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: λmax
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.
