Squaraines

Squaraines can be readily synthesized via the reaction of squaric acid (Treibs and Jacob, 1965) or its diester (Law and Bailey, 1986) with aromatic amines. For photoreceptor applications, Law and Bailey (1987) reported that the best results were obtained with squaraines prepared via the diester. This has been ascribed to the formation of a different crystal modification with lower impurity concentrations. Other purification methods have been reported by Lin and 

Dudek (1986). The synthesis of squaraines as generation materials has received considerable recent emphasis (Law and Bailey, 1988,1991,1992,1993). 

The effects of the generation-layer fabrication variables on the sensitometry of a dual-layer photoreceptor prepared with bis(4-dimethylaminophenyl) squaraine (X = H in Appendix 2) have been extensively investigated by Law (1987). The charge acceptance, dark discharge, sensitivity, and the residual 

Unsymmetrical squaraines (Yanus and Limberg, 1986 Kazmaier et al., 1988a) prepared from different aromatic amines have been compared with the corresponding symmetrical compounds (Kazmaier et al., 1988b). In the same 

A squaraine prepared from N-chlorobenzyl-N-methylaniline and squaric acid has found utility as a generation layer in a dual-layer photoreceptor with an inverted structure for positive charging applications (Yamamoto et al 1986). The photoreceptor showed full-process stability. 

Some complexes with croconate violet 17 display intramolecular interactions in which the metal transmits electronic properties for electrocatalytic or electrochromic applications [44b]. Such intramolecular interactions can be understood as the most important driving forces existing in the solid state for the coordination compound, where the electronic properties are localized over the oxocarbon moieties. Croconate violet 17, croconate blue 18, and lithium croconate are highly soluble in water and also in many nonaqueous solvents, thus restricting their practical applications [59]. However, this solubility can be circumvented in the case of croconate violet, for example, by incorporating it in a protonated film of poly(4-vinylpyridine), which, after adequate treatment, exhibits interesting electrocatalytic properties [60]. 

In addition to the previously mentioned pseudo-oxocarbons, derivatives of the squarate ion 2 (or l,2-dihydroxycyclebutene-3,4 dione 6 in the acid form) have been prepared with carbon chains, nitrogen, sulfur, and selenium as substituent species [61]. The study of thio-derivatives of pseudo-oxocarbons demonstrates the interest to understand the characteristics and chemical behavior of these derivatives, which could be useful for the preparation of reduced dimension materials with metallic or semiconductive properties [4]. 

Squaric acid 6 possesses a unique structure that may enable new physicochemical properties as well as enhance other preexisting characteristics, such as its highly delocalized electron density. 

Many authors agree that products with one or more nitrogen groups as substitutes of the oxygen atoms in the squarate 2 ion should be called rather squaraines. The first of this class of compounds was prepared in 1965 through the reaction of squaric acid 6 and pyrroles, which produced intensely colored condensation products [62]. But the literature also describes the synthesis of a squaraine obtained from resorcinol, which does not possess nitrogen atoms therefore, there is no consensus on the concept of squaraines in the literature [63]. 

Semi-empirical calculations of the molecular orbitals reveal that both the ground and the excited states of squaraines exhibit donor-acceptor-donor (D-A-D) intramolecular charge transfer [67]. This class of compounds possesses a resonance-stabilized zwitterionic structure. Typical squaraines have a four-membered central electron-deficient ring and two-electron donor groups. As monomers in solution, these compounds strongly absorb at wavelengths above 600 nm with high molar absorptions (e 10 1 mol cm ) and intense fluorescent emission with a small Stokes shift moreover, squaraines are photostable [66, 67]. 

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Squalene epoxidation Squaraine Square 50 Square 80 Square permalloy Square Permalloy 80 Squaric acid [2892-51-5]... 

Long-Wavelength Probes and Labels Based on Cyanines and Squaraines... 

Ring-Substituted Squaraines Probes and Labels with... 

Fig. 1 Absorption and emission spectra of Cy3, CyS, Cy7, oxo-squaraine 13b (part 3 of this chapter) and dicyanomethylene squaraine 41j (part 4 of this chapter) in water (pH 7.4)...

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... 

In general, symmetrical oxo-squaraines having the same end-groups are synthesized by reacting squaric acid with two equivalents of quatemized indolenine, 2-methyl-substituted benzothiazole, benzoselenazole, pyridine, quinoline [39, 45, 46] (Fig. 4) in a mixture of 1-butanol - toluene or 1-butanol - benzene with azeotropic removal of water in presence [39, 45] or absence [47] of quinoline as a catalyst. Other reported solvent systems include 1-butanol - pyridine [48], 1-propanol - chlorobenzene, or a mixture of acetic acid with pyridine and acetic anhydride [49]. Low CH-acidic, heterocyclic compounds such as quatemized aryl-azoles and benzoxazole do not react, and the corresponding oxo-squaraines cannot be obtained using this method [23, 50]. 

The key intermediates for the synthesis of unsymmetrical, heterocyclic oxo-squaraines are the mono-squaraines (semi-squaraines) shown in Fig. 5. These intermediates can be synthesized via condensation of dialkylsquarate with an equimolar amount of methylene base [51]. The obtained alkoxy-mono-squaraines are then reacted with the second methylene base to yield unsymmetrical oxo-squaraines. These mono-squaraine intermediates display a higher reactivity compared to squaric acid or its esters they allow the synthesis of the corresponding... 

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]. 

Mixed squaraines can be synthesized via heterocyclic or aromatic mono-squaraines [49]. Symmetrical and unsymmetrical oxo-squaraines and mono-squar-aine intermediates can be obtained also by formation of squaraine ring [42]. 

The synthesis of oxo-squaraines and related compounds, including their spectral properties and applications as biomedical probes, photoconducting materials, and photosensitizers are provided in a recent review [56]. 

The synthesis, spectral properties, and applications of symmetrical as well as unsymmetrical, hydrophobic oxo-squaraine probes for noncovalent interaction with proteins, lipids, cells, and other high-molecular-weight analytes are described in numerous publications and patents [52, 57, 58]. 

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 sulfo-gmup containing squaraine-taurine probe 8 [62] displayed a very high affinity for BSA and other blood proteins. This probe exhibits low quantum yields and short fluorescence lifetimes in water and a significant increase of these characteristics upon binding to proteins. 

The noncovalent binding of a series of oxo-squaraine dyes 9a-e to BSA was evaluated by measurement of absorption, emission, and circular dichroism [63]. The magnitude of the association constants (Ks) for the dye-BSA complexes depended on the nature of the side chains and ranged from 34 x 103 to 1 x 107 M-1. Depending on the side chains, the Ks increase in the order [R1 = R2 = butyl-phthalimide] < R1 = R2 = cetyl] <[RJ = R2 = ethyl] <<[R = butyl-phthalimide, R2 = butyl-sulfonate] <<[RJ = R2 = butyl-sulfonate]. These dyes seem to interact mainly with a hydrophobic cavity on BSA. However, the association constants Ks increase substantially when the side chains are selected from butyl sulfonate. 

Hydrophobic (10a) and hydrophilic (10b) squaraines show a noticeable increase in fluorescence intensity in presence of HSA and importantly dye 10b, containing a sulfo group, exhibits a large intensity increase when bound to avidin, a protein well-known to quench many fluorescent dyes [58]. 

The squaraine probe 9g was tested for its sensitivity to trace the formation of protein-lipid complexes [57]. The binding of dye 9g to model membranes composed of zwitter-ionic lipid phosphatidylcholine (PC) and its mixtures with anionic lipid cardiolipin (CL) in different molar ratios was found to be controlled mainly by hydrophobic interactions. Lysozyme (Lz) and ribonuclease A (RNase) influenced the association of 9g with lipid vesicles. The magnitude of this effect was much higher... 

Symmetrical and unsymmetrical quinaldine-based squaraines 14 linked to cellular recognition elements that exhibit near-infrared absorption (>740 nm) could have potential biological and photodynamic therapeutical applications [68]. 

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