Oligothiophenes incorporating MM quadruple bonds have been prepared from the reactions between Mo2(TiPB)4 (TiPB ‫؍‬ 2,4,6-triisopropyl benzoate) and 3,4-dihexyl-2,2-:5,2؆-terthiophene-5,5؆-dicarboxylic acid. The oligomers of empirical formula Mo2(TiPB)2(O2C(Th)-C4(n-hexyl)2S-(Th)CO2) are soluble in THF and form thin films with spin-coating (Th ‫؍‬ thiophene). The reactions between Mo2(TiPB)4 and 2-thienylcarboxylic acid (Th-H), 2,2-bithiophene-5-carboxylic acid (BTh-H), and (2,2:5,2؆-terthiophene)-5-carboxylic acid (TTh-H) yield compounds of formula trans-Mo2(TiPB)2L2, where L ‫؍‬ Th, BTh, and TTh (the corresponding thienylcarboxylate), and these compounds are considered as models for the aforementioned oligomers. In all cases, the thienyl groups are substituted or coupled at the 2,5 positions. Based on the x-ray analysis, the molecular structure of trans-Mo2(TiPB)2(BTh)2 reveals an extended L-M2␦-L conjugation. Calculations of the electronic structures on model compounds, in which the TiPB are substituted by formate ligands, reveal that the HOMO is mainly attributed to the M2␦ orbital, which is stabilized by back-bonding to one of the thienylcarboxylate * combinations, and the LUMO is an in-phase combination of the thienylcarboxylate * orbitals. The compounds and the oligomers are intensely colored due to M2␦-thienyl carboxylate * charge transfer transitions that fall in the visible region of the spectrum. For the molybdenum complexes and their oligomers, the photophysical properties have been studied by steady-state absorption spectroscopy and emission spectroscopy, together with time-resolved emission and transient absorption for the determination of relaxation dynamics. Remarkably, THF solutions the molybdenum complexes show room-temperature dual emission, fluorescence and phosphorescence, originating mainly from 1 MLCT and 3 MM(␦␦*) states, respectively. With increasing number of thienyl rings from 1 to 3, the observed lifetimes of the 1 MLCT state increase from 4 to 12 ps, while the phosphorescence lifetimes are Ϸ80 s. The oligomers show similar photophysical properties as the corresponding monomers in THF but have notably longer-lived triplet states, Ϸ200 s in thin films. These results, when compared with metallated oligothiophenes of the later transition elements, reveal that M 2␦-thienyl conjugation leads to a very small energy gap between the 1 MLCT and 3 MLCT states of <0.6 eV.photophysics ͉ dual emission B ecause of their potential applications in optoelectronic and magnetic devices, conjugated organic polymers have received much attention (1-3), and oligothiophenes, which constitute one important subclass of these materials, have been found to show excellent hole transport properties (3-7). We have been interested for some time in incorporating MM quadruple bonds into these oligomers by use of the carboxylate tether (8-10), such that the M 2 ␦ electrons can be brought into conjugation with an extended -framework of the thiophene moiety. As shown in Scheme 1, the out-of-phase combination of the carbo...

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Syntheses of CdTe/CdSe type‐II quantum dots (QDs) using CdO and CdCl2 as precursors for core and shell, respectively, are reported. Characterization was made via near‐IR interband emission, transmission electron microscopy (TEM), energy dispersive spectroscopy (EDX), and X‐ray diffraction (XRD). Femtosecond fluorescence upconversion measurements on the relaxation dynamics of the CdTe core (in CdTe/CdSe) emission and CdTe/CdSe interband emission reveal that as the size of the core increases from 5.3, 6.1 to 6.9 nm, the rate of photoinduced electron separation decreases from 1.96, 1.44 to 1.07 ×1012 s−1. The finite rates of the initial charge separation are tentatively rationalized by the small electron–phonon coupling, causing weak coupling between the initial and charge‐separated states.

Syntheses of CdSe/ZnTe/ZnS (core/shell/shell) type-II quantum dots (QDs) are reported. Structural characterization was made via TEM, EDX, XPS and XRD. Photophysical properties were investigated via the interband emission (CdSe A ZnTe) and its associated quantum efficiency as well as relaxation dynamics. In comparison to the weak emission (W f y 4 6 10 23 in toluene) of CdSe/ZnTe (3.9/0.5 nm), capping ZnS (0.4 nm in thickness) enhances the CdSe A ZnTe interband emission by y30 fold (W f y 0.12), whereas the peak wavelength shifts only slightly to the red. By encapsulating dihydrolipoic acid (DHLA), water-soluble CdSe/ZnTe/ZnS QDs were also prepared, and they exhibited an interband emission at y930 nm with an emission yield of y0.1. Femtosecond pulse excitation (l ex y 1200 nm) measurement estimated a two-photon absorption cross section, s, of y70 6 10 250 cm 4 s photon 21 for DHLA-capped CdSe/ZnTe/ZnS in water, supporting its suitability for the use as near-infrared (NIR) dyes in biomedical imaging.

The compounds M(2)(TiPB)(2)(OSC-2-Th)(2) have been prepared from the reactions between M(2)(TiPB)(4) and Th-2-COSH (2 equiv) in toluene solution, where M = Mo (Mo(2)ThCOS) or W (W(2)ThCOS), TiPB = 2,4,6-triisopropylbenzoate and Th = thienyl. The molybdenum and tungsten compounds are pink and blue, air-sensitive, ether soluble solids that show M(+) ions in the mass spectrometer and metal and ligand based reversible oxidation and reduction waves, respectively, by cyclic voltammetry. Electronic structure calculations on the model compounds M(2)(O(2)CH)(2)(OSC-2-Th)(2) indicate that the highest occupied molecular orbital (HOMO) is principally M(2)delta and the lowest unoccupied molecular orbital (LUMO) is thienylthiocarboxylate pi* but with significant metal-sulfur mixing. The intense visible absorptions arise from (1)MLCT, M(2)delta to thienylthiocarboxylate. The photoexcited states of these molecules have been studied by transient absorption spectroscopy and steady state emission. These properties are compared with those of previously reported thienylcarboxylate compounds, M(2)(TiPB)(2)(O(2)C-2-Th)(2), where M = Mo (Mo(2)ThCO(2)) or W (W(2)ThCO(2)).