Second-harmonic generation from Langmuir–Blodgett films of centrosymmetric squaraines with heterocyclic donor groups

Abstract

Langmuir–Blodgett (LB) monolayers of squaraine dyes of general formula D–(C₄O₂)–D, where D is a heterocyclic donor, exhibit second-harmonic generation (SHG) with, for example, χ_zzz⁽²⁾=100–150 pm V^–1 at 1.064 µm for 2,4-bis[(3,3-dimethylindolin-2-ylidene)methyl]squaraine. The behaviour is not an inherent property of the molecule and the second-order susceptibility is too high to arise from polarisation of the molecular film at the interface. Instead, the SHG is attributed to non-centrosymmetric aggregation with intermolecular charge transfer between the donor (heterocycle) and acceptor (C₄O₂) groups of adjacent molecules playing a significant role and contributing to the susceptibility. The nonlinear optical properties are discussed with reference to films of 1,4-bis(pyrrol-2-yl)squaraine derivatives which show either very weak or no SHG. The behaviour is dependent upon the type of aggregation and the extent of the donor–acceptor interaction.