Quadratic nonlinear optical properties of N-aryl stilbazolium dyes

B. J. Coe, J. A. Harris, I. Asselberghs, K. Clays, G. Olbrechts, A. Persoons, J. T. Hupp, R. C. Johnson, S. J. Coles, M. B. Hursthouse, K. Nakatani

    Research output: Contribution to journalArticlepeer-review


    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 hexafluoro-phosphate ([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.
    Original languageEnglish
    Pages (from-to)110-116
    Number of pages6
    JournalAdvanced Functional Materials
    Issue number2
    Publication statusPublished - Feb 2002


    • Electromagnetic wave scattering (hyper-; quadratic nonlinear optical properties of N-aryl stilbazolium dyes); Molecular structure; NMR; Nonlinear optical materials; Nonlinear optical properties; Optical hyperpolarizability; Oxidation potential; Reduction potential; Second-harmonic generation; Stark effect; UV and visible spectra (quadratic nonlinear optical properties of N-aryl stilbazolium dyes); Dyes (stilbazolium; quadratic nonlinear optical properties of N-aryl stilbazolium dyes)


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