Zinc ( II ) as a Versatile Template for Efficient Dipolar and Octupolar Second-Order Nonlinear Optical Molecular Materials

This short review outlines the main results obtained in the field of molecular materials based on zinc coordination compounds for second-order nonlinear optics. It presents an overview of the main classes of second-order nonlinear optical (NLO) active complexes bearing monodentate, bidentate, tridentate, or tetradentate π-delocalized ligands such as substituted stilbazoles, bipyridines, phenanthrolines, terpyridines, and Schiff bases. Macrocyclic ligands such as porphyrins and phthalocyanines are not covered. This paper shows how coordination to the Zn(II) center of π-delocalized nitrogen donor ligands produces a significant enhancement of their quadratic hyperpolarizability. Dipolar complexes are mainly presented, but octupolar zinc complexes are also presented. The coverage is mainly focused on NLO properties that are measured at the molecular level, working in solution, by means of the electric field-induced second harmonic generation (EFISH) or the hyper-Rayleigh scattering (HRS) techniques.

In the panorama of NLO active coordination compounds, low-cost zinc complexes have progressively occupied a role of primary interest for their third-order [22][23][24][25] and second-order [19][20][21] NLO properties.Due to the d 10 configuration of the metal center, zinc(II) complexes do not exhibit stereochemistry preference arising from ligand field stabilization effects.Therefore, they can adopt a variety of geometries and coordination numbers, depending on the nature and structure of the ligand
NLO is due to the interaction of an applied electromagnetic field with a molecule or material leading to the emission of a new electromagnetic field which differs in frequency, phase, or other physical properties from the incident one [1][2][3].One important nonlinear optical process is the second harmonic generation (SHG), which is due to the interaction of two incident waves, having ω frequency, with the molecule characterized by a quadratic hyperpolarizability value β to produce a new wave with the frequency of 2ω.
In order to obtain molecular materials characterized by high second-order NLO effects, it is necessary to have high values of β.Oudar gave an interpretation of the electronic origin of β [29][30][31].The hyperpolarizability of a compound depends on the mobility of electrons under the effect of an electric field → E related to light; therefore, it requires electronic transitions with high charge-transfer character.Oudar assumed that, in suitable organic chromophores, the second-order NLO response is dominated by one charge-transfer process, so that: where z is the direction of the charge transfer, ν eg is the frequency of the charge-transfer transition, r eg is the transition dipole moment, ∆µ eg is the difference between the excited state and ground state molecular dipole moment (µ e − µ g ) and ν L is the frequency of the incident radiation.This equation represents the so-called "two-level" model.Extrapolation to zero frequency (ν L = 0.0 eV; λ = ∞) gives the static quadratic hyperpolarizability β 0 [32], which is the figure of merit to determine the second-order NLO properties of a molecule, and can be calculated by using the following equation: where β λ is the quadratic hyperpolarizability value at λ incident wavelength, and λ max is the absorption wavelength of the major charge-transfer transition that is considered [33].
This "two-level" model helps to prepare new efficient molecule-based NLO materials.In fact, one can extrapolate the requirements that a molecule must have to show a large second-order NLO response: it should be non-centrosymmetric, with charge-transfer transitions with high ∆µ eg and r eg values.This can be accomplished by the separation of an electron donor and an electron acceptor moiety with a π-conjugated polarizable bridge, for example, in D-π-A systems.It appeared that not only dipolar structures but also multipolar systems, such as octupoles, may be of interest for SHG.The octupolar molecules, which are characterized by the presence of multidirectional CT excitations, should have a twofold (D 2 ) or threefold (D 3 ) rotational axis.The description of the nonlinearity of such systems implies a three-level approach.
Two techniques, the electric field-induced second harmonic generation (EFISH) and hyper-Rayleigh scattering (HRS), which is also called harmonic light scattering (HLS), allow the determination of β in solution.The EFISH method [34] is suitable for dipolar molecules only, and gives the quadratic hyperpolarizability through the following equation: where µβ λ /5kT is the dipolar orientational contribution and γ (−2ω;ω,ω,0), a third-order term at frequency ω of the incident light, is the electronic contribution, which is negligible for molecules with a low electronic polarizability.β λ is the projection along the dipole moment axis of β VEC , which is the vectorial component of the tensor of the quadratic hyperpolarizability, working with an incident wavelength λ of a pulsed laser.To obtain the value of β λ , the ground state dipole moment µ of the molecule should be known.
In the HRS technique [35][36][37], the detection of the incoherently scattered second harmonic, generated by the molecule in solution under irradiation with a laser of wavelength λ, leads to the mean value of the β × β tensor product.From the polarization dependence of the second harmonic signal, which was evaluated by selecting the polarization of the incident and scattered radiation, one has information on the components of the quadratic hyperpolarizability tensor β.Contrarily to EFISH, HRS is used also for octupolar molecules.In this review, the quadratic hyperpolarizability determined with an incident wavelength λ by the EFISH and HRS techniques will be indicated as β λ (EFISH) and β λ (HRS), respectively.

Monodentate Nitrogen Ligands: Stilbazoles
The effect of coordination to a zinc center on the second-order NLO response of stilbazole ligands bearing a strong electron-donating group such as NMe 2 has been deeply studied by Ugo et al. by means of the EFISH technique in solution [38][39][40].In these systems, the quadratic hyperpolarizability is dominated by the stilbazole ILCT transition.The metal center behaves as an inductive electron acceptor, which can increase the acceptor strength of the π* orbitals of the pyridine, and therefore produce a red-shift of the ILCT transition with an increase of the quadratic hyperpolarizability.The relevant data for this family of complexes are collected in Table 1.
Remarkably, the quadratic hyperpolarizability of both Zn(II) triflate complexes (3, 7) increases abruptly by decreasing the concentration, up to very large values.For example, the β1.91(EFISH) value of 3 is 163 × 10 −30 , 220 × 10 −30 and 404 × 10 −30 esu working in CHCl3 solution at 5 × 10 −4 , 1 × 10 −4 and 0.5 × 10 −4 M, respectively.This behavior, which is not observed for the related acetate or trifluoroacetate complexes, can be attributed to an increased concentration of the [Zn(CF3SO3)(4,4′-trans-Me2N-C6H4(CH=CH)nC5H4N)2] + (n = 1, 2) cation produced by solvolysis of the triflate ligand.This explanation was confirmed by electrical conductivity measurements, which showed a sharp conductivity increase at concentrations below 10 −4 M for triflate complexes [39].A similar behavior of the quadratic hyperpolarizability in CHCl3 solution upon dilution was observed in the case of Zn(II) complexes with the methanesulfonate or para-toluene sulfonate anions instead of the triflate anion as ancillary ligands [40].Therefore, ionic dissociation by working in diluted CHCl3 solution is a behavior that is typical of all Zn(II) complexes bearing an ancillary sulfonate ligand.Similarly, the quadratic hyperpolarizability of 4,4 -trans,trans-NMe 2 -C 6 H 4 (CH=CH) 2 C 5 H 4 N is increased upon coordination to a Zn(II) center, in which the enhancement is much larger with "Zn(CF 3 SO 3 ) 2 " (7) than with "Zn(CH 3 CO 2 ) 2 " (6).This is in agreement with a higher Lewis acidity of the Zn(II) center, as confirmed by the much larger red-shift of the ILCT transition upon coordination (∆λ max = 123 and 10 nm, for CF 3 SO 3 and CH 3 CO 2 , respectively; Table 1) [39].
Remarkably, the quadratic hyperpolarizability of both Zn(II) triflate complexes (3, 7) increases abruptly by decreasing the concentration, up to very large values.For example, the β 1.91 (EFISH) value of 3 is 163 × 10 −30 , 220 × 10 −30 and 404 × 10 −30 esu working in CHCl 3 solution at 5 × 10 −4 , 1 × 10 −4 and 0.5 × 10 −4 M, respectively.This behavior, which is not observed for the related acetate or trifluoroacetate complexes, can be attributed to an increased concentration of the [Zn(CF 3 SO 3 )(4,4 -trans-Me 2 N-C 6 H 4 (CH=CH) n C 5 H 4 N) 2 ] + (n = 1, 2) cation produced by solvolysis of the triflate ligand.This explanation was confirmed by electrical conductivity measurements, which showed a sharp conductivity increase at concentrations below 10 −4 M for triflate complexes [39].A similar behavior of the quadratic hyperpolarizability in CHCl 3 solution upon dilution was observed in the case of Zn(II) complexes with the methanesulfonate or para-toluene sulfonate anions instead of the triflate anion as ancillary ligands [40].Therefore, ionic dissociation by working in diluted CHCl 3 solution is a behavior that is typical of all Zn(II) complexes bearing an ancillary sulfonate ligand.

Bidentate Nitrogen Ligands: Bipyridines, Phenanthrolines, and Diazafluorens
The second-order NLO response of various bidentate chelating nitrogen ligands such as bipyridine, phenanthroline, and diazafluoren increases upon coordination to a zinc center, as observed in the case of stilbazoles (see Section 3.1).The enhancement factor of the quadratic hyperpolarizability depends on both chelating ligands and ancillary ones [19].

Bidentate Nitrogen Ligands: Bipyridines, Phenanthrolines, and Diazafluorens
The second-order NLO response of various bidentate chelating nitrogen ligands such as bipyridine, phenanthroline, and diazafluoren increases upon coordination to a zinc center, as observed in the case of stilbazoles (see Section 3.1).The enhancement factor of the quadratic hyperpolarizability depends on both chelating ligands and ancillary ones [19].
The development of switchable nonlinear optical (NLO) materials is of recent interest.An elegant approach to the reversible switching of NLO properties is the use of photochromic moieties such as dithienylethene (DTE) [20].In fact, DTE derivatives undergo reversible interconversion between an unconjugated open form and a π-conjugated closed form when irradiated in the UV and visible spectral ranges, respectively.Therefore, with the aim of photoswitching the NLO properties, Le Bozec et al. prepared novel dipolar Zn(II) complexes bearing a 4,4′-bis(ethenyl)-2,2′-bipyridine ligand functionalized by phenyl and dimethylaminophenyl DTE groups [46] (Figure 2).The NLO response for the open forms is low (μβ1.91(EFISH)= 90 and 200 × 10 −48 esu, for D = H and NMe2, respectively; 10) due to the lack of π-conjugation between the thiophene moieties.As expected, upon conversion to the closed form by irradiation with a suitable wavelength, the NLO response increases (μβ1.91(EFISH)= 2020 and 4220 × 10 −48 esu for D = H and NMe2, respectively; 11) due to the more efficient delocalization of the π-electron system.As expected, the complex featuring the strongly electron-donating dimethylamino substituent shows the largest NLO response.These novel Besides, Le Bozec et al. reported that the ZnCl 2 complex bearing a 4,4 -bis(p(dibutylamino)styryl)[2,2 ]-bipyridine (complex 9, Figure 1) is characterized by a µβ 1.34 (EFISH) of 1420 × 10 −48 esu [44].Remarkably, this value is much higher than that of the related nonchelated complex [ZnCl 2 (4,4 -trans-NMe 2 -C 6 H 4 CH=CHC 5 H 4 N) 2 ] [38], due to the planar arrangement of the bidentate ligand upon coordination, and a shift of the ILCT transition at lower energy [19,45].Substitution of the two HC=CH groups of the bipyridine ligand by azo moieties leads to an increase of the NLO response, but this effect is due to resonant enhancement [44].Upon coordination of these bipyridines to the ZnCl 2 moiety, a red-shift of the intense ILCT (intraligand charge transfer) band is observed (∆λ max = 45-60 nm), as expected from the inductive acceptor strength of the Lewis acid [44].
The development of switchable nonlinear optical (NLO) materials is of recent interest.An elegant approach to the reversible switching of NLO properties is the use of photochromic moieties such as dithienylethene (DTE) [20].In fact, DTE derivatives undergo reversible interconversion between an unconjugated open form and a π-conjugated closed form when irradiated in the UV and visible spectral ranges, respectively.Therefore, with the aim of photoswitching the NLO properties, Le Bozec et al. prepared novel dipolar Zn(II) complexes bearing a 4,4 -bis(ethenyl)-2,2 -bipyridine ligand functionalized by phenyl and dimethylaminophenyl DTE groups [46] (Figure 2).The NLO response for the open forms is low (µβ 1.91 (EFISH) = 90 and 200 × 10 −48 esu, for D = H and NMe 2 , respectively; 10) due to the lack of π-conjugation between the thiophene moieties.As expected, upon conversion to the closed form by irradiation with a suitable wavelength, the NLO response increases (µβ 1.91 (EFISH) = 2020 and 4220 × 10 −48 esu for D = H and NMe 2 , respectively; 11) due to the more efficient delocalization of the π-electron system.As expected, the complex featuring the strongly electron-donating dimethylamino substituent shows the largest NLO response.These novel photochromic zinc complexes are fascinating, allowing an excellent on/off switching of the NLO responses [46].2) [38].There is a twofold increase of the dipole moment upon coordination to the metal center.Interestingly, the enhancement factor (EF) of the β1.34(EFISH) value of the phenanthroline upon coordination is higher for the better donor group NMe2 (EF = β1.34EFISH)complex/β1.34(EFISH)free phenanthroline = 4.6; 13) than for OMe (EF = 3.2; 12).It becomes less and less relevant by increasing the length of the π-delocalized bridge between the donor group NMe2 and the phenanthroline moiety, (EF = 1.9 and 1.5 for trans-CH=CHC6H4-4′-NMe2 (14) and trans,trans-(CH=CH)2C6H4-4′-NMe2 (15), respectively).It should be pointed out that the increase of the quadratic hyperpolarizability of planar 5-X-1,10-phenanthrolines upon coordination to a Zn(II) center is lower than that of the nonplanar and flexible ligand 4-(p(dibutylamino)styryl)-4′-methyl-2,2′-bipyridine, which is probably because this latter ligand becomes planar and rigid upon coordination [38,42].2) [38].There is a twofold increase of the dipole moment upon coordination to the metal center.Interestingly, the enhancement factor (EF) of the β 1.34 (EFISH) value of the phenanthroline upon coordination is higher for the better donor group NMe 2 (EF = β 1.34 EFISH) complex/β 1.34 (EFISH) free phenanthroline = 4.6; 13) than for OMe (EF = 3.2; 12).It becomes less and less relevant by increasing the length of the π-delocalized bridge between the donor group NMe 2 and the phenanthroline moiety, (EF = 1.9 and 1.5 for trans-CH=CHC 6 H 4 -4 -NMe 2 (14) and trans,trans-(CH=CH) 2 C 6 H 4 -4 -NMe 2 (15), respectively).It should be pointed out that the increase of the quadratic hyperpolarizability of planar 5-X-1,10-phenanthrolines upon coordination to a Zn(II) center is lower than that of the nonplanar and flexible ligand 4-(p(dibutylamino)styryl)-4 -methyl-2,2 -bipyridine, which is probably because this latter ligand becomes planar and rigid upon coordination [38,42].trans-CH=CHC6H4-4′-NMe2 (14)  419 More recently it was observed that, with respect to 5-trans-CH=CHC6H4-4′-NMe2-1,10phenanthroline (Table 2), N,N-dibutyl-( 4,5-diazafluorenyl-9-ylidene-penta-1,3-dienyl)-amine (Table 3) is characterized by a much lower value of the λmax of the ILCT transition (Δλmax = 114 nm), reflecting the large π-delocalization of its structure, and suggesting a large NLO response [47].This was confirmed by EFISH measurements (Table 3) [47].In fact, the μβ1.91 value for this novel free ligand is rather large (998 × 10 −48 esu), and it increases upon coordination to a "Zn(CH3CO2)2" moiety ( 16), due to an increase of the dipole moment and to the red-shift of the ILCT transition.This increase is easily controlled by the Lewis acceptor properties of the metal moiety, and therefore by the ancillary ligands that tune its acceptor properties.Thus, the μβ1.91 value increases upon the substitution of CH3CO2 − (16) by CF3CO2 − (17) or CF3SO3 − (18), as expected from the λmax value dominating the NLO response of the related Zn(II) complexes (Table 3).As previously observed for the stilbazole Zn(II) complexes (Section 3.1) [39], the second-order NLO response of the triflate complex 18 increases exponentially with decreasing concentration (Table 3), which is an effect that is not observed for the related complexes with the acetate or trifluoroacetate ligand, and can be attributed to the formation of cationic Zn(II) species, given the low nucleophilicity of the triflate anion [47].Similarly, the coordination of 9-[(1-azulenyl)methylene]-4,5-diazafluorene to "Zn(CF3CO2)2" (19) leads to an enhancement of the second-order NLO response and the red-shift of the ILCT transition (Table 3).As expected, the μβ1.91 value increases upon the substitution of CF3CO2 − (19) with CF3SO3 − (20).The μβ1.91 value of the triflate complex 20 is also dependent on concentration, but in a minor way with respect to the zinc complex with N,N-dibutyl-(4,5-diazafluorenyl-9-ylidene-penta-1,3-dienyl)-amine (18) [47].More recently it was observed that, with respect to 5-trans-CH=CHC6H4-4′-NMe2-1,10phenanthroline (Table 2), N,N-dibutyl-( 4,5-diazafluorenyl-9-ylidene-penta-1,3-dienyl)-amine (Table 3) is characterized by a much lower value of the λmax of the ILCT transition (Δλmax = 114 nm), reflecting the large π-delocalization of its structure, and suggesting a large NLO response [47].This was confirmed by EFISH measurements (Table 3) [47].In fact, the μβ1.91 value for this novel free ligand is rather large (998 × 10 −48 esu), and it increases upon coordination to a "Zn(CH3CO2)2" moiety ( 16), due to an increase of the dipole moment and to the red-shift of the ILCT transition.This increase is easily controlled by the Lewis acceptor properties of the metal moiety, and therefore by the ancillary ligands that tune its acceptor properties.Thus, the μβ1.91 value increases upon the substitution of CH3CO2 − (16) by CF3CO2 − (17) or CF3SO3 − (18), as expected from the λmax value dominating the NLO response of the related Zn(II) complexes (Table 3).As previously observed for the stilbazole Zn(II) complexes (Section 3.1) [39], the second-order NLO response of the triflate complex 18 increases exponentially with decreasing concentration (Table 3), which is an effect that is not observed for the related complexes with the acetate or trifluoroacetate ligand, and can be attributed to the formation of cationic Zn(II) species, given the low nucleophilicity of the triflate anion [47].Similarly, the coordination of 9-[(1-azulenyl)methylene]-4,5-diazafluorene to "Zn(CF3CO2)2" (19) leads to an enhancement of the second-order NLO response and the red-shift of the ILCT transition (Table 3).As expected, the μβ1.91 value increases upon the substitution of CF3CO2 − (19) with CF3SO3 − (20).The μβ1.91 value of the triflate complex 20 is also dependent on concentration, but in a minor way with respect to the zinc complex with N,N-dibutyl-(4,5-diazafluorenyl-9-ylidene-penta-1,3-dienyl)-amine (18) [47].More recently it was observed that, with respect to 5-trans-CH=CHC 6 H 4 -4 -NMe 2 -1,10phenanthroline (Table 2), N,N-dibutyl-(4,5-diazafluorenyl-9ylidene-penta-1,3-dienyl)-amine (Table 3) is characterized by a much lower value of the λ max of the ILCT transition (∆λ max = 114 nm), reflecting the large π-delocalization of its structure, and suggesting a large NLO response [47].This was confirmed by EFISH measurements (Table 3) [47].In fact, the µβ 1.91 value for this novel free ligand is rather large (998 × 10 −48 esu), and it increases upon coordination to a "Zn(CH 3 CO 2 ) 2 " moiety ( 16), due to an increase of the dipole moment and to the red-shift of the ILCT transition.This increase is easily controlled by the Lewis acceptor properties of the metal moiety, and therefore by the ancillary ligands that tune its acceptor properties.Thus, the µβ  3).As previously observed for the stilbazole Zn(II) complexes (Section 3.1) [39], the second-order NLO response of the triflate complex 18 increases exponentially with decreasing concentration (Table 3), which is an effect that is not observed for the related complexes with the acetate or trifluoroacetate ligand, and can be attributed to the formation of cationic Zn(II) species, given the low nucleophilicity of the triflate anion [47].Similarly, the coordination of 9-[(1-azulenyl)methylene]-4,5-diazafluorene to "Zn(CF 3 CO 2 ) 2 " (19) leads to an enhancement of the second-order NLO response and the red-shift of the ILCT transition (Table 3).As expected, the µβ 1.91 value increases upon the substitution of CF 3 CO 2 − (19) with CF 3 SO 3 − (20).The µβ 1.91 value of the triflate complex 20 is also dependent on concentration, but in a minor way with respect to the zinc complex with N,N-dibutyl-(4,5-diazafluorenyl-9-ylidene-penta-1,3-dienyl)-amine (18) [47].Interestingly, it was also reported that the β 0 (HRS) of push-pull carboxylate ligands increases upon coordination to the "(1,10-phenanthroline)Zn(II)" moiety.Thus, complex 21 (Figure 3) has a β 0 (HRS) of 39 × 10 −30 esu, which is a value that is six times higher than that of the free carboxylate ligand [48].Interestingly, it was also reported that the β0(HRS) of push-pull carboxylate ligands increases upon coordination to the "(1,10-phenanthroline)Zn(II)" moiety.Thus, complex 21 (Figure 3) has a β0(HRS) of 39 × 10 −30 esu, which is a value that is six times higher than that of the free carboxylate ligand [48].Interestingly, it was also reported that the β0(HRS) of push-pull carboxylate ligands increases upon coordination to the "(1,10-phenanthroline)Zn(II)" moiety.Thus, complex 21 (Figure 3) has a β0(HRS) of 39 × 10 −30 esu, which is a value that is six times higher than that of the free carboxylate ligand [48].Interestingly, it was also reported that the β0(HRS) of push-pull carboxylate ligands increases upon coordination to the "(1,10-phenanthroline)Zn(II)" moiety.Thus, complex 21 (Figure 3) has a β0(HRS) of 39 × 10 −30 esu, which is a value that is six times higher than that of the free carboxylate ligand [48].Interestingly, it was also reported that the β0(HRS) of push-pull carboxylate ligands increases upon coordination to the "(1,10-phenanthroline)Zn(II)" moiety.Thus, complex 21 (Figure 3) has a β0(HRS) of 39 × 10 −30 esu, which is a value that is six times higher than that of the free carboxylate ligand [48].Interestingly, it was also reported that the β0(HRS) of push-pull carboxylate ligands increases upon coordination to the "(1,10-phenanthroline)Zn(II)" moiety.Thus, complex 21 (Figure 3) has a β0(HRS) of 39 × 10 −30 esu, which is a value that is six times higher than that of the free carboxylate ligand [48].

Tridentate Nitrogen Ligands: Terpyridines
The second-order NLO properties of zinc complexes with 4 -(1-C 6 H 4 -p-R)-2,2 :6 ,2 -terpyridines (R = NBu 2 , trans-CH=CHC 6 H 4 -p-NBu 2 , trans,trans-(CH=CH) 2 C 6 H 4 -p-NMe 2 ) ligands have also been studied by Ugo et al. [49,50].By looking at the free terpyridines, and increasing the length of the π-conjugated spacer between the amino group and the chelated system of the terpyridine, a significant increase of β 1.34 (EFISH) occurs (from 22 to 95 × 10 −30 esu, Table 4).Coordination to a "ZnY 2 " moiety (Y = Cl, CF 3 CO 2 ) leads to an enhancement of both the dipole moment and the β 1.34 (EFISH) value, in which the enhancement is higher for the more electron-withdrawing ligand CF 3 CO 2 (22)(23)(24) [49].This behavior is in agreement with the red-shift of the ILCT transition of the terpyridine due to an increase of the acceptor properties of the π* orbitals upon chelation (Table 4).= Cl, CF3CO2) leads to an enhancement of both the dipole moment and the β1.34(EFISH) value, in which the enhancement is higher for the more electron-withdrawing ligand CF3CO2 (22)(23)(24) [49].This behavior is in agreement with the red-shift of the ILCT transition of the terpyridine due to an increase of the acceptor properties of the π* orbitals upon chelation (Table 4).
Lacroix et al. [62] first investigated the second-order NLO properties of a Zn(II) bis[4-(diethylamino)salicylaldiminato] complex-a derivative of the 2,3-diaminomaleonitrile as a diamine bridge-in comparison with the free ligand and the related Ni(II) and Cu(II) complexes.As expected, the enhancement is higher for the more electron-withdrawing ligand CF3CO2 (22-24) [49].This behavior is in agreement with the red-shift of the ILCT transition of the terpyridine due to an increase of the acceptor properties of the π* orbitals upon chelation (Table 4).
Lacroix et al. [62] first investigated the second-order NLO properties of a Zn(II) bis [4-(diethylamino)salicylaldiminato] complex-a derivative of the 2,3-diaminomaleonitrile as a diamine bridge-in comparison with the free ligand and the related Ni(II) and Cu(II) complexes.As expected, Zn(II) complexation involves an increased nonlinearity and, surprisingly, larger hyperpolarizability values (β 1.34 (EFISH) up to 400 × 10 −30 esu), compared to Ni(II) and Cu(II) analogues, despite the d 10 metal configuration of the Zn(II) ion.This was related to the existence of strong ILCT transitions in which the zinc(II) center acts as a bridge in a square-pyramidal arrangement with an apical molecule of solvent.
The NLO properties of analogous 4-alkoxy substituted-instead of 4-diethylamino-Zn(II) complexes were recently studied in relation to their interesting aggregation/deaggregation properties [63].Actually, tetradentate [N 2 O 2 ] Schiff-base complexes are coordinatively unsaturated Lewis acidic species because of the inability of the Zn(II) ion to reach a tetrahedral coordination, which is a consequence of the ring strain generated by the diamine bridge [64].Therefore, in the absence of Lewis bases, these complexes are aggregated by intermolecular Zn•••O interactions, while, in the presence of Lewis bases, they deaggregate with the formation of monomeric adducts [65,66].This phenomenon was appropriately exploited to explore NLO changes upon deaggregation.It was found that starting from concentrated dichloromethane solutions of complex 25, deaggregation with a strong Lewis base, such as pyridine, leads to a switch-on of the quadratic hyperpolarizability (β 1.91 (EFISH) = −518 × 10 −30 esu; Figure 4).This represents an unprecedented mode of NLO switching in molecular materials.
values (β1.34(EFISH) up to 400 × 10 esu), compared to Ni(II) and Cu(II) analogues, despite the d metal configuration of the Zn(II) ion.This was related to the existence of strong ILCT transitions in which the zinc(II) center acts as a bridge in a square-pyramidal arrangement with an apical molecule The NLO properties of analogous 4-alkoxy substituted-instead of 4-diethylamino-Zn(II) complexes were recently studied in relation to their interesting aggregation/deaggregation properties [63].Actually, tetradentate [N2O2] Schiff-base complexes are coordinatively unsaturated Lewis acidic species because of the inability of the Zn(II) ion to reach a tetrahedral coordination, which is a consequence of the ring strain generated by the diamine bridge [64].Therefore, in the absence of Lewis bases, these complexes are aggregated by intermolecular Zn•••O interactions, while, in the presence of Lewis bases, they deaggregate with the formation of monomeric adducts [65,66].This phenomenon was appropriately exploited to explore NLO changes upon deaggregation.It was found that starting from concentrated dichloromethane solutions of complex 25, deaggregation with a strong Lewis base, such as pyridine, leads to a switch-on of the quadratic hyperpolarizability (β1.91(EFISH) = −518 × 10 −30 esu; Figure 4).This represents an unprecedented mode of NLO switching in molecular materials.In order to increase the NLO properties of bis(salicylaldiminato) M(II) complexes, Gradinaru et al. [67] performed a detailed study on a series of Ni(II), Cu(II), and Zn(II) complexes with two unsymmetrical tetradentate [N2O2] Schiff-base ligand derivatives from Smethylisothiosemicarbazone (Figure 5).These complexes were structurally characterized, revealing in the case of Zn(II) complexes a distorted square pyramid geometry around the Zn(II) center, in which the oxygen atom of a methanol molecule occupies the apical position.Again, the Zn complex shows the largest hyperpolarizability values along the series (μβ1.91(EFISH)= −1650 × 10 −48 esu; β1.91(EFISH) = −280 × 10 −30 esu), even though the metal ion is not involved in the charge transfer transitions that are responsible for the NLO response of these complexes.Unsymmetrical, alkoxy-derivatized Schiff-base Zn(II) complexes, having on one side an alkyl ammonium bromide as a Lewis base, were recently investigated to probe their aggregation and NLO In order to increase the NLO properties of bis(salicylaldiminato) M(II) complexes, Gradinaru et al. [67] performed a detailed study on a series of Ni(II), Cu(II), and Zn(II) complexes with two unsymmetrical tetradentate [N 2 O 2 ] Schiff-base ligand derivatives from S-methylisothiosemicarbazone (Figure 5).These complexes were structurally characterized, revealing in the case of Zn(II) complexes a distorted square pyramid geometry around the Zn(II) center, in which the oxygen atom of a methanol molecule occupies the apical position.Again, the Zn complex shows the largest hyperpolarizability values along the series (µβ 1.91 (EFISH) = −1650 × 10 −48 esu; β 1.91 (EFISH) = −280 × 10 −30 esu), even though the metal ion is not involved in the charge transfer transitions that are responsible for the NLO response of these complexes.
metal configuration of the Zn(II) ion.This was related to the existence of strong ILCT transitions in which the zinc(II) center acts as a bridge in a square-pyramidal arrangement with an apical molecule of solvent.
The NLO properties of analogous 4-alkoxy substituted-instead of 4-diethylamino-Zn(II) complexes were recently studied in relation to their interesting aggregation/deaggregation properties [63].Actually, tetradentate [N2O2] Schiff-base complexes are coordinatively unsaturated Lewis acidic species because of the inability of the Zn(II) ion to reach a tetrahedral coordination, which is a consequence of the ring strain generated by the diamine bridge [64].Therefore, in the absence of Lewis bases, these complexes are aggregated by intermolecular Zn•••O interactions, while, in the presence of Lewis bases, they deaggregate with the formation of monomeric adducts [65,66].This phenomenon was appropriately exploited to explore NLO changes upon deaggregation.It was found that starting from concentrated dichloromethane solutions of complex 25, deaggregation with a strong Lewis base, such as pyridine, leads to a switch-on of the quadratic hyperpolarizability (β1.91(EFISH) = −518 × 10 −30 esu; Figure 4).This represents an unprecedented mode of NLO switching in molecular materials.Unsymmetrical, alkoxy-derivatized Schiff-base Zn(II) complexes, having on one side an alkyl ammonium bromide as a Lewis base, were recently investigated to probe their aggregation and NLO properties [68].These complexes are characterized by a significant nonlinearity (µβ 1.91 (EFISH) = 400 × 10 −48 esu), even if the values are lower when compared to that of the 25 analogue.This can be related to the substitution of pyridine by a bromide ion and the relative cation-anion ion pair configuration [69], which are expected to involve major changes in the ground state dipole and in ∆µ eg values, and hence in the second-order nonlinearity.Experimental results and NLO data suggest the existence of acentric dimeric species such as 27 (Figure 6), in which each molecular unit mutually interacts with another unit through intermolecular Zn•••Br − interactions.
10 −48 esu), even if the values are lower when compared to that of the 25 analogue.This can be related to the substitution of pyridine by a bromide ion and the relative cation-anion ion pair configuration [69], which are expected to involve major changes in the ground state dipole and in Δμeg values, and hence in the second-order nonlinearity.Experimental results and NLO data suggest the existence of acentric dimeric species such as 27 (Figure 6), in which each molecular unit mutually interacts with another unit through intermolecular Zn•••Br − interactions.ZnL2 complexes of bidentate Schiff-base ligands derived from a chiral amine offered the opportunity to investigate their bulk nonlinearity [70].These complexes, in fact, crystallize in noncentrosymmetric space groups, with a pseudo-tetrahedral geometry around the zinc(II) ion (28, Figure 7).It was found that these compounds give an intense powder SHG signal at 1.064 μm, which is between that of 3-methyl-4-nitropyridine-1-oxide and N-(4-nitrophenyl)-(S)-prolinol.

Octupolar Complexes
As previously stated, the Zn(II) center has a strong propensity to expand its coordination sphere with ligands having an appropriate structure.This is especially true in the case of 2,2′-bipyridine ligands, for which the Zn(II) ion is an excellent template to build octupolar structures.Thus, a variety of octupolar, tetrahedral, and octahedral coordinated Zn(II) complexes have been synthesized and investigated for their second-order NLO properties.Most of this work has been developed and reviewed in an excellent paper [14] by Le Bozec et al.
The first work on this family of compounds was communicated in 2002 by these authors [71] on a series of 4,4′-bis(dibutylaminostyryl)-[2,2′]-bipyridine Zn(II) derivatives (Figure 8).It was found that the switching from the dipolar complex 29 to the pseudo-tetrahedral octupolar complex 30 and ZnL 2 complexes of bidentate Schiff-base ligands derived from a chiral amine offered the opportunity to investigate their bulk nonlinearity [70].These complexes, in fact, crystallize in noncentrosymmetric space groups, with a pseudo-tetrahedral geometry around the zinc(II) ion (28, Figure 7).It was found that these compounds give an intense powder SHG signal at 1.064 µm, which is between that of 3-methyl-4-nitropyridine-1-oxide and N-(4-nitrophenyl)-(S)-prolinol.
10 −48 esu), even if the values are lower when compared to that of the 25 analogue.This can be related to the substitution of pyridine by a bromide ion and the relative cation-anion ion pair configuration [69], which are expected to involve major changes in the ground state dipole and in Δμeg values, and hence in the second-order nonlinearity.Experimental results and NLO data suggest the existence of acentric dimeric species such as 27 (Figure 6), in which each molecular unit mutually interacts with another unit through intermolecular Zn•••Br − interactions.ZnL2 complexes of bidentate Schiff-base ligands derived from a chiral amine offered the opportunity to investigate their bulk nonlinearity [70].These complexes, in fact, crystallize in noncentrosymmetric space groups, with a pseudo-tetrahedral geometry around the zinc(II) ion (28, Figure 7).It was found that these compounds give an intense powder SHG signal at 1.064 μm, which is between that of 3-methyl-4-nitropyridine-1-oxide and N-(4-nitrophenyl)-(S)-prolinol.

Octupolar Complexes
As previously stated, the Zn(II) center has a strong propensity to expand its coordination sphere with ligands having an appropriate structure.This is especially true in the case of 2,2′-bipyridine ligands, for which the Zn(II) ion is an excellent template to build octupolar structures.Thus, a variety of octupolar, tetrahedral, and octahedral coordinated Zn(II) complexes have been synthesized and investigated for their second-order NLO properties.Most of this work has been developed and reviewed in an excellent paper [14] by Le Bozec et al.
The first work on this family of compounds was communicated in 2002 by these authors [71] on a series of 4,4′-bis(dibutylaminostyryl)-[2,2′]-bipyridine Zn(II) derivatives (Figure 8).It was found that the switching from the dipolar complex 29 to the pseudo-tetrahedral octupolar complex 30 and

Octupolar Complexes
As previously stated, the Zn(II) center has a strong propensity to expand its coordination sphere with ligands having an appropriate structure.This is especially true in the case of 2,2 -bipyridine ligands, for which the Zn(II) ion is an excellent template to build octupolar structures.Thus, a variety of octupolar, tetrahedral, and octahedral coordinated Zn(II) complexes have been synthesized and investigated for their second-order NLO properties.Most of this work has been developed and reviewed in an excellent paper [14] by Le Bozec et al.
The first work on this family of compounds was communicated in 2002 by these authors [71] on a series of 4,4 -bis(dibutylaminostyryl)-[2,2 ]-bipyridine Zn(II) derivatives (Figure 8).It was found that the switching from the dipolar complex 29 to the pseudo-tetrahedral octupolar complex 30 and the octahedral octupolar complex 31 is accompanied by an increased nonlinearity (Figure 8) and optical transparency, thus demonstrating the effective role of the octupolar strategy in improving the NLO properties of these molecular materials.
the octahedral octupolar complex 31 is accompanied by an increased nonlinearity (Figure 8) and optical transparency, thus demonstrating the effective role of the octupolar strategy in improving the NLO properties of these molecular materials.This approach was further developed through investigating octupolar D3 and D2d Zn(II) complexes with different functionalized bipyridyl ligands, in comparison with other D3 (Fe(II), Ru(II), Hg(II)) [21], and D2d (Cu(I), Ag(I)) metal complexes [21,72].The pseudo-tetrahedral D2d geometry in these complexes is stabilized by either alkyl or aryl substituents at the 6,6′-positions in the bipyridyl ligand.Very large hyperpolarizability values are achieved, especially for octahedral D3 complexes, in relation to the nature of the ligands (donor end-groups and π-linkers) and the nature of the metal center.Interestingly, Zn(II) complexes always involve at least comparable or even larger β(HRS) values in comparison with the other metal complexes, despite the absence of MLCT transitions in the former species.Thus, the nonlinearity of these complexes seems governed by ILCT transitions, which, in turn, are correlated to the Lewis acidity of the metal ion.The length of the π-conjugated backbone of the ligand is a key parameter to maximize the NLO activity, more than the strength of the donor group.The tris(bipyridyl) Zn(II) derivative having the π-conjugated 4,4′-oligophenylenevinylenefunctionalized 2,2′-bipyridine (32, Figure 9) exhibits the highest NLO activity, with a β1.91(HRS) value of 870 × 10 −30 esu and β0 = 657 × 10 −30 esu.
Analogous octupolar D3 Zn(II) complexes, in which the six aminostyryl π-electron donor substituents are replaced by ferrocenyl groups, were developed by Coe et al. [73].These complexes are almost unique in combining very large quadratic and cubic NLO effects in structures featuring multiple redox-switchable metal centers.Although a comparison of the experimental β(HRS) values for these complexes with those previously obtained for octupolar D3 Zn(II) species cannot be done, because the former were obtained in resonant conditions, the Stark-derived β0 values for these species are very large: as high as approximately 10 −27 esu.In order to achieve the macroscopic noncentrosymmetric orientation of these octupolar chromophores, the so-called "all-optical poling" technique [74], which is an interference process between one and two-photon excitations that locally induces macroscopic second-order effects in polymer films, has been employed.To this end, octupolar tris(bipyridyl)Zn(II) complexes containing three photoisomerizable ligands, such as 4,4′-bis-(styryl)-2,2′-bipyridine functionalized with a dialkylamino-azobenzene, 33, were synthesized and studied [75,76].These complexes allowed the synthesis of star-shaped polymers, in which the octupolar molecules are covalently linked to the organic backbone, 34.It has been demonstrated that grafted polymers show an improved stability of the macroscopic photoinduced nonlinearity, in comparison with that of doped films (Figure 10).
More recently, Le Bozec and Jacquemin et al. communicated the first example of hexadithienylethene substituents on octahedral tris(bipyridine)M (M = Fe(II), Zn(II)) complexes [77].Analogous octupolar D 3 Zn(II) complexes, in which the six aminostyryl π-electron donor substituents are replaced by ferrocenyl groups, were developed by Coe et al. [73].These complexes are almost unique in combining very large quadratic and cubic NLO effects in structures featuring multiple redox-switchable metal centers.Although a comparison of the experimental β(HRS) values for these complexes with those previously obtained for octupolar D 3 Zn(II) species cannot be done, because the former were obtained in resonant conditions, the Stark-derived β 0 values for these species are very large: as high as approximately 10 −27 esu.
In order to achieve the macroscopic noncentrosymmetric orientation of these octupolar chromophores, the so-called "all-optical poling" technique [74], which is an interference process between one and two-photon excitations that locally induces macroscopic second-order effects in polymer films, has been employed.To this end, octupolar tris(bipyridyl)Zn(II) complexes containing three photoisomerizable ligands, such as 4,4 -bis-(styryl)-2,2 -bipyridine functionalized with a dialkylamino-azobenzene, 33, were synthesized and studied [75,76].These complexes allowed the synthesis of star-shaped polymers, in which the octupolar molecules are covalently linked to the organic backbone, 34.It has been demonstrated that grafted polymers show an improved stability of the macroscopic photoinduced nonlinearity, in comparison with that of doped films (Figure 10).
polymer films, has been employed.To this end, octupolar tris(bipyridyl)Zn(II) complexes containing three photoisomerizable ligands, such as 4,4′-bis-(styryl)-2,2′-bipyridine functionalized with a dialkylamino-azobenzene, 33, were synthesized and studied [75,76].These complexes allowed the synthesis of star-shaped polymers, in which the octupolar molecules are covalently linked to the organic backbone, 34.It has been demonstrated that grafted polymers show an improved stability of the macroscopic photoinduced nonlinearity, in comparison with that of doped films (Figure 10).
More recently, Le Bozec and et al. communicated the first example of hexadithienylethene substituents on octahedral tris(bipyridine)M (M = Fe(II), Zn(II)) complexes [77].The photostability of the isomeric forms of the Fe(II) complexes allowed to demonstrate the photoswitching of their second-order NLO responses.
To probe the effect of the metal center on the electronic density of π-extended ligands, a series of D3-symmetry octupolar Zn(II) complex derivatives of the phenanthroline were synthesized and studied [78].Although their derived β1.06(HRS) values are relatively low, especially in comparison with that of their 2,2′-bipyridine analogues, this work further demonstrated the active role of the Zn(II) center as a template for the octupolar coordination and as a tool for inducing polarization in the electronic density of the π-extended ligands.More recently, Le Bozec and Jacquemin et al.
communicated the first example of hexadithienylethene substituents on octahedral tris(bipyridine)M (M = Fe(II), Zn(II)) complexes [77].The photostability of the isomeric forms of the Fe(II) complexes allowed to demonstrate the photoswitching of their second-order NLO responses.
To probe the effect of the metal center on the electronic density of π-extended ligands, a series of D 3 -symmetry octupolar Zn(II) complex derivatives of the phenanthroline were synthesized and studied [78].Although their derived β 1.06 (HRS) values are relatively low, especially in comparison with that of their 2,2 -bipyridine analogues, this work further demonstrated the active role of the Zn(II) center as a template for the octupolar coordination and as a tool for inducing polarization in the electronic density of the π-extended ligands.

Conclusions
An overview on the second-order NLO properties of the main families of Zn(II) complexes with π-delocalized ligands has been outlined.Thanks to their easy synthetic accessibility, a variety of dipolar and octupolar complexes have been explored as low-cost and efficient second-order NLO materials.Since the stereochemistry of Zn(II) complexes is not affected by ligand field stabilization, the Zn(II) ion can easily accommodate monodentate, bidentate, tridentate, or tetradentate π-delocalized ligands, in order to satisfy its coordination sphere.Thus, pyramidal, tetrahedral, and octahedral acentric structures have been achieved with relevant second-order NLO properties.Complexation almost always involves an enhancement of the nonlinearity, even in comparison with homologous metal complexes, due to intense ILCT transitions at low energy.Thus, large hyperpolarizability values have been reached within each investigated family of complexes.Moreover, for monodentate, bidentate,

a
Working at a concentration 5 × 10 −4 M.
photochromic zinc complexes are fascinating, allowing an excellent on/off switching of the NLO responses[46].
1.91 value increases upon the substitution of CH 3 CO 2 − (16) by CF 3 CO 2 − (17) or CF 3 SO 3 − (18), as expected from the λ max value dominating the NLO response of the related Zn(II) complexes (Table

Figure 4 .
Figure 4. Switching on the second-order NLO response for complex 25 by the addition of pyridine.Reproduced from Ref.[63] with permission from The Royal Society of Chemistry.

Figure 5 .
Figure 5. Structure of unsymmetrical Schiff bases of the S-methylisothiosemicarbazide derivative.

Figure 4 .
Figure 4. Switching on the second-order NLO response for complex 25 by the addition of pyridine.Reproduced from Ref.[63] with permission from The Royal Society of Chemistry.

Figure 4 .
Figure 4. Switching on the second-order NLO response for complex 25 by the addition of pyridine.Reproduced from Ref.[63] with permission from The Royal Society of Chemistry.

Figure 6 .
Figure 6.Proposed structure for the acentric dimeric species 27.Reproduced from Ref. [68] with permission from The Royal Society of Chemistry.

Figure 6 .
Figure 6.Proposed structure for the acentric dimeric species 27.Reproduced from Ref. [68] with permission from The Royal Society of Chemistry.

Figure 6 .
Figure 6.Proposed structure for the acentric dimeric species 27.Reproduced from Ref. [68] with permission from The Royal Society of Chemistry.

Figure 8 . 32 2+Figure 9 .
Figure 8. Dipolar and octupolar architectures based on zinc(II) complexes.This approach was further developed through investigating octupolar D 3 and D 2d Zn(II) complexes with different functionalized bipyridyl ligands, in comparison with other D 3 (Fe(II), Ru(II), Hg(II))[21], and D 2d (Cu(I), Ag(I)) metal complexes[21,72].The pseudo-tetrahedral D 2d geometry in these complexes is stabilized by either alkyl or aryl substituents at the 6,6 -positions in the bipyridyl ligand.Very large hyperpolarizability values are achieved, especially for octahedral D 3 complexes, in relation to the nature of the ligands (donor end-groups and π-linkers) and the nature of the metal center.Interestingly, Zn(II) complexes always involve at least comparable or even larger β(HRS) values in comparison with the other metal complexes, despite the absence of MLCT transitions in the former species.Thus, the nonlinearity of these complexes seems governed by ILCT transitions, which, in turn, are correlated to the Lewis acidity of the metal ion.The length of the π-conjugated backbone of the ligand is a key parameter to maximize the NLO activity, more than the strength of the donor group.The tris(bipyridyl) Zn(II) derivative having the π-conjugated 4,4 -oligophenylenevinylene-functionalized 2,2 -bipyridine (32, Figure9) exhibits the highest NLO activity, with a β 1.91 (HRS) value of 870 × 10 −30 esu and β 0 = 657 × 10 −30 esu.Inorganics 2018, 6, x FOR PEER REVIEW 13 of 18

Figure 9 .
Figure 9. Structure of 32 displaying the highest NLO activity.

Figure 10 .
Figure 10.Optical orientation and relaxation of films F1 and F2 under resonant one-photon and twophoton excitations (open circles and diamonds) compared to a biexponential plot (lines).F1 = doped film of 33; F2 = grafted polymer film of 34.Adapted with permission from Ref. [75].Copyright (2004) American Chemical Society.

Figure 10 .
Figure 10.Optical orientation and relaxation of films F1 and F2 under resonant one-photon and two-photon excitations (open circles and diamonds) compared to a biexponential plot (lines).F1 = doped film of 33; F2 = grafted polymer film of 34.Adapted with permission from Ref. [75].Copyright (2004) American Chemical Society.