Symmetrical squaraines

A long-wavelength probe 29 signaling carbohydrates in aqueous solutions by increasing of fluorescence was developed by Akkaya and Kukre on the basis of a symmetrical squaraine dye containing two phenylboronic acid functions [89]. The emission maximum of this probe is at 645 nm. A maximal response of about 25% was found for fructose. 

Synthesis of symmetric squaraines is generally achieved by the condensation of one equivalent of squaric acid, with two equivalents of the respective nucleophiles, under reflux in an azeotropic solvent. In this extremely versatile one-pot synthesis, same substituents are attached on the 1,3-position of squaric acid to yield symmetric squaraines. Nucleophiles like pyrroles [7], phenols [7], azulenes [14] and N,N-dialkylanilines [15] condense with squaric acid, and a representative example is given in Scheme 1. Nucleophiles obtained by the elimination of HI from 2-methyl substituted quinolium, benzothiazolium, and benzoselenazolimn iodide also react with squaric acid to yield 1,3-disubstituted squaraines [16], as shown in Scheme 2 for the synthesis of 5. 

In a recent study, Ashwell et al. have observed that centrosymmetric squaraine dyes incorporated into LB monolayers showed second harmonic generation efficiencies, which compare favourably with the highest values hitherto reported for LB monolayers of noncentrosymmetric dyes [126]. Based on comparison of the absorption spectra and SHG characteristics of the LB films of squaraine 45 (Structure 15) as well as the effect of deposition pressure on these properties, it has been suggested that formation of noncentrosymmetric aggregates is responsible for these effects. Nonlinear optical studies have also shown that symmetric squaraines have quite large molecular second hyperpolarizabilities [127-131]. 

Table 1 Absorption Spectral Data of Symmetrical Squaraines in CHjClj...

Fluorescence Emission and Fluorescence Lifetime Data of Symmetrical Squaraines... 

The fluorescence lifetime data of symmetrical squaraines are also summarized in Table 3. For each squaraine, monoexponential decays with similar lifetimes... 

V. EFFECTS OF SOLVENT AND TEMPERATURE ON THE ABSORPTION AND FLUORESCENCE EMISSION OF SYMMETRICAL SQUARAINES... 

Similar to symmetrical squaraines, pseudo-unsymmetrical squaraines also exhibit multiple fluorescence emission. Representative emission spectra are given in Figs. 18 and 19. Spectral assignments can be made by comparing the Stokes shifts and the spectral characteristics with those of symmetrical squaraines. The fluorescence emission data, together with the measured lifetimes are included in Table 7. 

Alex, S., Santhosh, U and Das, S. (2005) Dye sensitization of nanocrystalline Ti02 enhanced efficiency of unsymmetrical versus symmetrical squaraine dyes. 

In an attempt to improve the synthetic yield of high-sensitivity squaraines, e.g., Sq-1 and Sq-8, mixtures of squaraines were synthesized by reacting either squaric acid or dialkyl squarate with a mixture of N,N-dimethylanilines [171-173]. The resulting squaraine mixtures show very similar solution and solid state properties as comparable symmetrical squaraines. More importantly, they also exhibit acceptable xerographic sensitivity. Structural analyses indicated that each of these mixtures consists of two symmetrical squaraines and one unsymmetrical squaraine. The observation of favorable photoconductive properties from these compositions was initially a surprise because it is generally... 

In xerographic applications, the major technology shortfalls for symmetrical squaraines have been the low synthetic yield, high dark-decay and poor spectral response in the visible region, e.g., < 600 nm. Law and Bailey reported efforts to address these issues via unsymmetrical squaraines, such as USq-5 to -15 in Table 10.5 [178]. The precursors for the synthesis, III-V (structures in Scheme 10.3), were synthesized by a [2 + 2] cycloaddition reaction between p-methoxy-phenylketene, or 3,4-dimethoxyphenylketene or 3,4,5-trimethoxyphenylketene with tetraethoxyethylene, followed by a hydrolysis reaction. Unsymmetrical squaraines can then be prepared by condensing III or IV or V with an aniline derivative. The synthetic and absorption spectral data for the synthesized unsymmetrical squaraines are tabulated in Table 10.5. Studies of... 

C. A. T. Laia and S. M. B. Costa, Phys. Chem. Chem. Phys., 1, 4409 (1999). Probing the Interface Polarity of AOT Reversed Micelles Using Centro-Symmetrical Squaraine Molecules. 

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