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.

This paper reports the experimental and theoretical investigations of two trigonal bipyramidal Ni(II) complexes, [Ni(Me(6)tren)Cl](ClO(4)) (1) and [Ni(Me(6)tren)Br](Br) (2). High-field, high-frequency electron paramagnetic resonance spectroscopy performed on a single crystal of 1 shows a giant uniaxial magnetic anisotropy with an experimental D(expt) value (energy difference between the M(s) = ± 1 and M(s) = 0 components of the ground spin state S = 1) estimated to be between -120 and -180 cm(-1). The theoretical study shows that, for an ideally trigonal Ni(II) complex, the orbital degeneracy leads to a first-order spin-orbit coupling that results in a splitting of the M(s) = ± 1 and M(s) = 0 components of approximately -600 cm(-1). Despite the Jahn-Teller distortion that removes the ground term degeneracy and reduces the effects of the first-order spin-orbit interaction, the D value remains very large. A good agreement between theoretical and experimental results (theoretical D(theor) between -100 and -200 cm(-1)) is obtained.

X-ray crystallography has traditionally been limited to the study of the groundstate structure of molecules and solids. 2À ], performed at helium temperatures with synchrotron radiation. The shortening of the PtÐPt bond by 0.28 (9) A Ê upon excitation is compatible with the proposed mechanism involving promotion of a PtÐPt antibonding d'* electron to a weakly bonding p orbital. The contraction is accompanied by a 3 molecular rotation. The time-resolved diffraction technique described here is applicable to reversible light-driven processes in the crystalline solid state.

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.

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.

We investigated by optical microscopy the propagation of the thermal spin transition in [Fe(btr) 2 (NCS) 2 ]·H 2 O·(btr = bis-triazole) single crystals. Using high-quality fresh crystals embedded in oil we could follow the transformation front of the on-cooling transition. We observed in detail a linear front propagating through the entire crystal at extremely slow velocity, ∼2.5 μm/s on average. The analysis of the temporal dependences of the optical densities (OD) at given positions within a Kolmogorov-Johnson-Mehl-Avrami (KJMA) law led to an exponent ∼0.6, typical for a process governed by planar diffusion. It is inferred that the spin transition is a multiscale process. The agreement with a previous structural investigation, analyzed in terms of an average KJMA law, is shown. We conclude that the stresses generated by the volume change associated with the spin transformation are the driving force for the propagation of the nucleation and growth mechanism. The role of defects is discussed and optical effects specific of the experiment are evidenced.

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