Star-shaped multifunctional organic emitters based on N-(2-cyanophenyl) carbazole frameworks: Effects of steric hindrance fluorene and heavy-atom bromine.

To investigate the effects of steric hindrance fluorene and heavy-atom bromine on the general optoelectronic properties of star-shaped organic emitters based on 9-(2-cyanophenyl) carbazole (OCzPhCN) frameworks, heavy element of bromine and steric hindrance fluorene were introduced into OCzPhCN to produce four derivatives of 2-(3-bromo-9H-carbazol-9-yl)benzonitrile (BrCzPhCN), 2-(3-bromo-6-(9-(4-ethoxyphenyl)-9H-fluoren-9-yl)-9H-carbazol-9-yl)benzonitrile (BrFCzPhCN), 2-(3-(9-(4-ethoxyphenyl)-9H-fluoren-9-yl)-9H-carbazol-9-yl)benzonitrile (FCzPhCN) and 2-(3,6-bis(9-(4-ethoxyphenyl)-9H-fluoren-9-yl)-9H-carbazol-9-yl)benzonitrile (2FCzPhCN). The fluorene units obviously improve the thermal stability of the obtained compounds, and 2FCzPhCN has the highest thermal stability with 5 % mass heat loss temperature reaching 447 °C. In different polar solvents, the absorption peaks wavelength of OCzPhCN, FCzPhCN and 2FCzPhCN are basically unchanged, and the redshifted emission peaks are positively correlated with solvent polarity. The photoluminescence quantum yields (PLQYs) of OCzPhCN, BrCzPhCN, FCzPhCN, BrFCzPhCN and 2FCzPhCN powders were 20.17 %, 5.43 %, 30.75 %, 3.27 % and 23.56 %. The fluorescence and phosphorescent quantum efficiencies of OCzPhCN, BrCzPhCN, FCzPhCN, BrFCzPhCN and 2FCzPhCN powders are 9.76 % and 10.41 %, 1.2 % and 3.23 %, 28.45 % and 2.3 %, 3.27 % and 0 %, 23.56 % and 0 %. OCzPhCN, BrCzPhCN and FCzPhCN powders show obvious room temperature phosphorescent emission, and the phosphorescent emission lifetime of OCzPhCN, BrCzPhCN and FCzPhCN powders at 561 nm, 576 nm and 568 nm are 193.17 ms, 18.65 ms and 7.25 ms. Compared with OCzPhCN, the introduction of bromine decreases the PLQY and the phosphorescent lifetime of BrCzPhCN powder, while the fluorescence quantum efficiency of the compound FCzPhCN powder has been improved. The corresponding single-triplet energy splitting (ΔEST) of OCzPhCN, FCzPhCN and 2FCzPhCN in solutions are 0.49 eV, 0.63 eV and 0.63 eV, and the corresponding ΔEST values of OCzPhCN, BrCzPhCN FCzPhCN powders are 1.19 eV, 0.74 eV and 0.55 eV. The steric hindrance fluorene units result in smaller and stabilized ΔEST in the solid powder states, and the same situation is opposite in the unimolecular solutions. The maximum external quantum efficiency of organic light-emitting diode based on 10,10'-(4,4'-sulfonylbis (4,1-phenylene)) bis (9,9-dimethyl-9,10-dihydroacridine) hosted by OCzPhCN reaches 12.7 %, and the external quantum efficiency at 100 cd/m2 rolls down to 11 %. OCzPhCN is the best emitters in terms of room temperature phosphorescent emission and host applications.

Unique Organic Crystal Skeleton Based on N-Phenyl-Carbazole Derivatives with Adjustable Room Temperature Phosphorescence and Highly Stable Inclusion Ability to Dichloromethane.

Monosubstituted 9-(2-bromophenyl)-carbazole (1Br1CZ) and disubstituted 9,9'-(2,4-dibromo-1,3-phenylene) bis(9H-carbazole) (2Br2CZ) are synthesized by introducing bromine into ortho-phenyl position of 9-phenyl-carbazole (PhCZ). The decomposition temperature with 5% mass loss and melting point of 2Br2CZ crystal are 360 and 230 °C. The highest occupied molecular orbital energy level of PhCZ is the highest, and that of 2Br2CZ is the lowest. The crystals of PhCZ, 1Br1CZ, and 2Br2CZ are monoclinic, orthorhombic, and triclinic system, which exhibit room temperature phosphorescence with lifetimes of 171.81, 37.15, and 28.77 ms, and their corresponding phosphorescence quantum yields are 0.83%, 0.16%, and 4.58%. It theoretically reveals that six triplet energy levels (T1-T6) exist under S1 in 2Br2CZ crystal, and the spin orbit coupling constants between S1 and Tn in 2Br2CZ are also greater than those in PhCZ and 1Br1CZ, which promotes the intersystem crossing. Meanwhile, through crystal structure and Hirshfeld surface analysis, the torsion angles between the carbazole unit of 2Br2CZ and the central phenyl group reached 85.28°. The 2Br2CZ crystal exhibits the richest intermolecular interactions. A cavity of 4.498 Å is formed within the crystal skeleton of 2Br2CZ, which can precisely fixe dichloromethane with a record-high desorption temperature over 145 °C.

Three-year clinical observation of the outcomes of transepithelial and epithelial-off accelerated corneal collagen crosslinking treatment for different types of progressive keratoconus.

In the present study, the clinical and long-term effects of accelerated transepithelial corneal collagen crosslinking (ATE-CXL) and accelerated epithelial-off corneal collagen crosslinking (A-CXL) for the treatment of different types of progressive keratoconus were compared. A total of 70 patients, including 96 eyes with advanced keratoconus, were enrolled in the study. ATE-CXL or A-CXL was performed on one or two eyes of each subject according to corneal thickness, keratoconus type and surgical approach. Patients were divided into the following four groups: Group A, ATE-CXL for central keratoconus; group B, A-CXL for central keratoconus; group C, ATE-CXL for peripheral keratoconus; and group D, A-CXL for peripheral keratoconus. Uncorrected distant visual acuity (UDVA), best-corrected distant (BD)VA and corneal astigmatism (CA) were evaluated in all patients by routine ophthalmology pre-operatively and 3 years post-operatively. Topographical features, including maximum corneal curvature (Kmax), thinnest corneal thickness (TCT), anterior corneal elevation (ACE) and corneal endothelial cell density (ECD) were also compared across groups. The results suggested that pre- and post-operative UDVA, BDVA, Kmax, CA and ACE values differed in all four groups (P<0.05), whereas no differences were observed between pre- and post-operative TCT and ECD (P>0.05). Concordant results were obtained between groups A and C and groups B and D. ATE-CXL achieved better control of central keratoconus UDVA, Kmax and CA as compared with A-CXL. The difference between pre- and post-operative UDVA, Kmax and CA as compared with A-CXL was highly correlated with the change in intraocular pressure and treatment effectiveness. There was a statistically significant improvement in BDVA with ATE-CXL for treatment of central keratoconus compared with that after A-CXL treatment (P=0.032). There were statistically significant improvements in BDVA (P=0.047), CA (P=0.045) and ACE (P=0.012) with A-CXL treatment of peripheral keratoconus when compared with ATE-CXL treatment. Central, and to a lesser extent, peripheral, keratoconus may be effectively controlled by either approach, with disease stabilization 3 years later. ATE-CXL is suggested to be the most suitable treatment for keratoconus of <400 µm with a corneal thickness of >400 µm; however, A-CXL yields superior long-term outcomes.

Chemical preparation of graphene-based nanomaterials and their applications in chemical and biological sensors.

Graphene is a flat monolayer of carbon atoms packed tightly into a 2D honeycomb lattice that shows many intriguing properties meeting the key requirements for the implementation of highly excellent sensors, and all kinds of proof-of-concept sensors have been devised. To realize the potential sensor applications, the key is to synthesize graphene in a controlled way to achieve enhanced solution-processing capabilities, and at the same time to maintain or even improve the intrinsic properties of graphene. Several production techniques for graphene-based nanomaterials have been developed, ranging from the mechanical cleavage and chemical exfoliation of high-quality graphene to direct growth onto different substrates and the chemical routes using graphite oxide as a precusor to the newly developed bottom-up approach at the molecular level. The current review critically explores the recent progress on the chemical preparation of graphene-based nanomaterials and their applications in sensors.

Tuning the optoelectronic properties of 4,4'-N,N'-dicarbazole-biphenyl through heteroatom linkage: new host materials for phosphorescent organic light-emitting diodes.

Five carbazole end-capped heterofluorenes (CzHFs) designed by structurally mimicking 4,4'-N,N'-dicarbazole-biphenyl (CBP) via connecting the biphenyl core of CBP with the linking atom of C, P, N, O, and S, respectively, were synthesized successfully, and their optoelectronic properties were investigated. The theoretical calculations and experimental results demonstrate that CzHFs are potential green, red, and even blue hosts for phosphorescent light-emitting diodes (PHOLEDs) with more desirable localization and energy levels of HOMO and LUMO and also higher triplet energy than CBP.

Organic ambipolar conjugated molecules for electronics: synthesis and structure-property relationships.

The field of organic electronics has been developed vastly in the past two decades, and the performance and lifetime of these devices are critically dependent on the materials development, device design, deposition processes, and modeling, among which the active materials of organic semiconductor play a crucial role. The unique properties of organic semiconductor are largely based on the versatility to synthesize multifunctional organic conjugated materials by judicious molecular design. To effectively adjust the optoelectronic properties, especially energy levels, of organic semiconductor, the scientists have presented a synthesis methodology of organic ambipolar conjugated molecules, in which typical p-dope type and n-dope type segments are incorporated into one molecule. The present review summarizes the progress on organic ambipolar conjugated molecules for electronics in the past few years. Some issues to be addressed are also highlighted and discussed.