Samik Jhulki

Twist Does a Twist to the Reactivity: Stoichiometric and Catalytic Oxidations with Twisted Tetramethyl-IBX

The methyl groups in TetMe-IBX lower the activation energy corresponding to the rate-determining hypervalent twisting (theoretical calculations), and the steric relay between successive methyl groups twists the structure, which manifests in significant solubility in common organic solvents. Consequently, oxidations of alcohols and sulfides occur at room temperature in common organic solvents. In situ generation of the reactive TetMe-IBX from its precursor iodo-acid, i.e., 3,4,5,6-tetramethyl-2-iodobenzoic acid, in the presence of oxone as a co-oxidant facilitates the oxidation of diverse alcohols at room temperature.

Amorphous Host Materials Based on Tröger’s Base Scaffold for Application in Phosphorescent Organic Light-Emitting Diodes

Trögers bases (TBs) functionalized with carbazoles (TB-Czs) and phosphine oxides (TB-POs) were designed and synthesized as host materials for application in phosphorescent organic light-emitting diodes. The TB scaffold is shown to impart thermal stability with high Tg values (171-211 °C) as well as high triplet energies in the range of 2.9-3.0 eV. With a limited experimentation of the devices, it is shown that the TBs doped with a green phosphor, namely, Ir(ppy)3, permit impressive external efficiencies on the order of ca. 16% with a high brightness of ca. 3000-4000 cd/m2. Better device performance results are demonstrated by a small structural manipulation of the TB scaffold involving substitution of methyl groups in the core scaffold.

Hole-Transporting Materials Based on Twisted Bimesitylenes for Stable Perovskite Solar Cells with High Efficiency

A new class of hole-transport materials (HTMs) based on the bimesitylene core designed for mesoporous perovskite solar cells is introduced. Devices fabricated using two of these derivatives yield higher open-circuit voltage values than the commonly used spiro-OMeTAD. Power conversion efficiency (PCE) values of up to 12.11% are obtained in perovskite-based devices using these new HTMs. The stability of the device made using the highest performing HTM (P1) is improved compared with spiro-OMeTAD as evidenced through long-term stability tests over 1000 h.

Benzophenones as Generic Host Materials for Phosphorescent Organic Light-Emitting Diodes

Despite the fact that benzophenone has traditionally served as a prototype molecular system for establishing triplet state chemistry, materials based on molecular systems containing the benzophenone moiety as an integral part have not been exploited as generic host materials in phosphorescent organic light-emitting diodes (PhOLEDs). We have designed and synthesized three novel host materials, i.e., BP2-BP4, which contain benzophenone as the active triplet sensitizing molecular component. It is shown that their high band gap (3.91-3.93 eV) as well as triplet energies (2.95-2.97 eV) permit their applicability as universal host materials for blue, green, yellow, and red phosphors. While they serve reasonably well for all types of dopants, excellent performance characteristics observed for yellow and green devices are indeed the hallmark of benzophenone-based host materials. For example, maximum external quantum efficiencies of the order of 19.2% and 17.0% were obtained from the devices fabricated with yellow and green phosphors using BP2 as the host material. White light emission, albeit with rather poor efficiencies, has been demonstrated as a proof-of-concept by fabrication of co-doped and stacked devices with blue and yellow phosphors using BP2 as the host material.

Organic amorphous hole-transporting materials based on Tröger's Base: alternatives to NPB

Trogers Base (TB) scaffold has been exploited to create two novel amorphous hole-transporting materials (HTMs), i.e., TB1 and TB2, with high Tgs (152–156 °C) as alternatives to the popular NPB (Tg ∼ 95 °C). The hole-transporting properties of both TB1–2 are shown to be comparable to or better than that of NPB by contrasting the results of device fabrications with two different emissive materials that are TB-based and non-TB-based. This in conjunction with the ease of synthesis of TB1–2 should render them appealing choices as HTMs.

Deep blue-emissive bifunctional (hole-transporting + emissive) materials with CIEy ∼ 0.06 based on a ‘U’-shaped phenanthrene scaffold for application in organic light-emitting diodes

Bifunctional diamines (HTM + EM), namely, PTPA, PDPA and PCZL, have been designed based on a U-shaped phenanthrene scaffold and synthesized in a single step with good isolated yields. In particular, PTPA is shown to exhibit deep blue emission (CIEx,y ∼ 0.16, 0.06) with respectable efficiencies in a simple double layer device.

Benzophenone-imbedded benzoyltriptycene with high triplet energy for application as a universal host material in phosphorescent organic light-emitting diodes (PhOLEDs)

Benzoyltriptycene (TRP-BP) – accessed readily by Fridel–Crafts benzoylation reaction – represents a molecular system with an imbedded benzophenone chromophore. The concave features inherent to a triptycene scaffold impart amorphous properties, which is an important consideration in OLEDs. Due to very weak homoconjugation in TRP-BP, the high triplet energy of the parent benzophenone chromophore (ca. 2.99 eV) is conserved. This in conjunction with a high band gap (3.80 eV) permits application of TRP-BP as a universal host material for blue, green, yellow and red dopants. An external quantum efficiency of 19.8% and the brightness of 11 800 cd m−2 were elicited from the simple devices fabricated with yellow and green dopants, namely, PO-01 and Ir(ppy)3 respectively. Ready synthetic access and impressive device performance results make TRP-BP an important addition to the scantily explored host materials based on benzophenone, which has traditionally served as a prototype carbonyl compound in the development of organic photochemistry in general to delineate the triplet-excited state properties.

Tri- and tetraarylanthracenes with novel λ, χ and ψ topologies as blue-emissive and fluorescent host materials in organic light-emitting diodes (OLEDs)

Three unique λ-, χ- and ψ-shaped tri- and tetaarylanthracenes, i.e., ANT1–3, have been designed and synthesized by Suzuki coupling as a key reaction for application as emissive and host materials in OLEDs. The regioisomeric anthracenes ANT2 and ANT3 display intriguingly similar photophysical, electrochemical and thermal properties, while their lower analog, i.e., ANT1, exhibits different properties. All the three arylanthracenes show respectable thermal stabilities with high Tds in the range of 372–427 °C and moderate Tgs in the range of 90–114 °C. Their fluorescence quantum yields are in the range of 45–67%. Their utility in OLEDs has been demonstrated by fabrication of doped- and non-doped devices. In particular, ANT1 is shown to emit deep-blue light with moderate efficiencies and CIE coordinates that are very close to those demanded by the NTSC (National Television System Committee). It has also been shown to serve as a fluorescent host for sky-blue and green dopants, with the luminance being higher for the latter.

Twisted biaryl-amines as novel host materials for green-emissive phosphorescent organic light-emitting diodes (PhOLEDs)

Insertion of rigid biaryl moieties, namely, 2,2′,6,6′-tetramethylbiphenyl and 2,2′,6,6′-tetramethoxylbiphenyl, between two phenyl rings of CBP leads to novel host materials with improved thermal stabilities (Tgs ∼ 111–148 °C). Their high triplet energies (2.63–2.83 eV) permit application as host materials for green phosphors, i.e., Ir(ppy)3. External quantum efficiency and brightness of the order of ca. 15% and ca. 6000 cd m−2 have been realized with TetMeTPA and TetOMeCZL, respectively.

Carbo[5]helicene versus planar phenanthrene as a scaffold for organic materials in OLEDs: the electroluminescence of anthracene-functionalized emissive materials

Aesthetically enticing helical structures are inextricable systems in biology, and have rapidly begun to pervade several aspects of materials science. The entry of helicenes in electroluminescent materials is very recent. In the present study, we have designed and synthesized two twisted carbo[5]helicenes functionalized with fluorescent phenylanthryl moieties, namely, CHMANT and CHDANT, for their applications as blue-emissive materials in nondoped OLED devices. Their lower analogs, i.e., PMANT and PDANT, based on a planar phenanthrene scaffold were also designed and synthesized to contrast the effect of helical versus planar cores in the development of emissive materials based on phenylanthracenes. It is shown that the helicenes functionalized with phenylanthryl group(s) not only display better fluorescence properties and thermal stabilities than analogous phenanthrenes, but also serve as superior emissive materials as well as host materials in electroluminescent devices. The comparative results bring out the fact that helicity (carbo[5]helicene) as a design element fares much better than a planar scaffold (phenanthrene) in developing anthracene-based emissive materials for OLEDs. The brightness (9820 cd m−2) and power and luminous efficiencies (3.48 lm W−1 and 4.22 cd A−1) obtained from a nondoped device fabricated with CHDANT are the highest by quite some margin among those of all helical materials reported so far.