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Review

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

by
Santo Di Bella
1,*,
Alessia Colombo
2,
Claudia Dragonetti
2,
Stefania Righetto
2 and
Dominique Roberto
2,*
1
Dipartimento di Scienze Chimiche, Università di Catania, Viale A. Doria 8, I-95125 Catania, Italy
2
Dipartimento di Chimica dell’Università degli Studi di Milano, UdR-INSTM, Via Golgi 19, I-20133 Milano, Italy
*
Authors to whom correspondence should be addressed.
§
This review is dedicated to Prof. Renato Ugo, on the occasion of his 80th birthday, for his essential contribution to the field of metal complexes with second-order nonlinear optical properties.
Inorganics 2018, 6(4), 133; https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics6040133
Submission received: 20 November 2018 / Revised: 6 December 2018 / Accepted: 8 December 2018 / Published: 11 December 2018
(This article belongs to the Special Issue Metal Complexes as Nonlinear Optical Molecular Materials)
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Abstract

:
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.

Graphical Abstract

1. Introduction

Second-order nonlinear optical (NLO) coordination complexes are of great interest for nonlinear optics [1,2,3], mostly due to the unique characteristics associated with the metal center [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]. Compared to organic compounds, metal complexes offer a greater variety of electronic structures, in relation to the coordination sphere, electronic configuration, and oxidation state of the metal. They possess intense, low-energy metal-to-ligand charge-transfer (MLCT), ligand-to-metal charge-transfer (LMCT), or intraligand charge-transfer (ILCT) transitions, and therefore, the metal can act as a donor, acceptor, or polarizable bridge of a dipolar donor–π–bridge–acceptor system. Besides, some metal centers are ideal templates to build compounds based on octupolar coordination with D2 or D3 symmetry [21].
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 d10 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 framework. Coordination numbers 4, 5, and 6, which are associated to tetrahedral, square pyramidal, and octahedral geometry, respectively, are often encountered. The unique characteristics of zinc allow the preparation not only of NLO-active dipolar compounds, thanks to intense ILCT transitions at low energy, but also of NLO-active octupolar compounds, owing to the capacity of the zinc center to act as a versatile template, allowing a suitable coordination sphere. Moreover, the absence of low-energy metal-to-ligand or metal d–d electronic transitions makes these zinc complexes generally more optically transparent than the metal dn congeners.
In the following sections, we report an overview on the dipolar and octupolar second-order NLO active zinc complexes bearing stilbazole, bipyridine, phenanthroline, terpyridine, or Schiff bases as ligands, focusing on the relevant aspects associated to each class of complexes. Macrocyclic ligands such as porphyrins and phthalocyanines are not presented, being already covered in excellent papers and reviews [15,19,26,27,28].

2. Principles of Second-Order Nonlinear Optics

In this section, we briefly present the principles of second-order nonlinear optics. More details can be found in various books [1,2,3] and reviews [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20].
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:
β z z z = 3 2 h 2 c 2 ν e g 2 r e g 2 Δ μ e g ( ν e g 2 − ν L 2 ) ( ν e g 2 − 4 ν L 2 )
where z is the direction of the charge transfer, νeg is the frequency of the charge-transfer transition, reg is the transition dipole moment, Δµeg is the difference between the excited state and ground state molecular dipole moment (µe − µg) and νL is the frequency of the incident radiation. This equation represents the so-called “two-level” model. Extrapolation to zero frequency (νL = 0.0 eV; λ = ∞) gives the static quadratic hyperpolarizability β0 [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:
β0 = βλ [1 − (2λmax/λ)2][1 − (λmax/λ)2]
where βλ is the quadratic hyperpolarizability value at λ incident wavelength, and λmax 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 Δµeg and reg 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 (D2) or threefold (D3) 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:
γEFISH = (μβλ/5kT) + γ (−2ω;ω,ω,0)
where μβλ/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. βλ is the projection along the dipole moment axis of βVEC, 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 βλ, 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 βλ(EFISH) and βλ(HRS), respectively.

3. Dipolar Complexes

3.1. 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 NMe2 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.
The µβ1.91(EFISH) value of 4,4′-trans-NMe2-C6H4CH=CHC5H4N (CHCl3, 5 × 10−4 M) [41] increases by a factor of 2.3 upon coordination to the “Zn(CH3CO2)2” moiety [38], which is mainly due to the large increase of the dipole moment (complex 1, Table 1). In fact, the quadratic hyperpolarizability is quite similar, which is in agreement with the irrelevant red-shift upon coordination of the stilbazole ILCT (Δλmax = 2 nm) [38]. However, an increase of the acceptor properties of the Zn(II) ancillary ligands (CH3CO2 < CF3CO2 < CF3SO3) tunes the acceptor properties of the Zn(II) center, leading to an increase of the ILCT red-shift observed upon coordination (with “Zn(CF3CO2)2” (2), Δλmax = 46 nm [38]; with “Zn(CF3SO3)2” (3), Δλmax = 116 nm) [39]. Such a behavior is reflected by an enhancement of the quadratic hyperpolarizability. Thus, on passing from (1) to (2) and (3), the β1.91(EFISH) value increases by factors of 1.3 and 4.2, respectively [39]. It turned out that with other less electron-withdrawing non-fluorinated sulfonate ligands, such as CH3SO3− (4) or p-CH3C6H4SO3− (5), the second-order NLO response is much lower than with CF3SO3− (3), despite the significantly high red-shift of the ILCT transition of the stilbazole ligand (Δλmax = 101–102 nm), showing the unique property of the triflate ancillary ligand, which increases the second-order NLO response due to its extremely low nucleophilic character [40].
Similarly, the quadratic hyperpolarizability of 4,4′-trans,trans-NMe2-C6H4(CH=CH)2C5H4N is increased upon coordination to a Zn(II) center, in which the enhancement is much larger with “Zn(CF3SO3)2” (7) than with “Zn(CH3CO2)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 (Δλmax = 123 and 10 nm, for CF3SO3 and CH3CO2, 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 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.

3.2. 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].
Le Bozec et al. studied by means of the EFISH technique (1.34 μm incident wavelength) the second-order NLO response of various Zn(II) complexes with 4-(p-R-styryl)-4′-methyl-2,2′-bipyridine where R is a donor substituent [42,43]. They found that the quadratic hyperpolarizability increases with the Lewis acidity of the metal center (“ZnCl2” > “Zn(OAc)2”) and with the strength of the electron donor group R on the bipyridine (Ooctyl is less efficient than NBu2). The best value (β1.34(EFISH) = 152 × 10−30 esu; µβ1.34(EFISH) = 1780 × 10−48 esu) was obtained for the ZnCl2 complex with 4-(p(dibutylamino)styryl)-4′-methyl-2,2′-bipyridine (complex 8; Figure 1) [43]. The quadratic hyperpolarizability and the dipole moment were 11 and 2.3 times higher, respectively, than those determined for the related free bipyridine [42].
Besides, Le Bozec et al. reported that the ZnCl2 complex bearing a 4,4′-bis(p(dibutylamino)styryl)[2,2′]-bipyridine (complex 9, Figure 1) is characterized by a µβ1.34(EFISH) of 1420 × 10−48 esu [44]. Remarkably, this value is much higher than that of the related nonchelated complex [ZnCl2(4,4′-trans-NMe2-C6H4CH=CHC5H4N)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 ZnCl2 moiety, a red-shift of the intense ILCT (intraligand charge transfer) band is observed (Δλmax = 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 (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 photochromic zinc complexes are fascinating, allowing an excellent on/off switching of the NLO responses [46].
Ugo et al. showed that the second-order NLO response of 5-R-1,10-phenanthroline (R = donor group such as OMe, NMe2, trans-CH=CHC6H4-4′-NMe2, and trans,trans-(CH=CH)2C6H4-4′-NMe2) increases upon coordination to the “Zn(CH3CO2)2” moiety, which is in agreement with the ILCT red-shift (Table 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].
More recently it was observed that, with respect to 5-trans-CH=CHC6H4-4′-NMe2-1,10-phenanthroline (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].
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].

3.3. Tridentate Nitrogen Ligands: Terpyridines

The second-order NLO properties of zinc complexes with 4′-(1-C6H4-p-R)-2,2′:6′,2′′-terpyridines (R = NBu2, trans-CH=CHC6H4-p-NBu2, trans,trans-(CH=CH)2C6H4-p-NMe2) 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 β1.34(EFISH) occurs (from 22 to 95 × 10−30 esu, Table 4). Coordination to a “ZnY2” moiety (Y = 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 (22–24) [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).
As previously observed for other metal complexes [15,19], the quadratic hyperpolarizability enhancement factor upon coordination to a “Zn(CF3CO2)2” moiety (EF given by β1.34(EFISH) complex/β1.34(EFISH) free terpyridine) decreases when the length of the π-conjugated system of the linker connecting the donor group increases (EF = 4.0, 3.5, and 1.4 for R = NBu2 (22), trans-CH=CHC6H4-p-NBu2 (23), and trans,trans-(CH=CH)2C6H4-p-NMe2 (24), respectively; Table 4). Such enhancement factors can be attributed to the red-shift of the ligand ILCT transition and the stabilization of the cisoid conformation of the terpyridine due to chelation [50].
Interestingly, the second-harmonic generation (1064-nm incident wavelength) of Langmuir Blodgett films of a Zn(II) complex bearing a 4′-(1-C6H4-p-NMe(C16H33)-2, 2′: 6′,2′′-terpyridine has been measured. A fair χ(2) value of 8.1 pm/V was obtained [51].

3.4. Schiff-Bases

Although Schiff-base metal complexes—mainly involving Ni(II) [52,53,54,55] and Cu(II) [56,57] ions—have been widely investigated and reviewed as second-order NLO materials [58,59,60,61], the great potential of their Zn(II) analogues has not been fully explored.
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 (β1.34(EFISH) up to 400 × 10−30 esu), compared to Ni(II) and Cu(II) analogues, despite the d10 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.
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 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 (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 properties [68]. These complexes are characterized by a significant nonlinearity (μβ1.91(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 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.

4. 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.
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′-oligophenylenevinylene-functionalized 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]. 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.

5. 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, and tridentate nitrogen donor ligands, a tunable NLO response, in relation to the nature of the ancillary ligands, has been found. In some cases, the switching of their NLO properties has been accomplished. In summary, zinc(II) complexes are resourceful and promising for the development of second-order NLO molecular materials.

Author Contributions

S.D.B. and D.R. have written the first version of the review; A.C., C.D. and S.R. have critically and scientifically revised the original manuscript and prepared the figures, tables and graphical abstract. The final revision was made by all the authors.

Funding

This research was funded by MIUR (PRIN, FIRB), Università degli Studi di Catania e di Milano (Piano della Ricerca di Ateneo 2016–2018: Linea di intervento 1 e 2), and National Interuniversity Consortium of Materials Science and Technology (Project INSTMMI012).

Acknowledgments

We wholeheartedly thank Professor Renato Ugo not only for his constructive guidance in the field of second-order nonlinear optics, but also for his kindness, support and thoughts on life. We treasure everything he taught us.

Conflicts of Interest

The authors declare no conflict of interest.

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Inorganics 06 00133 g001 550
Figure 1. Second-order nonlinear optical (NLO)-active Zn(II) bipyridine complexes.
Figure 1. Second-order nonlinear optical (NLO)-active Zn(II) bipyridine complexes.
Inorganics 06 00133 g001
Inorganics 06 00133 g002 550
Figure 2. Zn(II) bipyridine complexes with a photoswitchable second-order NLO-response; Ac = acetyl.
Figure 2. Zn(II) bipyridine complexes with a photoswitchable second-order NLO-response; Ac = acetyl.
Inorganics 06 00133 g002
Inorganics 06 00133 g003 550
Figure 3. Zn(II) phenanthroline complex bearing a push–pull carboxylate ligand.
Figure 3. Zn(II) phenanthroline complex bearing a push–pull carboxylate ligand.
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Inorganics 06 00133 g004 550
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. 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.
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Inorganics 06 00133 g005 550
Figure 5. Structure of unsymmetrical Schiff bases of the S-methylisothiosemicarbazide derivative.
Figure 5. Structure of unsymmetrical Schiff bases of the S-methylisothiosemicarbazide derivative.
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Inorganics 06 00133 g006 550
Figure 6. Proposed structure for the acentric dimeric species 27. Reproduced from Ref. [68] with permission from The Royal Society of Chemistry.
Figure 6. Proposed structure for the acentric dimeric species 27. Reproduced from Ref. [68] with permission from The Royal Society of Chemistry.
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Figure 7. Structure of chiral ZnL2 complexes.
Figure 7. Structure of chiral ZnL2 complexes.
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Figure 8. Dipolar and octupolar architectures based on zinc(II) complexes.
Figure 8. Dipolar and octupolar architectures based on zinc(II) complexes.
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Inorganics 06 00133 g009 550
Figure 9. Structure of 32 displaying the highest NLO activity.
Figure 9. Structure of 32 displaying the highest NLO activity.
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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.
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.
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Table
Table 1. Electronic spectra, dipole moments, and β1.91(EFISH) of Zn(II) stilbazole complexes in CHCl3 solution [38,39,40]. EFISH: electric field-induced second harmonic generation.
Table 1. Electronic spectra, dipole moments, and β1.91(EFISH) of Zn(II) stilbazole complexes in CHCl3 solution [38,39,40]. EFISH: electric field-induced second harmonic generation.
MoleculeY (Complex)λmax (nm)µβ1.91(EFISH) (10−48 esu) aμ (10−18 esu)β1.91(EFISH) (10−30 esu)
Inorganics 06 00133 i001 3741363.935
Inorganics 06 00133 i002CH3CO2 (1)3763168.039
CF3CO2 (2)42051210.549
CF3SO3 (3)490271516.7163
CH3SO3 (4)47545015.529
p-CH3C6H4SO3 (5)47642816.027
Inorganics 06 00133 i003 3963104.569
Inorganics 06 00133 i004CH3CO2 (6)4066806.7101
CF3SO3 (7)519384014.7261
a Working at a concentration 5 × 10−4 M.
Table
Table 2. Electronic spectra, dipole moments, and β1.34(EFISH) of Zn(II) phenanthroline complexes in CHCl3 solution [38].
Table 2. Electronic spectra, dipole moments, and β1.34(EFISH) of Zn(II) phenanthroline complexes in CHCl3 solution [38].
MoleculeR (Complex)λmax (nm)µβ1.34(EFISH) (10−48 esu) aµ (10−18 esu)β1.34(EFISH) (10−30 esu)
Inorganics 06 00133 i005OMe272164.04
NMe2328273.87.2
trans-
CH=CHC6H4-4′-NMe2		3712014.941
trans,trans-
(CH=CH)2C6H4-4′-NMe2		3993684.975
Inorganics 06 00133 i006OMe (12)284997.613
NMe2 (13)3442547.733
trans-
CH=CHC6H4-4′-NMe2 (14)		4196168.077
trans,trans-
(CH=CH)2C6H4-4′-NMe2 (15)		4328627.7112
a working at a concentration 5 × 10−4 M. Ac = Acetyl.
Table
Table 3. Electronic spectra and μβ1.91(EFISH) of Zn(II) diazafluoren complexes in CHCl3 solution [47].
Table 3. Electronic spectra and μβ1.91(EFISH) of Zn(II) diazafluoren complexes in CHCl3 solution [47].
MoleculeY (Complex)λmax (nm)Concentration (10−4 M)µβ1.91(EFISH) (10−48 esu)
Inorganics 06 00133 i007 48510998
Inorganics 06 00133 i008CH3CO2 (16)490101230
CF3CO2 (17)548101900
CF3SO3 (18)560102230
53170
15750
0.512,000
Inorganics 06 00133 i009 43910760
Inorganics 06 00133 i010CF3CO2 (19)447101320
CF3SO3 (20)486101640
0.53570
Table
Table 4. Electronic spectra, dipole moments, and β1.34(EFISH) of Zn(II) terpyridine complexes in CHCl3 solution [49,50].
Table 4. Electronic spectra, dipole moments, and β1.34(EFISH) of Zn(II) terpyridine complexes in CHCl3 solution [49,50].
MoleculeR (Complex)λmax (nm)µβ1.34(EFISH) (10−48 esu) aµ (10−18 esu)β1.34(EFISH) (10−30 esu)
Inorganics 06 00133 i011NBu2360462.122
trans-
CH=CHC6H4-p-NBu2		3951873.652
trans,trans-
(CH=CH)2C6H4-p-NMe2		3993703.995
Inorganics 06 00133 i012NBu2 (22)4278801088
trans-
CH=CHC6H4-p-NBu2 (23)		45415028.3181
trans,trans-
(CH=CH)2C6H4-p-NMe2 (24)		44412198.9137

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Di Bella, S.; Colombo, A.; Dragonetti, C.; Righetto, S.; Roberto, D. Zinc(II) as a Versatile Template for Efficient Dipolar and Octupolar Second-Order Nonlinear Optical Molecular Materials §. Inorganics 2018, 6, 133. https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics6040133

AMA Style

Di Bella S, Colombo A, Dragonetti C, Righetto S, Roberto D. Zinc(II) as a Versatile Template for Efficient Dipolar and Octupolar Second-Order Nonlinear Optical Molecular Materials §. Inorganics. 2018; 6(4):133. https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics6040133

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Di Bella, Santo, Alessia Colombo, Claudia Dragonetti, Stefania Righetto, and Dominique Roberto. 2018. "Zinc(II) as a Versatile Template for Efficient Dipolar and Octupolar Second-Order Nonlinear Optical Molecular Materials §" Inorganics 6, no. 4: 133. https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics6040133

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