Magnetism and Molecular Nonlinear Optical Second-Order Response

Article pubs.acs.org/JPCC

Magnetism and Molecular Nonlinear Optical Second-Order Response Meet in a Spin Crossover Complex Sébastien Bonhommeau,† Pascal G. Lacroix,*,‡ David Talaga,† Azzedine Bousseksou,‡ Maksym Seredyuk,§,∥ Igor O. Fritsky,§ and Vincent Rodriguez*,† †

Université de Bordeaux, Institut des Sciences Moléculaires, CNRS UMR 5255, F-33400 Talence, France Laboratoire de Chimie de Coordination, CNRS UPR 8241, 205 route de Narbonne, 31077 Toulouse, France § National Taras Shevchenko University, Department of Chemistry, Volodymyrska Street 64, 01033 Kyiv, Ukraine ∥ Universidad de Valencia, Instituto de Ciencia Molecular, Edificios Institutos de Paterna, Catedrático José Beltrán Martínez no. 2, 46980 Paterna, Spain ‡

S Supporting Information *

ABSTRACT: The quadratic hyperpolarizability of two inorganic Schiff base metal complexes which differ from each other by the nature of the central metal ion (FeII or ZnII) is estimated using hyper-Rayleigh light-scattering (HRS) measurements. The investigated FeII microcrystals exhibit a thermal spin-crossover (SCO) from a diamagnetic to a paramagnetic state centered at T1/2 = 233 K that can be reproduced by the HRS signal whose modest intensity is mainly due to their centrosymmetric packing structure. Diamagnetic ZnII microcrystals even lead to much weaker (∼400 times) HRS intensities which are in addition temperature-independent. These observations allow us to ascribe the change in HRS of the FeII complex to two contributions, namely, the molecular SCO phenomenon and the crystal orientation with respect to the light polarization. A connection between the SCO and a nonlinear optical property has thus been demonstrated for the first time, with potential future applications in photonics. by the nature of the central metal ion, namely, {FeII[tren(6Mepy)3]}(ClO4)2 (noted FeTren) and {ZnII[tren(6-Mepy)3]}(ClO4)2 (noted ZnTren), where the tren(6-Mepy)3 ligand is the tris[3-aza-4-(6-methyl-2-pyridyl)but-3-enyl]amine.14,15 FeTren shows a gradual thermal SCO from a low-temperature diamagnetic low-spin (LS) state to a high-temperature paramagnetic high-spin (HS) state at the inversion temperature T1/2 = 233 K as revealed by magnetic susceptibility and Mössbauer measurements carried out on microcrystalline powder14 but also confirmed by Raman spectroscopy measurements (Figures S1−S3 in Supporting Information (SI)). In contrast, ZnTren remains diamagnetic irrespective of the temperature.14

1. INTRODUCTION Since the discovery that a quartz crystal could double the frequency of the output radiation of a ruby laser in 1961, nonlinear optics has rapidly developed and led to the emergence of photonics, where light is used for data processing, transportation, and storage. 1−4 In this field, sustained researches aim notably to design optoelectronic devices based on specific hybrid materials and combining electronic (e.g., magnetic and semiconducting) and nonlinear optical (NLO) properties, with the constant constraint to improve their versatility.5−7 The interplay between nonlinear optics and magnetism has been described recently in several transition metal compounds,8−10 and it has occasionally been discussed theoretically at the molecular level considering quadratic (β)11 and cubic (γ)12 hyperpolarizabilities. However, such a correlation has never been observed experimentally in spincrossover (SCO) complexes, although they are widely addressable materials of high technological relevance due to their photomagnetic and dielectric properties and their preparation as nanopatterned films.13 Furthermore, these materials constitute excellent candidates for the direct observation of NLO switching induced by magnetic switching at the molecular scale. We report here on the first evidence that the first-order hyperpolarizability in a SCO complex can reflect its magnetic behavior. The two selected inorganic complexes differed only © 2012 American Chemical Society

2. EXPERIMENTAL METHODS FeTren and ZnTren microcrystals were synthesized from perchlorate salts Fe(ClO4)2·6H2O and Zn(ClO4)2·6H2O added to an aqueous solution of tris(2-ethylamine)amine and 6methylpicolinaldehyde following a cautious procedure14 and experimentally characterized as prepared. The microabsorption measurements were carried out using a tailor-made setup endowed with a deuterium tungsten-halogen Received: February 16, 2012 Revised: March 27, 2012 Published: March 28, 2012 11251

dx.doi.org/10.1021/jp301552u | J. Phys. Chem. C 2012, 116, 11251−11255

The Journal of Physical Chemistry C

Article

fiber optic light source Mikropack dh-2000 as the sample excitation source. The emerging unpolarized light was guided by an optical fiber of 100 μm core diameter, was collimated by a 10× objective, traversed a 45° beam splitter 50/50, and was finally focused onto the sample by a 50× objective. The spot size at the sample was 20 μm. The transmitted light was collected by a 10× objective and analyzed by a Horiba Jobin Yvon HR800 confocal spectrometer. This spectrometer equipped with a 150 grooves/mm grating and a Symphony detector provided a 2 nm spectral resolution. SHG measurements were performed using a micro-SHG setup based on a modified micro-Raman spectrometer (Horiba HR800) allowing the analysis of backscattered light and described in detail elsewhere.16−18 Briefly, they were carried out using a diode-pumped picosecond laser (EKSPLA LP2200: pulse duration 65 ps, repetition rate 2kHz) operating at λ = 1064 nm focused at the surface of the sample with a 50× NIR objective (NA = 0.42). The energy per pulse was adjusted by a power unit composed of a rotating half-wave plate in front of a GLAN Taylor prism and was always kept much lower than 1 μJ to avoid sample photodegradation. The incident pulse power was monitored by a fast InGaAs photodiode for quantitative power calibration purposes. The measurement of the HRS intensity I2ω was realized using three different light polarization configurations, namely, the dubbed parallel (//) and perpendicular (⊥) configurations, corresponding to incident and scattered lights with collinear or orthogonal polarizations, respectively (Figure 1), and another one where the incident

3. THEORETICAL CALCULATIONS In the frame of our Density Functional Theory (DFT) approach, the spin-dependent hyperpolarizability of FeTren and the hyperpolarizability of ZnTren were computed using the finite fields (FF) procedure available in Gaussian03 (reference in SI). The gas phase geometries were first computed at the B3LYP/6-31G* level. The starting geometries were the X-ray data previously reported in the literature.14 Calculations were performed in the C1 point group, to be compared with X-ray data. The spin states were selected introducing the multiplicity in the input files for each computation. A quintuplet ground state was considered for FeIITren in the HS state (4s03d6 ≡ t2g4eg2; S = 2), and two singlet ground states were taken into account for FeIITren in the LS state (4s03d6 ≡ t2g6eg0; S = 0) and ZnIITren (4s03d10 ≡ t2g6eg4; S = 0). In a second step, the static quadratic hyperpolarizabilities were computed at the B3LYP/6-31G** level, from the derivative procedure βijk = −(∂3W/∂Ei∂Ej∂Ek)E=0, where W is the energy, E the electric field, and i, j, k can be each of the x, y, z molecular Cartesian coordinates. This expression is only valid in the static field limit. Experimental HRS intensity I2ω as a function of the polarization configuration and the sample rotation angle θ in HS FeTren were fitted using an in-house generalized NLO ellipsometry method19,20 taking into account static hyperpolarizability tensor components determined by DFT calculations (Table S4 in SI) but adjusting the crystal orientation (three Euler angles). The same procedure was employed to predict the behavior in the LS state considering DFT calculations in this specific state but keeping the same molecular orientation as in the HS state owing to the absence of experimental data in this case. To assess the IHRS intensities and facilitate comparison with experiments, the simulated I2ω values were divided by the square of the constant Iω incident light intensity applied to excite the sample. 4. RESULTS AND DISCUSSION Elastic second-order NLO properties of the aforementioned inorganic complexes have been investigated by means of hyperRayleigh light scattering (HRS) measurements. 21 The excitation wavelength was 1064 nm, while light scattered at 532 nm was collected. As illustrated by absorption spectra of FeTren microcrystals in the HS and LS states (Figure 2), the absorption at 532 nm remains nearly the same irrespective of the spin state. In addition, FeTren shows a marked thermochromism typified by a strong absorption band centered at about 540 nm in the HS state and at 600 nm in the LS state. This band was assigned to metal-to-ligand charge transfer (MLCT) in compounds belonging to the same family.22 On the contrary, ZnTren seems almost transparent and does not exhibit any noticeable absorption change in the 400−1000 nm range (Figure 2). Furthermore, since FeTren and ZnTren microcrystals retain the space group P21/c whatever the temperature,14 they are centrosymmetric, and no (macroscopic) coherent SHG signal is expected. Therefore, only (molecular) HRS occurs. The signals at 532 nm in HS and LS FeTren should be similarly enhanced due to absorption and dispersion effects, whereas much weaker NLO molecular signatures should appear in ZnTren. Figure 3 provides the normalized HRS intensity IHRS as a function of temperature for FeTren and ZnTren. This intensity is defined as the ratio I2ω/Iω2 of the HRS intensity I2ω and the square of the incident laser intensity Iω. It is immediately

Figure 1. Orientation conventions for the backscattered parallel (//) and perpendicularly (⊥) polarized light (I2ω intensity) with respect to the incident light (Iω intensity). The sample rotation angle θ is also indicated.

excitation laser light remains linearly polarized but the scattered light is the sum of the // and ⊥ contributions. Note that the response I2ω with respect to the sample rotation angle θ was recorded in the // and ⊥ light polarization configurations only for FeTren microcrystals in the HS state. Indeed, measurements with respect to the sample rotation angle θ required us to mount the glass slide onto which the microcrystalline powder was deposited on a rotation plate. As it was not possible to fix the cryostat on the rotation plate and move the whole device during the measurements because of experimental hindrance due to electrical cables and cryogenic pipes, the determination of the HRS intensity as a function of the sample rotation angle θ could not be achieved in the LS state. The origin of the θ angle (θ = 0°) was arbitrary. 11252



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molecules in FeTren microcrystals14 and an orientational one depending on the molecular crystal positioning with respect to the light polarization. To describe the molecular contribution in the observed experimental trends, the hyperpolarizability tensor components of FeTren and ZnTren have been numerically assessed using a DFT-based approach (Table S4 in SI). From these components, we calculated an isotropic average to extract the dipolar and octupolar contributions in the hyperpolarizability (Table 1).23 Albeit a crude approximation for a crystal, this Table 1. Isotropic Averages of Static Quadratic Hyperpolarizability β Tensor Components [au] and Nonlinear Anisotropy ρ for FeTren and ZnTren Determined by DFT Calculations a βHRS

compound

Figure 2. Absorption spectra of single microcrystals of FeTren in the LS and HS states and ZnTren, measured at 150, 310, and 310 K, respectively. The inset shows the ZnTren (bottom) and FeTren (top) single crystals (pictures on the left side) as well as the small area selected for the absorption measurement (pictures on the right side).

FeTren (LS) FeTren (HS) ZnTren a c

βdipolar

b

617 751 214

d

βoctupolar

1143 690 406

1 au = 8.641 × 10−33 esu.

b

e

972 2198 307

ρ

c

0.85 3.18 0.76

βHRS = βdipolar[2/3(1/3 + ρ2/7)]1/2.

ρ = βoctupolar /βdipolar d

βdipolar 2 =

3 5

x ,y ,z 2 ∑ βζζζ + ζ

6 5

x ,y ,z

∑ βζζζ βζηη + ζ≠η

3 5

x ,y ,z 2 ∑ βηζζ ζ≠η

x ,y ,z

3 + ∑ β β 5 ζ ≠ η ≠ δ ηζζ ηδδ e

x ,y ,z

βoctupolar 2 =

Figure 3. Temperature dependence of the normalized HRS intensity IHRS (in arbitrary units) of the probed area in a single microcrystal of FeTren (filled circles) and ZnTren (open circles) and magnetic susceptibility measurements of FeTren microcrystalline powder (filled squares). Solid lines are guides for the eye. The inset provides a sketch of the local symmetry around the metal center in the complexes.

2 ∑ β2 − 6 5 ζ ζζζ 5 x ,y ,z

x ,y ,z

∑ βζζζ βζηη + ζ≠η

12 5

x ,y ,z 2 ∑ βηζζ ζ≠η

x ,y ,z

3 2 − ∑ βηζζ βηδδ + 1 ∑ βζηδ 5 ζ≠η≠δ 4 ζ≠η≠δ

method has the merit of highlighting molecular effects. Such a determination is crucial since inorganic complexes may show either dominant dipolar or octupolar characters depending on the directionality of the MLCT.24,25 In our case, both dipolar and octupolar characters coexist. The LS FeTren and ZnTren complexes exhibit a rather dipolar hyperpolarizability (ρ < 1), while HS FeTren appears strongly octupolar (ρ > 3), as reflected by the nonlinear anisotropy ρ (Table 1). Therefore, a significant multidirectional MLCT should occur in the HS FeTren molecule, replaced by a much more unidirectional one in the others. All the investigated molecules display an approximate local 3-fold pseudosymmetry C3 (Tables S1−S3 in SI).21 Moreover, ZnTren and HS FeTren present similar structures according to X-ray diffraction data,14 and the HS FeII complex is the only one showing half-filled eg orbitals26 and a strong MLCT band in the visible range (Figure 2). This indicates that the electronic structure, and in particular the MLCT from the metal eg orbitals, should play a major role in the observed octupolar character. It is also apparent that βHRS is larger in HS FeTren than in LS FeTren and much larger than in ZnTren (Table 1), which is in good qualitative accordance with observations reported in Figure 3. However, the increase in βHRS predicted by DFT calculations is too small to be

proportional to the square of the quadratic hyperpolarizability βHRS thereby.21 ZnTren presents a very low IHRS of about 0.1 (arbitrary units) as expected from the lack of MLCT transition (Figure 2). In addition, this intensity does not show any noticeable temperature dependence. On the contrary, FeTren exhibits both a high IHRS and significant variations with temperature. IHRS is about 23 (arbitrary units) from 150 to 210 K, starts increasing subsequently, and eventually reaches a plateau at about 63 (arbitrary units) from 260 K. The stronger SHG signature in HS and LS FeTren is consistent with the enhanced signal expected at 532 nm due to dispersion effects close to the MLCT resonance (Figure 2).21 It is particularly remarkable that the observed inversion temperature is 237 K, which is extremely close to the 233 K value determined from magnetic susceptibility measurements (Figure 3).14 Nevertheless, it is worth pointing out that reproducing many times the experiment with single microcrystals of various sizes and shapes showed that a HRS intensity higher in the LS than in the HS state might also be obtained in a few cases. This latter result suggests the coexistence of two contributions, namely, a molecular one related to the thermal SCO phenomenon ascribed to the relatively weak intermolecular contacts between 11253



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simulated curves presented in Figure 4 take account of FeTren structures calculated by DFT (Tables S1−S3 in SI) and of the spin state change thereby. The summation of // and ⊥ contributions of the HRS intensity in the LS and HS states can be easily deduced (Figure 4c) and directly compare data plotted in Figure 3. It reveals that, depending on the microcrystal orientation, it is possible to obtain a HRS intensity stronger in the HS than in the LS state (for rotation angles in the 0−94° and 152−180° ranges) or the exact opposite (for rotation angles in the 94−152° range). Thus, if the orientation angles of a crystal are drawn randomly, there is a higher (angular) probability to obtain a stronger HRS intensity in the HS state in comparison with the LS state, which nicely corroborates our experimental observations. In particular, we can infer from Figure 4c that normalized HRS intensities reported in Figure 3 originate from the excitation of a FeTren sample tilted by a θ = 40° angle. The most striking observation lies in the systematic difference in IHRS in HS and LS FeTren whatever the sample orientation except for two specific angles (θ = 94° and 152° here) where HRS values are the same.

exclusively responsible for the experimentally measured behaviors. The molecular crystal orientation should also affect the resulting HRS intensity. To test this hypothesis, we evaluated the normalized HRS intensity IHRS with respect to the sample rotation angle θ in HS FeTren (Figure 4a) for parallel (//) and perpendicular (⊥)

5. CONCLUSION In summary, the SCO phenomenon and quadratic hyperpolarizability have been proven to be correlated to each other. The HRS signal can reflect the thermal spin state change and give insight into molecular structural dipolar/octupolar characters, even though its intensity is modulated by the crystalline sample orientation. Despite the relatively low second-order NLO response compared with other NLO systems,16,27 our findings turn out to be of an obvious fundamental interest and promising for the design of highly addressable photonic devices based on SCO materials. To achieve this goal, the SHG signal could be markedly enhanced in noncentrosymmetric SCO crystals that exhibit inherent (macroscopic) SHG signatures and are prone to be excited on resonance with specific electronic transitions to optimize dispersion effects.

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Figure 4. (a) Experimental (circles) and simulated (lines) normalized HRS intensities IHRS (in arbitrary units) as a function of the HS FeTren sample rotation angle θ for // (open circles and dashed line) and ⊥ (black circles and solid line) polarization configurations. (b) Simulated normalized HRS intensities IHRS (in arbitrary units) as a function of the LS FeTren sample rotation angle θ for // (dashed line) and ⊥ (solid line) polarization configurations. (c) Total simulated normalized HRS intensity IHRS (in arbitrary units) in the LS (blue solid line) and HS (red solid line) states and experimental IHRS in the HS state (red open circles) as a function of the FeTren sample rotation angle θ. Filled red and blue circles indicate the IHRS value extracted from Figure 3 in the HS and LS states, respectively.

ASSOCIATED CONTENT

S Supporting Information *

Raman spectroscopy data revealing the change in spin state upon thermal SCO; calculated structures and static quadratic hyperpolarizabilities for FeTren and ZnTren complexes using DFT; full reference of computational chemistry software program Gaussian03. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] and [email protected].

light polarization configurations. As expected, minima for IHRS (//) are associated with maxima for IHRS(⊥) and vice versa. Fitting these experimental intensities allowed us to estimate the molecular orientation in HS FeTren microcrystals with respect to the light polarization. Considering that this orientation is preserved upon thermal SCO, we were able to predict the nonmeasurable normalized HRS intensities in the LS state (Figure 4b). These intensities markedly differ from their counterparts in the HS state because the calculated Cartesian components of quadratic hyperpolarizability tensors in the two spin states strongly depend on the dipolar/octupolar structural changes involving the ligand (Table 1). It is also worth pointing out that

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge Dr. Y. Danten for fruitful discussions and CALMIP (Calcul en Midi-Pyrénées) for computation facilities. This work was supported by the University of Bordeaux and the CNRS (Centre National de la Recherche Scientifique) through the excellence chair of S. Bonhommeau and by the Marie Curie IIF Scheme of the seventh EU Framework Program for M. Seredyuk. 11254



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REFERENCES

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Magnetism and Molecular Nonlinear Optical Second-Order Response

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