We report measurements of the 63 Cu nuclear spin echo decay rate 63 \/T2G in the paramagnetic state of a quasi-two-dimensional antiferromagnet La2CuC>4 up to 900 K, and from it deduce the temperature dependence of the spin-spin correlation length £. £ shows a crossover from ^expC/A^r) in the renormalized classical regime (7;$600 K) to £~~J//CBT in the quantum critical regime (7S;600 K), where J is the exchange interaction. Our finding that the ratio 63 T\T/ 63 T2G is temperature independent, where 63 T\ is the 63 Cu nuclear spin-lattice relaxation time, gives clear evidence for quantum critical scaling.PACS numbers: 74.72.Dn, 75.40.Cx, 75.40.Gb The discovery of high-T c superconductivity in doped La2CuC>4 promoted strong interest in the physical properties of the undoped antiferromagnetic parent material because of its low dimensionality and strong quantum phenomena [1]. It is firmly established that the magnetism of La2CuC>4 can be described very well based on the twodimensional Heisenberg model with a nearest neighbor exchange interaction J (-1500 K) [2],where Si is a copper d spin (S = j ) at a lattice site /. A weak intralayer coupling induces a three-dimensional antiferromagnetic ordering at T N~~3 00 K. Neutron scattering experiments by the Brookhaven Collaboration carried out up to 540 K demonstrated that the copper spin-spin antiferromagnetic correlation length £ shows a very strong temperature dependence even far above T^ [3]. Chakravarty, Halperin, and Nelson [4] showed theoretically that there are two regimes in the paramagnetic state of La2CuC>4. One is the renormalized classical regime, T N

No abstract

Recently, π-conjugated molecules containing a 9-silafluorene moiety have attracted considerable attention owing to their potential applications in organic electronics. While most studies focus on the oligomers and (co)polymers of 9-silafluorenes, functionalized 9-silafluorenes themselves remain unexplored. In this paper, we describe the preparation, photophysical properties, and theoretical calculations of 2-amino-7-acceptor-9-silafluorenes and we show that these silafluorenes can potentially be used as novel chromophores for functional organic materials. The D-π-A type silafluorenes were prepared by Pd-catalyzed intramolecular coupling of 2-(3-aminophenyldiisopropylsilyl)aryl triflates and the subsequent functional group conversion through reduction of the cyano group to a formyl and Knoevenagel condensation of the formyl group with malonitrile. The UV−visible absorption and fluorescence spectra of the D-π-A type silafluorenes exhibited a red-shift when the electron-withdrawing nature of the acceptor increased. The emission maxima of the fluorescence were highly dependent on the solvent. In the cases of formyl- and dicyanoethenyl-substituted silafluorenes, the emission colors ranged from blue to yellow for the formyl derivatives and from green to red for the dicyanoethenyl derivatives. The noticeable fluorescence solvatochromism suggests the intramolecular charge-transfer character of the excited states. The silafluorenes also exhibited fluorescence in the solid state (e.g., a neat thin film and a doped polymer film), and the emission color was dependent on the polarity of the polymer. In the solid state, the quantum yields of diphenylamino derivatives were generally higher than those of dimethylamino derivatives, presumably because the bulky diphenylamino group prevented chromophores from assembling close to each other. A comparison of the photophysical properties and theoretical calculations of D-π-A type silafluorenes with those of the corresponding fluorenes revealed that the silicon bridge contributed to the extension of the effective conjugation length of the biphenyl moiety when the acceptor was either hydrogen, trifluoromethyl, or a cyano group, whereas there was no contribution of the silicon bridge to the π-extension in silafluorenes substituted by strong electron acceptors such as formyl and dicyanoethenyl groups. White photoluminescence was demonstrated with an excellent quantum yield of 0.81 in the solid state from the poly(methyl methacrylate) (PMMA) film doped with Ph2N/CHO- and Ph2N/CH(CN)2-substituted silafluorenes. This work reveals the potential of D-π-A type silafluorenes as versatile organic emitting materials.

Benzosiloles fused to heterocycles such as thiophene, benzothiophene, and benzofuran, and indole-and benzosilole-fused dibenzosiloles were prepared by palladium-catalyzed intramolecular coupling of the corresponding 2-(arylsilyl)aryl triflates in good to high yields. Molecular and crystal structures of 5,7-dihydro-5,5,7,7-tetrakis(1-methylethyl)bis[1]benzosilolo-[2,3-b:3', 2'-d]thiophene, 6-methyl-12,12-diisopropyl-12H-indololo[3,2-b][1]silafluorene, and 5,5,11,11-tetraisopropyl-5,11H-benzosilolo[3,2-c]silafluorene were determined by X-ray diffraction analysis. The UV absorption spectra of the (di)benzosilole derivatives in cyclohexane red-shifted when compared to 1,1-diisopropyldibenzosilole, indicating that replacing a benzene ring of dibenzosilole by the heterocycles as well as fusion of indole and benzosilole moieties onto dibenzosilole narrowed the HOMO-LUMO gaps of the -conjugation system. The thiophene-fused benzosiloles were faintly fluorescent in solution and in the solid state, whereas the dibenzosiloles exhibited luminescence with moderate and high quantum yields in cyclohexane and in microcrystals, respectively. In other words, aggregation-induced emission was observed for the dibenzosiloles. Notably,5,5,11,silafluorene in microcrystals exhibited violet fluorescence ( max = 396 nm) with a quantum yield of 0.70. Density functional theory (DFT) calculations of the prepared (di)benzosiloles were also performed. absorption, benzosilole, fluorescence, silicon, solid-state emission