Wai-Kwok Wong

Synthesis and near-infrared luminescence of 3d-4f bi-metallic Schiff base complexes

The reaction of the zinc(II) Schiff base complex ZnL [H2L=N,N′-bis(3-methoxysalicylidene)ethylene-1,2-diamine] with one equivalent of Ln(NO3)3·xH2O (Ln=Nd, Ho, Er or Yb) gives the neutral 3d-4f bi-metallic Schiff base complexes [Zn(NO3)(μ-L)Ln(NO3)2(H2O)], which in solution exhibit emission corresponding to the Ln(III) ions (Ln=Nd, Er and Yb) in the near-infrared region.

High-efficiency and color-stable white organic light-emitting devices based on sky blue electrofluorescence and orange electrophosphorescence

Highly efficient and color-stable two-wavelength white organic light-emitting devices (WOLEDs) combining an orange phosphor [Ir(Cz–CF3)] and a sky blue fluorescent dye BUBD-1 are fabricated where the host singlet is resonant with the fluorophore singlet state and the host triplet is resonant with the phosphor triplet level. A thin layer of 1,3,5-tris[N-(phenyl)benzimidazole]benzene between the phosphorescent and the fluorescent regions confines both singlet and triplet excitons efficiently and suppress Dexter transfer of the phosphor excitons to the nonradiative triplet state of BUBD-1. The best device reaches peak efficiencies of 19.3cd∕A and 11.1lm∕W which are superior to common two-color all-fluorescent or all-phosphor WOLEDs.

Water-Soluble Mitochondria-Specific Ytterbium Complex with Impressive NIR Emission

A water-soluble porphyrinato ytterbium complex linked with rhodamine B (Yb-2) showed mitochondria-specific subcellular localization and strong two-photon-induced NIR emissions (λ(em) = 650 nm, porphyrinate ligand π → π* transition; λ(em) = 1060 nm, Yb(III) (5)F(5/2) → (5)F(7/2) transitions; σ(2) = 375 GM in DMSO) with an impressive Yb(III) NIR emission quantum yield (1% at λ(ex) = 340 nm; 2.5% at λ(ex) = 430 nm) in aqueous solution.

Tetranuclear NIR luminescent Schiff-base Zn–Nd complexes

With the simple dinuclear Zn–Nd complex [ZnNdL(H2O)(NO3)3] (1) (H2L = N,N′-bis(3-methoxysalicylidene)ethylene-1,2-diamine) as the precursor, tetranuclear [Zn2Nd2L2(4,4′-bpy)(NO3)6]·Et2O (2) and [Zn2Nd2L2(4,4′-bpe)]·2H2O (3) (4,4′-bpy = 4,4′-bipyridine, 4,4′-bpe = trans-bis(4-pyridyl)ethylene) complexes are formed: suitable choice of bidentate linkers could construct multinuclear heterometallic 3d–4f Schiff-base complexes with improved luminescent properties, and the controlling of linker character shows a potentially effective way to the fine-tuning properties of NIR luminescence from Nd ions.

Molecular Engineering of Simple Phenothiazine-Based Dyes To Modulate Dye Aggregation, Charge Recombination, and Dye Regeneration in Highly Efficient Dye-Sensitized Solar Cells

A series of simple phenothiazine-based dyes, namely, TP, EP, TTP, ETP, and EEP have been developed, in which the thiophene (T), ethylenedioxythiophene (E), their dimers, and mixtures are present to modulate dye aggregation, charge recombination, and dye regeneration for highly efficient dye-sensitized solar cell (DSSC) applications. Devices sensitized by the dyes TP and TTP display high power conversion efficiencies (PCEs) of 8.07 (Jsc = 15.2 mA cm(-2), Voc =0.783 V, fill factor (FF) = 0.679) and 7.87 % (Jsc = 16.1 mA cm(-2), Voc = 0.717 V, FF = 0.681), respectively; these were measured under simulated AM 1.5 sunlight in conjunction with the I(-)/I3(-) redox couple. By replacing the T group with the E unit, EP-based DSSCs had a slightly lower PCE of 7.98 % with a higher short-circuit photocurrent (Jsc) of 16.7 mA cm(-2). The dye ETP, with a mixture of E and T, had an even lower PCE of 5.62 %. Specifically, the cell based on the dye EEP, with a dimer of E, had inferior Jsc and Voc values and corresponded to the lowest PCE of 2.24 %. The results indicate that the photovoltaic performance can be finely modulated through structural engineering of the dyes. The selection of T analogues as donors can not only modulate light absorption and energy levels, but also have an impact on dye aggregation and interfacial charge recombination of electrons at the interface of titania, electrolytes, and/or oxidized dye molecules; this was demonstrated through DFT calculations, electrochemical impedance analysis, and transient photovoltage studies.

Conformational engineering of co-sensitizers to retard back charge transfer for high-efficiency dye-sensitized solar cells

We demonstrate that the post-adsorption of small molecules (a phenothiazine-based dye) on the porphyrin-sensitized TiO2 anode surface plays dual roles: (1) to greatly retard the back reaction between conduction-band electrons in TiO2 and the oxidized species (I3−) in the electrolyte and (2) to enhance the spectral response of solar cells. These two effects finally give rise to device efficiencies exceeding 10%, which are superior to those of individual dye-sensitized devices by either porphyrin (7.4%) or phenothiazine (8.2%) under the same conditions. Experimental analyses show that the incoming small molecules are adsorbed in the interstitial site of porphyrin dyes, forming densely surface packed molecules and thus impeding the I3− species from approaching the TiO2 surface. Since a broad range of ruthenium-based dyes and porphyrin-based photosensitizers possess relatively large molecular volumes, this method is anticipated to be applicable for further improving the energy conversion efficiency of devices sensitized by these two classes of dyes.

Comparative Studies of the Cellular Uptake, Subcellular Localization, and Cytotoxic and Phototoxic Antitumor Properties of Ruthenium(II)-Porphyrin Conjugates with Different Linkers

Six water-soluble free-base porphyrin-Ru(II) conjugates, 1-3, and Zn(II) porphyrin-Ru(II) conjugates, 4-6, with different linkers between the hydrophobic porphyrin moiety and the hydrophilic Ru(II)-polypyridyl complex, have been synthesized. The linear and two-photon-induced photophysical properties of these conjugates were measured and evaluated for their potential application as dual in vitro imaging and photodynamic therapeutic (PDT) agents. Conjugates 1-3, with their high luminescence and singlet oxygen quantum yields, were selected for further study of their cellular uptake, subcellular localization, and cytotoxic and photocytotoxic (under linear and two-photon excitation) properties using HeLa cells. Conjugate 2, with its hydrophobic phenylethynyl linker, was shown to be highly promising for further development as a bifunctional probe for two-photon (NIR) induced PDT and in vitro imaging. Cellular uptake and subcellular localization properties were shown to be crucial to its PDT efficacy.

Hetero-trinuclear near-infrared (NIR) luminescent Zn2Ln complexes from Salen-type Schiff-base ligands

With the Zn–Schiff-base [ZnL11(Py)] or [ZnL22(Py)] from the simple Salen-type Schiff-base ligand H22L11 or H22L22 (H22L11: N,N′-bis(salicylidene)ethylene-1,2-diamine; H22L22: N,N′-bis(salicylidene)phenylene-1,2-diamine) without the outer O2O2 portion as the precursor, two series of eight trinuclear Zn2Ln arrayed complexes (Ln = Nd (1 or 5), Yb (2 or 6), Er (3 or 7) or Gd (4 or 8)) have been obtained by the further reaction with Ln(NO3)3·6H2O, respectively. The results of photophysical studies show that the strong and characteristic NIR luminescence with emissive lifetimes in microsecond ranges, has been sensitized from the excited state (1LC and 3LC or only 1LC) of the ligand, whereas the Zn(II)-based visible luminescence is mostly quenched because of quite effective intramolecular energy transfer from the ligand-centered excited state of the Zn(II)–Schiff base complex to Ln(III) ions.

Novel host materials for single-component white organic light-emitting diodes based on 9-naphthylanthracene derivatives

A series of 9-naphthylanthracene based dimers and trimers were synthesized and characterized. TGA studies reveal that they are all thermally stable with decomposition temperatures well above 500 °C. Upon optical excitation, all of these dimers and trimers show intense blue emission in common organic solvents, accompanied by a new peak at long wavelength in the solid state. They exhibit nanosecond transient lifetimes consisting of two decay components, suggesting the formation of excimers. Single-component light-emitting electroluminescent devices based on these robust materials have been fabricated. The device based on 4,4′-bis(9-(1-naphthyl)anthracen-10-yl)biphenyl exhibits a maximum luminance efficiency of 7.0 cd A−1 with CIE coordinates of (0.31, 0.36) and luminance of 1396 cd m−2 at 6.9 V. The device based on 1,3,5-tris(9-(1-naphthyl)anthracen-10-yl)benzene exhibits a maximum luminance efficiency of 5.78 cd A−1 with CIE coordinates of (0.33, 0. 43) and luminance of 1156 cd m−2 at 7.8 V.

New Co(OH)2/CdS nanowires for efficient visible light photocatalytic hydrogen production

Through a facile impregnation synthesis, new noble-metal-free Co(OH)2/CdS nanowires (NWs) have been developed for photocatalytic hydrogen production. The loading of Co(OH)2 on CdS NWs can effectively accelerate the charge separation and transfer in photocatalytic reactions, leading to an enhanced H2 production rate (HPR). The optimum HPR based on Co(OH)2/CdS reaches 14.43 mmol h−1 g−1 under visible light (λ ≥ 420 nm) irradiation, which is 206 times higher than for the pristine CdS NWs and even 3 times higher than for 1 wt% Pt/CdS NWs as a benchmark. Impressively, core–shell structural Co(OH)2/CdS NWs formed by visible-light-induced self-assembly during the photocatalytic reaction. And the core–shell structural characteristics of the Co(OH)2/CdS NWs can effectively avoid light corrosion, leading to a stable HPR in 12 hours duration.