculum Vitae
