Squaraines aniline-based

Symmetrical pyrrole- and phloroglucinol-based oxo-squaraines 2 and 3 were first synthesized by Treibs and Jacob in 1965 [43, 44] and aniline-based squaraines 4... 

Fig. 6 Synthesis of symmetrical and unsymmetrical aniline-based oxo-squaraines...

Symmetrical, aniline-based, and aromatic oxo-squaraines are synthesized via a one-step reaction by heating two equivalents of the appropriate /V,/V-dialkylaniline or other reactive aromatic or heteroaromatic derivatives with squaric acid (Fig. 6) [38, 41]. Unsymmetrical aniline-type squaraines can be synthesized in two steps first one component is reacted with squaric acid dichloride to yield a mono-squaraine intermediate, which in a subsequent step is then reacted with the second component to yield the unsymmetrical squaraine dye [53]. 

A series of unsymmetrical oxo-squaraines 7, containing both heterocyclic and aniline-based end-groups, was synthesized and their absorption spectra were investigated [61]. Some of these dyes, absorbing between 680-820 nm with high molar... 

The aniline-based squaraines 4 with hydroxyalkyl, carboxypropyl, and ethylene glycol groups exhibit absorption maxima between 640-649 nm (eM = 100,000-300,000 M 1cm x) and emission maxima between 663 and 675 nm in aqueous solutions... 

The quantum yields are 0.15-0.21 in ethanol and 0.01-0.02 in an aqueous medium, but in micelles, the quantum yields are five to tenfold increased. The aggregation of these dyes was studied in [53]. The amphiphilic squaraines 4 combine favorable photophysical properties and good solubility in aqueous media and in addition interact efficiently with micelles, and therefore have the potential to be used as NIR fluorescent sensors. However, our own investigations show that aniline-based squaraines lack chemical and photochemical stability when compared to oxo-squaraines with heterocyclic end-groups. 

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). 

The anlhracene-conVd n ng tetralactam macrocycles 16a and 16b were found to have an extremely high affinity for aniline-based squaraine dyes in chloroform [56]. 

A variety of hydrophobic and hydrophilic squaraine rotaxane probes and labels such as 21a-21e c Rp and 22a-22e c Rp, containing reactive carboxylic functionalities and hydrophilic sulfo groups, are disclosed in a recent patent application [60]. It was shown that not only aniline-based squaraines 21a-21e but also heterocyclic squaraines 22a-22e can form stable pseudorotaxane complexes. The indo-lenine-based squaraine 22a forms rotaxane 22a C Rp. Importantly, also the sulfonated squaraine 22b could be successfully encapsulated in a Leigh-type, phenylene-based, tetralactam macrocycle to yield the water-soluble rotaxane 22b C Rp. Quatemized, indolenme-based squaraines do not form pseudorotaxanes probably because of sterical hindrance caused by /V-alkyl and 3,3 -dimethyl groups. On the other hand, quatemized benzothiazole (22c) and benzoselenazole (22d) squaraines could be embedded in a Leigh-type macrocycle to yield rotaxanes 22c C Rp and 22d C Rp, respectively. The hydrophilic, mono-reactive rotaxane 22e-NHS C Rp based on asymmetric squaraine, synthesized by a cross-reaction of squaric acid with the two different indolenines, was also obtained. 

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