Aggregation-induced linear and non-linear optical properties of four hydroxy-substituted analogues of 2,4-bis[4-(dibutylamino)phenyl]squaraine

Langmuir films of the title compound, 2,4-bis[4-(dibutylamino) phenyl]squaraine, Sq0, and its substituted analogues, Sq1 (monohydroxy) to Sq4 (tetrahydroxy), exhibit three well defined phases at the air-water interface: lambda(max) 525-550 nm (H-aggregate); 660-670 nm (intermediate); 760-770 nm (J-aggregate). The monolayer spectra are influenced by the surface pressure and the number of hydroxy substituents and, when the monolayers are transferred to a solid substrate, there is a correlation between the linear and non-linear optical behaviour of the Langmuir-Blodgett films. Second-harmonic generation has been observed even though the unit molecular structures of two analogues, Sq0 and Sq4, are centric. The behaviour is aggregation-induced and there is evidence that the dyes associate in dilute solution. The electrospray ionization mass spectra exhibit peaks with high mass/charge (m/z) ratios; the data conform to the dimeric aggregate and, for solutions of two or more squaraines, corroborate the existence of heteromolecular associates.