Like with a string of pearls, four molecular "beads" are threaded on a molecular rectangle to form a molecular necklace. This rectangular species is synthesized from two L-shaped, preorganized pseudorotaxanes with two molecular beads each (cucurbituril, schematically symbolized by the barrels), held together by Cu ions [Eq. (1)].
The fluorescence emission properties of 2-(2'-hydroxy-4'-R-phenyl)benzothiazole (HBT-R) nanoparticles with different substituents (R = -COOH, -H, -CH(3), -OH, and -OCH(3)) were investigated using spectroscopic and theoretical methods. HBT-Rs displayed dual enol and keto (excited-state intramolecular proton transfer (ESIPT)) emissions in nonpolar solvents. The spectral change of their ESIPT emissions was affected differently by the electron donating (or withdrawing) power of the substituents; a bathochromic shift for the electron donating group and a hypsochromic shift in electron withdrawing group. In addition, the changes in energy levels calculated by the ab initio method were consistent with the spectral shifts of HBT-R in solution. We prepared aggregated HBT-R nanoparticles using a simple reprecipitation process in tetrahydrofuran-water solvents. The ESIPT emission of aggregated HBT-R nanoparticles was strongly enhanced (over 45 times) compared to those of monomer HBT-Rs in toluene, as markedly shifted ESIPT emissions are observed at longer wavelength without any quenching by self-absorption. Aggregated HBT-R nanoparticles showed longer lifetimes than those of monomer molecules. The temperature effect on the aqueous dispersion of the aggregated HBT-R nanoparticles was also explored. It shows a fluorescent ratiometric change in a range of temperature from 7 to 65 degrees C. A mechanism of a temperature-dependent equilibrium between the nanoparticles and the solvated enols is proposed for the emission color change.
New Zn(II)‐chelated complexes based on benzothiazole derivatives, including substituted functional groups such as methyl (MeZn), methoxy (MeOZn), or fluorenyl unit (FuZn), are investigated to produce white‐light emission. 2‐(2‐Hydroxyphenyl)benzothiazole derivatives in toluene and DMSO exhibit excited‐state intramolecular proton transfer (ESIPT), leading to a large Stokes shift of the fluorescence emission. However, in methanol they exhibit no ESIPT due to the intermolecular hydrogen bonding between the 2‐(2‐hydroxyphenyl)benzothiazole derivative and methanol. Their Zn(II)‐chelated complexes exhibit the absorption band red‐shifted at 500 nm in nonpolar solvent and the absorption band blue‐shifted at about 420 nm in protic solvent. In multilayer electroluminescent devices, methyl‐substituted Zn(II)‐chelated complex (MeZn) exhibits excellent power efficiency and fluorene‐substituted Zn(II)‐chelated complex (FuZn) has a high luminance efficiency (1 cd m−2 at 3.5 V, 10 400 cd m−2 at 14 V). The EL spectra of Zn(II)‐chelated complexes based on benzothiazole derivatives exhibit broad emission bands. In addition, their electron‐transport property for red–green–blue (RGB) organic light‐emitting diodes (OLEDs) is systematically studied, in comparison with that of Alq3. The results demonstrate the promising potential of MeZn as an electron‐transporting layer (ETL) material in preference to Alq3, which is widely used as an ETL material.
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