Crystals of the title compound exist as a green monoclinic phase [space group P21/c with u=9.046( l), b= 19.615(2), c =9.055( 1) A, p = 116.107( 5)", 2 = 21 and a purple triclinic phase. The chromophore is both planar and centrosymmetric and its dimensions indicate a tendency towards a quinoidal structure with extensive delocalisation. The Langmuir-Blodgett (LB) films show two types of aggregation with absorption maxima at 655 nm and 540 nm. These monolayers also exhibit strong second harmonic generation (SHG) comparable to the intensity from films of hemicyanine dyes. The anomalous non-linear optical properties are attributed to a serendipitous non-centrosymmetric packing arrangement within the films and to an intermolecular charge transfer contribution to the bulk second-order susceptibility.Current interest in organic materials for non-linear optics'-' stems from the high molecular hyperpolarizabilities of donor-(z-bridge)-acceptor molecules and the potential applications of such materials in the electronics and communications industries. The non-linearities arise from the dependence of the polarization on the electric field of the incident radiation:
The linear and nonlinear optical properties of 2,4-bis[4-(N-methyl-N-alkylamino)-2,6-dihydroxyphenyl]squaraine (I), 2,4-bis[4-(N,N-dialkylamino)-2-hydroxyphenyl]squaraine (II), and 2,4-bis(8-hydroxy-9julolidinyl)squaraine (III) are reported. Electrospray ionization mass spectrometry (ESI-MS) has provided evidence of association in dilute solution (ca. 2 × 10 -5 mol dm -3 ) with m/z values which may be assigned to dimeric aggregates for dye II. Solid solutions of this dye in poly(vinyl acetate) show second-harmonic generation (SHG), albeit very weak, and therefore indicate that the dimeric aggregate is noncentrosymmetric. In contrast, dyes I and III form heptameric aggregates with their solid solutions being SHG-inactive. There is some correlation between the type of aggregate from ESI-MS and the Langmuir film spectra, but van der Waals interactions between the hydrophobic groups play a more significant role in determining the structure at the air-water interface. The absorption maxima of the Langmuir and deposited Langmuir-Blodgett films vary from 530 nm to the near-infrared, but only those films with a maximum at 650-700 nm, or a significant shoulder in this range, are SHG-active. The second-order behavior is not an inherent property of the molecule itself but, instead, is a result of intermolecular effects and is dependent upon the noncentrosymmetry of the aggregate and the film.
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.
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