Quadratic Hyperpolarizability Research Articles

Effect of substitution on second-order nonlinear optical properties of ferrocene appended donor–π–acceptor Y-shaped trifluoromethyl imidazole chromophores

Substitution of an imidazole ring creates a steric effect and restricts the intramolecular charge-transfer (ICT) process, resulting in the feeble quadratic hyperpolarizability of the chromophore.

Heteroanionic Melilite Oxysulfide: A Promising Infrared Nonlinear Optical Candidate with a Strong Second-Harmonic Generation Response, Sufficient Birefringence, and Wide Bandgap.

The achievement of balanced performance with a strong second-harmonic generation (SHG) response, proper birefringence, and wide band gap concurrently is a crucial but challenging task in infrared nonlinear optical (IR-NLO) crystals. Here, we theoretically confirmed that the heteroanionic oxysulfide tetrahedron would produce improved polarizability anisotropy and quadratic hyperpolarizability compared with the monoanionic oxide or sulfide tetrahedra. When this anion-mixing strategy was applied, melilite oxysulfide with the four representative members A2GeGa2OS6 (A = Ca, Sr) and Sr2MGe2OS6 (M = Zn, Cd) was successfully discovered as a promising IR-NLO material system. Remarkably, compared with the monoanionic melilite oxides, these compounds exhibited unbiased performances of proper birefringences (0.106-0.143), strong SHG responses (>10× melilite oxide Ba2CdGe2O7, 1.3-2.1× AgGaS2 (AGS) @1570 nm) with phase-matchable ability, wide band gaps (2.95-3.15 eV), and large laser-induced damage thresholds (LIDTs, 5.6-13.4× AGS). The high structure tolerance of melilite offers a great possibility to improve the SHG response via tuning the orbital composition and hybridization near band edges. This work provides an effective approach for the design of high-performance IR-NLO materials.

Progress and perspectives on strategies to control photochemical properties in Metallo-Dithiolene Donor-Acceptor systems

In this mini-review recent progress in studies on heteroleptic d8-metal dithiolene complexes with a D-M-A donor–π–acceptor electronic structure showing non-linear and linear optical properties, are presented. The ligands, formally dithiolenes, consist in a variety of donors (dithiolato) and acceptors (dithioxamide and dithioxamidate), differing for electronic properties and/or structural features, varying from rigid and planar systems to conformationally-flexible ones. It is shown how the components of these D-M-A systems modulate both the energy and topology of the frontier molecular orbitals involved in the charge transport mechanism to reach a fine tuning of the optical properties and achieve high values of the quadratic hyperpolarizability. Further tailoring of the ligands by integrating specific functional groups in their periphery, such as NH and/or a quinoxaline ring, is presented as a versatile tool to achieve a reversible response to external stimuli. Accordingly, selected cases where exchange color proton and silver ions tunable properties, switching of the nonlinear optical response (with a 2.5–6.5 contrast for HCl exchange) and also uncommon emission color reversibly tunable by different stimuli, such as proton and silver ions, are presented. In the bulk, crystals gave a poor or null second harmonic generation (SHG) response, while a stable and large response has been obtained in a limited number of cases of complexes dispersed in polymethylmethacrylate films. Reported results are addressed to stimulate further work to exploit the potential of this class of complexes as molecular sensors and switches and to optimize the processing procedures to achieve good SHG responses in the bulk.

Analytical theory for three-wave mixing processes in a slightly deformed nanowire

The second order optical response of centrosymmetric materials manifests itself mostly at their surface, being strongly suppressed in their bulk. However, the overall surface response is also suppressed in nanoparticles with a centrosymmetric geometry subjected to homogeneous fields. Nevertheless, nanoparticles with a noncentrosymmetric geometry do exhibit second order optical properties. We develop an analytical theory to investigate the second order optical response of a noncentrosymmetric thin cylinder with a slightly deformed cross-section made up of a centrosymmetric material subjected to two monochromatic fields. We calculate the linear and nonlinear near fields perturbatively using the extent of the deformation away from a circular cross-section as the perturbation parameter. We obtain expressions for the quadratic hyperpolarizabilities in terms of the linear response evaluated at the three frequencies involved. We analyze the spectral features of the nonlinear response functions and explore their resonant structure for a model dielectric cylinder. Furthermore, we evaluate the second order radiated fields, the radiation patterns and efficiency of the different quadratic processes. We obtain a strong competition between electric dipolar, magnetic dipolar and electric quadrupolar contributions even for very small deformations.

Transition metal(II) complexes featuring push-pull dianionic Schiff base ligands: synthesis, crystal structure, electrochemical, and NLO studies

The present study reports the synthesis and characterization of a family of three new neutral Zn(II), Cu(II), Co(II) bimetallic (2–4), and one ionic Co(II) trimetallic (5) complexes of ferrocene-containing unsymmetrically-substituted N2O2 tetradentate Schiff base ligands. They were all prepared via a three-component one-pot template reaction involving the 4-hydroxyphenyl substituted ferrocenyl-containing half unit Fc-C(=O)CH = C(4-C6H4OH)NH-CH2CH2NH2 (Fc = ferrocenyl = (η5-C5H5)Fe(η5-C5H4)), metal(II) acetate, and 5-nitrosalicylaldehyde (for 2–4) or the organometallic salicylaldehyde [Cp*Ru(η6-2-OH-C6H4CHO)]PF6 (for 5) (Cp* = η5-C5Me5). The four complexes were isolated as air and moisture insensitive solids in yields > 80%. They were characterized using several analytical techniques such as CHN analyses, ESI mass, FT-IR, UV-vis, and NMR spectroscopy, including single crystal X-ray diffraction analysis. The crystal structure of 3 reveals that the four-coordinate copper ion adopts a square planar geometry with two nitrogen and two oxygen atoms as donors occupying cis positions. An electrochemical study utilizing cyclic voltammetry showed for each compound a reversible redox process ascribed to the Fe(II)/Fe(III) couple of the appended donor ferrocene. The largest anodic potentials were measured for the two Co(II) species. The second-order NLO responses of the push-pull chromophores 2–5 have been determined by harmonic light scattering measurements in N,N-dimethylformamide solutions at 1.91 μm incident wavelength, and rather high, non-resonant quadratic hyperpolarizability β values ranging from 140 to 280 × 10−30 esu were determined.

Second Order Nonlinear Optical Properties of 4-Styrylpyridines Axially Coordinated to A4 ZnII Porphyrins: A Comparative Experimental and Theoretical Investigation

In this research, two 4-styrylpyridines carrying an acceptor –NO2 (L1) or a donor –NMe2 group (L2) were axially coordinated to A4 ZnII porphyrins displaying in 5,10,15,20 meso position aryl moieties with remarkable electron withdrawing properties (pentafluorophenyl (TFP)), and with moderate to strong electron donor properties (phenyl (TPP) < 3,5-di-tert-butylphenyl (TBP) < bis(4-tert-butylphenyl)aniline) (TNP)). The second order nonlinear optical (NLO) properties of the resulting complexes were measured in CHCl3 solution by the Electric-Field-Induced Second Harmonic generation technique, and the quadratic hyperpolarizabilities βλ were compared to the Density Functional Theory (DFT)-calculated scalar quantities β||. Our combined experimental and theoretical approach shows that different interactions are involved in the NLO response of L1- and L2-substituted A4 ZnII porphyrins, suggesting a role of backdonation-type mechanisms in the determination of the negative sign of Electric-Field-Induced Second Harmonic generation (EFISH) βλ, and a not negligible third order contribution for L1-carrying complexes.

NLO-active Y-shaped ferrocene conjugated imidazole chromophores as precursors for SHG polymeric films.

New Y-shaped ferrocene conjugated imidazole chromophores were prepared and fully characterized. The Y-shaped structure was confirmed by the single crystal X-ray diffraction technique. The chromophores show interesting second-order nonlinear optical (NLO) properties in solution, as determined by the Electric-Field Induced Second Harmonic generation (EFISH) technique. Remarkably, the trifluoro substituted compound 3 is characterized by a high μβEFISH value and has good potential as a molecular building block for composite films with Second Harmonic Generation (SHG) properties. For all compounds, the dipole moments and frontier orbital energies were calculated by the Density Functional Theoretical method.

Thioethyl Porphyrazines: Attractive Chromophores for Second-Order Nonlinear Optics and DSSCs

The first study on the second-order NLO properties of unconventional push–pull systems constituted by asymmetrically monoaryl-substituted thioalkyl porphyrazines, carried out by EFISH measurements and DFT calculations, is here reported. The results obtained show that the porphyrazine macrocycle behaves as a better electron donor and acceptor with respect to structurally similar porphyrin and phthalocyanine tetrapyrroles. Noteworthy, the highest quadratic hyperpolarizability (β1907) values are displayed by the pyrene-substituted thioalkyl porphyrazines, both as free-base and as PdII complex, without the presence of either electron-withdrawing or electron-donating groups. In these cases, the pyrene moiety behaves as an electron-donating group, giving rise to charge transfer HOMO–LUMO electronic transitions. These outcomes show, for the first time, the great potential of the thioalkyl porphyrazine macrocycles for second-order NLO applications. Moreover, the hydroxyphenyl-substituted porphyrazine 2 dye has been also tested as a sensitizer in a TiO2–DSSC, representing the second example reported so far of a DSSC with thioalkyl porphyrazines.

Structural Characterization and Second-Order Nonlinear Optical Behavior of Metal Complexes of Ferrocene Derivative

In this manuscript, a ferrocene-linked vic-dioxime ligand (LH2) and its metal complexes [Ni(LH)2, Cu(LH)2, Co(LH)2(H2O)2, Cd(LH)(H2O)(Cl), and Zn(LH)(H2O)(Cl)] are synthesized, and their structures are studied by spectral methods. Redox behavior of the compounds is studied by cyclic voltammetry (CV). For approaching microscopic second-order nonlinear optical (NLO) behavior of the synthesized ligand and its diamagnetic Cd(II), Zn(II), Ni(II) complexes, the electric dipole moment μ, static dipole polarizability α, and first hyperpolarizability β values are computed using ab-initio quantum chemical procedure [finite field (FF)]. The accumulated data indicate that the compounds exhibit non-zero quadratic hyperpolarizability tensor components, implying microscopic NLO phenomena. The HOMOs, LUMOs and the HOMO—LUMO band gaps for first and second frontier orbitals of LH2 ligand and its Cd(II), Zn(II), Ni(II) complexes are evaluated by means of the Hartree-Fock (HF) method.

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.

Improved electro-optical property by introducing stronger acceptor to thermal stable chromophores using modified julolidine as donor

Two novel second order nonlinear optical chromophores based on large steric hindrance group modified julolidine donor and CF3-Ph-TCF acceptor linked via thiophene bridge were synthesized and characterized. Density functional theory (DFT) calculations suggested that the molecular quadratic hyperpolarizability (β) value of CF3-Ph-TCF based chromophore is 18.1% and 18.8% larger than that of TCF based chromophores, respectively. The optimized structure shows that chromophore z4-3f with stronger acceptor and larger modification group presents the best molecular configuration. UV–Vis results show great red-shift about 130 nm for chromophores z2-3f and z4-3f compared with z2 and z4. According to cyclic voltammetry measurements, chromophores z2-3f and z4-3f showed lower energy gap indicating better intramolecular charge-transfer (ICT) transition. Especially, chromophore z4-3f displayed good thermal stability with onset decomposition temperature at about 265 °C. The poled films containing 25 wt% chromophores in amorphous polycarbonate (APC) afforded electro-optical(EO) coefficients (r33) of 125 and 128 pm/V at 1310 nm. Further, film containing larger group modified chromophore was more efficient to realize good EO property with an order parameter (Φ) up to 18.4%. It's confirmed that chromophores z2-3f and z4-3f, which had good thermal stability and high r33 values after poling in films, was a promising candidate for practical use.

Design of Near-Infrared-Absorbing Unsymmetrical Polymethine Dyes with Large Quadratic Hyperpolarizabilities

Fifteen heptamethine dyes in which a tricyanofuran acceptor is linked to donors of different strengths were synthesized, and their absorption, thermal, electrochemical, and second-order nonlinear optical properties were investigated. While the variation of the peripheral bulky substituents allowed a decrease of the intermolecular interactions in the solid state, subtle engineering of the heterocyclic donor provided dyes with electronic structures that varied between dipolar and cyanine-like (i.e., with little bond length alternation and with ground and excited states having similar dipole moment) and remarkably high quadratic hyperpolarizabilities (μβ1.9) of up to 115 000 × 10–48 esu.

Quadratic and cubic hyperpolarizabilities of nitro-phenyl/-naphthalenyl/-anthracenyl alkynyl complexes.

1-Nitronaphthalenyl-4-alkynyl and 9-nitroanthracenyl-10-alkynyl complexes [M](C[triple bond, length as m-dash]C-4-C10H6-1-NO2) ([M] = trans-[RuCl(dppe)2] (6b), trans-[RuCl(dppm)2] (7b), Ru(PPh3)2(η5-C5H5) (8b), Ni(PPh3)(η5-C5H5) (9b), Au(PPh3) (10b)) and [M](C[triple bond, length as m-dash]C-10-C14H8-9-NO2) ([M] = trans-[RuCl(dppe)2] (6c), trans-[RuCl(dppm)2] (7c), Ru(PPh3)2(η5-C5H5) (8c), Ni(PPh3)(η5-C5H5) (9c), Au(PPh3) (10c)) were synthesized and their identities were confirmed by single-crystal X-ray diffraction studies. Electrochemical studies and a comparison to the 1-nitrophenyl-4-alkynyl analogues [M](C[triple bond, length as m-dash]C-4-C6H4-1-NO2) ([M] = trans-[RuCl(dppe)2] (6a), trans-[RuCl(dppm)2] (7a), Ru(PPh3)2(η5-C5H5) (8a), Ni(PPh3)(η5-C5H5) (9a), Au(PPh3) (10a)) reveal a decrease in oxidation potential for ruthenium and nickel complexes on proceeding from the phenyl- to naphthalenyl- and then anthracenyl-containing bridge. HOMO → LUMO transitions characteristic of MC[triple bond, length as m-dash]C-1-C6H4 to 4-C6H4-1-NO2 charge transfer red-shift and gain in intensity on proceeding to the ruthenium complexes; the low-energy transitions have increasing ILCT character on proceeding from the phenyl- to naphthalenyl- and then anthracenyl-containing bridge. Spectroelectrochemical studies of the Ru-containing complexes reveal the appearance of low-energy bands corresponding to chloro-to-RuIII charge transfer that red-shift on proceeding from the phenyl- to naphthalenyl- and then anthracenyl-containing bridge. Second-order nonlinear optical (NLO) studies at 1064 nm employing ns pulses and the hyper-Rayleigh scattering technique reveal an increase in quadratic optical nonlinearity upon introduction of metal to the precursor alkyne to afford alkynyl complexes and on proceeding from ligated-gold to -nickel and then to -ruthenium for a fixed alkynyl ligand. Quadratic NLO data of the gold complexes optically transparent at the second-harmonic wavelength reveal an increase in βHRS on proceeding from the phenyl- to the naphthalenyl-containing complex. Broad spectral range third-order nonlinear optical studies employing fs pulses and the Z-scan technique reveal an increase in two-photon absorption cross-section on replacing ligated-gold by -nickel and then -ruthenium for a fixed alkynyl ligand. Computational studies undertaken using time-dependent density functional theory have been employed to assign the nature of the key optical transitions and suggest that the significant optical nonlinearities observed for the ruthenium-containing complexes correlate with the low-energy formally Ru → NO2 band which possesses strong MLCT character, while the more moderate nonlinearities of the gold complexes correlate with a band higher in energy that is primarily ILCT in character.

An investigation on the second-order nonlinear optical response of cationic bipyridine or phenanthroline iridium(iii) complexes bearing cyclometallated 2-phenylpyridines with a triphenylamine substituent.

The synthesis and characterisation of six new cationic iridium(iii) complexes bearing either 4,4'-di-tert-butyl-2,2'-bipyridine (dtBubpy) or 5-NO2-1,10-phenanthroline along with two cyclometallated 2-phenylpyridine derivative ligands, decorated with triphenylamine groups either meta or para to the Ir-CC^N bond or para to the Ir-NC^N bond, are reported. The second-order nonlinear optical (NLO) properties of all the compounds have been determined by the electric field induced second harmonic generation technique and show that the μβEFISH value can be tuned by the nature of the iridium coordination sphere. The dipole moment, μ, necessary to evaluate the quadratic hyperpolarizability βEFISH, was theoretically determined. The linear optical properties of the complexes are also presented and rationalised by quantum-chemical calculations. One of the prepared iridium compounds was incorporated into a polystyrene film, affording the first example of a second-order NLO active polymeric film based on a cationic organometallic complex.

On the contribution of f electrons to the quadratic hyperpolarizability: the case of lanthanide terpyridyl complexes.

Over the last decades, trivalent lanthanide ions (Ln3+) have gained much attention due to their peculiar luminescence, which opened the way to a broad range of applications, from medical diagnostic to lasers. Their impact on nonlinear optical (NLO) properties also attracted interest, especially in the framework of lanthanide complexes. Several experimental studies demonstrated that the quadratic hyperpolarizability varies with the number of 4f-electrons, with a stronger effect on dipolar than octupolar components. The main interpretation put forward to explain the observed trends relied on the polarizable character of the 4f-electrons. We report here a first step towards understanding the role of 4f-electrons in NLO responses, considering a series of dipolar terpyridyl-trinitro lanthanide complexes LLn(NO3)3 (Ln = Gd, Dy, Yb, Lu as well as La and Y; L = terpyridil-like ligand). Using DFT and TD-DFT we investigate their linear and non-linear optical properties. Consistently with earlier experimental findings, simulated UV-visible spectra show minor changes by varying Ln. The same holds for dipole moments and polarizabilities, whereas the nature of the lanthanide affects hyperpolarizabilities. It is shown that the observed changes are not a direct effect of the 4f-electrons that behave like core electrons.

Nonlinear optical response of octupolar Zn(II) complexes incorporating highly aromatic polypyridinic ligands: Insights into the role of the metal center

In this work, the linear and nonlinear optical properties of a series of octupolar Zn(II) complexes with highly aromatic polypyridine ligands are investigated. The effect of the metal center on the spectroscopic properties of octupolar Zn (II) complexes are explored and compared to its respective free ligands. DFT and TD-DFT calculations were performed to gain more insights about the electronic and structural properties of these compounds. The aromaticity index of ligands and its modulation by Zn(II) in complexes was also theoretically studied. Quadratic hyperpolarizabilities (β) were determined by using the Hyper-Light Scattering (HLS) technique at 1.06μm.Results show that β values of the octupolar Zn(II) complexes are around three times higher than those of their respective ligands, which correlates with the number of ligands directly coordinated to the metal center. These results show how Zn(II) plays a positive structural role in the elaboration of highly transparent and active nonlinear optical molecules, without a significant electronic effect on the individual ligand NLO response, as expected when increasing the aromaticity of conjugated molecules.

New Internal-Charge-Transfer Second-Order Nonlinear Optical Chromophores Based on the Donor Ferrocenylpyrazole Moiety

A series of new N-arylated ferrocenepyrazole structures, carrying different donor or acceptor substituents in the para position of the aryl ring, has been synthesized by the Chan–Lam cross-coupling reaction. The nonplanar geometric molecular structure of some of these chromophores together with their crystal packing was determined by X-ray diffraction, and the HOMO and LUMO energy levels were evaluated by electrochemical and optical measurements and by density functional theoretical (DFT) calculations. By the investigation of solvent effects and time-dependent DFT (TD-DFT) calculations, the intense electronic absorption band around 270–310 nm was confirmed to be an internal-charge-transfer (ICT) band, showing a significant red shift by increasing the electron withdrawing properties of the substituent on the para position of the aryl ring. TD-DFT calculations and electric field induced second harmonic generation (EFISH) measurements of the quadratic hyperpolarizability have shown that also the second-order...

Unprecedented intramolecular cyclization in strongly dipolar extended merocyanine dyes: A route to novel dyes with improved transparency, nonlinear optical properties and thermal stability

Novel magenta dyes were obtained via an unprecedented base-promoted intramolecular cyclization of strongly dipolar elongated merocyanine dyes bearing a phthalimide substituted tricyanopropylidene terminal segment. These shorter diarylpolyene derivatives retain a dipolar character and show a typical intramolecular charge transfer (ICT) transition. The cyclization noticeably modifies the electronic structure inducing both onset of bond length alternation and reduction of dipole moment as well as a marked hypsochromic and hypochromic shift of the ICT absorption band compared to their merocyanine precursors. Yet the new dyes show improved quadratic hyperpolarizability and thermal stability as compared to the merocyanine dyes having the same number of conjugated double bonds in the polyenic linker. As such these derivatives hold promise as new class of dyes for NLO materials and SHG probes.

The design of nonlinear optical chromophores exhibiting large electro-optic activity and high thermal stability: The role of donor groups

A novel chromophore based on N-(4-dibutylaminophenyl)tetrahydroquinolinyl donor group was synthesized to compare with traditional chromophores based on either diethylaminophenyl or triarylaminophenyl donor groups. Cyclic voltammetry measurements showed that the new chromophore had a much smaller energy gap than the structurally related chromophores. Density functional theory calculations suggested that the molecular quadratic hyperpolarizability (μβ) value of tetrahydroquinolinyl chromophore is 65% and 110% larger than that of either the diethylaminophenyl or triarylaminophenyl chromophores, respectively. The doped film containing the tetrahydroquinolinyl chromophore showed an r33 value of 126 pm/V at the concentration of 25 wt% which is much higher than the electro-optic activity of the diethylaminophenyl derived chromophore (39 pm/V) and six times higher than that of the traditional triarylaminophenyl chromophore (19 pm/V). The tetrahydroquinolinyl chromophore possesses much higher thermal stability (with its onset decomposition temperature above 280 °C) than those of commonly used alkyl chromophores.

Intramolecular Cooperative Effects in Multichromophoric Cavitands Exhibiting Nonlinear Optical Properties

We report on the design, synthesis, and characterization of a new class of multichromophoric cavitands based on resorcin[4]arenes. The novel compounds have exhibited high values of second-order nonlinear optical (NLO) properties, as evidenced by electric-field-induced second harmonic generation (EFISHG) measurements. Theoretical calculations indicate the presence of edge-to-face T-shaped interactions between the aromatic building blocks within these multichromophoric systems, which is further supported by the detection of hypsochromic shifts in UV-vis and upfield aromatic chemical shifts in 1H NMR. We proved for the first time that the gain in the quadratic hyperpolarizabilities of multichromophoric NLO macrocycles, originating from the near parallel orientations of the subchromophores, can be partially suppressed if the distance between the dipolar subunits falls into a specific range, where intramolecular cooperative and/or collective effects are operative. Our finding will contribute to the better understanding of the phenomenon of cooperativity in new molecular materials with promising NLO properties. (Figure Presented). © 2015 American Chemical Society.