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In photochromic materials, we demonstrated that non-linear optical (NLO) properties such as second harmonic generation (SHG) can be switched reversibly by alternate UV and visible irradiations. We report here on the design of materials fulfilling the prerequisites for SHG activity (push-pull and acentric space group) and photochromic properties. Photochromic and NLO properties of a series of anils are presented, along with the ability of some of them to switch SHG.

Two new anil molecules exhibiting photochromism in the crystalline state, N-(4-hydroxy)-salicylidene-amino-4-(methylbenzoate) (2) and N-(3,5-di-tert-butylsalicylidene)-4-aminopyridine (3), are obtained. Upon irradiation in the UV, the yellow crystals change color to red, owing to enol-keto intramolecular tautomerism. The red color disappears, when crystals are left in the dark or irradiated with visible light. 3 has the most stable keto form among all anil-type photochromic compounds (τ = 460 days at room temperature). Both exhibit nonlinear optical (NLO) properties and show powder second harmonic generation (SHG) of respectively 10 and 3 times vs urea. X-ray diffraction shows acentric structures where molecules line up “head-to-tail” through hydrogen bonds for 2 (space group Pc), or form a chiral helix 3 (space group P32). Evidence of reversible structural change is given for 3, and we demonstrate the functionality of this crystal as an NLO switching material, as SHG can be photomodulated by about 30%.

The light-induced crossover in a series of iron(II) compounds has been investigated by irradiating the sample at 10 K with a Kr + laser coupled to an optical fiber within a SQUID cavity. The temperature dependence of the molar fraction of the light-induced metastable HS state has been recorded for 22 compounds. The critical LIESST temperature, T c (LIESST), has been defined as the temperature for which the light-induced HS information was erased in the SQUID cavity. The dependence of T c (LIESST) as a function of the thermal spin crossover temperature, T 1/2 , has been discussed. The effect of cooperation has been analyzed and the influence of horizontal and vertical displacements of the LS and HS potential wells has been discussed on the basis of the non-adiabatic multiphonon theory.

Using two complementary techniques, the relaxation of a photo-induced spin state transition in a strongly cooperative compound has been studied from 10 to 170 K. Up to 150 K, the relaxation rate increases with temperature. At higher temperature, the stabilization of the high-spin state results in a decrease of the relaxation rate. Within the hysteresis loop, as one approaches the switching temperature branch, a critical slowing down of the kinetics and a constant decrease of the cooperative contribution are recorded. This study evidences, for the first time, the complex temperature evolution of the relaxation of photo-induced spin state transition.

Within the thermal hysteresis loop of the [Fe(PM-BiA) 2 (NCS) 2 ] compound, a single laser pulse of 14 mJ cm −2 induces a photo-conversion from the low spin (LS, S = 0) to the high spin (HS, S = 2) state of the Fe 2+ metallic center. The temporal dynamic of this phenomena indicates that the system is firstly photo-excited into the HS state and then slowly relaxes to a mixture of HS/LS state. Subsequent laser pulses do not affect the HS/LS ratio. The system can be brought back to its initial LS state by adjusting the temperature of the compound. A simple model accounts qualitatively for the observed phenomenon.

[EN] The compounds [Fe(abpt)(2)(NCS)(2)] (1) and [Fe(abpt)(2)(NCSe)(2)] (2) with abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole have been synthesized. The X-ray structures have been determined at 293 K. 1 and 2 are isostructural and crystallize in the monoclinic space group P2(1)/n with Z = 2, a = 8.538(8), b = 10.246(8), c = 16.45(2) Angstrom, beta = 93.98(9)degrees for 1 and a = 8.623(2), b = 10.243(3), c = 16.585(3) Angstrom, beta = 93.19(2)degrees for 2. In both complexes, the coordination core has a similar pseudo-octahedral geometry with the NCS- (1) and NCSe- (2) groups in the trans-position. Variable-temperature magnetic susceptibility data give evidence for a low-spin (LS)high-spin (HS) conversion centered at T-1/2 around 180 and 224 K for 1 and 2, respectively. The spin conversion takes place gradually, without hysteresis. The enthalpy and entropy changes associated with the spin conversion are evaluated from the DSC measurements: Delta H = 5.8 +/- 0.5 (1) and 8.6 +/- 0.8 kJ mol(-1) (2); BS = 32.5 +/- 3 (1) and 38 +/- 4 J mol(-1) K-1 (2). At 10 K the light-induced excited spin state trapping (LIESST) effect has been observed within the SQUID magnetometer cavity, by irradiating powder samples with a Kr+ laser coupled to an optical fiber. The critical LIESST temperatures T-liesst are around 40 and 32 K for 1 and 2, respectively. The magnetic behavior recorded under light irradiation in the warming and cooling modes has revealed a light-induced thermal hysteresis (LITH) effect. The HS-->LS relaxation kinetics have been investigated in the temperature range 6-40 K. A thermally activated mechanism at elevated temperatures and a nearly temperature independent relaxation behavior at low temperatures can be observed for 1. The very fast relaxation precludes the estimation of the kinetic parameters for 2 at temperatures higher than 10 K. (C) 1999 Elsevier Science S.A. All rights reserved., We thank the financial assistance from the Spanish DGICYT and CICYT through Projects PB97-1397 and MAT97-0987. We acknowledge the European Commission for granting the TMR-Network Thermal and Optical Switching of Molecular Spin States (TOSS) , contract no. ERB-FMRX-CT98-0199EEC/TMR.

A l'aide de deux techniques complementaires, nous avons etudie sur une large gamme de temperature la dynamique de relaxation de composes a transition d'etat de spin photo-excites. Ces composes sont particulierement interessants pour le stockage optique d'information. Nous avons ainsi montre qu'il etait possible de stocker durablement une information dans la boucle d'hysteresis de ces composes.