Fluorescence-free hyperpolarizability values by near-infrared, femtosecondhyper-Rayleigh scattering

It is shown that femtosecond modulated hyper-Rayleigh scattering (HRS) can be used to suppress multi-photon fluorescence (MPF) contributions to the hy per-Rayleigh scattering signal. A high frequency modulation of the fundamen tal beam modulates both the immediate scattering and the time-delayed fluor escence. Due to the finite lifetime of the excited state, the fluorescence exhibits essentially complete demodulation (zero amplitude) while the scatt ering is unaffected. The fundamental wavelength of 1300 nm is accessible by the use of a Ti:Sapphire femtosecond laser together with an Optical Parame tric Oscillator. Even at the latter non-resonant wavelength, fluorescence i s observed for several chromophores. 4-(N,N-dimethylamino)-4'-nitrostilbene (DANS) and 4-methoxy-4'-nitrostilbene (MONS) were used to demonstrate the fluorescence-issue in HRS experiments. The ability to measure the fluoresce nce-free hyperpolarizability value beta of an ionic compound was demonstrat ed on 4-[4-(N,N-methyl-octadecylamino)styryl]-1-methyl-pyridinium bromide ( MO). Moreover, two examples are discussed in which the technique proved suc cessful. First, the molecular second-order properties of some subphthalocya nine derivatives, i.e. chloro[2,3,9,10,16,17-hexa-alkylthiosubphthalocyanin ato]-boron(III) homologues with as the alkyl substituent, an octyl-, decyl- , dodecyl-, or hexadecyl-chain, were studied. For all homologues, identical beta -values of (190 +/- 30) x 10(-30) esu were found. Second, the beta -v alues of a series of five fluorescent and ionic N,N-dimethylamino stilbazol ium (DAST) homologues of increasing conjugation length (from ethenyl to dec apentaenyl) have been measured. The inherent B-value of these chromophores shows a maximum of (2045 +/- 35) x 10(-30) esu for the hexatrienyl conjugat ion length. (C) 2000 Elsevier Science S.A. All rights reserved.