Dual Influence of H-Bonding on the Solid-State Second-Harmonic Generation of a Chiral Quinonoid Compound

A new chiral push-pull quinonoid compound, 7,7-bis(3(R)-hydroxypyrrolidino)-8,8-dicyanoquinodimethane (BHPDQ), has been synthesized. Powder studies indicate an appreciable second harmonic generation (SHG) capability, and crystal structure analysis reveals extensive intermolecular H-bonding in BHPDQ. The influence of H-bonding and molecular chirality on the dipole alignment in the crystal is described. The H-bonds play an unusual role of modifying the molecular structure by producing an enhanced molecular twist in BHPDQ; this in turn affects the molecular hyperpolarizability. Thus we describe a novel instance wherein the powder SHG in a molecular material is determined by the dual influence of intermolecular H-bonding on the crystal structure as well as on the molecular structure and hyperpolarizability. The new dimension that this observation adds to the general theme of molecular materials design is highlighted.