Charge distortion by sparkles can explain strong SHG by centrosymmetric squaraine dyes

Abstract

Ashwell and his co-workers have published a report describing second harmonic generation (SHG) from a laser beam (Nd:YAG, 1.064 µm) by a Langmuir-Blodgett (LB) monolayer of a symmetrically substituted diaryl squaraine dye. The molecular geometry of the squaraine, and its unit cell is known to be centrosymmetric in the crystalline state. It has been stated that interfacial effects (arising from the twin-step function in the refractive index) cannot account for the magnitude of the observed SHG. It has also been assumed that molecular centrosymmetry has been retained in the LB film, with the observed SHG being attributed to the presence of non-centrosymmetric ensembles of these centrosymmetric molecules. We show that an electronically distorted (and therefore non-centrosymmetric) squaraine molecule can exhibit large SHG coefficients, these distortions being induced by the proximity of the highly polar species that constitute the molecular environment. In our model of a T-shaped dimer, both monomer components become non-centrosymmetric, the mutual interaction between the two monomers being modelled by using a negative or a positive sparkle. We attribute the noteworthy SHG output from LB films of Ashwells centrosymmetric squaraines to intramolecular charge transfer within the distorted monomers in the T-dimer.