The new salts trans‐4′‐(dimethylamino)‐N‐R‐4‐stilbazolium hexafluorophosphate (R = methyl, Me 1, phenyl, Ph 2, 2,4‐dinitrophenyl, DNPh 3, 2‐pyrimidyl, Pym 4, Scheme 1) have been prepared. Their electronic absorption spectra show intense, visible intramolecular charge‐transfer bands, the energy (Emax) of which decreases in the order R = Me > Ph > DNPh > Pym. This trend arises from the steadily increasing electron deficiency of the pyridinium ring, a phenomenon also observed in cyclic voltammetric and 1H nuclear magnetic resonance (NMR) data. Fluorescence‐free first hyperpolarizability β values of [1–4]PF6 were measured by using femtosecond hyper‐Rayleigh scattering (HRS) with acetonitrile solutions and a 1300 nm laser, and static first hyperpolarizabilities β0 were obtained by application of the two‐state model. The HRS results indicate that the N‐aryl chromophores in [2–4]PF6 have considerably larger β0 values than their N‐methyl counterpart in [1]PF6, with a ca. 10‐fold increase in β0 observed in moving from [1]PF6 to [4]PF6 (25 → 230 × 10–30 esu). Stark (electroabsorption) spectroscopic studies in butyronitrile glasses at 77 K allowed the derivation of dipole moment changes Δμ12 (10.9–14.8 D), which have been used to calculate β0 according to the two‐state equation β0 = 3Δμ12(μ12)2/2(Emax)2 (μ12 = transition dipole moment). With the exception of [1]PF6, the Stark‐derived β0 values are in reasonable agreement with those from HRS. However, the increase in β0 in moving from [1]PF6 to [4]PF6 is only 2‐fold for the Stark data (90 → 185 × 10–30 esu). The observed trend of increasing β0 in the order [1]PF6 < [3]PF6 < [2]PF6 < [4]PF6 arises from a combination of decreasing Emax and increasing Δμ12, with only a slight increase in μ12 between [1]PF6 and [4]PF6. It is likely that the β0 values for [3]PF6 are lower than expected due to the steric effect of the ortho‐NO2 group, which causes twisting of the DNPh ring out of the plane of the stilbazolium unit. A single crystal X‐ray structure shows that [2]PF6 crystallizes in the space group Cc, with head‐to‐tail alignment and almost parallel stacking of the pseudo‐planar stilbazolium portions of the cations to form polar sheets within a polar bulk structure. [2]PF6 is essentially isostructural with the related Schiff base salt trans‐4‐[(4‐dimethylaminophenyl)iminomethyl]‐N‐phenylpyridinium hexafluorophosphate ([8]PF6). Second harmonic generation (SHG) studies on [2]PF6 and [8]PF6 using a 1907 nm laser and sieved powdered samples (53–63 μm) afforded efficiencies of 470 and 240 times that of urea, respectively. Under the same conditions, the well‐studied compound [1]p‐MeC6H4SO3 gave an SHG efficiency of 550 times that of urea.

A novel technique for suppression of the multi-photon fluorescence contribution in second-order nonlinear optical hyper-Rayleigh scattering experiments is described. The technique takes advantages of the demodulation and the phase shift in the frequency domain of the time-delayed (multi-photon) fluorescence in the time domain. We demonstrate the effectiveness of demodulation at high modulation frequencies of the fundamental laser beam by determining the molecular second-order nonlinear polarizability for a reference molecule under fluorescent conditions. The value that was obtained for crystal-violet in methanol with 9,10-diphenylanthracene added as a centrosymmetric fluorophore compares very well with the values that were previously obtained. The possibility of complete suppression of all fluorescence, based on phase-sensitive measurements in quadrature with the fluorescence, is also discussed.

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.

Suppression of the multiphoton fluorescence contribution to the hyper-Rayleigh (second-order nonlinear optical) scattering signal was recently achieved by intrinsic demodulation of the fluorescence at high amplitude-modulation (AM) frequencies [Olbrechts et al., Rev. Sci. Instrum. 69, 2233 (1998)]. These high AM frequencies were obtained from the high harmonic content in the Fourier spectrum of a repetitive train of femtosecond pulses from a Ti:sapphire laser emitting at 800 nm. We have used a femtosecond parametric oscillator to shift the fundamental wavelength to 1.3 mum . By further improving the detection electronics, we can now obtain fluorescence suppression at AM frequencies up to 600 MHz. Fluorescence-free hyperpolarizability values were obtained for fluorescent dipolar compounds as well as for an ionic fluorophore. The results also indicate that shifting the fundamental wavelength to the near infrared only is not a general solution to the multiphoton fluorescence problem in hyper-Rayleigh scattering.

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