A novel selenoalkenyl-isoxazole based donor–acceptor nonlinear optical material

A novel selenium-based donor–acceptor molecule with high second harmonic generation efficiency is presented.

The following reaction was performed in oven-dried glassware. Reagents were purchased from commercial sources and used without prior purification. Analytical TLC was performed on Merck silica gel 60 F254 plates. Chromatographic separations at preparative scale were carried out on silica gel (Merck silica gel 60, 40 -63 µm). UV/Vis absorption spectra were recorded in tetrahydrofuran solutions (5 µM) or for powder samples using Suprasil glass slides as sample holder with a Perkin Elmer Lambda 750 spectrometer. The thermal behaviour of substances 2 and 3 was studied with differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) using a Netzsch simultaneous thermal analyzer (STA 449 F1 Jupiter) under N 2 flow (40 mL/min) with a heating rate of 10 K/min. The STA 449 Type-K thermocouples were calibrated using indium, tin, bismuth and zinc metals. The syntheses of trimethyl[(3Z)-4-(methylseleno)-3-penten-1-yn-1-yl]silane 1 1 and Nhydroxybenzimidoyl chloride 4 2 were performed according to literature.
Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance III HD 600 MHz spectrometer equipped with a cryo probe Prodigy™ at 600.2 MHz ( 1 H), 150.9 MHz ( 13 C) and 114.5 MHz ( 77 Se). The 1 H and 13 C chemical shifts are reported in  units, parts per million (ppm) downfield from tetramethylsilane using residual solvent signals for calibration. 77 Se chemical shifts are given in ppm relative to dimethylselenide, using selenophene ( = 605 ppm) 3 as an external secondary standard. Coupling constants are reported in Hertz; multiplicity of signals is indicated by using following abbreviations: s=singlet, d=doublet, t=triplet, q=quartet, quin=quintet. The multiplicity of 13

Crystallographic Data
X-ray diffraction intensities of a suitable crystal of the title compound 3 were collected at 100 K in a dry stream of nitrogen on a Bruker KAPPA APEX II diffractometer system using graphitemonochromatized Mo-Kα radiation (λ = 0.71073 Å) and fine sliced φ-and ω-scans. Frame data were reduced to intensity values with SAINT-Plus 4 and a correction for absorption effects was applied using the multi-scan approach implemented in SADABS 4 . More details of the data collection are summarized in Table 1. The crystal structure was solved by charge-flipping using SUPERFLIP. 5 All non-H atoms were located in the resulting electron density map. The structure was refined with JANA2006. 6 The protons were placed at calculated positions and refined as riding on their parent atoms. All non-H atoms were modelled with anisotropic displacement parameters.

Powder SHG Studies
For a qualitative evaluation of the NLO properties of 3, the process of frequency doubling or second harmonic generation (SHG) was employed utilizing the powder technique developed by Kurtz and Perry. 7 This method allows the relative measurement of the NLO coefficients since the SH efficiency scales quadratically with the nonlinear coefficient for samples with a particle size that is significantly less than the so-called nonlinear coherence length (which is more than 10 μm). detector in the range between 450 and 600 nm. The angle between the optical axis of the monochromator and the sample surface normal was 40°, so that no specular reflection from the glass slides was incident on the monochromator input. The resulting SH spectrum was identical to that obtained by inserting a commercial SH crystal into the beam path, displaying the peak at 1034 nm / 2 = 517 nm and an FWHM of 6 nm, which is due to the relatively broad spectrum of the femtosecond laser source. The sample plane was positioned somewhat out of the focal plane (toward the lens) so as to prevent any damage to the sample. After each measurement, the samples were carefully checked for the absence of damage or thermal modification. The SH signal of our sample was compared to that from a similarly prepared powder sample of another organic compound 8 with point group 222, which exhibits a second-order nonlinear coefficient of 0.6 x 10 -12 m V -1 , somewhat higher than potassium dihydrogen phosphate (KDP).

Theoretical calculations
DFT calculations were performed using the Gaussian 09 package revision D.01. 9 applying the long range corrected CAM-B3LYP functional 10 in combination with Pople basis sets 6-311++G(d,p) 11,12 in order to include polarized and diffuse functions. Geometry optimizations were performed in the gas phase. First-order hyperpolarizability tensors were visualized using a method adapted from Tuer et al. 13 The graphical representations were rendered using the POVRAY software package. 14 6. References