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.     

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.     

Syntheses and quadratic optical nonlinearities of ruthenium(II) complexes with ethynyl-connected N-methylpyridinium electron acceptors

We have prepared a number of new dipolar complexes containing ethynyl or buta-1,3-diynyl units linking electron-rich {Ru(II)(NH3)5}2+, trans-{Ru(II)(NH3)4L}+ (L = pyridine or N-methylimidazole), or trans-{Ru(II)Cl(pdma)2}+ [pdma = 1,2-phenylenebis(dimethylarsine)] centers to pyridinium electron acceptors. In acetonitrile solutions at 295 K, the new complexes display unusual blue-shifting of their metal-to-ligand charge-transfer (MLCT) bands as the conjugation is extended, in a fashion similar to that of the corresponding ethenyl systems. Hyper-Rayleigh scattering (HRS) and Stark spectroscopic measurements provide direct and indirect estimates of static first hyperpolarizabilities beta0, and both the linear and nonlinear optical (NLO) properties are temperature- and medium-dependent. Thus, at 77 K in butyronitrile glasses, the MLCT bands display more normal red shifts upon conjugation extension. While the Stark-derived beta0 values generally increase as n (the number of ethynyl units) increases from 0 to 2, the HRS data show maximization at n = 1 for two of the ammine series but an increase upon moving from n = 1 to 2 for the pdma complexes. Comparisons with the analogous ethenyl chromophores show that the latter generally display larger beta0 values, whether determined via HRS or Stark data, and the inferiority of the ethynyl systems in terms of NLO response is more pronounced when n = 2. This differing behavior is attributable primarily to larger increases in the transition dipole moment mu12 (and, hence, donor-acceptor pi-electronic coupling) on elongation in the ethenyl chromophores.     

Exceptionally large second-order nonlinear optical response in donor-graphene nanoribbon-acceptor systems

Graphene nanoribbon (GNR) has been used, for the first time, as an excellent conjugated bridge in a donor-conjugated bridge-acceptor (D-B-A) framework to design high-performance second-order nonlinear optical materials. Owing to the unique diradical planar conjugated bridge of GNR, D(NH(2))-GNR-A(NO(2)) exhibits exceptionally large static first hyperpolarizability (β(0)) up to 2.5×10(6) a.u. (22000×10(-30) esu) for H(2)N-(7,3)ZGNR-NO(2) (ZGNR=zigzag-edged GNR), which is about 15 times larger than the recorded value of β(0) (1470×10(-30) esu) for the D-A polyene reported by Blanchard-Desce et al. [Chem. Eur. J. 1997, 3, 1091]. Interestingly, we have found that the size effect of GNR plays a key role in increasing β(0) for the H(2)N-GNR-NO(2) system, in which the width effect of GNR perpendicular to the D-A direction is superior to the length effect along the D-A direction.     

Theoretical analyses of the effects on the linear and quadratic nonlinear optical properties of N-arylation of pyridinium groups in stilbazolium dyes

N-Arylation of the pyridinium electron acceptor unit in stilbazolium chromophores has been found by previous experimental hyper-Rayleigh scattering and electronic Stark effect (electroabsorption) spectroscopic studies to lead to substantial increases in the static first hyperpolarizability beta(0). We show here that INDO/SCI calculations on the isolated cations trans-4'-(dimethylamino)-N-R-4-stilbazolium (R = methyl 1, phenyl 2, 2,4-dinitrophenyl 3, or 2-pyrimidyl 4) predict only slight red-shifts in the energy of the intramolecular charge-transfer (ICT) transition and accompanying relatively small changes in beta(0) on moving along the series. The inclusion of acetonitrile solvent using a polarizable continuum model affords a somewhat better agreement with the experimental data, especially the red-shifting of the ICT transition and the increase in beta(0) on going from 1 to 4. Time-dependent density functional theory (TD-DFT), finite field, and coupled perturbed Hartree-Fock calculations reproduce even more closely the empirical data and trends; the latter two approaches lead to the highest quadratic nonlinear optical (NLO) response of the studied chromophores for 3, for which the predicted beta(0) is ca. 50-100% larger than that of the analogous N-methylated cation 1. Although the TD-DFT and INDO/SCI approaches give quite different results for ground- and excited-state dipole moments, the overall conclusions of these two methods regarding the ICT absorption and NLO responses are similar.     

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.     

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.     

One-dimensional energy/electron transfer through a helical channel

We have synthesized A-D-A-type linear chain chromophores based on oligo(phenylene vinylene) as an electron donor (D) and several electron/energy acceptors (A), which are linked by an alkyl spacer with various lengths and are processed for a helical encapsulation with amylose. Photoinduced electron/energy transfers (eT/ET) of the chromophores are investigated in the presence and absence of the helical encapsulation with respect to D-A distance. Fluorescence intensity of the free chromophores is unusually small, not due to the advancement of eT/ET but most likely to self-quenching by aggregation and/or conformational flexibility in solution. By contrast, the helically encapsulated chromophores exhibit highly efficient eT/ET over a long D-A distance and a well-defined distance effect depending on the acceptor strength.     

Vibrational and quantum-chemical study of nonlinear optical chromophores containing dithienothiophene as the electron relay

A series of nonlinear optical (NLO) donor-acceptor (D-A) chromophores containing a fused terthiophene, namely dithienothiophene (DTT), as the electron relay, the same donor group, and acceptors of various strengths, has been investigated by means of infrared and Raman spectroscopies, both in the solid state as well as in a variety of solvents, to evaluate the effectiveness of the intramolecular charge transfer from the electron-donor to the electron-acceptor end groups. The Raman spectral profiles of these NLO-phores measured from their dilute solutions have been found to be rather similar to those of the corresponding solids, and thus their intramolecular charge transfer (ICT) shows very little dependence on the solvent polarity. The experimental results obtained for the DTT-containing NLO-phore with a 4-(N,N-dibutylamino)styryl end group as the donor and a 2,2-dicyanoethen-1-yl end group as the acceptor differ from those previously obtained for two parent "push-pull" chromophores with the same D-A pair but built-up around either a bis(3,4-ethylenedioxythienyl) (BEDOT) or a bithienyl (BT) electron relay. Vibrational spectroscopy shows that DTT is significantly more efficient as an electron relay than BT (which has the same number of conjugated C=C bonds) or BEDOT (which can be viewed as a rigidified version of BT on account of noncovalent intramolecular interactions between heteroatoms of adjacent rings). Density functional theory (DFT) calculations have also been performed on these NLO-phores to assign their main electronic and vibrational features and to obtain information about their structures. An additional merit of these molecular materials was revealed by the infrared spectra of the DTT-based NLO-phores recorded at different temperatures. Thus, spectra recorded between -170 and 150 degrees C did not show any substantial change, indicating that the materials have a high thermal stability, which is of significance for their use as active components in optoelectronic devices.     

Cooperative interactions in supramolecular aggregates: linear and nonlinear responses in calix[4]arenes.

The linear and nonlinear optical polarizabilities of donor-acceptor (D-pi-A) chromophores in confined geometries of calix[4]arenes are investigated through a model for interacting polar-polarizable molecules. Both the linear polarizability (alpha) and the first hyperpolarizability (beta) decrease with increasing the interdipolar angle, as expected in the oriented-gas picture. However, within the polar-polarizable model we predict deviations from the additive result, irrespective of the interdipolar angle. Depending on the nature of the chromophore, electrostatic intermolecular interactions between polar and polarizable chromophores lead to cooperative damping or enhancement of the optical responses. Specifically, for chromophores whose ground state is dominated by the neutral D-pi-A structure both alpha and beta are suppressed with respect to the prediction of the oriented-gas model, whereas the opposite holds true for chromophores whose ground state is dominated by the zwitterionic D(+)-pi-A(-). These results explain recent experimental data on a calix[4]arene functionalized with a donor-acceptor dye for nonlinear optical applications. Density functional theory calculations on the relevant crystal structure further support our interpretation.     

Quantum-chemical investigation of second-order nonlinear optical chromophores: comparison of strong nitrile-based acceptor end groups and role of auxiliary donors and acceptors

We report a detailed quantum-chemical investigation of donor-acceptor substituted dipolar nonlinear optical chromophores incorporating the 4-(dimethylamino)phenyl donor end group and a variety of strong heterocyclic acceptor end groups, including tricyanofurans and tricyanopyrroles. In particular, we study the variation of the molecular second-order polarizability (beta) with the acceptor end group and when inserting auxiliary donors (thiophene) and acceptors (thiazole) into the pi bridge. Both finite-field calculations (in the context of local contributions) and sum-over-states calculations were carried out in order to probe the relationship between beta and the chemical structure of the various chromophores. The trends obtained with these two methods are fully consistent. The large beta values (up to 700 x 10(-30) esu) as well as the observed tunability of the optical absorption maximum (lambda(max)) make the chromophores investigated here interesting candidates for use in electro-optic applications at telecommunications wavelengths.     

Theoretical investigation of the large nonlinear optical properties of (HCN)n clusters with Li atom

Two new classes of (HCN)(n)...Li and Li...(HCN)(n) (n = 1, 2, 3) clusters with the electride characteristic are formed in theory by the metal Li atom attaching to the (HCN)(n) (n = 1, 2, 3) clusters. Because of the interaction between the Li atom and the (HCN)(n) part, the 2s valence electron of the Li atom becomes a loosely bound excess electron. Our high-level ab initio calculations show that these new clusters with the excess electron have large first hyperpolarizabilities, for example, beta(0) = -15,258 au for (HCN)...Li and beta(0) = -3401 au for Li...(HCN) at the QCISD/6-311++G(3df,3pd) level (only beta(0) = -2.8 au for HCN monomer(26)). Obviously, the excess electron from the Li atom plays a crucial role in the large first hyperpolarizabilities of these clusters. The beta(0) value of (HCN)(n)...Li (beta(0) > 10(4) au, from sigma --> pi* transition) is larger than that of Li...(HCN)(n) (beta(0) > 10(3) au, from sigma --> sigma* transition) for n = 1, 2, or 3. In addition, two interesting rules have been observed. They are that |beta(0)| decreases with lengthening of the HCN chain for (HCN)(n)...Li clusters and that |beta(0)| increases as n increases for Li...(HCN)(n) clusters. In this paper, we discuss two classes of clusters that are highly similar to the electride structure model, of which the structural characteristics are that alkali metal atoms ionize to form cations and trapped electrons under the action of other polar molecules. Thus, the investigation on the large first hyperpolarizabilities of (HCN)(n)...Li and Li...(HCN)(n) (n = 1, 2, 3) may prompt one to study the unusual nonlinear optical responses of some electrides.     

Linear and nonlinear optical properties of a macrocyclic trichromophore bundle with parallel-aligned dipole moments

A macrocyclic trichromophore bundle 1 with parallel-aligned dipole moments has been synthesized to study the influence of aggregation and orientation of a nonlinear optical (NLO) chromophore on its optical properties. The linear and nonlinear optical properties of 1 and a single chromophore standard 2 have been studied by UV-vis absorption, fluorescence, solvatochromic spectrometry, and hyper-Rayleigh scattering (HRS). Reduced first-order hyperpolarizability beta, hypsochromic shift, enhanced solvatochromic shifts, and fluorescence quenching for individual chromophores were observed when 1 was compared with 2. Analysis of the data showed that the transition dipole moment changes only slightly when the chromophores are parallel aligned in the bundle architecture. However, the apparent hyperpolarizability of the individual chromophores decreased significantly by about 20%. The reduction in beta for the individual chromophores in 1 is largely due to the hypsochromic shift, i.e., excitation energy increase of the interband (charge-transfer) energy gap and the reduced difference between the ground-state and excited-state dipole moments. The hypsochromic shift and fluorescence quenching are consistent with exciton theory. Possible reasons for the enhanced solvatochromic shift are discussed.     

Antiparallel-aligned neutral-ground-state and zwitterionic chromophores as a nonlinear optical material

Efficient noncentrosymmetric arrangement of nonlinear optical (NLO) chromophores with high first-order hyperpolarizability (beta) for increased electro-optical (EO) efficiency has proven challenging as strong dipolar interactions between the chromophores encourage antiparallel alignment, attenuating the macroscopic EO effect. This work explores a novel approach to simultaneously achieve large beta values while providing an adjustable dipole moment by linking a strong neutral-ground-state (NGS) NLO chromophore with positive beta to a zwitterionic (ZWI) chromophore with negative beta in an antiparallel fashion. It is proposed that the overall beta of such a structure will be the sum of the absolute values of the two types of chromophores while the dipole moment will be the difference. Molecules 1-3 were synthesized to test the feasibility of this approach. Molecular dynamics calculations and NMR data supported that the NGS chromophore component and the ZWI chromophore component self-assemble to an antiparallel conformation in chloroform. Calculations showed that the dipole moment of 1 is close to the difference of the two component chromophores. Hyper-Rayleigh scattering (HRS) studies confirmed that the first hyperpolarizability of 1 is close to the sum of the two component chromophores. These results support the idea that an antiparallel-aligned neutral-ground-state chromophore and a zwitterionic chromophore can simultaneously achieve an increase in beta and a decrease of the dipole moment.     

高活性、高产率非线性偶氮化合物的合成及其线性、非线性光学性能

合成了一类高产率的基于偶氮的新型非线性活性化合物. 初步测定其薄膜的折射率可在1.55—1.64范围内任意调节, 薄膜的Tg在130 ℃ 以上, 其未经优化的非线性光学系数γ33为16 pm/V(1310 nm), 该材料在通讯波长窗口的光学损耗较小, 化合物的热分解温度在239 ℃ 以上.     

Novel spiro-configured PET chromophores incorporating 4,5-diazafluorene moiety as an electron acceptor

[reaction: see text] We have synthesized novel spiro-bridged chromophores incorporating a 4,5-diazafluorene unit as the electron acceptor (A) and aryl amino groups as donor (D) moieties. The orthogonal D-A configuration limits ground-state D-A interactions but allows efficient PET reactions. The PET efficiency was modulated by the electronic characteristics of the donor and by the D-A distance. Preliminary studies showed that the PET process can be manipulated through metal ion complexation of the 4,5-diazafluorene moiety.     

Property tuning in charge-transfer chromophores by systematic modulation of the spacer between donor and acceptor

A series of donor-acceptor chromophores was prepared in which the spacer separating 4-dimethylanilino (DMA) donor and C(CN)(2) acceptor moieties is systematically varied. All of the new push-pull systems, except 4 b, are thermally stable molecules. In series a, the DMA rings are directly attached to the central spacer, whereas in series b additional acetylene moieties are inserted. X-ray crystal structures were obtained for seven of the new, intensely colored target compounds. In series a, the DMA rings are sterically forced out of the mean plane of the residual pi system, whereas the entire conjugated pi system in series b is nearly planar. Support for strong donor-acceptor interactions was obtained through evaluation of the quinoid character of the DMA ring and by NMR and IR spectroscopy. The UV/Vis spectra feature bathochromically shifted, intense charge-transfer bands, with the lowest energy transitions and the smallest optical gap being measured for the two-dimensionally extended chromophores 6 a and 6 b. The redox behavior of the push-pull molecules was investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). In the series 1 b, 2 b, 4 b, 5 b, in which the spacer between donor and acceptor moieties is systematically enlarged, the electrochemical gap decreases steadily from 1.94 V (1 b) to 1.53 V (5 b). This decrease is shown to be a consequence of a reduction in the D-A conjugation with increasing spacer length. Degenerate four-wave mixing experiments reveal high third-order optical nonlinearities, pointing to potentially interesting applications of some of the new chromophores in optoelectronic devices.     

Photochemical stability of nonlinear optical chromophores in polymeric and crystalline materials

We compare the photochemical stability of the nonlinear optical chromophore configurationally locked polyene 2-{3-[2-(4-dimethylaminophenyl)vinyl]-5,5-dimethylcyclohex-2-enylidene} malononitrile (DAT2) embedded in a polymeric matrix and in a single-crystalline configuration. The results show that, under resonant light excitations, the polymeric compound degrades through an indirect process, while the DAT2 crystal follows a slow direct process. We show that chromophores in a crystalline environment exhibit three orders of magnitude better photostability as compared to guest-host polymer composites.     

Band gap tunable for near-infrared absorbing chromophores with multi-triphenylamine and tris (thieno)hexaazatriphenylenes acceptors

Two disk-like D-A type chromophores with multi-triphenylamine donors and hexaazatriphenylenes acceptors were synthesized and fully characterized by ¹H and ¹³C NMR, elemental analysis and mass spectrometry. The effects of the hexaazatriphenylene on the optical and electrochemical properties and band gap of the chromophores were investigated. As the hexaazatriphenylene core fused with three thiophene rings, the band gaps of the compounds could be tuned from 1.65 eV to 1.15 eV. The π-π* absorption peak and charge-transfer absorption peak of the hexaazatriphenylene compounds were red shifted from visible spectrum region (393 and 530 nm) to near-infrared spectrum region (542 and 756 nm). In addition, due to an increase in the π electronic coupling between electron donor and electron acceptor, the extinction coefficient (charge-transfer absorption) of the hexaazatriphenylene compound decreases 85% from 3.4 × 10⁴ mol⁻¹ dm⁻³ cm⁻¹ to 0.5 × 10⁴ mol⁻¹ dm⁻³ cm⁻¹.     

Molecular design of porphyrin-based nonlinear optical materials

Nonlinear optical chromophores containing (porphyrinato)Zn(II), proquinoid, and (terpyridyl)metal(II) building blocks were optimized in a library containing approximately 10(6) structures using the linear combination of atomic potentials (LCAP) methodology. We report here the library design and molecular property optimizations. Two basic structural types of large beta(0) chromophores were examined: linear and T-shaped motifs. These T-shaped geometries suggest a promising NLO chromophoric architecture for experimental investigation and further support the value of performing LCAP searches in large chemical spaces.