Quadratic Optical Nonlinearities of N‐Methyl and N‐Aryl Pyridinium Salts

Abstract

Four series of dyes with dimethylamino electron donor groups and N‐R‐pyridinium (R = methyl Me, phenyl Ph, 2,4‐dinitrophenyl 2,4‐DNPh, or 2‐pyrimidyl 2‐Pym) electron acceptors are studied as their hexafluorophosphate salts. The intramolecular charge‐transfer (ICT) energies (Emax) of these compounds decrease within each of the series in the order R = Me > Ph > 2,4‐DNPh > 2‐Pym, as the electron‐accepting ability of the pyridinium ring increases. Hyper‐Rayleigh scattering with femtosecond 1300 or 800 nm lasers yields fluorescence‐free first hyperpolarizabilities β, and static first hyperpolarizabilities β0[H] are obtained via the two‐state model. Dipole moment changes Δμ12 for the ICT transitions obtained from Stark spectroscopy afford β0[S] values by using β0 = 3Δμ12(μ12)2/2(Emax)2 (μ12 = transition dipole moment). The β0[S] data show that the combination of pyridyl N‐arylation with conjugation extension affords large increases in β0. The β0[H] data generally agree with this conclusion, but resonance effects may explain some apparent anomalies. X‐ray structural studies on various salts reveal that the use of tosylate anions is not a generally applicable approach to engineering noncentrosymmetric structures of pyridinium salts. However, trans‐N‐phenyl‐4‐(4‐dimethylaminophenyl‐4‐buta‐1,3‐dienyl)pyridinium hexafluorophosphate adopts the polar space group Cc, and shows a very large powder second harmonic generation efficiency from a 1907 nm laser, which is similar to that of the well‐studied material trans‐4′‐(dimethylamino)‐N‐methyl‐4‐stilbazolium tosylate (DAST).

Azulene‐substituted pyridines and pyridinium salts. Synthesis and structure. 1. Azulene‐substituted pyridines

“…The last route is, however, preferable for the preparation of 2,6-diphenyl derivatives, especially with a substituent at the 3-position (the compounds 4-6) because, here, the azulenyl-pyranylium salts, used as reactant, resulted from the synthesis contaminated with starting material making separation difficult [2]. Whereas the yields of pyridines 3-substituted with Me were very good, those with Ph or Br resulted in somewhat lower yields.…”

“…Whereas route (a) can be used both for 2,6-diphenyl or 2,6-dimethyl-pyridines, only the 2,6-diphenyl derivatives were obtained by route (b) because the 4-chloro-2,6-dimethyl-pyranylium salts can not be separated from the reaction mixture after halogenation of the corresponding pyranone [2]. The last route is, however, preferable for the preparation of 2,6-diphenyl derivatives, especially with a substituent at the 3-position (the compounds 4-6) because, here, the azulenyl-pyranylium salts, used as reactant, resulted from the synthesis contaminated with starting material making separation difficult [2].…”

“…This property could be valuable in technical purposes, such as for optical applications (e.g. materials with non-linear optical properties) [2,3]. The possibility to obtain the new heterocyclic systems, namely pyridines and pyridinium salts, starting from these synthons [4] motivated us to undertake a systematic study regarding the generation of such compounds.…”

Molecular Structures and Second-Order Nonlinear Optical Properties of Ionic Organic Crystal Materials

“…Almost all of the salts designed crystallize centrosymmetrically, leading to crystals without second-order NLO effects. Fortunately, molecules (6) and (10) adopt the polar space group C C and show SHG activity of 0.8 and 1.0 times that of DAST [66,67]. Second-order NLO susceptibility of crystals made of chromophore (6) with PF 6 -counter anion higher than that of DAST have been reported [68].…”

Synthesis, crystal growth, spectral, thermal and optical properties of acenaphthene picrate

“…Thanks to the extremely high optical nonlinearity possessed by the organic molecular crystals, they steal a march over the inorganic analogues in the potential areas of applications akin to electro optic modulation, frequency doubling of lasers, etc. The former class of materials is associated with large non linear optical coefficients, suitable transparency and excellent comprehensive properties [6][7][8][9].…”