Centrosymmetric molecules for second harmonic generation

The requirement of a non‐centrosymmetric structure for second harmonic generation (SHG) was until recently assumed necessarily to apply to te smallest building block, the molecule. However, non‐centrosymmetric aggregates of centrosymmetric molecules can exist, and their SHG properties are considered here. Derivatives of squaraines are taken as examples. Finally, the role of intermolecular charge transfer in Langmuir–Blodgett film‐forming materials for SHG—both within the monolayers and across the boundary—is discussed.