A Chiral Bis(salicylaldiminato)zinc(II) Complex with Second-Order Nonlinear Optical and Luminescent Properties in Solution

Whereas there is an increasing amount of reports on the second-order nonlinear optical (NLO) and luminescence properties of tetradentate [N2O2] Schiff base–zinc complexes, the study of zinc complexes having two bidentate [NO] Schiff-base ligands is relatively unexplored from an NLO point of view. This work puts in evidence that the known chiral bis{2-[(R)-(+)-1-phenylethyliminomethyl]phenolato-N,O}zinc(II) complex is a fascinating multifunctional molecular inorganic–organic hybrid material characterized by interesting second-order NLO and luminescent properties in solution. The emissive properties of the organic 2-(R)-(+)-1-phenylethyliminomethyl]phenol proligand are greatly enhanced upon coordination to the inorganic Zn(II) center.

An increasing amount of work has been devoted to low-cost zinc(II) complexes for their interesting third-order [37][38][39][40] and second-order NLO [41] and luminescent [42] properties. Due to its d 10 configuration, the zinc(II) center does not have a favorite stereochemistry caused by ligand field stabilization effects. Therefore, depending on the nature of the ligands, Zn(II) complexes can have various geometries (tetrahedral, square pyramidal, octahedral) and number of coordination (4, 5, and 6), affording NLO-active dipolar and octupolar compounds. Besides, they are particularly

Results and Discussion
The known 2-(R)-(+)-1-phenylethyliminomethyl]phenol was prepared as previously reported by reaction of (R)-1-phenylethylamine with salicylaldehyde in refluxing methanol, as shown in Scheme 1 [84]. Deprotonation with NaHCO3 and reaction with zinc(acetate)2 in refluxing ethanol afforded the related bis{2-[(R)-(+)-1-phenylethyliminomethyl]phenolato-N,O}zinc(II) complex [78,80]. The second-order NLO properties of 2-(R)-(+)-1-phenylethyliminomethyl]phenol and of the related zinc(II) complex were studied by means of the EFISH method. This technique [81][82][83] affords information on the molecular second-order NLO properties, through the following Equation (1): in which µβλ/5kT represents the dipolar orientational contribution to the molecular nonlinearity, whereas γ (−2ω;ω,ω,0) is the third-order polarizability, an electronic cubic contribution to γEFISH which is usually neglected when investigating the second-order NLO properties of dipolar molecules. βλ represents the projection along the ground-state dipole moment (µ) axis of the vectorial component of the tensor of the quadratic hyperpolarizability (βVEC), when the incident wavelength of the pulsed laser is λ. In order to have a compound of interest for second-order NLO applications, one needs a high µβEFISH value. Extrapolation to zero frequency (νΛ = 0.0 eV; λ = ∞) allows the determination of µβ0, the product of the ground-state dipole moment by the static quadratic hyperpolarizability β0, a useful figure of merit to evaluate the basic second-order NLO properties of a molecular material. The µβ0 value can be obtained by using the following Equation (2): where βλ is the quadratic hyperpolarizability value at the incident wavelength λ, and λmax is the absorption wavelength of the major charge-transfer transition considered. Besides, it is essential to avoid overestimation of the quadratic hyperpolarizabilty value due to resonance enhancements. For this reason, one has to use an incident wavelength whose second harmonic is remote from any absorption of the molecule investigated. In the present study, we chose

Results and Discussion
The known 2-(R)-(+)-1-phenylethyliminomethyl]phenol was prepared as previously reported by reaction of (R)-1-phenylethylamine with salicylaldehyde in refluxing methanol, as shown in Scheme 1 [84]. Deprotonation with NaHCO3 and reaction with zinc(acetate)2 in refluxing ethanol afforded the related bis{2-[(R)-(+)-1-phenylethyliminomethyl]phenolato-N,O}zinc(II) complex [78,80]. The second-order NLO properties of 2-(R)-(+)-1-phenylethyliminomethyl]phenol and of the related zinc(II) complex were studied by means of the EFISH method. This technique [81][82][83] affords information on the molecular second-order NLO properties, through the following Equation (1): in which µβλ/5kT represents the dipolar orientational contribution to the molecular nonlinearity, whereas γ (−2ω;ω,ω,0) is the third-order polarizability, an electronic cubic contribution to γEFISH which is usually neglected when investigating the second-order NLO properties of dipolar molecules. βλ represents the projection along the ground-state dipole moment (µ) axis of the vectorial component of the tensor of the quadratic hyperpolarizability (βVEC), when the incident wavelength of the pulsed laser is λ. In order to have a compound of interest for second-order NLO applications, one needs a high µβEFISH value. Extrapolation to zero frequency (νΛ = 0.0 eV; λ = ∞) allows the determination of µβ0, the product of the ground-state dipole moment by the static quadratic hyperpolarizability β0, a useful figure of merit to evaluate the basic second-order NLO properties of a molecular material. The µβ0 value can be obtained by using the following Equation (2): where βλ is the quadratic hyperpolarizability value at the incident wavelength λ, and λmax is the absorption wavelength of the major charge-transfer transition considered. Besides, it is essential to avoid overestimation of the quadratic hyperpolarizabilty value due to resonance enhancements. For this reason, one has to use an incident wavelength whose second harmonic is remote from any absorption of the molecule investigated. In the present study, we chose The second-order NLO properties of 2-(R)-(+)-1-phenylethyliminomethyl]phenol and of the related zinc(II) complex were studied by means of the EFISH method. This technique [81][82][83] affords information on the molecular second-order NLO properties, through the following Equation (1): in which µβ λ /5kT represents the dipolar orientational contribution to the molecular nonlinearity, whereas γ (−2ω;ω,ω,0) is the third-order polarizability, an electronic cubic contribution to γ EFISH which is usually neglected when investigating the second-order NLO properties of dipolar molecules. β λ represents the projection along the ground-state dipole moment (µ) axis of the vectorial component of the tensor of the quadratic hyperpolarizability (β VEC ), when the incident wavelength of the pulsed laser is λ. In order to have a compound of interest for second-order NLO applications, one needs a high µβ EFISH value. Extrapolation to zero frequency (ν Λ = 0.0 eV; λ = ∞) allows the determination of µβ 0 , the product of the ground-state dipole moment by the static quadratic hyperpolarizability β 0 , a useful figure of merit to evaluate the basic second-order NLO properties of a molecular material. The µβ 0 value can be obtained by using the following Equation (2): where β λ is the quadratic hyperpolarizability value at the incident wavelength λ, and λ max is the absorption wavelength of the major charge-transfer transition considered.
Besides, it is essential to avoid overestimation of the quadratic hyperpolarizabilty value due to resonance enhancements. For this reason, one has to use an incident wavelength whose second harmonic is remote from any absorption of the molecule investigated. In the present study, we chose an incident wavelength of 1.907 µm, obtained by Raman-shifting the fundamental 1.064 µm wavelength produced by a Q-switched, mode-locked Nd:YAG laser.
We found that both 2-(R)-(+)-1-phenylethyliminomethyl]phenol and the related zinc(II) complex are characterized by a positive value of µβ λ , in agreement with a positive value of ∆µ eg (difference of the dipole moment in the excited and ground states) upon excitation, according to the "two-level" model [85,86].
In order to determine the β 1.907 and β 0 values, it is necessary to know µ. Therefore, the geometry of the zinc complex was optimized, and its dipole moment was calculated by means of the Density Functional Theory. We used the B3LYP exchange correlation functional, the 6-311g** basis set for all atoms except for Zn, which has been described with the LANL2DZ basis set, along with the corresponding pseudopotentials. The CHCl 3 solution effects were included by means of the conductor-like polarizable continuum model. Geometry optimization, performed with Gaussian09 [88] without any symmetry constraints, showed a pseudo-C 2 symmetry (Figure 2). In agreement with the structure determined by X-ray crystallography, the dihedral angle between plane O 1 ZnN 1 and plane O 2 ZnN 2 is 84.4 • , so that the geometry around the zinc atom is almost tetrahedral, with a small distortion on the way to a cis-planar geometry [78]. The computed dipole moment, 6.72 D, is aligned on the bisector of O 1 ZnO 2 angle from the oxygen atoms to the metal center. an incident wavelength of 1.907 µm, obtained by Raman-shifting the fundamental 1.064 µm wavelength produced by a Q-switched, mode-locked Nd:YAG laser. We found that both 2-(R)-(+)-1-phenylethyliminomethyl]phenol and the related zinc(II) complex are characterized by a positive value of µβλ, in agreement with a positive value of Δµeg (difference of the dipole moment in the excited and ground states) upon excitation, according to the "two-level" model [85,86].
In order to determine the β1.907 and β0 values, it is necessary to know µ. Therefore, the geometry of the zinc complex was optimized, and its dipole moment was calculated by means of the Density Functional Theory. We used the B3LYP exchange correlation functional, the 6-311g** basis set for all atoms except for Zn, which has been described with the LANL2DZ basis set, along with the corresponding pseudopotentials. The CHCl3 solution effects were included by means of the conductor-like polarizable continuum model. Geometry optimization, performed with Gaussian09 [88] without any symmetry constraints, showed a pseudo-C2 symmetry (Figure 2). In agreement with the structure determined by X-ray crystallography, the dihedral angle between plane O1ZnN1 and plane O2ZnN2 is 84.4°, so that the geometry around the zinc atom is almost tetrahedral, with a small distortion on the way to a cis-planar geometry [78]. The computed dipole moment, 6.72 D, is aligned on the bisector of O1ZnO2 angle from the oxygen atoms to the metal center. As a further step, we were curious to investigate the luminescence properties of the complex. Indeed, there is interest in the study of organic-inorganic hybrid materials consisting of Schiff base-Zn(II) complexes in polymethyl methacrylate (PMMA) to impart new features such as luminescence As a further step, we were curious to investigate the luminescence properties of the complex. Indeed, there is interest in the study of organic-inorganic hybrid materials consisting of Schiff base-Zn(II) complexes in polymethyl methacrylate (PMMA) to impart new features such as luminescence properties [89]. Akitsu et al. prepared various organic-inorganic materials containing an organic photochromic dye and a chiral Schiff base-zinc(II) complex in PMMA in order to obtain multi-input and multi-output digital logic circuits [90]. They proposed that a chiral Schiff base-zinc(II) complex could be used in logic circuits in combination with a photochromic dye by using intermolecular quenching of emission [91]. Of interest for this application was bis{2-[(R)-(+)-1-phenylethyliminomethyl]phenolato-N,O}zinc(II), which emits at 451 nm, in combination with spiropyran (or its photoisomerized form merocyanine) [91], but the quantum yield of this complex was not reported. These observations prompted us to reinvestigate its photophysical properties.
Whereas 2-(R)-(+)-1-phenylethyliminomethyl]phenol is not luminescent, the related zinc(II) complex is intensely luminescent in dichloromethane solution, displaying a band at 451 nm upon excitation at 372 nm ( Figure 3), a behavior that puts in evidence the strong luminescent effect of complexation to a Zn(II) center. The luminescent quantum yield (ϕ lum = 0.17) was very good, being similar to that of the most luminescent Schiff base-zinc(II) complexes [70]. properties [89]. Akitsu et al. prepared various organic-inorganic materials containing an organic photochromic dye and a chiral Schiff base-zinc(II) complex in PMMA in order to obtain multi-input and multi-output digital logic circuits [90]. They proposed that a chiral Schiff base-zinc(II) complex could be used in logic circuits in combination with a photochromic dye by using intermolecular quenching of emission [91]. Of interest for this application was bis{2-[(R)-(+)-1phenylethyliminomethyl]phenolato-N,O}zinc(II), which emits at 451 nm, in combination with spiropyran (or its photoisomerized form merocyanine) [91], but the quantum yield of this complex was not reported. These observations prompted us to reinvestigate its photophysical properties. Whereas 2-(R)-(+)-1-phenylethyliminomethyl]phenol is not luminescent, the related zinc(II) complex is intensely luminescent in dichloromethane solution, displaying a band at 451 nm upon excitation at 372 nm ( Figure 3), a behavior that puts in evidence the strong luminescent effect of complexation to a Zn(II) center. The luminescent quantum yield (φlum= 0.17) was very good, being similar to that of the most luminescent Schiff base-zinc(II) complexes [70].
EFISH measurements were carried out in CHCl 3 solutions at a concentration of 10 -3 M, with a non-resonant incident wavelength of 1.907 µm, obtained by Raman-shifting the fundamental 1.064 µm wavelength produced by a Q-switched, mode-locked Nd:YAG laser manufactured by Atalaser (see Supplementary Materials). The reported µβ EFISH values are the mean values of 16 measurements performed on the same sample, the error is ca. 10%.

Conclusions
In conclusion, this work unveils the interesting properties in solution of the chiral bis{2-[(R)-(+)-1-phenylethyliminomethyl]phenolato-N,O}zinc(II) complex. It was previously reported that powder samples of this complex exhibit second-harmonic generation [78]. The present study shows that this hybrid inorganic-organic compound is also characterized by good second-order NLO properties at the molecular level in solution. This is an interesting aspect, from an applicative point of view, because it suggests that dispersion of this complex in organic matrices is a promising route for the preparation of NLO-active polymeric films. This fascinating zinc complex is characterized by multifunctional properties. As a matter of fact, it is also intensely luminescent at 451 nm in dichloromethane, with a quantum yield (ϕ lum = 0.17) similar to that of the most luminescent tetradentate [N 2 O 2 ] Schiff base-zinc(II) complexes. It is worth noting that the free ligand 2-(R)-(+)-1-phenylethyliminomethyl]phenol is not luminescent, a behavior that puts in evidence the strong effect of complexation of the organic ligand to the inorganic Zn(II) center on the luminescent properties. This work opens the door to the exploration of related bidentate [NO]-zinc(II) complexes with various substituents on the phenolato moiety to understand their effects on the second-order NLO and luminescence properties.