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We investigate the structural, magnetic, and dielectricpropertiesof the organic–inorganic hybrid material CuCl4(C6H5CH2CH2NH3)2and demonstrate that spontaneous ferroelectric order setsin below 340 K, which coexists with ferromagnetic ordering below 13K. We use X-ray diffraction to show that the electric polarizationresults from the spatial ordering of hydrogen bonds that link theorganic block comprised of phenylethylammonium cations to the inorganiccopper chloride block. The hydrogen bond ordering is driven by bucklingof the corner-linked copper chloride octahedra. Because the magneticexchange pathways are also determined by this octahedral buckling,a potentially large magnetoelectric coupling is induced. Our resultsimply that such hybrids form a new family of multiferroic materials. [ABSTRACT FROM AUTHOR]

Magnetic dioxygen molecules can be used as building blocks of model systems to investigate spin-polarization that arises from unpaired p-electrons, the scientific potential of which is evidenced by phenomena such as spin-polarized transport in graphene. In solid elemental oxygen and all of the known ionic salts comprised of magnetic dioxygen anions and alkali metal cations, the dominant magnetic interactions are antiferromagnetic. We have induced novel ferromagnetic interactions by introducing oxygen deficiency in rubidium superoxide (RbO2). The anion vacancies in the resulting phase with composition RbO1.72provide greater structural flexibility compared to RbO2and facilitate a Jahn−Teller-driven order−disorder transition involving the anion orientations at ∼230 K, below which their axes become confined to a plane. This reorganization gives rise to short-range ferromagnetic ordering below ∼50 K. A ferromagnetic cluster-glass state then forms below ∼20 K, embedded in an antiferromagnetic matrix that orders at ∼5 K. We attribute this inhomogeneous magnetic order to either subtly different anion geometries within different structural nanodomains or to the presence of clusters in which double exchange takes place between the anions, which are mixed-valence in nature. We thus demonstrate that nonstoichiometry can be employed as a new route to induce ferromagnetism in alkali metal oxides. [ABSTRACT FROM AUTHOR]

The charge photogeneration and early recombination in MEH-PPV-based charge-transfer complexes (CTCs) and in MEH-PPV/PCBM blend as a reference are studied by ultrafast visible-pump−IR-probe spectroscopy. After excitation of the CTC band, an immediate (<100 fs) electron transfer is observed from the polymer chain to the acceptor with the same yield as in the MEH-PPV/PCBM blend. The forward charge transfer in the CTCs is followed by an efficient (∼95%) and fast (<30 ps) geminate recombination. For comparison, the recombination efficiency obtained in the MEH-PPV/PCBM blend does not exceed a mere 50%. Polarization-sensitive experiments demonstrate high (∼0.3) values of transient anisotropy for the CTCs polaron band. In contrast, in the MEH-PPV/PCBM blend the dipole moment orientation of the charge-induced transition is less correlated with the polarization of the excitation photon. According to these data, photogeneration and recombination of charges in the CTCs take place locally (i.e., within a single pair of a polymer conjugation segment and an acceptor) while in the MEH-PPV/PCBM blend exciton migration precedes the separation of charges. Results of the ultrafast experiments are supported by photocurrent measurements on the corresponding MEH-PPV/acceptor photodiodes. [ABSTRACT FROM AUTHOR]

A periodic density functional theory method using the B3LYP hybrid exchange-correlation potential is applied to the Prussian blue analogue RbMn[Fe(CN) 6] to evaluate the suitability of the method for studying, and predicting, the photomagnetic behavior of Prussian blue analogues and related materials. The method allows correct description of the equilibrium structures of the different electronic configurations with regard to the cell parameters and bond distances. In agreement with the experimental data, the calculations have shown that the low-temperature phase (LT; Fe 2+(t 62g, S= 0)−CN−Mn 3+(t 32ge 1g, S= 2)) is the stable phase at low temperature instead of the high-temperature phase (HT; Fe 3+(t 52g, S= 1/2)−CN−Mn 2+(t 32ge 2g, S= 5/2)). Additionally, the method gives an estimation for the enthalpy difference (HT ⇔ LT) with a value of 143 J mol −1K −1. The comparison of our calculations with experimental data from the literature and from our calorimetric and X-ray photoelectron spectroscopy measurements on the Rb 0.97Mn[Fe(CN) 6] 0.98·1.03H 2O compound is analyzed, and in general, a satisfactory agreement is obtained. The method also predicts the metastable nature of the electronic configuration of the high-temperature phase, a necessary condition to photoinduce that phase at low temperatures. It gives a photoactivation energy of 2.36 eV, which is in agreement with photoinduced demagnetization produced by a green laser. [ABSTRACT FROM AUTHOR]

Energy transfer properties of novel coumarin−perylene bisimide dendrimer are studied by means of steady state and time-resolved UV/vis spectroscopy. At low donor excitation density fast (transfer rate ∼10 ps-1) and efficient (quantum yield ∼99.5%) donor−acceptor energy transfer is observed. The random distributions of donor−acceptor orientations and distances result in nonexponential energy transfer kinetics. The energy transfer remains independent of excitation density up to densities corresponding to one absorbed photon per 10 dendrimer molecules. At higher excitation densities the transfer rate is found to increase due to excitation of multiple donors per dendrimer. Control of the donor−acceptor energy transfer rate is achieved by pre-excitation of the acceptor and monitored by prepump−pump−probe experiments, which show that the energy transfer rate can be decreased by a factor of 2. The relative orientations of transition dipole moments in the donor and acceptor molecules are found to be one of the key factors determining the energy transfer dynamics at high excitation densities. [ABSTRACT FROM AUTHOR]

Recent experimental and theoretical work on hydrogen-like absorption spectra of excitons in external magnetic fields revealed new effects when the Coulomb interaction becomes comparable to the magnetic perturbation. We present a theoretical approach that allows for calculation of absorption spectra for any value of magnetic field. This approach opens the possibility to compute the optical functions i.e. reflectivity, transmission and absorption including the excitonic effects for various strength of external magnetic field. [ABSTRACT FROM AUTHOR]