Chiho Lee

High-performance bipolar host materials for blue TADF devices with excellent external quantum efficiencies

New bipolar host molecules composed of carbazole, pyridoindole, and dibenzothiophene (DBT) were synthesized for blue thermally activated delayed fluorescence (TADF)-based organic light-emitting diodes (OLEDs). 2,8-Di(9H-carbazol-9-yl)dibenzo[b,d]thiophene, 9-(8-(9H-carbazol-9-yl)dibenzo[b,d]thiophen-2-yl)-9H-pyrido[2,3-b]indole, and 2,8-bis(9H-pyrido[2,3-b]indol-9-yl)dibenzo[b,d]thiophene were prepared based on the selective reactivity at the 2,8-positions of DBT. The new symmetric and asymmetric host materials exhibited high triplet energies (2.89–2.95 eV). 4,5-Di(9H-carbazol-9-yl)phthalonitrile (2CzPN) was selected as an emitting dopant for achieving sky-blue emissions in TADF-OLEDs. 2CzPN-doped TADF-OLEDs, whose configuration is ITO (50 nm)/HATCN (7 nm)/TAPC (75 nm)/host:6% 2CzPN (20 nm)/TmPyPB (50 nm)/LiF (15 nm)/Al (100 nm), showed low driving voltages and high external quantum efficiencies (EQEs). These results are attributed to the well-controlled bipolar character of the host giving a better charge balance in the emitting layer. In particular, the device containing ZDN:6% 2CzPN showed an unprecedentedly high EQE of 25.7% (at 0.074 mA cm−2).

Solution-processed thermally activated delayed fluorescence organic light-emitting diodes using a new polymeric emitter containing non-conjugated cyclohexane units

The importance of thermally activated delayed fluorescence (TADF) materials in organic light-emitting diode (OLED) applications continues to grow as a consequence of their unique properties and excellent performance. Herein, a new green-emitting TADF polymer, P(DMTRZ-Cp), for solution-processable OLEDs was designed and synthesized based on the structure of DMAC-TRZ that shows typical TADF characteristics as a small molecule. The 1,1-diphenylcyclohexane (Cp) moiety introduced into the structure of the polymeric emitter not only acts as a linker between the conjugated monomeric units, but also helps enhancing solubility while disconnecting conjugation along the polymer backbones. P(DMTRZ-Cp) exhibits obvious TADF features such as a small energy gap (0.023 eV) between its lowest singlet and triplet excited states, and obvious delayed photoluminescence (PL) decay behavior. Moreover, this new polymeric emitter exhibits high PL quantum yield over 90% in the film state. By applying solution-processed TADF-OLED devices, P(DMTRZ-Cp) exhibited the maximum external quantum efficiency of up to 15.4% with green emission. As far as we know, this is the first report to introduce a non-conjugated linker, Cp, in the main-chain type polymeric emitter structure that presents TADF characteristics. The Cp linker is considered to be a promising moiety for the development of solution-processable polymeric emitters with high efficiencies.

Vibrational probing of the hydrogen-bond structure and dynamics of water in aqueous NaPF6 solutions

Direct measurements of the individual dynamics of water in bulk and ionic hydration shells in aqueous ionic solutions are quite experimentally challenging because the different subsets of water in bulk and ionic hydration shells are not spectrally well-resolved in most aqueous ionic solutions. In contrast, the different subsets of water in the bulk, cationic hydration shell, and anionic hydration shell in aqueous NaPF6 solutions were found to be spectrally well distinct. Such spectral features allowed us to study the individual dynamics of the different subsets of water in aqueous NaPF6 solutions. In this work, we studied the hydrogen-bond (H-bond) structure and dynamics of water in aqueous NaPF6 solutions at different NaPF6 concentrations by FTIR, Raman, and IR pump–probe spectroscopy. Three different subsets of water in the bulk, cationic hydration shell, and anionic hydration shell were found to have their own characteristic hydroxyl stretch peaks (eigen spectra) in FTIR and Raman spectra and have unique vibrational lifetimes independent of NaPF6 concentration. However, the orientational relaxation dynamics, r(t), were not able to be separately measured for three different subsets of water. The overall orientational relaxation times were found to be linearly dependent on the solution viscosity and were reasonably well described by the Debye–Stokes–Einstein equation. Finally, the frequency-dependent transition dipole moments of the hydroxyl (–OD) stretch vibration obtained in neat water and aqueous 3.0 M NaPF6 solution were compared and found to be dependent on the nature of H-bonds.

Unconventional Three-Armed Luminogens Exhibiting Both Aggregation-Induced Emission and Thermally Activated Delayed Fluorescence Resulting in High-Performing Solution-Processed Organic Light-Emitting Diodes

In this work, three-armed luminogens IAcTr-out and IAcTr-in were synthesized and used as emitters bearing triazine and indenoacridine moieties in thermally activated delayed fluorescence organic light-emitting diodes (OLEDs). These molecules could form a uniform thin film via the solution process and also allowed the subsequent deposition of an electron transporting layer either by vacuum deposition or by an all-solution coating method. Intriguingly, the new luminogens displayed aggregation-induced emission (AIE), which is a unique photophysical phenomenon. As a nondoped emitting layer (EML), IAcTr-in showed external quantum efficiencies (EQEs) of 11.8% for the hybrid-solution processed OLED and 10.9% for the all-solution processed OLED with a low efficiency roll-off. This was evident by the higher photoluminescence quantum yield and higher rate constant of reverse intersystem crossing of IAcTr-in, as compared to IAcTr-out. These AIE luminogens were used as dopants and mixed with the well-known host material 1,3-bis( N-carbazolyl)benzene (mCP) to produce a high-efficiency OLED with a two-component EML. The maximum EQE of 17.5% was obtained when using EML with IAcTr-out doping (25 wt %) into mCP, and the OLED with EML bearing IAcTr-in and mCP showed a higher maximum EQE of 18.4% as in the case of the nondoped EML-based device.

Fluorescent Labeling of Protein Using Blue-Emitting 8-Amino-BODIPY Derivatives

Abstract8-Amino-BODIPY (boron-dipyrromethane) dyes show bright blue fluorescence. Disclosed here are synthesis and characterization of the photophysical properties of a series of functionalized 8-Amino-BODIPY (BP1–4) for protein labeling. The compact structure and solvent-insensitive absorption property of the dye are desirable features for protein labeling. For the model protein, bovine serum albumin (BSA), the labeling proceeds under mild condition via amide bond formation or thiol-ene conjugation with maintaining the bright blue fluorescence. The chromatography and mass spectroscopy analysis clearly support the labeling of the BODIPY dye on the BSA. The protein labeling with blue-emitting BODIPY would be applicable for studying protein dynamics and fluorescence resonance energy transfer (FRET) with intrinsic biomolecules.

Selective Recognition of Fluoride by using a Benzobisimidazolium Derivative through Aggregation‐Induced Fluorescence

Abstract A new benzobisimidazolium derivative (1) bearing four naphthalene moieties was synthesized and demonstrated as an F− ion‐selective fluorescent chemosensor. The fluorescence of 1 in acetonitrile (CH3CN) is significantly stronger with F− and acetate (CH3CO2 −), but not with other anions (Cl−, Br−, I−, HSO4 −, and H2PO4 −). The fluorescence of 1 is enhanced selectively with F− in the presence of a small amount of water. Our DFT calculations indicate that the electrostatic interactions between the positively charged benzobisimidazolium moieties and F− play an important role in the formation of stable aggregates. The formation of stable aggregates of 1 with F− in CH3CN is a key step for the selective sensing of F−, and the fluorescence of the aggregates is further enhanced in a mixture of 95 % CH3CN and 5 % water, which can be attributed to the aggregation‐induced emission.

Effect of Hydrogen Bonds on the Vibrational Relaxation and Orientational Relaxation Dynamics of HN3 and N3(-) in Solutions.

Hydrogen bonds (H-bonds) play an important role in determining the structures and dynamics of molecular systems. In this work, we investigated the effect of H-bonds on the vibrational population relaxation and orientational relaxation dynamics of HN3 and N3(-) in methanol (CH3OH) and N,N-dimethyl sulfoxide (DMSO) using polarization-controlled infrared pump-probe spectroscopy and quantum chemical calculations. Our detailed analysis of experimental and computational results reveals that both vibrational population relaxation and orientational relaxation dynamics of HN3 and N3(-) in CH3OH and DMSO are substantially dependent on the strength of the H-bonds between the probing solute and its surrounding solvent. Especially in the case of N3(-) in CH3OH, the vibrational population relaxation of N3(-) is found to occur by a direct intermolecular vibrational energy transfer to CH3OH due to large vibrational coupling strength. The orientational relaxation dynamics of HN3 and N3(-), which are well fit by a biexponential function, are analyzed by the wobbling-in-a-cone model and extended Debye-Stokes-Einstein equation. Depending on the intermolecular interactions, the slow overall orientational relaxation occurs under slip, stick, and superstick boundary conditions. For HN3 and N3(-) in CH3OH and DMSO, the vibrational population relaxation becomes faster but the orientational relaxation becomes slower as the H-bond strength is increased. Our current results imply that H-bonds have significant effects on the vibrational population relaxation and orientational relaxation dynamics of a small solute whose size is comparable to the size of the solvent.