Qunbo Mei

Theoretical Studies of Electron Transport in Thiophene Dimer: Effects of Substituent Group and Heteroatom

The electron-transport properties of various substituted molecules based on the thiol-ended thiophene dimer (2Th1DT) are investigated through density functional theory (DFT) combined with nonequilibrium Greens function (NEGF) method. The current-voltage (I-V) curves of all the Au/2Th1DT/Au systems in this work display similar steplike features, while their equilibrium conductances show a large difference and some of these I-V curves are asymmetric distinctly. The results reveal the dependence of conductance on the energy level of the substituted 2Th1DT molecules. Rectification ratios are computed to examine the asymmetric properties of the I-V curves. The rectifying behavior in the 2Th1DT molecule containing the amino group close to the molecular end is more prominent than that in the other molecules. The rectifying behavior is analyzed through transmission spectra and molecular projected self-consistent Hamiltonian (MPSH) states. Slight negative differential resistance (NDR) can be observed in some of the systems. The electron-transport properties of 2Th1DT molecules containing different heteroatoms are also investigated. The results indicate that the current in heteroatom-containing molecules is larger than that in their pristine analogues, and lighter heteroatoms are more favorable than heavier heteroatoms for electron transport of the thiophene dimer.

A highly selective and naked-eye sensor for Hg2+ based on quinazoline-4(3H)-thione

A highly selective colorimetric and fluorescent sensor for Hg2+, TPS (TPS = 2-(4-(diphenyl-amino)phenyl)quinazoline-4(3H)-thione), based on a 2-substituted quinazoline-4(3H)-thione derivative, was designed and characterized in this paper. TPS displays relatively strong fluorescence centered at about 468 nm. When mixing with Hg2+, TPS interacts with Hg2+ in a 2 : 1(TPS-Hg2+) stoichiometry via a coordination bond interaction between the sulfur atom and Hg2+ with an association constant of 4.17 × 108 M−2 (R2 = 0.96). Upon addition of Hg2+, TPS showed a remarkable decrease in fluorescence spectra (468 nm) with a clear color change from yellow to red, so TPS could serve as a naked-eye sensor for Hg2+. The sensor allow determination of Hg2+ in the working range 2.0 μM–12.0 μM with a detection limit of 1.5 μM. Upon addition of KI to the solution of TPS-Hg2+ species, the spectra can be restored to the original spectrum in both absorption and fluorescence and indicates that this sensor could be reused. The sensor exhibited excellent reproducibility, reversibility and selectivity.

First-Principles Study of Rectification in Bis-2-(5-ethynylthienyl)ethyne Molecular Junctions

Using density functional theory (DFT) combined with the first-principles nonequilibrium Greens function (NEGF), we investigated the electron-transport properties and rectifying behaviors of several molecular junctions based on the bis-2-(5-ethynylthienyl)ethyne (BETE) molecule. To examine the roles of different rectification factors, asymmetric electrode-molecule contacts and donor-acceptor substituent groups were introduced into the BETE-based molecular junction. The asymmetric current-voltage characteristics were obtained for the molecular junctions containing asymmetric contacts and donor-acceptor groups. In our models, the computed rectification ratios show that the mode of electrode-molecule contacts plays a crucial role in rectification and that the rectifying effect is not enhanced significantly by introducing the additional donor-acceptor components for the molecular rectifier with asymmetric electrode-molecule contacts. The current-voltage characteristics and rectifying behaviors are discussed in terms of transmission spectra, molecular projected self-consistent Hamiltonian (MPSH) states, and energy levels of MPSH states.

Effect of pH on the photophysical properties of two new carboxylic-substituted iridium(III) complexes.

Two cyclometalated iridium(III) complexes have been prepared based on 2-(4-diphenylamino-phenyl)-quinoline and incorporating carboxylic acid ethyl ester (–COOC(2)H(5), (TPAQCE)(2)Irpic and carboxylic acid (–COOH, (TPAQCOOH)(2)Irpic) substituents at the 4-position of the quinoline ligand, respectively. The absorption, emission and (1)H NMR spectra of (TPAQCE)(2)Irpic and (TPAQCOOH)(2)Irpic under alkaline or acidic conditions demonstrate that they respond to the pH of the surrounding solvent environment. The deprotonation of the carboxylic acid group significantly blue-shifts the metal-to-ligand charge transfer absorption band of (TPAQCOOH)(2)Irpic by 48 nm and enhances the emission quantum-yield in DMSO. In addition, (1)H-NMR titration reveals that (TPAQCOOH)(2)Irpic is deprotonated into negatively charged (TPAQCOO(−))(2)Irpic in free DMSO-d(6) solution, and the acid-induced N^O ancillary ligands cleavage or replacement in (TPAQCOOH)(2)Irpic could be ignored. A water-soluble near-neutral optical pH probe (TPAQCOOH)(2)Irpic with pK(a) of ~7 is also reported. In aqueous buffer, (TPAQCOOH)(2)Irpic possesses an obvious emission response with an excellent linearity in the pH range of 6.50–8.00, showing a promising application in bioprocessing.

Synthesis, Crystal Structure and Luminescence Properties of a Cyclometalated Ir(III) Complex of 3,4-Diphenylcinnoline

An iridium(III) complex, [(dpci-H)2Ir(dafo)](PF6) (dpci-H = deprotonated 3,4-diphenylcinnoline, dpci; dafo = 4,5-diazafluoren-9-one), was synthesized from the reaction of the iridium complex [Ir(dpci-H)2(Cl)]2 and dafo in methanol, and characterized by single-crystal X-ray diffraction along with FT-IR, UV/Vis, 1H NMR and mass spectroscopy. The luminescence properties and the decay of the cyclometalated iridium(III) complex were also investigated. Excitation at the absorption wavelength (469 nm) resulted in a strong emission band centered at 591 nm with a lifetime of 0.9 μs. Graphical Abstract Synthesis, Crystal Structure and Luminescence Properties of a Cyclometalated Ir(III) Complex of 3,4-Diphenylcinnoline

Novel phosphorescent iridium(III) complexes containing 2-thienyl quinazoline ligands: synthesis, photophysical properties and theoretical calculations

The easy tailoring of organic ligands of iridium(III) complexes provides a facile way to tune their opto-electronic properties for applications in high efficiency phosphorescent light emitting diodes. Herein, a series of yellow and red emitting phosphorescent iridium complexes based on 2-thienyl quinazoline derivatives are successfully synthesized and systematically characterized with various opto-electronic properties. The X-ray crystal structures demonstrate that the iridium centers in the complexes with bulky substituents on the 4-position of quinazolyl rings prefer to chelate with the N atoms in the 1-position of quinazolyl rings. Both experiment and theoretical studies indicate that the steric hindrance along with the electron-donating effect of substituents on the C^N ligands enhances the emission quantum yields, accompanied by significant emission shifts. Two yellow phosphorescent iridium complexes (Ir2 and Ir3) are successfully designed and exhibit moderate emission efficiencies, through the incorporation of bulky ligands with strong electron-donating abilities (piperidine for Ir2 and 2,6-dimethyl-phenoxy for Ir3, respectively). The synergistic effect of electron structure and hindrance of ligand is believed to be a promising strategy for tuning the opto-electronic properties of iridium complexes.

Novel Cyclometalated Iridium(III) Xanthate Complexes and Their Phosphorescence Behavior in the Presence of Metal Ions

Two cyclometalated iridium(III) complexes, Ir(dpppz)(ppz)(ipx) and Ir(ppz)2(ipx) (dpppzH=1- (2,6-dimethylphenoxy)-4-phenylphthalazine, ppzH=4-phenylphthalazinone, ipx=isopropyl xanthate), have been synthesized and characterized by single-crystal X-ray diffraction. The photophysical properties of the two complexes were also investigated. Ir(dpppz)(ppz)(ipx) shows orange-red emission at around 606 nm with a phosphorescence quantum yield of ca. 0.0032 and an emission lifetime of 188 ns, while Ir(ppz)2(ipx) shows orange-red emission at around 599 nm with a phosphorescence quantum yield of ca. 0.00318 and an emission lifetime of 259 ns. The phosphorescence behavior of Ir(ppz)2(ipx) towards different metal cations has also be studied. Its strong phosphorescence is quenched by Hg2+, Cu2+ and Ag+ cations. The interaction ratio with Hg2+ is 1:1 Graphical Abstract Novel Cyclometalated Iridium(III) Xanthate Complexes and Their Phosphorescence Behavior in the Presence of Metal Ions

Tuning the Photophysical Properties of Cyclometalated Ir(III) Complexes by a Trifluoroacetyl Group

Four cationic Ir(III) complexes, [Ir(dpq)2(bpy)]PF6 (1), [Ir(dpq)2(phen)]PF6 (2), [Ir(tfapq)2- (bpy)]PF6 (3), and [Ir(tfapq)2(phen)]PF6 (4) (dpqH = 2,4-diphenylquinoline, tfapqH = 2-(4ʹ-trifluoroacetylphenyl)- 4-phenylquinoline, bpy = 2,2ʹ-bipyridine, phen = 1,10-phenanthroline) have been synthesized and fully characterized. The structure of 4 was also confirmed by single-crystal X-ray diffraction. The electron-acceptor character of the trifluoroacetyl unit leads to a reduced HOMO-LUMO gap and consequently a red-shift of the UV/Vis absorption and luminescence spectra. The solvophobic character of the trifluoroacetyl unit gives rise to a molecule assembly in solution. Graphical Abstract Tuning the Photophysical Properties of Cyclometalated Ir(III) Complexes by a Trifluoroacetyl Group

A New Phosphorescent Cyclometalated Iridium(III) Complex for the Selective Detection of Silver(I) Ion

A new cyclometalated iridium(III) complex, Ir(ppz)2(dtp) (Ir1) (ppzH = 4-phenylphthalazinone, dtp = diethyl dithiophosphate), has been synthesized and characterized by single-crystal X-ray diffraction. The photoluminescence spectrum of Ir1 shows an emission maximum at 597 nm and its quantum yield is ca. 0.072. Complex Ir1 exhibits a strong and fast decrease of emission upon addition of Ag+ in aqueous media. The ratio of Ir1 responding to Ag+ was determined to be 1:1 by UV–Vis absorption and phosphorescent emission measurements. Complex Ir1 is a highly selective chemosensor for Ag+ over other transition metal ions.