Pachaiyappan Rajamalli

A New Molecular Design Based on Thermally Activated Delayed Fluorescence for Highly Efficient Organic Light Emitting Diodes.

Two benzoylpyridine-carbazole based fluorescence materials DCBPy and DTCBPy, bearing two carbazolyl and 4-(t-butyl)carbazolyl groups, respectively, at the meta and ortho carbons of the benzoyl ring, were synthesized. These molecules show very small ΔEST of 0.03 and 0.04 eV and transient PL characteristics indicating that they are thermally activated delayed fluorescence (TADF) materials. In addition, they show extremely different photoluminescent quantum yields in solution and in the solid state: in cyclohexane the value are 14 and 36%, but in the thin films, the value increase to 88.0 and 91.4%, respectively. The OLEDs using DCBPy and DTCBPy as dopants emit blue and green light with EQEs of 24.0 and 27.2%, respectively, and with low efficiency roll-off at practical brightness level. The crystal structure of DTCBPy reveals a substantial interaction between the ortho donor (carbazolyl) and acceptor (4-pyridylcarbonyl) unit. This interaction between donor and acceptor substituents likely play a key role to achieve very small ΔEST with high photoluminescence quantum yield.

Diastereoselective [3+2] Annulation of Aromatic/Vinylic Amides with Bicyclic Alkenes through Cobalt‐Catalyzed C−H Activation and Intramolecular Nucleophilic Addition

A highly diastereoselective method for the synthesis of dihydroepoxybenzofluorenone derivatives from aromatic/vinylic amides and bicyclic alkenes is described. This new transformation proceeds through cobalt-catalyzed C-H activation and intramolecular nucleophilic addition to the amide functional group. Transition-metal-catalyzed C-H activation reactions of secondary amides with alkenes usually lead to [4+2] or [4+1] annulation; to the best of our knowledge, this is the first time that a [3+2] cycloaddition is described in this context. The reaction proceeds under mild conditions and tolerates a wide range of functional groups. Mechanistic studies imply that the C-H bond cleavage may be the rate-limiting step.

New Molecular Design Concurrently Providing Superior Pure Blue, Thermally Activated Delayed Fluorescence and Optical Out-Coupling Efficiencies

Simultaneous enhancement of out-coupling efficiency, internal quantum efficiency, and color purity in thermally activated delayed fluorescence (TADF) emitters is highly desired for the practical application of these materials. We designed and synthesized two isomeric TADF emitters, 2DPyM-mDTC and 3DPyM-pDTC, based on di(pyridinyl)methanone (DPyM) cores as the new electron-accepting units and di(tert-butyl)carbazole (DTC) as the electron-donating units. 3DPyM-pDTC, which is structurally nearly planar with a very small ΔEST, shows higher color purity, horizontal ratio, and quantum yield than 2DPyM-mDTC, which has a more flexible structure. An electroluminescence device based on 3DPyM-pDTC as the dopant emitter can reach an extremely high external quantum efficiency of 31.9% with a pure blue emission. This work also demonstrates a way to design materials with a high portion of horizontal molecular orientation to realize a highly efficient pure-blue device based on TADF emitters.

Highly efficient orange and deep-red organic light emitting diodes with long operational lifetimes using carbazole–quinoline based bipolar host materials

Orange and deep red-emitting phosphorescent organic light-emitting diodes (PhOLEDs) are important for OLED displays and lighting; however, high-performance with long operational lifetime hosts designed for orange/deep red PhOLEDs are very rare. Three new carbazole–quinoline hybrids are synthesized and used as the host materials for orange and deep-red PhOLEDs. These bipolar hosts show high glass transition temperatures of 90–146 °C and triplet energy gaps of 2.51–2.95 eV. The optimized orange PhOLEDs using 9-(4-(4-phenylquinolin-2-yl)phenyl)-9H-carbazole (CzPPQ) as the host show the highest external quantum efficiency (EQE) of 25.6% and a power efficiency of 68.1 lm W−1, which are the highest values for orange PhOLEDs. More importantly, the efficiency roll-off is extremely small for both the orange and deep-red devices. For example, an orange device showed an EQE of 25.1% at 100 cd m−2 and 23.6% at 1000 cd m−2; the result appears to be the lowest efficiency roll-off for orange PhOLEDs to date. Additionally, the operational lifetime of both the orange and deep-red devices gave a T50 of more than 26412 and 11450 h, respectively, at an initial luminance of 500 cd m−2. The values are 12 times (orange) and 6 times (red) longer than those of the corresponding devices using CBP as the host.

Rhodium(III)-Catalyzed [4+1] Annulation of Aromatic and Vinylic Carboxylic Acids with Allenes: An Efficient Method Towards Vinyl-Substituted Phthalides and 2-Furanones.

A highly regio- and stereoselective synthesis of 3,3-disubstituted phthalides from aryl carboxylic acids and allenes using a rhodium(III) catalyst has been demonstrated. The reaction features broad functional group tolerance and provides a simple and straightforward route to the synthesis of various 3-vinyl-substituted phthalides. Furthermore, the catalytic reaction can also be applied to the synthesis of biologically active 5-vinyl-substituted 2-furanones from α,β-unsaturated carboxylic acids and allenes. The reactions proceed through a carboxylate-assisted ortho-CH activation and [4+1] annulation. The preliminary mechanistic studies suggest that a CH cleavage is the rate-determining step.

A high triplet energy, high thermal stability oxadiazole derivative as the electron transporter for highly efficient red, green and blue phosphorescent OLEDs

A high glass transition temperature (Tg = 220 °C), high triplet energy gap (ET = 2.76 eV) and high electron mobility material bis(m-terphenyl)oxadiazole was readily synthesized. It can serve as a universal electron transporter for blue, green and red phosphorescent OLEDs with excellent efficiencies. The material shows high current density compared to other electron transport materials and exhibits reduced driving voltage for all color PhOLEDs irrespective of the energy level of the host materials, due to efficient electron injection from 2,5-di([1,1′:3′,1″-terphenyl]-5′-yl)-1,3,4-oxadiazole (TPOTP) to the host material. For the green PhOLED, maximum external quantum efficiency (EQE) over 25%, current efficiency of 97.6 cd A−1 and power efficiency of 100.6 lm W−1 were achieved. The red and blue devices using TPOTP as the electron transporter also show EQE higher than 23% with very low roll-off in efficiencies in practical brightness level.

Bromo induced reversible distinct color switching of a structurally simple donor–acceptor molecule by vapo, piezo and thermal stimuli

Altering the luminescence properties of a material through external factors is an attractive feature that has the potential for various luminescence-related applications. Here, we report the synthesis and luminescence properties of two anthracene-based donor acceptor compounds, N,N-di-p-tolylanthracen-9-amine (TAA) and 10-bromo-N,N-di-p-tolylanthracen-9-amine (TAAB). In the solid state, the bromo-substituted compound TAAB shows reversible visible switching of the emission by external stimuli such as solvent, mechanical grinding and temperature. Single crystal X-ray studies, powder-XRD analysis and theoretical calculations reveal that switchable emission originated from the different stacking modes of TAAB. Furthermore, we have found that the bromo group interaction in the solid state plays a crucial role in this tunable emission. The other anthracene-based compound TAA does not show such switching of the emission by external stimuli. The observed changes in the luminescence of TAAB by external stimuli suggest potential applications in rewritable optical media, sensors, and optoelectronic devices.

A thermally activated delayed blue fluorescent emitter with reversible externally tunable emission

A thermally activated delayed fluorescent (TADF) material containing two meta carbazolyl groups on the phenyl ring of the benzoyl-4-pyridine moiety was prepared and its luminescence showed a colour change from blue to green by external stimuli. The organic-blue-light-emitting device using this TADF dopant provided a high external quantum efficiency of 18.4%.

Thermally activated delayed fluorescence emitters with a m,m-di-tert-butyl-carbazolyl benzoylpyridine core achieving extremely high blue electroluminescence efficiencies

Thermally activated delayed fluorescence (TADF) emitters are attractive for display and lighting applications. Here, a series of highly efficient blue TADF emitters including 3,5-bis((3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)(pyridin-4-yl)methanone (4BPy-mDTC), (3,5-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)(pyridin-3-yl)methanone (3BPy-mDTC), (3,5-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)(pyridin-2-yl)methanone (2BPy-mDTC) and (3,5-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)(phenyl)methanone (BP-mDTC) were designed and synthesized. The molecular structures feature two meta carbazole substituents attached to a benzoylpyridine (BPy) group or to a benzophenone (BP) group. These compounds show high thermal stability (Td = 371–439 °C), blue emissions (458–488 nm), high photoluminescence quantum yields (PLQY) (75–96%) in thin films and very small energy gaps between S1 and T1 (ΔEST) of 0.01–0.05 eV. In addition, they all reveal TADF properties including small ΔEST, two components in the transient PL decays, prompt emission and temperature-dependent delayed emission. The BPy series appears to give much higher photoluminescence quantum yields (PLQY > 92%) than BP-mDTC (75%) plausibly due to the more rigid structure caused by the interaction between pyridine nitrogen and the aromatic C–H bond. Furthermore, 4BPy-mDTC shows a more delayed component compared to 3BPy-mDTC and 2BPy-mDTC. The electroluminescent devices based on 4BPy-mDTC and 2BPy-mDTC as the dopant emitters exhibit sky blue emission with maximum external quantum efficiencies (EQEs) over 28%, and current and power efficiency and maximum luminance up to 67.0 cd A−1, 60.1 lm W−1 and 20 000 cd m−2, respectively. The presence of the pyridine ring and the position of the nitrogen atom in the molecules are critical for the high quantum yield and device efficiency. The PLQY EQE and luminance are dramatically improved by changing the phenyl into a pyridine group in the dopant in these devices.

A concise synthesis of quinolinium, and biquinolinium salts and biquinolines from benzylic azides and alkenes promoted by copper(II) species

A novel copper-promoted multiple aza-[4 + 2] cycloaddition reaction of N-methyleneanilines in situ generated from benzylic azide and alkenes afforded quinolinium salts, biquinolinium salts, biquinolines or substituted quinolines depending on the substitution on the phenyl ring of benzylic azide. The reaction of para substituted benzylic azides and 2 equivalents of alkenes afforded the corresponding substituted quinolinium salts, while benzylic azides without a para substituent provided biquinolinium salts. The copper-promoted cycloaddition reaction also allows biquinoline products to be obtained from ortho-substituted benzylic azides. These reactions work well with both terminal and internal alkenes. Unsymmetrical internal alkene reactions proceed with high regioselectivity. The reaction is likely started by Lewis acidic CuII-assisted rearrangement of benzylic azide to N-methyleneaniline, followed by a [4 + 2] cycloaddition with alkene. Detailed mechanistic studies suggest that the biquinoline and biquinolinium salts are likely formed via radical processes.