A new class of charge-neutral, bis-tridentate Ir(III) metal complexes is designed, synthesized and applied as emitters in the fabrication of organic light emitting diodes. Their basic properties are ...discussed vs. their tris-bidentate counterparts.
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•Relationship of the terpyridine complexes Ru(tpy)22+ and Ir(tpy)23+ are reviewed.•Dianionic chelates are constructed using pyrazole, pyridine and phenyl fragments.•N^C^N, C^N^C and C^C^C type ancillaries are employed to obtain monoanionic chelates.•Properties of the charge-neutral bis-tridentate Ir(III) complexes are discussed.•Bis-tridentate Ir(III) metal complexes are useful emitters for OLEDs.
This review is an update on current advances in metal complexes with d6-electronic configuration and bearing two tridentate chelates. We first elaborate the basic properties of the famous cationic complexes Ru(tpy)22+ and Ir(tpy)23+, where tpy represents 2,2′:6′,2″-terpyridine. The emphasis is then switched to various charge-neutral, bis-tridentate Ir(III) complexes, with emission spanning the whole visible region from blue, green to red. These Ir(III) metal complexes are capable of exhibiting high luminescence efficiency, reduced radiative lifetime, and adequate volatility and stability, similar to those of their traditional tris-bidentate Ir(III) counterparts, destined to be viable OLED phosphors.
A new series of molecules, T1–T4, possessing thermally activated delayed fluorescence (TADF) have been strategically designed and synthesized. Molecules T1–T4 contain the dimethyl acridine as the ...electron donor, which is linked to either symmetrical or unsymmetrical diphenyl pyrimidine as an acceptor. In comparison to the ubiquitous triazine acceptor, the selection of pyrimidine as an acceptor has advantages of facile functionalization and less stabilized unoccupied π orbitals, so that the energy gap toward the blue region can be accessed. Together with acridine donors, the resulting donor–acceptor functional materials reveal remarkable TADF properties. In the solid state, molecules T1–T4 all exhibit intriguing mechanochromism. The crystal structures, together with spectroscopy and dynamics acquired upon application of stressing, lead us to propose two types of structural arrangement that give distinct emission properties, one with and the other without TADF. Upon fabricating organic light‐emitting diodes, the T1–T4 films prepared from sublimation all exhibit dominant TADF behavior; this accounts for their high performance: an electroluminescent emission at λ=490 nm, with an external quantum efficiency of 14.2 %, can be attained when T2 is used as an emitter.
Functional pyrimidine acceptors: A new series of molecules, T1–T4, possessing thermally activated delayed fluorescence (TADF) have been strategically designed and synthesized. Together with acridine donors, the resulting donor– acceptor functional materials reveal remarkable TADF properties. In the solid state, molecules T1–T4 also exhibit intriguing mechanochromism (see figure).
The effect of electron withdrawing anchoring group on optoelectronic properties of pyrene derivatives and their interaction with TiO2 semiconductor nanoparticles is investigated.
•Optical properties ...of pyrene sensitizers were tuned by introducing electron withdrawing groups.•Bathochromic absorption was observed by introducing CN group in the pyrene sensitizers.•Carboxylate anchoring group having electron withdrawing CN group shows better binding with TiO2.•Electron injection from pyrene sensitizers to TiO2 was confirmed through laser flash photolysis.
A series of pyrene sensitizers (PC, PN, PMN, PR) with various electron withdrawing anchoring groups have been synthesized to understand their consequence on the optoelectronic properties. All the sensitizers have been characterized by NMR, mass spectroscopic and IR techniques. Absorption measurements revealed that charge transfer transition was enhanced by introducing electron withdrawing groups. Effect of solvents on the absorption and emission properties of the sensitizers was probed through multi-parameter Catalán solvent scales. Thermal stability of the sensitizers was found to be influenced by the presence of COOH group. DFT/TDDFT calculations were performed to gain insight into the structure and electronic properties of the pyrene sensitizers. To investigate the binding affinity of pyrene sensitizers with TiO2, absorption measurement was performed and the results suggest that the sensitizer having COOH group along with a CN group (PC) show higher binding affinity over other sensitizers containing COOH group (PR) and NO2 group (PN). Further, laser flash photolysis measurement was carried out to study the electron transfer process between the sensitizers and TiO2. Electron injection into the conduction band of TiO2 was confirmed by the detection of cation radical of the sensitizer. We envisage that the results from this work will pave the way to design new efficient sensitizers with predetermined electron withdrawing anchoring groups and their photophysical properties for photovoltaic applications.
Two D-π-A dyes, one with a strong electron withdrawing cyanoacrylic acid DPP 20 and one with a weak acceptor carboxylic acid DPP 21 have been synthesized and characterized for their application in ...dye-sensitized solar cells. This allows us to understand the role of electron withdrawing strength of the acceptor anchoring groups on the optoelectronic properties of chromophoric π spacer molecules, e.g., the diketopyrrolopyrrole (DPP) moiety. The low energy absorption maxima of DPP 20 is red-shifted by only 7 nm compared to DPP 21, implying a minimal role for the strong acceptor in the light harvesting properties of colored π spacer molecules. Because of this small red shift, J SC of DPP 20 is marginally higher than that of DPP 21. However, because of the higher fill factor of DPP 21, the overall power conversion efficiency (PCE = 7.65%) is higher than that of DPP20 (7.34%). The data reveal that a weak acceptor is adequate to achieve good light harvesting as well as provide good photovoltaic efficiencies for colored π spacer sensitizers.
Organic light‐emitting transistors (OLET) evolved from the fusion of the switching functionality of field‐effect transistors (FET) with the light‐emitting characteristics of organic light‐emitting ...diode (OLED) that can simplify the active‐matrix pixel device architecture and hence offer a promising pathway for future flat panel and flexible display technology. This review systematically analyzes the key device/molecular engineering tactics that assist in improving the electrode edge narrow emission to wide‐area emission for display applications via three different topics, that is, narrow to wide‐area emission, vertical architecture, and impact of high‐κ dielectric on the device performance. Source–drain electrode engineering such as symmetric/asymmetric, planar/non‐planar arrangement, semitransparent nature, multilayer approach comprising charge transport, and work function modification layers enable widening the emission zone. Vertical OLET architecture offers short channel lengths with a high aperture ratio, pixel type area emission, and stable light‐emitting area. Transistors utilizing high‐κ dielectric materials have assisted in lowering the operating voltage, enhancing luminance and air stability. The promising development in achieving wide‐area emission provides a solid basis for constructing OLET research toward display applications; however, it relies on developing highly luminescent and fast charge transporting materials, suitable semitransparent source/drain electrodes, high‐κ ‐dielectrics, and device architectural engineering.
The fusion of light‐emitting OLED technology with the switching functionality of OFET in a single device provokes a new research direction, organic light‐emitting transistors (OLET). This review summarizes the progress of OLET for their potential applications toward display technology via three promising tactics such as narrow to wide‐area emission, vertical architecture, and impact of high‐κ dielectrics.
Imidazole-based donor–acceptor materials are well known for their polarity-controlled trade-off phenomenon between the localized excitation-based short-wavelength (SW) emission in nonpolar solvents ...and charge transfer dominated long-wavelength (LW) emission in polar solvents. To attain concurrent SW- and LW-based dual-emission characteristics, a series of imidazole-based donor–acceptor fluorophores (CBImDCN, TPImDCN, PZImDCN) possessing different electron-donating groups such as carbazole, triphenylamine, and phenothiazine linked via the N-position of the imidazole core unit were synthesized and verified by NMR and mass spectroscopic techniques. As a result, the strong donating TPImDCN and PZImDCN exhibited dual emission in different solvents of varying polarity, covering the blue (SW) and green/orange (LW) regions. On the other hand, in contrast, only an SW emission band is observed with the weak donating CBImDCN. Moreover, PZImDCN shows panchromatic emission under 365 nm illumination, while only orange color emission is observed under visible light excitation, revealing two different origins of SW and LW emissions, as also evidenced from DFT calculations. Overall, this work reveals a new approach for attaining concurrent SW and LW emission characteristics from imidazole-based D–A materials and sheds light on the design and development of novel panchromatic emitters with intriguing properties for lighting and display applications.
We report two new molecularly engineered push–pull dyes, i.e., YA421 and YA422, based on substituted quinoxaline as a π-conjugating linker and bulky-indoline moiety as donor and compared with ...reported IQ4 dye. Benefitting from increased steric hindrance with the introduction of bis(2,4-dihexyloxy)benzene substitution on the quinoxaline, the electron recombination between redox electrolyte and the TiO2 surface is reduced, especially in redox electrolyte employing Co(II/III) complexes as redox shuttles. It was found that the open circuit photovoltages of IQ4, YA421, and YA422 devices with cobalt-based electrolyte are higher than those with iodide/triiodide electrolyte by 34, 62, and 135 mV, respectively. Moreover, the cells employing graphene nanoplatelets on top of gold spattered film as a counter electrode (CE) show lower charge-transfer resistance compared to platinum as a CE. Consequently, YA422 devices deliver the best power conversion efficiency due to higher fill factor, reaching 10.65% at AM 1.5 simulated sunlight. Electrochemical impedance spectroscopy and transient absorption spectroscopy analysis were performed to understand the electrolyte influence on the device performances with different counter electrode materials and donor structures of donor−π–acceptor dyes. Laser flash photolysis experiments indicate that even though the dye regeneration of YA422 is slower than that of the other two dyes, the slower back electron transfer of YA422 contributes to the higher device performance.
The short wavelength (SW) emission region of the materials employing strong donor (DS) phenothiazine (PTZ) is generally correlated to its quasi‐axial (QAPTZ) conformer arrangement. This propels to ...investigate the specific role of weak donors (DW) such as carbazole (CZ) in determining the emission characteristics of molecular heredity (MH) type structural design encompassing dual donors of varying strength. For this purpose, a common DS(PTZ)–Acceptor unit is linked to the DW(CZ) unit via different linkage positions to form M1, M2, and M3, which not only alters the absorption and dual emission characteristics but also influences the delayed fluorescence phenomenon. The SW emission originates by minimizing the aggregation factor rather than by literature envisaged QAPTZ conformer. Incorporating M1 as a single molecular white light emitter (SMWLE) in near UV light emitting diode (LED) devices results in cold white light emission with CRI of 68 and CIE of (0.24, 0.33), while its corresponding solution‐processed organic light‐emitting diode devices delivers warm white electroluminescence with an EQE of 5.8%. Overall, this work demonstrates the significant contribution of the DW in attaining the white light emission characteristics of MH molecular design not only at the molecular level but also in practical light‐emitting devices.
Systematic incorporation of a weak donor unit to form molecular heredity‐type DS‐A‐DW materials can achieve single molecular white light‐emitting material not only at the molecular level but also in light‐emitting devices.
Abstract The short wavelength (SW) emission region of the materials employing strong donor (D S ) phenothiazine (PTZ) is generally correlated to its quasi‐axial (QA PTZ ) conformer arrangement. This ...propels to investigate the specific role of weak donors (D W ) such as carbazole (CZ) in determining the emission characteristics of molecular heredity (MH) type structural design encompassing dual donors of varying strength. For this purpose, a common D S (PTZ)–Acceptor unit is linked to the D W (CZ) unit via different linkage positions to form M1, M2, and M3, which not only alters the absorption and dual emission characteristics but also influences the delayed fluorescence phenomenon. The SW emission originates by minimizing the aggregation factor rather than by literature envisaged QA PTZ conformer. Incorporating M1 as a single molecular white light emitter (SMWLE) in near UV light emitting diode (LED) devices results in cold white light emission with CRI of 68 and CIE of (0.24, 0.33), while its corresponding solution‐processed organic light‐emitting diode devices delivers warm white electroluminescence with an EQE of 5.8%. Overall, this work demonstrates the significant contribution of the D W in attaining the white light emission characteristics of MH molecular design not only at the molecular level but also in practical light‐emitting devices.
Four 2-oxo-1,2-dihydroquinoline-3-carbaldehyde N-substituted thiosemicarbazone ligands (H(2)-OQtsc-R, where R = H, Me, Et or Ph) and their corresponding new copper(II) complexes ...CuCl(2)(H(2)-OQtsc-H)·2H(2)O (1), CuCl(2)(H(2)-OQtsc-Me)·2H(2)O (2), CuCl(2)(H(2)-OQtsc-Et)(CH(3)OH)Cl (3) and CuCl(H-OQtsc-Ph)·CH(3)OH (4) have been synthesized in order to correlate the effect of terminal N-substitution on coordination behaviour, structure and biological activity. Single crystal X-ray diffraction studies revealed that the complexes 1, 2 and 3 have square pyramidal geometry around the central metal ion. In the complexes 1 and 2, the copper ion is coordinated by the ligand with ONS donor atoms, one chloride ion in apical position and the other chloride in the basal plane. Complex 3 consists of CuCl(2)(H(2)-OQtsc-Et)(CH(3)OH)(+) cation and a chloride as counter ion. The copper ion is coordinated by the ligand with ONS donor atoms and by one chloride ion in the basal plane. One methanol molecule is bonded through its neutral oxygen in the apical position. Complex 4 is square planar with the ligand coordinating through uni-negative tridentate ONS(-) and by one chloride ion in the basal plane. The binding of complexes with lysozyme protein was carried out by fluorescence spectroscopy. Investigations of antioxidation properties showed that all the copper(II) complexes have strong radical scavenging properties. The cytotoxicity of the complexes 3 and 4 against NIH 3T3 and HeLa cell lines showed that synergy between the metal and ligands results in a significant enhancement in the cell death with IC(50) of ~10-40 μM. A size dependence of substitution at terminal N in the thiosemicarbazones on the biological activities of the complexes has been observed.
