Nonlinear optical properties in calix[n]arenes: orientation effects of monomers

We investigate the nonlinear optical (NLO) polarizations for various geometric arrangements of the dipolar chromophores in a calix[n]arene geometry. The interactions were studied by using (HF)3 as model compounds both in parallel and frustrated orientations. The interdipolar angle between the monomer molecules is varied so that many different arrangements of the dipoles can be realized which is associated with the opening up of the calix[n]arene rings. Quantum-chemical calculations at both ab initio and semiempirical levels, show that the all-parallel arrangement of the dipoles exhibit the maximum magnitude for the nonlinear optical coefficients at small interdipolar angles for all interdipolar distances. In the frustrated orientations however, the NLO response is maximum at large interdipolar angles corresponding to the flattened calix[n]arene rings. The role of hydrogen bonding in stabilizing these assemblies in various conformations has been investigated. Crystallographic database analysis for calix[3]arenes shows that optical properties in real molecular systems can be modeled based on our simple interaction theory.     

Design, synthesis, linear, and nonlinear optical properties of conjugated (porphinato)zinc(II)-based donor-acceptor chromophores featuring nitrothiophenyl and nitrooligothiophenyl electron-accepting moieties

An extensive series of conjugated (porphinato)zinc(II)-based chromophores featuring nitrothiophenyl and nitrooligothiophenyl electron-accepting moieties has been synthesized using metal-catalyzed cross-coupling reactions involving [5-bromo-15-triisopropylsilylethynyl-10,20-diarylporphinato]zinc(II) and an unusual electron-rich Suzuki-porphyrin synthon, [5-(4-dimethylaminophenylethynyl)-15-(4',4',5',5'-tetramethyl[1',3',2']dioxaborolan-2'-yl)-10,20-diarylporphinato]zinc(II), with appropriately functionalized aryl and thienyl precursors. These donor-acceptor chromophores feature thiophenyl, [2,2']bithiophenyl, and [2,2';5',2' ']terthiophenyl units terminated with a 5-nitro group; one series of structures features these acceptor moieties appended directly to the porphyrin macrocycle meso-carbon position, while a second set utilizes an intervening meso-ethynyl moiety to modify porphyrin-to-thiophene conjugation. The dynamic hyperpolarizability of these compounds was determined from hyper-Rayleigh light scattering (HRS) measurements carried out at fundamental incident irradiation wavelengths (lambda(inc)) of 800 and 1300 nm; interestingly, measured beta(1300) values ranged from 650 --> 4350 x 10(-30) esu. The combined linear and nonlinear optical properties of these compounds challenge the classical concept of the nonlinearity/transparency tradeoff in charge-transfer chromophores: the magnitude of the molecular hyperpolarizability is observed to vary substantially despite approximately uniform ground-state absorptive signatures for a given porphyrin-to-thiophene linkage topology. These data show that these neutral dipolar molecules can express substantial beta(1300) values; such conjugated, electronically asymmetric porphyrin-thiophene chromophores may thus find utility for electrooptic applications at telecom-relevant wavelengths.     

Nonlinear response properties of ultralarge hyperpolarizability twisted pi-system donor-acceptor chromophores. Dramatic environmental effects on response

State-average complete active space self-consistent field (SA-CASSCF) calculations are performed on the energetically lowest two electronic states of a novel alkyl-substituted 4-quinopyran twisted pi-system electro-optic chromophore. In the gas phase, the ground-state electronic configuration is diradicaloid (D), and the first excited state is zwitterionic (Z). When an external dipolar field is applied to simulate polar solvation, the relative energies of D and Z are dramatically perturbed. At sufficient field strengths, the relative ordering of the states is inverted so that Z becomes the ground state. As the energy difference between the D and Z states falls, the magnitudes of the longitudinal static polarizability (alpha) and hyperpolarizability (beta) increase appreciably--in certain cases, by 2 orders of magnitude. These computational results are interpreted and supported by qualitative state correlation diagrams constructed from qualitative molecular orbital theory and are in agreement with recent experimental results on twisted pi-system electro-optic chromophores (Kang, H. et al. J. Am. Chem. Soc. 2007, 129, 3267). The computational results also suggest that changing the environmental polarity is a promising strategy for tuning alpha and beta in such types of chromophores, which experimentally exhibit large nonlinear optical response.     

尿素分子团簇线性和一阶非线性光学极化率的理论计算

检验分子晶体非线性光学极化率的可加性,对于确认有向气体模型(Oriented-Gas Model) 具有重要的科学意义,对探索新型实用的分子非线性光学材料也有实际的意义。本文对尿素晶体中分子簇非线性光学极化率的可加性做了详尽的理论研究。首次做到了尿素分子最近邻排布团簇的非线性光学极化率的从头算,从而扩展了前人的工作。研究结果有效地确认了有向气体模型。同时,我们还提出了壳层结构模型 (Shell-Structure Model),用以解决有向气体模型中的缺陷,可实现更为有效可靠地计算晶体宏观非线性光学系数。     

Multichromophoric Calix[4]arenes: Effect of Interchromophore Distances on Linear and Nonlinear Optical Properties

Multichromophoric calix[4]arenes with two or four disperse red one (DR1) moieties linked to the lower rim have been synthesized. The second‐order nonlinear optical activity was measured by using the electric‐field‐induced second‐harmonic generation technique and there was a nearly linear increase of the μβ value with the number of chromophores in the molecule without affecting the charge‐transfer absorption wavelength. The effect that the number of DR1 units plays on the hyperpolarizability, the dipole moment, and the absorption maxima has been also studied by using quantum chemical calculations. It was found that it was necessary to synthesize multichromophores with distant chromophores to obtain large nonlinear optical responses.     

Ab initio diradical/zwitterionic polarizabilities and hyperpolarizabilities in twisted double bonds

We investigated the nature of the ground state and static response properties (mu, alpha, and beta) for a promising class of twisted pi-electron system nonlinear optical chromophores at the HF, B3LYP, MP2, and CASSCF levels. We report results for a substituted twisted ethylene and a larger tictoid analogue. Previous work has reported only a zwitterionic character for such tictoid species, however, (14,13) CASSCF calculations predict a ground-state diradical. At the HF, B3LYP, MP2, and (14,13) CASSCF levels, the values of beta are orders of magnitude smaller than those predicted by semiempirical methods.     

Evidence for Dual Pathway in Through‐Space Singlet Energy Transfers in Flexible Cofacial Bisporphyrin Dyads

Flexible “pacman” scaffolds built upon a calix[4]arene platform bearing a [18]crown‐6 ether and either two OH functions or two OPr groups at the lower rim have been used to generate donor–acceptor (D–A) dyads incorporating a zinc–porphyrin donor and a free‐base porphyrin acceptor. Through‐space singlet energy transfer (SET) in the D–A dyads was studied by time‐resolved fluorescence spectroscopy. Although the effects of conformational changes are well documented when the chromophores switch from a non‐cofacial to a cofacial arrangement, little is known about flexible pacman scaffolds in which the changes are limited to the distance between the chromophores. The known SET rates for reported, geometrically well‐defined, rigid pacman D–A dyads were used as calibration to estimate the D–A distances in the flexible pacman dyads. Due to the flexibility of the calix[4]arene spacer, the D–A dyads adopt a “closed” or “open” geometry that is tuned by intramolecular hydrogen bonds (O? H???[18]crown‐6 ether) and by solvent interactions. Changes in the SET rates between the open and closed geometries were surprisingly less dramatic than expected, and are explained by a dual SET pathway that is specific to the calix[4]arene platform. Time‐resolved fluorescence studies support the hypothesis that, for the “open” conformer, the preferred through space SET pathway (i.e., at the shortest distance) is located within the calix[4]arene cavity through the cofacial phenyl groups. For the “closed” conformer, the preferred through space SET route is located between the zinc and free‐base porphyrins.     

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.     

Alkynyl expanded donor-acceptor calixarenes: geometry and second-order nonlinear optical properties

A number of wide- and narrow-rimmed functionalized alkynylcalix[4]arenes have been synthesized by Sonogashira coupling. With respect to their optical properties, these donor-acceptor systems are treated as ensembles of covalently linked, electronically independent tolane subchromophores. Linear UV/visible and fluorescence spectroscopic investigations revealed that the charge-transfer character of the electronic transitions in calixarenes, and also the second-order nonlinear optical (NLO) properties depend on the electron-withdrawing nature of the terminal ethynylphenyl substituent (NO(2), CF(3), H). The nitro derivatives display high values of the quadratic hyperpolarizability beta. Not only do the (nonlinear) optical properties of the target compounds depend on the number and relative disposition of the subchromophores, but also on the geometry of the calixarenes. In particular, the opening angle of the calixarene cavity can be determined by the substitution pattern of the calixarene scaffold (wide- versus narrow-rim substitution) and the number of the acetylene functions introduced. Both the NLO properties and the conformational issues are conveniently assessed by using hyper-Rayleigh scattering (HRS) in solution, and supported by X-ray crystallography in the solid state.     

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.     

Tetrathiafulvalene derivatives as NLO-phores: synthesis, electrochemistry, Raman spectroscopy, theoretical calculations, and NLO properties of novel TTF-derived donor-pi-acceptor dyads.

Novel pi-conjugated donor-acceptor chromophores, based on the strong electron-donating tetrathiafulvalene moiety and different electron-withdrawing acceptors, exhibit large second-order optical nonlinearities. The effect of increasing the length of the polyenic spacer and the influence of the nature of the acceptor moiety on the NLO properties have been studied by using the electric field-induced second-harmonic generation (EFISH) technique as well as by semiempirical and ab initio theoretical calculations. A charge-transfer band has been observed in the absorption spectra of these D-pi-A compounds that undergoes an hypsochromic shift when increasing the number of vinylenic spacer units connecting both donor and acceptor moieties. The degree of the intramolecular charge transfer from the donor to the acceptor has also been analyzed by means of Raman spectroscopy.     

Synthesis and electronic properties of covalent assemblies of oligophenylenevinylene units arising from a calix

Assemblies of four oligophenylenevinylene moieties arising from a calix[4]arene core, i.e., calix[4]oligophenylenevinylenes, have been prepared by Heck-type cross-coupling reactions of styrene derivatives with a tetraiodinated cone-calix[4]arene precursor. Photophysical studies in solution have revealed that there are electronic ground state interactions between the covalently bonded OPV moieties. The absorption spectra of the calix[4]oligophenylenevinylenes are significantly different from those obtained by summing the spectra of four model units and their emission is red-shifted when compared to the corresponding model compounds. Electrochemical studies have shown that the redox processes of the four OPV subunits do not take place at the same potentials indicating also a strong electronic interaction among them in the calix[4]oligophenylenevinylenes.     

Energy transfer in calixarene-based cofacial-positioned perylene bisimide arrays

The synthesis of multichromophoric perylene bisimide-calix[4]arene arrays with up to five perylene units (containing orange, violet, and green perylene bisimide chromophores) and of monochromophoric model compounds was achieved by subsequent imidization of mono-Boc functionalized calix[4]arene linkers with three different types of perylene bisimide dye units. The optical properties of all compounds were studied with UV/vis absorption and steady state and time-resolved fluorescence spectroscopy. Upon excitation of the inner orange dye at 490 nm of array 3, strong fluorescence emission of the outer green perylene bisimide (PBI) chromophore at 744 nm is observed. The fluorescence excitation spectra of compounds 3 and 4 (lambdadet = 850 nm) show all absorption bands of the parent chromophores (e.g., all perylene units contribute to the emission from S1 state of the green PBI). Thus, the fluorescence emission and excitation spectra as well as time-resolved data of fluorescence lifetimes in the absence (tauD = 5.1 ns) and in the presence of an acceptor (tauDA = 0.8 ns) suggest efficient energy transfer processes between the perylene bisimide dye units. For the bichromophoric array 4, the energy transfer rate is calculated to a value of 1.05 x 109 s-1. These results demonstrate highly efficient energy transfer in cofacially assembled dye arrays.     

咪唑类二阶非线性光学活性分子的设计、合成及性能

设计、合成了两个新的咪唑类二阶非线性光学生色团，对其热性能、透明性和 二阶非线性光学性能进行了测定和研究，实验表明生色团为非线性光学性能-透明 性-热稳定性能综合优化的有机二阶非线性光学生色团。     

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.     

Charge-transfer interactions in a multichromophoric hexaarylbenzene containing pyrene and triarylamines

Two different hexaarylbenzenes with three pyrene and three triarylamine substituents in different positions (trigonal symmetric and asymmetric arrangement) were synthesized, and their charge-transfer states were investigated by optical spectroscopy. In these multichromophoric systems triarylamine acts as the electron donor and pyrene as the electron acceptor. A reference chromophore with only one donor-acceptor pair was also investigated. All these chromophores form charge-transfer states upon photoexcitation which relax with a moderate fluorescence quantum yield to the ground state. The compounds do not differ significantly concerning most of their fluorescence properties, which shows that the fluorescent charge-transfer state is very similar in all chromophores. This observation indicates symmetry breaking for the symmetric chromophore within fluorescence lifetime of several tens of ns. This interpretation was substantiated by fluorescence excitation anisotropy measurements in a sucrose octaacetate matrix.     

Calix[4]arenes as molecules for second order nonlinear optics

Abstract

The synthesis of a novel class of molecules for second order nonlinear optics, i.e. calix[4]arenes with extended π-systems, is described. These compounds are obtained via Wittig-Horner reactions of the formylated calix[4]arenes 5 and 6 to give the stilbene derivatives 7–9, or by diazotization of calix[4]arene, 1, followed by alkylation to give the phenylazocalix[4]arenes 11 and 12. The molecular second order nonlinear optical properties (β_z) of these calix[4]arenes have been measured by electric field-induced second harmonic generation. The influence of different acceptors as well as the influence of the different conformations of the calix[4]arenes on β_z values were determined. Surprisingly, the wavelength of the charge-transfer band λ_max is lower when β_z increases upon increasing the number of acceptors.     

Novel NLO-phores with proaromatic donor and acceptor groups

[reaction: see text] Novel D-pi-A NLO-phores based on the 1,3-dithiol-2-ylidene donor and the thiobarbituric acceptor moieties have been prepared. Modification of the length and rigidity of the pi-spacer allows the first systematic study of the second-order nonlinear optical properties of doubly proaromatic merocyanines. The pi-electron donor efficiency of the 1,3-dithiol-2-ylidene group is superior to that of the tetrathiafulvalenyl group.     

Challenging the auxiliary donor effect on molecular hyperpolarizability in thiophene-containing nonlinear chromophores: X-ray crystallographic and optical measurements on two new isomeric chromophores

To reexamine the established "auxiliary donor" effect of thiophene in nonlinear optical (NLO) chromophores, we have prepared two isomeric donor-acceptor azo dyes, differing only in the position of the thiophene. Experimental analysis of these chromophores, including electric field-induced second harmonic generation (EFISH) and X-ray crystallography, contradicts previous experimental findings on similar chromophores but is consistent with the majority of computational precedents. We have found that the thiophene on the donor side produces a compound with a larger dipole moment; however, the isomer with the thiophene on the acceptor side is more nonlinear and has a higher figure of merit for NLO device applications.     

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.