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The wavelength-dependent molecular first hyperpolarizability of the zwitterionic nonlinear optical (NLO) chromophore picolinium quinodimethane (PQDM) is determined by hyper-Rayleigh scattering (HRS) and used to test and improve theoretical dispersion models. Experimental HRS data are obtained over a very wide fundamental wavelength range (780−1730 nm), spanning the entire range of two-photon resonance with the intramolecular charge-transfer (ICT) transition and reaching the onset of a higher energy resonance. Reliable calibration against the pure solvent (dimethylformamide, DMF, and DMF-d7at the longest wavelengths) was achieved over the full spectral range as a result of the high sensitivity of the HRS setup. Extremely high resonant values are obtained (up to 4560 × 10-30esu at 1360 nm) and also away from resonance remains very large (1210 × 10-30esu at 1730 nm). The two-photon resonance with the ICT band shows a pronounced red shift (∼33 nm in second-harmonic wavelength) relative to the absorption maximum. The various two-level dispersion models available in the literature are considered, and some important improvements are introduced. Furthermore, a vibronic model including a single vibrational mode and incorporating inhomogeneous broadening is developed and contrasted to the other extreme of a continuum of vibronic lines without inhomogeneous broadening. The red shift of the maximum can be largely explained by either an improved inhomogeneous broadening model or by vibronic coupling. However, the vibronic models are physically more realistic and lead to a better description of the observed dispersion. In general, models with more inhomogeneous broadening result in a narrower resonance, whereas incorporating more homogeneous broadening yields a broader resonance. Hence, the derived static electronic hyperpolarizability 0depends very critically on the precise modeling of the broadening mechanisms. Upper and lower bounds to the true 0are estimated from the two limiting vibronic models.

The crystal structures of chiral aminomethylpyrrolidine-substituted 7,7,8,8-tetracyanoquinodimethane (TCNQ) and dipyrrolidine-substituted 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (TCNQF4) have been determined by single crystal X-ray crystallography. The TCNQ derivative crystallised in a noncentrosymmetric space group (P212121) and showed powder second harmonic generation (SHG) comparable to that of urea. An interesting case of H-bonding observed in its crystal structure is discussed. The TCNQF4 derivative is the first in the class of amino-substituted tetrafluoro-TCNQ derivatives. It has been observed to crystallise in a centrosymmetric space group (P21/c). Semi-empirical quantum chemical calculations using crystal structure geometries indicate moderate and negative (30–50×10-30 esu) hyperpolarisabilities. Crystal structural features and computational results are compared with known analogous systems.