Amino-squaraines

Amino-squaraines 39 (Z1 = O, Z2 = R1R2) are synthesized by etherification of the oxo-squaraine oxygen using dimethylsulfate or methyl triflate followed by substitution of the methoxy group with amines [52] (Fig. 10). 

Aniline-based amino-squaraines 40 can be obtained via reduction of oxo-squar-aines with NaBFLj in methanol followed by substitution and oxidation with chlor-anil or lead dioxide [106] (Fig. 11). 

Squaraine dyes 10b, 39a, 39b, 41a, 41c, 41d, and 41e were used to measure different proteins such as BSA, HSA, ovalbumin, avidin from hen egg white, lysozyme, and trypsin (Fig. 12) [58]. It is difficult to predict correlations between the dyes structures and the affinity or sensitivity of the dyes for different proteins. All squaraine probes exhibit considerable fluorescence increases in the presence of BSA. Dicyanomethylene-squaraine 41c is the brightest fluorescent probe and demonstrates the most pronounced intensity increase (up to 190 times) in presence of BSA. At the same time, the fluorescent response of the dyes 10b, 39a, 39b, 41a, 41c, 41d, and 41e in presence of other albumins (HSA and ovalbumin) is, in general, significantly lower (intensity increases up to 24 times). Dicyanomethylene-squaraine 41a and amino-squaraines 39a and 39b are the most sensitive probes for ovalbumin. Dyes 41d, 10b, and 41e containing an A-carboxyalky I -group demonstrate sufficient enhancement (up to 16 times) in the presence of avidin. Nevertheless, the presence of hydrolases like lysozyme or trypsin has only minor effects on the fluorescence intensity of squaraine dyes. 

Compared to oxo-squaraines or other ring-substituted squaraines, amino-squaraines 39 [45, 52, 112] have ionic character, similar to open-chain cyanine dyes, and due to the positive net charge, these dyes are to some extend water-soluble. Amino-squaraines absorb and emit at longer wavelength than the corresponding oxo-squaraines the absorption maxima are between 650-710 nm (eM = 85,000-300,000 M-1cm-1) [45, 112], The increase in solvent polarity is accompanied by a hypsochromic shift of the absorption. Amino-squaraine dyes are potentially used as fluorescent probes but because their photostability is inferior to those of oxo-squaraines and other ring substituted squaraines of similar structure, their applications are rather limited. 

V. Kamat, S. Das, K. G. Thomas, and M. V. George, Photochemistry of squaraine dyes. 1. Excited singlet, triplet, and redox states of bis[4-(dimethylamino)phenyl]squaraine and bis[4-(dimethyl-amino)-2-hydroxyphenyl]squaraine./. Phys. Chem. 96, 195-199(1992). 

Squaraines are a class of 1,3-derivatives of squaric acid in which the substituents are amino or highly electron-releasing aromatic or heterocyclic groups. The squaraine ligand 106 (Ar = C6H3(i-Pr)2-2,6) yields with AlMe3 in thf a dimeric tetranuclear complex (107) . 

The dynamics of charge injection from the excited state of squaraines to the conduction band of semiconductors have been investigated by picosecond laser spectroscopy [45,67,101]. The sharp absorption band of bis[4-dimethyl-amino-2-hydroxy)phenyl]squaraine 21 (Strucmre 6) with at 636 nm became broad and red-shifted 670 nm) in the presence of colloidal Ti02 (Fig. 17). The red shift of 35 nm in the absorption band has been attributed to a strong... 

Dialkylaminothiophenes react at position 5 with squaric acid and croconic acid to form the squaraines (94) and croconines (95) <92CRV1227, 93LA935>. Other enaminic reactions of amino-thiophenes are discussed in Section 2.10.4.3.2. 

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