Second harmonic generation ability of vanillin polymorphs I and II

Stable Form-I and metastable Form-II polymorphs of the well-known organic nonlinear optical material vanillin, were separated successfully through nucleation control with selected templates from aqueous solution. Both the polymorphs crystallize in non-centrosymmetric space group. The structural confirmation of the grown poly -morphs was confirmed though powder X-ray diffraction (PXRD) analysis and the results reveal that Form-I crystallizes in monoclinic space group P21 and Form-II crystallizes in orthorhombic space group Pna21. The thermal stability of the polymorphs were studied by differential scanning calorimetry analysis and found that both the forms are highly stable until the melting. UV-Vis-NIR spectroscopic analysis shows that the stable Form -I polymorph has the lower cut-off wavelength at 305 nm and band gap value of 4.07 eV. Whereas the metastable Form-II has the lower cut-off wavelength at 300 nm and band gap value of 4.14 eV. The second harmonic generation (SHG) for the grown polymorphs were confirmed by Kurtz-Perry powder technique with 1064 nm Nd: YAG laser as a fundamental source and it was found that the SHG efficiency of Form-I was 14 times and Form-II was 14.5 times that of the standard KDP. The observed results reveal that the grown vanillin polymorphs have a very good SHG efficiency than the inorganic KDP and it can be used for nonlinear optical applications.

Automated summary

The stable Form-I and metastable Form-II polymorphs of vanillin were successfully separated through nucleation control with selected templates. Both polymorphs exhibit good thermal stability and high SHG efficiency, making them promising for nonlinear optical applications.