Theoretical Studies on the First Hyperpolarizabilities of One-dimensional Donor-bridge-acceptor Chromophores

The ground-state dipole moments and second-order nonlinear optical （NLO） properties of a series of one-dimensional （1D） chromophores with donor-bridge-acceptor （D-B-A） structures have been investigated by using the second-order MФller-Plesset （MP2） and density functional theory （DFT） methods with the basis set of 6-31＋G（d）. According to the calculated results, the relationship between the molecular static first hyperpolarizability （βμ） and the directions of electron transition has been summarized. In terms of the sign of βμ, these 1D organic chromophores were classified into two categories： type Ⅰ with negative βμ and type Ⅱ bearing positive βμ. The analyses show that the remarkable difference of the first hyperpolarizabilities between Ⅰ and Ⅱ chromophores is associated mainly with the electrostatic interaction between terminal groups and the transport electrons in excited states. Moreover, different from the popular viewpoint, the obtained results also show that most of this series of 1D D-B-A molecules are more charge-separated in the ground states than in the excited states. As a whole, this theoretical investigation, to some extent, can be considered as a useful reference in designing the NLO chromophores with large first hyperpolarizabilities.     

A comparison of the pentaammine(pyridyl)ruthenium(II) and 4-(dimethylamino)phenyl groups as electron donors for quadratic non-linear optics

Hyper-Rayleigh scattering and Stark spectroscopic studies show that the complex salts [1-4]PF6 have larger static first hyperpolarizabilities beta 0 than [5-8]PF6, because the higher HOMO energy of a (RuII(NH3)5)2+ centre more than offsets the superior pi-orbital overlap in the purely organic chromophores.     

Theoretical Studies on the First Hyperpolarizabilities of One-dimensional Donor-bridge-acceptor Chromophores and New Applications of BLA in Determining Molecular First Hyperpolarizabilities

We present a quantum-chemical analysis of the relationship between the bond length alteration （BLA） and the static first hyperpolarizability of a series of one-dimensional （1D） chromophores with donor-bridge-acceptor （D-B-A） structures. The calculated results show that the parameter BLA can be considered as an indicator to evaluate the molecular first hyper- polarizability. Along the direction of molecular ground-state dipole moments, the evolutions of BLA can be classified into three categories： the first is a non-monotonic line, which represents most chromophores; the second is monotonic increasing; and the third, contrarily, is monotonic decreasing. On the whole, the first hyperpolarizabilities of these studied chromophores are the monotonic functions of BLA along the direction of dipole moments. Therefore, the first hyperpolarizability of these 1D chromophores can be preliminarily evaluated in terms of the development of BLA without a rigorous computation. In other words, one can roughly estimate the relative magnitude of the first hyperpolarizability according to the optimized geometry.     

锂掺杂胺化物中氮原子配位数效应对结构和非线性光学性质的影响

采用MP2／6—311＋＋G（d,p）方法得到了含有不同氮原子配位数的系列锂掺杂胺化物的几何结构,并使用MP2／6．311＋＋G（2d,p）方法计算了该系列的非线性光学性质．结果表明：相对于未掺杂体系,锂掺杂极大地增加了体系的一阶超极化率（β0）,其中1的β0值是掺杂前的3238倍;体系的风随着氮原子配位数的增加而显著增加,具有3个配位氮原子的1的风值达到2．8×10^5a．u．另外,频率依赖的一阶超极化率（β（-ω;ω,0）和β（-2ω;ω,0））也表现了同样的趋势．     

Syntheses and spectroscopic and quadratic nonlinear optical properties of extended dipolar complexes with ruthenium(II) ammine electron donor and N-methylpyridinium acceptor groups

In this paper, we describe the extremely unusual optical properties of Ru(II)-based electron donor-acceptor (D-A) polyene and some closely related chromophores. For three different polyene series, the intense, visible d-->pi* metal-to-ligand charge-transfer bands unexpectedly blue-shift as the number of E-ethylene units (n) increases from 1 to 3, and the static first hyperpolarizabilities beta(0) determined via hyper-Rayleigh scattering and Stark spectroscopy maximize at n = 2, in marked contrast to other known D-A polyenes in which beta(0) increases steadily with n. Time-dependent density-functional theory and finite field calculations verify these empirical trends, which arise from the orbital structures of the complexes. This study illustrates that transition metal-based nonlinear optical chromophores can show very different behavior when compared with their more thoroughly studied purely organic counterparts.     

Quadratic and cubic nonlinear optical properties of salts of diquat-based chromophores with diphenylamino substituents

A series of chromophoric salts has been prepared in which 4-(diphenylamino)phenyl (Dpap) electron donor groups are connected to electron-accepting diquaternized 2,2'-bipyridyl (diquat) units. The main aim is to combine large quadratic and cubic nonlinear optical (NLO) effects in potentially redox-switchable molecules with 2D structures. The chromophores have been characterized as their PF(6)(-) salts by using various techniques including electronic absorption spectroscopy and cyclic voltammetry. The visible absorption spectra are dominated by intense π → π* intramolecular charge-transfer (ICT) bands, and all of the compounds show two reversible or quasireversible diquat-based reductions and partially reversible Dpap oxidations. Single crystal X-ray structures have been obtained for one salt and for the precursor compound (E)-4-(diphenylamino)cinnamaldehyde, both of which adopt centrosymmetric space groups. First hyperpolarizabilities β have been measured by using hyper-Rayleigh scattering (HRS) with a 800 nm laser, and Stark (electroabsorption) spectroscopy of the ICT bands affords estimated static first hyperpolarizabilities β(0). The directly and indirectly derived β values are large and generally increased substantially for the bis-Dpap derivatives when compared with their monosubstituted analogues. Polarized HRS studies show that the NLO responses of the disubstituted species are dominated by "off-diagonal" β(zyy) components. Lengthening the diquaternizing alkyl unit lowers the electron-acceptor strength and therefore increases the ICT energies and decreases the E(1/2) values for diquat reduction. However, compensating increases in the ICT intensity prevent significant decreases in the Stark-based β(0) responses. Cubic NLO properties have been measured by using the Z-scan technique over a wavelength range of 520-1600 nm, revealing relatively high two-photon absorption cross-sections of up to 730 GM at 620 nm for one of the disubstituted chromophores.     

A straightforward modular approach to NLO-active beta-amino vinyl nitrothiophenes

Secondary amines add very efficiently to 2-ethynyl-5-nitrothiophene to give beta-amino vinyl nitrothiophenes, a novel class of push-pull chromophores. According to first HRS measurements these highly solvochromic compounds with relatively short dipole axes display remarkably high static first hyperpolarizabilities = 29-31 x 10(-)(30) esu.     

Relationship between structure and nonlinear optical properties of new bisazo chromophores. Theoretical and experimental study

Two new bisazo chromophores—derivatives of bis-2,2-(4-hydroxyphenyl)propane (bisphenol A), heterocyclic amines (2-aminobenzothiazole, and 2-amino-6-nitrobenzothiazole) and aniline—were synthesized. Nonlinear optical properties of these chromophores were studied by solvatochromic method (obtained compounds have β_CT value ∼10⁻²⁹ esu). Dependence among the structure of molecules, their dipole moments, and first hyperpolarizabilities was considered on the basis of semi-empirical calculations. A correlation between the measured and calculated β values was studied. The NLO chromophores were used as special monomers to synthesize copolyarylates. The basic characterization of obtained polymers was carried out.     

Direct metal-metal interaction contributions to quadratic hyperpolarizability: a study on dirhenium complexes

We present a comparative study of the metal-metal interaction effect on the static quadratic hyperpolarizabilities of two typical dinuclear rhenium clusters. The electronic structures, excitation spectra, dipolar moments, static polarizabilities, and quadratic hyperpolarizabilities of the two complexes with direct metal-metal interactions have been computed and analyzed with the use of high-level DFT/TDDFT methods. The geometries and the first intense excitations agree with the relevant reported measurements. The orbital decomposition scheme ( J. Phys. Chem. A 2006, 110, 1014-1021) has been applied to analyze the relationship between the electronic structures and nonlinear optical (NLO) properties of these two complexes. We propose an unprecedented NLO response mechanism featuring the contribution of the direct metal-metal interaction transition process in these dinuclear rhenium complexes. This contribution positively enhances the quadratic hyperpolarizability and relates to the intensity of the metal-metal interactions of the complexes. The results are helpful to the development of NLO chromophores in polynuclear metal clusters through the molecular design technique.     

Synthesis and hyperpolarizabilities of high temperature triarylamine-polyene chromophores

The synthesis and hyperpolarizabilities of a series of push-pull chromophores containing bis-(4-methoxyphenyl)-amine donor and efficient acceptors bridged with ring locked polyene are presented. The chromophores are readily soluble in common organic solvents and exhibit high thermal decomposition temperatures (highest T_d = 330 °C). Molecular hyperpolarizabilities (β) of the chromophores were measured by Hyper Rayleigh Scattering (HRS) at 1604 nm (highest β = 20,000 × 10⁻³⁰ esu). These chromophores can be used to develop electro-optic (EO) materials due to their large optical nonlinearities, good absorption characteristics, high thermal decomposition temperatures, and excellent solubility with organic solvents and polymeric materials.     

Syntheses and properties of two-dimensional, dicationic nonlinear optical chromophores based on pyrazinyl cores

Six new dicationic 2D nonlinear optical (NLO) chromophores with pyrazinyl-pyridinium electron acceptors have been synthesized by nucleophilic substitutions of 2,6-dichloropyrazine with pyridyl derivatives. These compounds have been characterized as their PF(6)(-) salts by using various techniques including electronic absorption spectroscopy and cyclic voltammetry. Large red shifts in the intense, π → π* intramolecular charge-transfer (ICT) transitions on replacing -OMe with -NMe(2) substituents arise from the stronger π-electron donor ability of the latter. Each compound shows a number of redox processes which are largely irreversible. Single crystal X-ray structures have been determined for five salts, including two nitrates, all of which adopt centrosymmetric packing arrangements. Molecular first hyperpolarizabilities β have been determined by using femtosecond hyper-Rayleigh scattering at 880 and 800 nm, and depolarization studies show that the NLO responses of the symmetric species are strongly 2D, with dominant "off-diagonal" β(zyy) components. Stark (electroabsorption) spectroscopic measurements on the ICT bands afford estimated static first hyperpolarizabilities β(0). The directly and indirectly derived β values are large, and the Stark-derived β(0) response for one of the new salts is several times greater than that determined for (E)-4'-(dimethylamino)-N-methyl-4-stilbazolium hexafluorophosphate. These Stark spectroscopic studies also permit quantitative comparisons with related 2D, binuclear Ru(II) ammine complex salts.     

1,3-D,A取代方酸极化率和一阶超极化率的从头算研究

本文在RHF／6-311 G(3d，2p)水平上，对一系列1，3取代方酸衍生物的几何构型进行了全优化，得到分子最稳定构型，并用CPHF方法计算了分子线性极化率和一阶超极化率，分析了取代基对分子线性和非线性光学性质的影响。     

Highly unusual effects of pi-conjugation extension on the molecular linear and quadratic nonlinear optical properties of ruthenium(II) ammine complexes

We have used several techniques, including hyper-Rayleigh scattering and Stark spectroscopy, to investigate the effects of polyene chain length on the optical properties of complexes containing ruthenium(II) electron donor groups and pyridinium electron acceptors. In marked contrast with all other known donor-acceptor polyenes, conjugation extension beyond a single double bond in the dipolar complexes studied leads to blue-shifting of the intramolecular charge-transfer absorptions. Furthermore, the static first hyperpolarizabilities beta0 become maximized with trans-1,3-butadienyl linkages and then decrease in complexes with three CH=CH bonds. Our results clearly demonstrate that the molecular engineering criteria for metal-containing nonlinear optical chromophores can differ dramatically from those for purely organic compounds.     

NLO properties of metallabenzene-based chromophores: a time-dependent density functional study

The static and dynamic first hyperpolarizabilities for a series of substituted metallabenzene-based nonlinear optical (NLO) chromophores were determined by time-dependent density functional theory (TDDFT). The electronic excitation contributions to the first hyperpolarizability are rationalized in terms of the two-level model. The effects on the hyperpolarizabilities of (a) the metal center (Os, Ir, Pt); (b) the ligand environment (PH3, CO, Cl); (c) various donor and acceptor substituents (NH2, OH, Me, H, Cl, Br, I, COOMe, COOH, CN, NO2); and (d) the length of pi-conjugation were studied. Our calculations predict that metallabenzenes have significant second-order NLO susceptibilities, ranging from = 1.0 x 10(-29) to 5.6 x 10(-28) esu and from mu = 3.0 x 10(-47) to 1.1 x 10(-44) esu, that can be tuned by changing the metal center and/or ligand environment.     

Influence of pi-stacking on the resonant enhancement of the second-order nonlinear optical response of dipolar chromophores

The wavelength-dependent second-harmonic generation (SHG) efficiency of two simple dipolar chromophores, 4-NO2C6H4N(H)Bun (1) and 4-NO2C6H4SN(H)But (2), was compared in solution and in the solid state. Hyper-Rayleigh scattering measurements at 532 nm provided comparable molecular first hyperpolarizabilities. Both compounds crystallize in non-centrosymmetric space groups, but a more efficient arrangement of dipole moments results in a significantly larger deff value for 2. Kurtz-Perry experiments from 450 to 700 nm revealed an important difference in the resonant component of the nonlinear optical responses of these compounds; the SHG efficiency of crystalline 1 depends more strongly on the incident wavelength than that of 2. This would be in contradiction with the TD-DFT excitation energies calculated for these molecules, but the observation can be explained by the resonant contribution from low-energy interchromophore excitations enabled by pi-stacking in the crystal of 1.     

Wavelength dependence of first molecular hyperpolarizability of a dendrimer in solution

The frequency dependence of the first molecular hyperpolarizability of a dendrimer incorporated with thiophene-stilbene based charge-transfer chromophores is investigated by using a nanosecond 1907 nm laser and a number of wavelengths ranging from 1160 to 1760 nm emitted from an optical parametric amplifier pumped by a 1 kHz 130 fs Ti:sapphire laser. The measured hyperpolarizabilities are compared with those calculated from the charge-transfer absorption spectrum involving a Kramers-Kronig transformation scheme. The Kramers-Kronig transformation analysis provides a satisfactory account of the dispersion of the first molecular hyperpolarizability over the entire excitation wavelength range measured. The Kramers-Kronig technique extends the Oudar-Chemla two-level model previously proposed for the first molecular hyperpolarizability and it can be used in the nonresonance as well as the resonance region where the Oudar-Chemla model fails. The Kramers-Kronig transformation scheme allows a consistent intrinsic hyperpolarizability beta(0) to be obtained from the measured beta(HRS) using different excitation wavelengths for the dendrimer. The comparison of beta(0) for the dendrimer, which contains three chromophores, with that of corresponding monomer chromophore suggests that the chromophores inside the dendrimer are independent. This gives the evidence of the site isolation effect of the dendrimer and substantiates the larger macroscopic optical nonlinearity recently obtained for the dendrimer.     

Two-dimensional, pyrazine-based nonlinear optical chromophores with ruthenium(II) ammine electron donors

Six new nonlinear optical (NLO) chromophores with pyrazinyl-pyridinium electron acceptors have been synthesized by complexing a known pro-ligand with electron donating {Ru(II)(NH(3))(5)}(2+) or trans-{Ru(II)(NH(3))(4)(py)}(2+) (py = pyridine) centers. These cationic complexes have been characterized as their PF(6)(-) salts by using various techniques including electronic absorption spectroscopy and cyclic voltammetry. The visible d → π* metal-to-ligand charge-transfer (MLCT) absorptions gain intensity on increasing the number of Ru(II) centers from one to two, but remain at constant energy. One or two Ru(III/II) redox processes are observed which are reversible, quasi-reversible, or irreversible, while all of the ligand-based reductions are irreversible. Molecular first hyperpolarizabilities β have been determined by using hyper-Rayleigh scattering (HRS) at 1064 nm, and depolarization studies show that the NLO responses of the symmetric species are strongly two-dimensional (2D) in character, with dominant "off-diagonal" β(zyy) components. Stark (electroabsorption) spectroscopic measurements on the MLCT bands also allow the indirect determination of estimated static first hyperpolarizabilities β(0). Both the HRS and the Stark-derived β(0) values increase on moving from mono- to bimetallic complexes, and substantial enhancements in NLO response are achieved when compared with one-dimensional (1D) and 2D monometallic Ru(II) ammine complexes reported previously.     

Contrasting linear and quadratic nonlinear optical behavior of dipolar pyridinium chromophores with 4-(dimethylamino)phenyl or ruthenium(II) ammine electron donor groups

In this article, we contrast the optical properties of dipolar chromophores having 4-(dimethylamino)phenyl electron donor (D) and pyridinium acceptor (A) groups with those of closely related cations having pyridyl-coordinated Ru(II) donors. A range of physical data, including that from Stark (electroabsorption) spectroscopy, permits unprecedented quantitative comparisons, most notably regarding the effects of extension of bridging polyene chains. The purely organic compounds display normal optical properties in that their intense, visible pi --> pi intramolecular charge-transfer (ICT) bands red-shift as the number of E-ethylene units (n) increases from 1 to 3 and the associated static first hyperpolarizabilities beta(0) increase steadily with n. The related Ru(II) complexes show intense, visible d --> pi metal-to-ligand charge-transfer (MLCT) bands, which are found to lower energy when compared with the ICT transitions of the corresponding organics. Abnormally, these MLCT bands blue-shift as n increases, and beta(0) maximizes at n = 2. Time-dependent density-functional theory and finite field calculations verify these empirical trends for both types of compound, which can be rationalized as arising from the differing orbital structures of the chromophores and the associated degrees of D-A electronic coupling.