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Quadratic Nonlinear Optical Properties of N ‐Aryl Stilbazolium Dyes
Author(s) -
Coe B.J.,
Harris J.A.,
Asselberghs I.,
Clays K.,
Olbrechts G.,
Persoons A.,
Hupp J.T.,
Johnson R.C.,
Coles S.J.,
Hursthouse M.B.,
Nakatani K.
Publication year - 2002
Publication title -
advanced functional materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 6.069
H-Index - 322
eISSN - 1616-3028
pISSN - 1616-301X
DOI - 10.1002/1616-3028(20020201)12:2<110::aid-adfm110>3.0.co;2-y
Subject(s) - hyperpolarizability , hexafluorophosphate , pyridinium , dipole , materials science , stark effect , chromophore , aryl , acetonitrile , intramolecular force , femtosecond , analytical chemistry (journal) , photochemistry , spectral line , polarizability , chemistry , laser , physics , optics , organic chemistry , alkyl , ionic liquid , astronomy , molecule , catalysis
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 ( E max ) 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 1 H nuclear magnetic resonance (NMR) data. Fluorescence‐free first hyperpolarizability β values of [ 1 – 4 ]PF 6 were measured by using femtosecond hyper‐Rayleigh scattering (HRS) with acetonitrile solutions and a 1300 nm laser, and static first hyperpolarizabilities β 0 were obtained by application of the two‐state model. The HRS results indicate that the N ‐aryl chromophores in [ 2 – 4 ]PF 6 have considerably larger β 0 values than their N ‐methyl counterpart in [ 1 ]PF 6 , with a ca. 10‐fold increase in β 0 observed in moving from [ 1 ]PF 6 to [ 4 ]PF 6 (25 → 230 × 10 –30 esu). Stark (electroabsorption) spectroscopic studies in butyronitrile glasses at 77 K allowed the derivation of dipole moment changes Δμ 12 (10.9–14.8 D), which have been used to calculate β 0 according to the two‐state equation β 0 = 3Δμ 12 (μ 12 ) 2 /2( E max ) 2 (μ 12 = transition dipole moment). With the exception of [ 1 ]PF 6 , the Stark‐derived β 0 values are in reasonable agreement with those from HRS. However, the increase in β 0 in moving from [ 1 ]PF 6 to [ 4 ]PF 6 is only 2‐fold for the Stark data (90 → 185 × 10 –30 esu). The observed trend of increasing β 0 in the order [ 1 ]PF 6 < [ 3 ]PF 6 < [ 2 ]PF 6 < [ 4 ]PF 6 arises from a combination of decreasing E max and increasing Δμ 12 , with only a slight increase in μ 12 between [ 1 ]PF 6 and [ 4 ]PF 6 . It is likely that the β 0 values for [ 3 ]PF 6 are lower than expected due to the steric effect of the ortho ‐NO 2 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 ]PF 6 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 ]PF 6 is essentially isostructural with the related Schiff base salt trans ‐4‐[(4‐dimethylaminophenyl)iminomethyl]‐ N ‐phenylpyridinium hexafluorophosphate ([ 8 ]PF 6 ). Second harmonic generation (SHG) studies on [ 2 ]PF 6 and [ 8 ]PF 6 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 ‐MeC 6 H 4 SO 3 gave an SHG efficiency of 550 times that of urea.