Second-Order Optical Nonlinearities in Coordination Chemistry: The Case of Bis(salicylaldiminato)metal Schiff Base Complexes

Abstract

Since the mid‐1980s, there has been a growth of interest in the search and development of molecular second‐order nonlinear optical (NLO) materials that possess various device applications. Beside the most traditional donor‐acceptor (“push‐pull”) organic chromophores (e.g. the stilbene family), chemists have increasingly extended their field of investigations to newer generations of organic molecules of greater complexity. In recent years, coordination complexes through their unique characteristics such as various redox and magnetic properties, in addition to their great diversity of geometries, have introduced a new dimension to the area. This review attempts to summarize the NLO capabilities of bis(salicylaldiminato)metal Schiff base complexes: (i) chromophores with enhanced hyperpolarizabilities (β) can be obtained by virtue of the coordinated metal center; (ii) the use of chiral substituents allows the engineering of the chromophores into various non‐centrosymmetric environments and efficiencies of about 10 times that of urea in second‐harmonic generation; (iii) the general trend observed for the high temperature stability in this family of chromophores, while combined with the large β×μ products, makes these systems interesting candidates for their incorporation into poled polymers with high glass transition temperatures (Tg); (iv) the possibility for β modulation induced by magnetic transition is also seen with respect to the concept of molecular switching.

Solid-State Nonlinear Optical Properties of Mononuclear Copper(II) Complexes with Chiral Tridentate and Tetradentate Schiff Base Ligands

“…The solid-state NLO response of powder samples 1b , 1c , 2a , 2b , and 2c was investigated using the Kurtz–Perry technique [47] @1.907 µm, resulting in SHG efficiencies equal to 0.13, 0.44, 0.32, 0.19, and 0.30 times that of standard urea, respectively. Even if not high, these values are in line with those reported for the majority of similar Schiff base metal complexes [45,46,64,65,84], and lower than those reported only for [Mn(Cl)(4,4′-diNEt 2 -sal-(1 R ,2 R )-chxn)] [23,48] and [Ni(4,4′-diNEt 2 -sal-(1 R ,2 R )-dpen)] [23,49], whose SHG efficiencies are up to 8 and 13 times that of urea, respectively.…”

Solid-State Nonlinear Optical Properties of Mononuclear Copper(II) Complexes with Chiral Tridentate and Tetradentate Schiff Base Ligands

“…The solid-state NLO response of powder samples 1b , 1c , 2a , 2b , and 2c was investigated using the Kurtz–Perry technique [47] @1.907 µm, resulting in SHG efficiencies equal to 0.13, 0.44, 0.32, 0.19, and 0.30 times that of standard urea, respectively. Even if not high, these values are in line with those reported for the majority of similar Schiff base metal complexes [45,46,64,65,84], and lower than those reported only for [Mn(Cl)(4,4′-diNEt 2 -sal-(1 R ,2 R )-chxn)] [23,48] and [Ni(4,4′-diNEt 2 -sal-(1 R ,2 R )-dpen)] [23,49], whose SHG efficiencies are up to 8 and 13 times that of urea, respectively.…”

Structural, Spectral, Electric-Field-Induced Second Harmonic, and Theoretical Study of Ni(II), Cu(II), Zn(II), and VO(II) Complexes with [N2O2] Unsymmetrical Schiff Bases of S-Methylisothiosemicarbazide Derivatives

“…The Zn complex shows the largest μβ λ product, according to what was already evidenced by previous EFISH investigations on Schiff base complexes. , The other complexes are characterized by μβ λ and β λ values in the range normally observed for Schiff base complexes. , By comparing CuL 2 and NiL 2 complexes, which are structurally very similar and have virtually the same dipole moment, we note a strong decrease in β λ on going from Cu to Ni, confirming that open-shell Schiff base complexes can have larger hyperpolarizability compared with the corresponding closed-shell ones. , On the other hand, it is quite surprising the low NLO response of VOL 2 as given by EFISH measurements, on account of the similarity with the other ML 2 complexes as inferred from their UV−vis spectra (see Table ). This anomalous behavior deserves further investigation.…”

“…The corresponding experimental and theoretical β λ values were obtained by using respectively the experimental and the computed dipole moments. By analyzing the experimental results, we observe large μβ λ and β λ magnitudes for complexes CuL 2 and ZnL 2 , similar to those reported for the more promising bis(salicylaldiminato)metal complexes. ,, For comparison, the azo dye Disperse Red 1 (4-[ N -ethyl- N -(2-hydroxyethyl)]amino-4‘-nitroazobenzene) currently used as a dopant molecule in polymer matrices for electro-optic applications has values of μβ λ = 525 × 10 -48 esu at the same wavelength as that used in our measurements (λ = 1.907 μm) and μβ 0 = 409 × 10 -48 esu (errors estimated to be ±15%) 60b7 Experimental EFISH First Hyperpolarizabilities of Complexes 3 and 5 − 8 a compoundμβ λ β λ b compoundμβ λ β λ b ZnL 1 −340 −87 ZnL 2 −1650 −280 NiL 2 −176 −73 VOL 2 −200 −83 CuL 2 −538 −234 a Hyperpolarizabilities are given in 10 -30 esu, and the μβ λ product is given in 10 -48 esu.…”

“…The distinctive feature of the mass spectra of compounds 1-8 is the presence of peaks of molecular ions [M] + with 100% intensity and doubly charged ions [M] 2+ , both indicating a large stability of these complexes toward EI. Compounds 1-8 exhibit excellent thermal stability, comparable to that reported for bis(salicylaldiminato)Ni complexes 5 (see TGA curves in Supporting Information, Figure S1). Decomposition temperatures were 270, 280, 310, and 260 °C for NiL 1 , CuL 1 , ZnL 1 , and VOL 1 , respectively, and 295, 300, 240, and 300 °C for NiL 2 , CuL 2 , ZnL 2 , and VOL 2 , respectively.…”

Copper(II) Complexes of salen Analogues with Two Differently Substituted (Push−Pull) Salicylaldehyde Moieties. A Study on the Modulation of Electronic Asymmetry and Nonlinear Optical Properties

“…The absolute μ g β vec and β vec values are in the (100−700) × 10 -30 D cm 5 esu -1 and (18−115) × 10 -30 cm 5 esu -1 ranges, respectively, similar to those found for other Schiff base complexes. − The highest figures of μ g β vec are displayed by the NO 2 −NMe 2 derivatives ( 1h and 1p ); however, their β vec values are not the highest because this kind of substitution gives rise also to a large μ g .…”

Solid-State Nonlinear Optical Properties of Mononuclear Copper(II) Complexes with Chiral Tridentate and Tetradentate Schiff Base Ligands

“…The solid-state NLO response of powder samples 1b , 1c , 2a , 2b , and 2c was investigated using the Kurtz–Perry technique [47] @1.907 µm, resulting in SHG efficiencies equal to 0.13, 0.44, 0.32, 0.19, and 0.30 times that of standard urea, respectively. Even if not high, these values are in line with those reported for the majority of similar Schiff base metal complexes [45,46,64,65,84], and lower than those reported only for [Mn(Cl)(4,4′-diNEt 2 -sal-(1 R ,2 R )-chxn)] [23,48] and [Ni(4,4′-diNEt 2 -sal-(1 R ,2 R )-dpen)] [23,49], whose SHG efficiencies are up to 8 and 13 times that of urea, respectively.…”

Structural, Spectral, Electric-Field-Induced Second Harmonic, and Theoretical Study of Ni(II), Cu(II), Zn(II), and VO(II) Complexes with [N2O2] Unsymmetrical Schiff Bases of S-Methylisothiosemicarbazide Derivatives

“…The Zn complex shows the largest μβ λ product, according to what was already evidenced by previous EFISH investigations on Schiff base complexes. , The other complexes are characterized by μβ λ and β λ values in the range normally observed for Schiff base complexes. , By comparing CuL 2 and NiL 2 complexes, which are structurally very similar and have virtually the same dipole moment, we note a strong decrease in β λ on going from Cu to Ni, confirming that open-shell Schiff base complexes can have larger hyperpolarizability compared with the corresponding closed-shell ones. , On the other hand, it is quite surprising the low NLO response of VOL 2 as given by EFISH measurements, on account of the similarity with the other ML 2 complexes as inferred from their UV−vis spectra (see Table ). This anomalous behavior deserves further investigation.…”

“…The corresponding experimental and theoretical β λ values were obtained by using respectively the experimental and the computed dipole moments. By analyzing the experimental results, we observe large μβ λ and β λ magnitudes for complexes CuL 2 and ZnL 2 , similar to those reported for the more promising bis(salicylaldiminato)metal complexes. ,, For comparison, the azo dye Disperse Red 1 (4-[ N -ethyl- N -(2-hydroxyethyl)]amino-4‘-nitroazobenzene) currently used as a dopant molecule in polymer matrices for electro-optic applications has values of μβ λ = 525 × 10 -48 esu at the same wavelength as that used in our measurements (λ = 1.907 μm) and μβ 0 = 409 × 10 -48 esu (errors estimated to be ±15%) 60b7 Experimental EFISH First Hyperpolarizabilities of Complexes 3 and 5 − 8 a compoundμβ λ β λ b compoundμβ λ β λ b ZnL 1 −340 −87 ZnL 2 −1650 −280 NiL 2 −176 −73 VOL 2 −200 −83 CuL 2 −538 −234 a Hyperpolarizabilities are given in 10 -30 esu, and the μβ λ product is given in 10 -48 esu.…”

“…The distinctive feature of the mass spectra of compounds 1-8 is the presence of peaks of molecular ions [M] + with 100% intensity and doubly charged ions [M] 2+ , both indicating a large stability of these complexes toward EI. Compounds 1-8 exhibit excellent thermal stability, comparable to that reported for bis(salicylaldiminato)Ni complexes 5 (see TGA curves in Supporting Information, Figure S1). Decomposition temperatures were 270, 280, 310, and 260 °C for NiL 1 , CuL 1 , ZnL 1 , and VOL 1 , respectively, and 295, 300, 240, and 300 °C for NiL 2 , CuL 2 , ZnL 2 , and VOL 2 , respectively.…”

Copper(II) Complexes of salen Analogues with Two Differently Substituted (Push−Pull) Salicylaldehyde Moieties. A Study on the Modulation of Electronic Asymmetry and Nonlinear Optical Properties

“…The absolute μ g β vec and β vec values are in the (100−700) × 10 -30 D cm 5 esu -1 and (18−115) × 10 -30 cm 5 esu -1 ranges, respectively, similar to those found for other Schiff base complexes. − The highest figures of μ g β vec are displayed by the NO 2 −NMe 2 derivatives ( 1h and 1p ); however, their β vec values are not the highest because this kind of substitution gives rise also to a large μ g .…”

Solid-State Nonlinear Optical Properties of Mononuclear Copper(II) Complexes with Chiral Tridentate and Tetradentate Schiff Base Ligands

“…The solid-state NLO response of powder samples 1b , 1c , 2a , 2b , and 2c was investigated using the Kurtz–Perry technique [47] @1.907 µm, resulting in SHG efficiencies equal to 0.13, 0.44, 0.32, 0.19, and 0.30 times that of standard urea, respectively. Even if not high, these values are in line with those reported for the majority of similar Schiff base metal complexes [45,46,64,65,84], and lower than those reported only for [Mn(Cl)(4,4′-diNEt 2 -sal-(1 R ,2 R )-chxn)] [23,48] and [Ni(4,4′-diNEt 2 -sal-(1 R ,2 R )-dpen)] [23,49], whose SHG efficiencies are up to 8 and 13 times that of urea, respectively.…”

Structural, Spectral, Electric-Field-Induced Second Harmonic, and Theoretical Study of Ni(II), Cu(II), Zn(II), and VO(II) Complexes with [N2O2] Unsymmetrical Schiff Bases of S-Methylisothiosemicarbazide Derivatives

“…The Zn complex shows the largest μβ λ product, according to what was already evidenced by previous EFISH investigations on Schiff base complexes. , The other complexes are characterized by μβ λ and β λ values in the range normally observed for Schiff base complexes. , By comparing CuL 2 and NiL 2 complexes, which are structurally very similar and have virtually the same dipole moment, we note a strong decrease in β λ on going from Cu to Ni, confirming that open-shell Schiff base complexes can have larger hyperpolarizability compared with the corresponding closed-shell ones. , On the other hand, it is quite surprising the low NLO response of VOL 2 as given by EFISH measurements, on account of the similarity with the other ML 2 complexes as inferred from their UV−vis spectra (see Table ). This anomalous behavior deserves further investigation.…”

“…The corresponding experimental and theoretical β λ values were obtained by using respectively the experimental and the computed dipole moments. By analyzing the experimental results, we observe large μβ λ and β λ magnitudes for complexes CuL 2 and ZnL 2 , similar to those reported for the more promising bis(salicylaldiminato)metal complexes. ,, For comparison, the azo dye Disperse Red 1 (4-[ N -ethyl- N -(2-hydroxyethyl)]amino-4‘-nitroazobenzene) currently used as a dopant molecule in polymer matrices for electro-optic applications has values of μβ λ = 525 × 10 -48 esu at the same wavelength as that used in our measurements (λ = 1.907 μm) and μβ 0 = 409 × 10 -48 esu (errors estimated to be ±15%) 60b7 Experimental EFISH First Hyperpolarizabilities of Complexes 3 and 5 − 8 a compoundμβ λ β λ b compoundμβ λ β λ b ZnL 1 −340 −87 ZnL 2 −1650 −280 NiL 2 −176 −73 VOL 2 −200 −83 CuL 2 −538 −234 a Hyperpolarizabilities are given in 10 -30 esu, and the μβ λ product is given in 10 -48 esu.…”

“…The distinctive feature of the mass spectra of compounds 1-8 is the presence of peaks of molecular ions [M] + with 100% intensity and doubly charged ions [M] 2+ , both indicating a large stability of these complexes toward EI. Compounds 1-8 exhibit excellent thermal stability, comparable to that reported for bis(salicylaldiminato)Ni complexes 5 (see TGA curves in Supporting Information, Figure S1). Decomposition temperatures were 270, 280, 310, and 260 °C for NiL 1 , CuL 1 , ZnL 1 , and VOL 1 , respectively, and 295, 300, 240, and 300 °C for NiL 2 , CuL 2 , ZnL 2 , and VOL 2 , respectively.…”

Copper(II) Complexes of salen Analogues with Two Differently Substituted (Push−Pull) Salicylaldehyde Moieties. A Study on the Modulation of Electronic Asymmetry and Nonlinear Optical Properties

“…The absolute μ g β vec and β vec values are in the (100−700) × 10 -30 D cm 5 esu -1 and (18−115) × 10 -30 cm 5 esu -1 ranges, respectively, similar to those found for other Schiff base complexes. − The highest figures of μ g β vec are displayed by the NO 2 −NMe 2 derivatives ( 1h and 1p ); however, their β vec values are not the highest because this kind of substitution gives rise also to a large μ g .…”