Javier Pérez-Moreno

Linear and nonlinear optical properties of colloidal photonic crystals.

Linear and Nonlinear Optical Properties of Colloidal Photonic Crystals Luis Gonz alez-Urbina, Kasper Baert, Branko Kolaric,* Javier P erez-Moreno, and Koen Clays* Department of Chemistry and INPAC Institute of Nanoscale Physics and Chemistry, K. U. Leuven, Celestijnenlaan 200D, B-3001 Heverlee, Belgium Laboratoire Interfaces et Fluides Complexes, Centre d’Innovation et de Recherche en Mat eriaux Polym eres, Universit e de Mons, 20 Place du Parc, 7000 Mons, Belgium

Modulated conjugation as a means for attaining a record high intrinsic hyperpolarizability

We report on a series of chromophores that have been synthesized with a modulated conjugation path between donor and acceptor. Hyper-Rayleigh scattering measurements of the best molecule show an enhanced intrinsic hyperpolarizability that breaches the apparent limit of all previously studied molecules.

Modulated conjugation as a means of improving the intrinsic hyperpolarizability.

A new strategy for optimizing the first hyperpolarizability based on the concept of a modulated conjugated path in linear molecules is investigated. A series of seven novel chromophores with different types of conjugated paths were synthesized and characterized. Hyper-Rayleigh scattering experiments confirmed that modulated conjugation paths that include benzene, thiophene, and/or thiazole rings in combination with azo and/or ethenyl linkages between dihydroxyethylamino donor groups and various acceptor groups result in enhanced intrinsic hyperpolarizabilities that exceed the long-standing apparent limit for two of the chromophores. The experimental results are analyzed and interpreted in the context of quantum limits, which show that conjugation modulation of the bridge in donor/acceptor molecules simultaneously optimizes the transition moments and the energy-level spacing.

A new dipole-free sum-over-states expression for the second hyperpolarizability

The generalized Thomas-Kuhn sum rules are used to eliminate the explicit dependence on dipolar terms in the traditional sum-over-states (SOS) expression for the second hyperpolarizability to derive a new, yet equivalent, SOS expression. This new dipole-free expression may be better suited to study the second hyperpolarizability of nondipolar systems such as quadrupolar, octupolar, and dodecapolar structures. The two expressions lead to the same fundamental limits of the off-resonance second hyperpolarizability; and when applied to a particle in a box and a clipped harmonic oscillator, have the same frequency dependence. We propose that the new dipole-free equation, when used in conjunction with the standard SOS expression, can be used to develop a three-state model of the dispersion of the third-order susceptibility that can be applied to molecules in cases where normally many more states would have been required. Furthermore, a comparison between the two expressions can be used as a convergence test of molecular orbital calculations when applied to the second hyperpolarizability.

Combined molecular and supramolecular bottom-up nanoengineering for enhanced nonlinear optical response: experiments, modeling, and approaching the fundamental limit.

The authors study the combination of two independent strategies that enhance the hyperpolarizability of ionic organic chromophores. The first molecular-level strategy is the extension of the conjugation path in the active chromophore. The second supramolecular-level strategy is the bottom-up nanoengineering of an inclusion complex of the chromophore in an amylose helix by self-assembly. The authors study a series of five (dimethylamino)stilbazolium-type chromophores with increasing conjugation length between the (dimethylamino)phenyl donor ring and the pyridinium acceptor ring in conjunction with four amylose helices of differing molecular weights. The first hyperpolarizabilities of the self-assembled inclusion complexes, as determined with frequency-resolved femtosecond hyper-Rayleigh scattering at 800 and 1300 nm, are compared with experimental values for the free chromophores in solution and with theoretical values. While the experimental values for the hyperpolarizability in solution are lower than the theoretically predicted values, an enhancement upon inclusion is observed, with the longest chromophore in the best amylose helix showing an enhancement by one order of magnitude. Molecular modeling of the inclusion of the chromophore suggests that the coplanarity of the two rings is more important than all-trans configuration in the conjugation path. The fundamental limit analysis indicates that the inclusion inside the amylose helix results in an optimal excited-level energy spacing that is responsible for breaching the apparent limit.

Synthesis and nonlinear optical properties of tetrahedral octupolar phthalocyanine-based systems.

Two series of tetrahedral phthalocyanine-based systems presenting a central carbon or silicon atom have been synthesized and fully characterized. Ethynyl spacers connect the peripheral Pc units to the central core. Some of the structures contain four identical Pc moieties, whereas other ones bear either an electron-withdrawing or an electron releasing group in the fourth subunit. The synthetic strategy consisted in metal mediated coupling reactions between tri-tert-butylethynylphthalocyanine and the corresponding methane or silane derivatives. A second-order nonlinear optical (NLO) study, through hyper-Rayleigh scattering measurements, reveals that, by combining centrosymmetrical moieties that are not second-order NLO active by themselves, in an octupolar fashion, a large second-order NLO response is achieved, in contrast to classical octupolar combinations of donor-acceptor NLO active dipolar moieties. In particular, the C-centered tetramer exhibits a large beta(HRS) value, which is among the highest reported so far for octupolar Pc-based molecules. Interestingly, carbon-centered molecules show a better NLO response with respect to the silicon-centered ones, probably due to a different effective symmetry, largely T(d) for the C-centered compounds and D(2d) for the Si-centered systems. While other design strategies for second-order NLO effects have always fundamentally kept on relying on the old dipolar paradigm (even though the resulting molecular structure was octupolar--the most striking exponent of this is the octupolar 1,3,5-triamino-2,4,6-trinitrobenzene molecule, a simple octupolar expansion of the dipolar p-nitroaniline), we here present for the first time that the octupolar symmetry by itself, realized by four nondipolar moieties in a tetrahedral arrangement, results also in a large second-order nonlinear response.

Comment on “Organometallic Complexes for Nonlinear Optics. 45. Dispersion of the Third-order Nonlinear Optical Properties of Triphenylamine-cored Alkynylruthenium Dendrimers” – Increasing the Nonlinear Optical Response by Two Orders of Magnitude

In a recent paper published in this journal, Roberts et. al. report on a “dramatic enhancement of the two-photon absorption merit upon embedding metal centers through a dendritic architecture” . [ 1 ] Roberts et. al. investigate the impact of π -system lengthening through metal centers for three systematically varied triphenylamine-cored alkynylruthenium dendrimers upon the two-photon absorption (TPA) properties of the structures. The authors recognize the need for scaling the TPA cross-section data with the “effective number of electrons” to allow a comparison of the “TPA merit of structurally and compositionally diverse molecules” . [ 1 ] We show that when analyzed under the effects of simple scaling, the properties of triphenylamine-cored alkynylruthenium dendrimers exhibit an unprecedented improvement over their non-metal-core cousins. Roberts et al. calculate the TPA merit of the metal-containing dendrimers by dividing the experimental TPA cross-sections by the number of effective electrons, N eff . In this way, the authors estimate the performance of inorganic dendrimers to be one order of magnitude greater than that of similar organic dendrimers. This procedure misses the fact that the TPA crosssection has been rigorously shown to scale quadratically with the number of electrons. [ 2 , 3 ] When the appropriate scaling is used, the performance of the organometallic dendrimers is two orders of magnitude greater than that of similar organic dendrimers. In this correspondence, we re-analyze the scaling of the TPA cross-section of these molecules to emphasize the truly remarkable nonlinear optical performance of triphenylaminecored alkynylruthenium dendrimers. We consider the degenerate on-resonance TPA case, where two photons, each of frequency ω are absorbed and excite the molecular Bohr frequency Ω = ω + ω which can be

Experimental verification of a self-consistent theory of the first-, second-, and third-order (non)linear optical response

We show that a combination of linear absorption spectroscopy, hyper-Rayleigh scattering, and a theoretical analysis using sum rules to reduce the size of the parameter space leads to a prediction of the imaginary part of the second hyperpolarizability of the dye AF-455 that agrees with the experimental data gathered through two-photon absorption spectroscopy. Our procedure, which demands self-consistency between several measurement techniques and does not use adjustable parameters, provides a means for determining transition moments between the dominant excited states based strictly on experimental characterization. This is made possible by our new approach that uses sum rules and molecular symmetry to rigorously reduce the number of required physical quantities.

“Push-no-pull” porphyrins for second harmonic generation imaging

The established approach to design a molecule with strong second-order nonlinear optical (NLO) activity is to connect an electron-donor to an electron-acceptor via a π-conjugated bridge, to generate push–pull system. Surprisingly, we have found that dyes with large first hyperpolarizabilities, and which exhibit strong second harmonic generation (SHG), can be created just by attaching an electron-donor to a porphyrin. The free-base porphyrin core is sufficiently electron-deficient that the hyperpolarizability does not increase on addition of a pyridinium electron-acceptor.

Fundamental limits: developing new tools for a better understanding of second-order molecular nonlinear optics

The generalized Thomas-Kuhn sum rules are used to characterize the nonlinear optical response of organic chromophores in terms of fundamental parameters that can be measured experimentally. The nonlinear optical performance of organic molecules is evaluated from the combination of hyper-Rayleigh scattering measurements and the analysis in terms of the fundamental limits. Different strategies for the enhancement of nonlinear optical behavior at the molecular and supramolecular level are evaluated and new paradigms for the design of more efficient nonlinear optical molecules are proposed and investigated.