We investigate the out-of-equilibrium switching dynamics of a molecular Fe(III) spin-crossover solid triggered by a femtosecond laser flash. The time-resolved x-ray diffraction and optical results show that the dynamics span from subpicosecond local photoswitching followed by volume expansion (nanosecond) and thermal switching (microsecond). We present a physical picture of the consecutive steps in the photoswitching of molecular materials.

We discuss here the important role of intermolecular coupling for the thermal-and light-induced molecular state switching in the solid state. Investigations were performed on the two crystalline polymorphs of the spin-crossover [Fe-(PM-BIA) 2 (NCS) 2 ] material. In addition to structural studies at thermal equilibrium, lightinduced phenomena were investigated through photocrystallography, photomagnetic, and dynamical optical measurements. Strong similarities between the thermal-equilibrium and the out-of-equilibrium light-induced transformations are observed in each polymorph: strong cooperative phenomena in one polymorph versus weak cooperative ones in the second polymorph. These different responses of the two crystalline forms of the compound to external perturbations are discussed at the microscopic level in terms of Ising-like model and two-mode description of on-site molecular potentials.

We have identified two polymorphs of the molecular complex [(TPA)Fe((III))(TCC)]PF(6) [TPA = tris(2-pyridylmethyl)amine and TCC = 3,4,5,6-tetrachlorocatecholate dianion]: one is monoclinic and the other is orthorhombic. By lowering the temperature both undergo a thermal spin-crossover between a high-spin (S = 5/2) and a low-spin (S = 1/2) state, which we detected by magnetic, optical and X-ray diffraction measurements. The thermal crossover is only slightly shifted between the polymorphs. Their crystalline structures consist of similar cation layers alternating with PF(6) anion layers, packed differently in the two polymorphs. The magnetic and optical properties of the polymorphs are presented.

Achieving control of photoinduced phase transitions requires understanding how materials work during transformation induced by a laser pulse. Here we investigate the precursors of a photoinduced phase transition in the highly cooperative charge-transfer molecular crystal tetrathiafulvalene-p-chloranil and provide key insights. The photogeneration of one-dimensional nanoscale clusters was detected by time-resolved diffuse x-ray scattering with 50-ps time resolution. Such clustering of structurally relaxed electronic excitations is expected to be a common process in many materials presenting photoinduced transformations.

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Actually a crossover behavior is observed at thermal equilibrium with however a quasiabrupt shape indicating significant cooperative effects. These aspects are compared between the temperature-and photoinduced spin crossovers. INTRODUCTION:

We investigated the thermo- and photo-induced phase transitions between low spin (LS) and high spin (HS) states of the molecular crystal of [Fe(PM-BiA)2(NCS)2] in the orthorhombic form, by using X-ray diffraction. The structure of the photoinduced HS state, generated from the LS state at low temperature, is compared to the structures of the HS and LS phases at thermal equilibrium and to the thermally trapped HS state. The preliminary results presented here show that the structural reorganization is similar in the different HS states.

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