Quadratic Nonlinear Optical Properties ofN-Aryl Stilbazolium Dyes

Autor: André Persoons, Benjamin J. Coe, Michael B. Hursthouse, Keitaro Nakatani, Robert C. Johnson, Joseph T. Hupp, Geert Olbrechts, James A. Harris, Inge Asselberghs, Simon J. Coles, Koen Clays
Rok vydání: 2002
Předmět:
Zdroj: Advanced Functional Materials. 12:110-116
ISSN: 1616-3028
1616-301X
DOI: 10.1002/1616-3028(20020201)12:2<110::aid-adfm110>3.0.co;2-y
Popis: Trans-4'-(dimethylamino)-N-R-4-stilbazolium hexafluorophosphate (R = Me, Me 1, Ph, Ph 2, 2,4-dinitrophenyl, DNPh 3, 2-pyrimidyl, Pym 4, Scheme 1) were prepd. Their electronic absorption spectra show intense, visible intramol. 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 obsd. in cyclic voltammetric and 1H NMR data. Fluorescence-free 1st hyperpolarizability .beta. values of [1-4]PF6 were measured by using femtosecond hyper-Rayleigh scattering (HRS) with MeCN solns. and a 1300. nm laser, and static 1st hyperpolarizabilities .beta.0 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 .beta.0 values than their N-Me counterpart in [1]PF6, with a .apprx. 10-fold increase in .beta.0 obsd. in moving from [1]PF6 to [4]PF6 (25 .fwdarw. 230 .times. 10-30 esu). Stark (electroabsorption) spectroscopic studies in butyronitrile glasses at 77 K allowed the derivation of dipole moment changes .DELTA..mu.12 (10.9-14.8 D), which were used to calc. .beta.0 according to the two-state equation .beta.0 = 3.DELTA..mu.12(.mu.12)2/2(Emax)2 (.mu.12 = transition dipole moment). With the exception of [1]PF6, the Stark-derived .beta.0 values are in reasonable agreement with those from HRS. However, the increase in .beta.0 in moving from [1]PF6 to [4]PF6 is only 2-fold for the Stark data (90 .fwdarw. 185 .times. 10-30 esu). The obsd. trend of increasing .beta.0 in the order [1]PF6 < [3]PF6 < [2]PF6 < [4]PF6 arises from a combination of decreasing Emax and increasing .DELTA..mu.12, with only a slight increase in .mu.12 between [1]PF6 and [4]PF6. Probably the .beta.0 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 powd. samples (53-63 .mu.m) afforded efficiencies of 470 and 240 times that of urea, resp. Under the same conditions, the well-studied compd. [1]p-MeC6H4SO3 gave an SHG efficiency of 550 times that of urea. [on SciFinder (R)]
Databáze: OpenAIRE
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