A melt-processable squaraine-based organic glass for nonlinear optics

The synthesis and properties of a new indole-based squaraine dye functionalized with ethylhexyl substituents are reported. The chromophore displays a large two-photon absorption cross-section of ∼1490 GM at a resonance wavelength of 1160 nm. The crystalline material exhibits a melting temperature of 222 °C, which is sufficiently low to allow for melt-processing without significant thermal decomposition. Crystallization of the dye from the melt is exceedingly slow, so that amorphous glasses with a glass transition temperature of 60 °C can easily be produced, even without particularly rapid cooling. This allows the fabrication of thin films and more complex shapes, which offer a maximum of dye content, yet lack the scattering effects associated with organic crystalline materials.