A series of five fluorescent and ionic dimethylamino stilbazolium homologues with increasing conjugation length (from ethenyl to decapentaenyl) is investigated by high-frequency, amplitude-modulated femtosecond hyper-Rayleigh scattering at 1300 nm. A hyperpolarizability value that is not overestimated by the presence of a multiphoton fluorescence contribution is obtained from the Fourier analysis of the hyper-Rayleigh scattering signal. The demodulation curve (decrease of Fourier amplitude versus modulation frequency) is characterized by both the hyperpolarizability value and the fluorescence decay parameters. The fluorescence decay parameters are accurately determined independently by single-photon counting. A detailed analysis of the fluorescence decay parameters from the hyper-Rayleigh scattering demodulation curve and of their relation to the fluorescence decay parameters obtained from single-photon counting experiments is presented. The inherent hyperpolarizability value for these chromophores shows a maximum of (2045 Ϯ 35) ϫ 10 Ϫ30 esu or (760 Ϯ 13) ϫ 10 Ϫ50 C 3 m 3 J Ϫ2 for the hexatrienyl conjugation length. A comparison with theoretical calculations suggests the importance of trans-cis isomerization in the excited state.
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.
We present the results of the combination of two independently valid optimization strategies for the first hyperpolarizability of ionic organic chromophores. The first strategy to enhance the nonlinear optical response, at the molecular level, is the extension of the conjugation path in the chromophore itself. The second strategy, at the supramolecular level, is the bottom-up nano-engineering of an inclusion complex of the chromophore in an amylose helix. We have studied a series of five (dimethylamino)stilbazolium-type chromophores with increasing conjugation length between the (dimethylamino)phenyl donor ring and the pyridinium acceptor ring in combination with four amylose helices of different molecular weights. The first hyperpolarizability of the self-assembled inclusion complexes has been experimentally determined by frequency-resolved femtosecond hyper-Rayleigh scattering. These values are compared with experimental values for the free chromophores in solution and with theoretical values. Where experimental values for the hyperpolarizability in solution were lower than theoretically predicted, an enhancement upon inclusion was observed -with the longest chromophore in the best amylose helix showing an enhancement by one order of magnitude. Molecular modelling of the inclusion of the chromophore shows that the coplanarity of the two rings is more important than all-trans configuration in the conjugation path. The degree of enhancement, however, is not enough to breach the apparent limit of the first hyperpolarizability which is about an order of magnitude below the fundamental limit calculated by Kuzyk. This analysis confirms the determining role of the arrangement of the excitedstate energy levels on the nonlinear response.
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