ABSTRACT. The performance of hybrid organic perovskite (HOP) for solar energy conversion is driving a renewed interest in their light emitting properties. The recent observation of broad visible emission in layered HOP highlights their potential as white light emitters. Improvement of the efficiency of the material requires a better understanding of its photophysical properties. We present in-depth experimental investigations of white light (WL) emission in thin films of the (C6H11NH3)PbBr4. The broadband, strongly Stokes shifted emission presents a maximum at 90K when excited at 3.815 eV, and below this temperature coexists with an excitonic edge emission.X-rays and calorimetry measurements excludes the existence of a phase transition as an origin of the thermal behavior of the WL luminescence. The free excitonic emission quenches at low temperature, despite a binding energy estimated to 280 meV. Time-Resolved Photoluminescence spectroscopy reveals the multicomponent nature of the broad emission. We analyzed the dependence of these components as function of temperature and excitation energy. The results are consistent with the existence of self-trapped states. The quenching of the free exciton and the thermal evolution of the WL luminescence decay time are explained by the existence of an energy barrier against self-trapping, estimated to ~10 meV.

We have analyzed an Ising-like model, in the mean-field approach, involving two “antiferromagnetically” coupled sublattices. This model simulates the so-called “two-step” spin-crossover transition, for which a precise definition is given. If both sublattices are equivalent, it implies a spontaneous breaking of symmetry which may occur within a temperature range limited by two “Néel températures”. It, also predicts a simultaneous reversal of the magnetization of the sublattices (if they are unequivalent) at a “characteristic” value of temperature. These features are analyzed simultaneously with some details. The present model fits and explains well the available experimental data concerning [ Fe(2-pic)3] Cℓ2- EtOH and FeII[ 5NO2 – sal – N(1, 4, 7, 10)]

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We investigate the behaviour of photo-excitable, bistable systems, under permanent light irradiation, in presence of relaxation towards the non-excited state. Cooperativity causes bistability of the steady state, leading to light-induced thermal and optical hysteresis (LITH and LIOH). The light-induced instability is expected to induce demixtion, i.e. the coexistence of domains of the two stable steady states. Such effects are evidenced by magnetic and reflectivity measurements on the spin-crossover solid solution: [Fex Co1−x (btr)2(NCS)2] · H2O, with x = 0.3, 0.5, 0.85. Experimental data are in quantitative agreement with a simple macroscopic model which includes a non-linear relaxation term in the master equation. PACS. 64.60.My Metastable phases -70.60.Es Relaxation effects -75.90.+w Other topics in magnetic properties and materials -78.20.Nv Thermooptical and photothermal effects

We report a two-dimensional Hofmann-like spin-crossover (SCO) material, [Fe(trz-py){Pt(CN)}]·3HO, built from [FePt(CN)] layers separated by interdigitated 4-(2-pyridyl)-1,2,4,4H-triazole (trz-py) ligands with two symmetrically inequivalent Fe sites. This compound exhibits an incomplete first-order spin transition at 153 K between fully high-spin (HS-HS) and intermediate high-spin low-spin (HS-LS) ordered states. At low temperature, it undergoes a bidirectional photoswitching to HS-HS and fully low-spin (LS-LS) states with green and near-IR light irradiation, respectively, with associated T(LIESST = Light-Induced Excited Spin-State Trapping) and T(reverse-LIESST) values of 52 and 85 K, respectively. Photomagnetic investigations show that the reverse-LIESST process, performed from either HS-HS or HS-LS states, enables access to a hidden stable LS-LS state, revealing the existence of a hidden thermal hysteresis. Crystallographic investigations allowed to identify that the strong metastability of the HS-LS state originates from the existence of a strong elastic frustration causing antiferroelastic interactions within the [FePt(CN)] layers, through the rigid NC-Pt-CN bridges connecting the inequivalent Fe sites. The existence of the stable LS-LS state paves the way for a multidirectional photoswitching and allows potential applications for electronic devices based on ternary digits.

Two-step and multistep spin transitions are frequently observed in switchable cooperative molecular solids. They present the advantage to open the way for three- or several-bit electronics. Despite extensive experimental studies, their theoretical description was to date only phenomenological, based on Ising models including competing ferro- and antiferro-magnetic interactions, even though it is recognized that the elastic interactions are at the heart of the spin transition phenomenon, due to the volume change between the low- and high-temperature phases. To remedy this shortcoming, we designed the first consistent elastic model, taking into account both volume change upon spin transition and elastic frustration. This ingredient was revealed to be powerful, since it was able to obtain all observed experimental configurations in a consistent way. Thus, according to the strength of the elastic frustration, the system may undergo first-order transition with hysteresis, gradual, hysteretic two-step or multistep transitions, and incomplete transitions. Furthermore, the analysis of the spatial organization of the HS and LS species in the plateau regions revealed the emergence of complex antiferro-elastic patterns going from simple antiferro-magnetic-like order to long-range spatial modulations of the high-spin fraction. These results enabled us to identify the elastic frustration as the fundamental mechanism at the origin of the very recent experimental observations showing the existence of organized spatial modulations of the high-spin fraction inside the plateau of two-step spin transitions.

We study the origin of the cooperative nature of spin crossover (SC) between low-spin and high-spin (HS) states from the viewpoint of elastic interactions among molecules. As the size of each molecule changes depending on its spin state, the elastic interaction among the lattice distortions provides the cooperative interaction of the spin states. We develop a simple model of SC with intra and intermolecular potentials which accounts for the elastic interaction including the effect of the inhomogeneity of the spin states and apply constant temperature molecular dynamics based on the Nosé-Hoover formalism. We demonstrate that, with increase of the strength of the intermolecular interactions, the temperature dependence of the HS component changes from a gradual crossover to a first-order transition.

Organic−inorganic hybrid perovskites (OIHP) are developing rapidly as high-performance semiconductors for solid-state solar cells and light emitting devices. Recently, lead-halide two-dimensional (2D) OIHP were found to present bright broadband visible emission, thus, highlighting their potential as single component white-light (WL) emitters. This contribution deals with the preparation of a new Cd-based 2D hybrid perovskite, of the chemical formula (C 6 H 11 NH 3 ) 2 CdBr 4 (abbreviated as compound 1), of which structural and optical properties have been studied and analyzed. Room temperature optical absorption (OA) measurements, performed on spin-coated film of compound 1, revealed a sharp excitonic absorption peak at 3.24 eV, and a large exciton binding energy of 377 meV, estimated from low temperature OA spectrum. Upon 325 nm irradiation, compound 1 showed a very broadband WL emission consisting of one peak at 2.94 eV, attributed to exciton confined in the [CdBr 4 ] 2− inorganic layers, and a second peak at 2.53 eV resulting from the cyclohexylammonium cations emission. Temperature dependence of PL spectra evidenced anomalous behavior accompanied by singularities around 50 and 150 K in the integrated intensity, the full width at half-maximum and the PL peaks positions. These singularities have been traced back to structural phase transitions, from temperature dependence powder and single crystal X-ray diffraction investigations, from which strong correlations had emerged between the structural distortion of the CdBr 6 pseudo-octahedron and the broadening characteristics of the WL emission band. These hitherto unrecognized properties turn this and similar OIHP into perspective candidates for potential applications as WL-emitting diodes.