Crystal structures of amino substituted dicyanoquinodimethanes with potential nonlinear optical applications
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.