Squaraines synthesis

Chandrasekaran Y, Dutta G, Kanth RB, Patil S (2009) Tetrahydroquinoxaline based squaraines Synthesis and photophysical properties. Dyes Pigm 83 162-167... 

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...

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]. 

Reddington MV (2007) Synthesis and properties of phosphonic acid containing cyanine and squaraine dyes for use as fluorescent labels. Bioconjugate Chem 18 2178-2190... 

Law KY, Bailey FC (1992) Squaraine chemistry. Synthesis, characterization, and optical properties of a class of novel unsymmetrical squaraines [4-(dymethylamino)phenyl](4 -methoxyphenyl)squaraine and its derivatives. J Org Chem 57 3278-3286... 

Terpetschnig E, Szmacinski H, Lakowicz JR (1993) Synthesis, spectral properties and photostabilities of symmetrical and unsymmetrical squaraines a new class of lluorophores with long-wavelength excitation and emission. Anal Chim Acta 282 633-641... 

Tatarets AL, Fedyunyaeva IA, Terpetschnig E, Patsenker LD (2005) Synthesis of novel squaraine dyes and their intermediates. Dyes Pigm 64 125-134... 

Terpetschnig E, Szmacinski H, Ozinskas A, Lakowicz JR (1994) Synthesis of squaraine-iV-hydroxysuccinimide esters and their biological application as long-wavelength fluorescent labels. Anal Biochem 217 197-204... 

Jyothish K, Avirah RR, Ramaiah D (2006) Synthesis of new cholesterol- and sugar-anchored squaraine dyes further evidence of how electronic factors influence dye formation. Org Lett 8 111-114... 

Arun KT, Ramaiah D (2005) Near-infrared fluorescent probes synthesis and spectroscopic investigations of a few amphiphilic squaraine dyes. J Phys Chem A 109 5571-5578... 

Basheer MC, Santhosh U, Alex S, Thomas KG, Suresh CH, Das S (2007) Design and synthesis of squaraine based near infrared fluorescent probes. Tetrahedron 63 1617-1623... 

Oswald B, Patsenker L, Duschl J, Szmacinski H, Wolfbeis OS, Terpetschnig E (1999) Synthesis, spectral properties, and detection limits of reactive squaraine dyes, a new class of diode laser compatible fluorescent protein labels. Bioconjugate Chem 10 925-931... 

Thomas J, Sherman DB, Amiss TJ, Andaluz SA, Pitner JB (2007) Synthesis and biosensor performance of a near-IR thiol-reactive fluorophore based on benzothiazolium squaraine. Bioconjugate Chem 18 1841-1846... 

This chapter describes the synthesis, properties, and biomedical applications of cyanine and squaraine dyes encapsulated in CDs, CBs, Leigh-type tetralactam macrocycles, aptamers, and micro- or nano-particles. The optical and photochemical properties of supramolecular guest-host nanostructures that are based on intra-and intermolecular complexes of crown-containing styryl dyes with metal cations, and aggregates of carbocyanine dyes are discussed in a separate review [18]. 

Encapsulation of squaraine 23a in diastereomeric triptycene-based tetralactam macrocycles 24a and 24b was described in [61], The synthesis of the macrocyclic hosts was done by the reaction of pyridine-2,6-dicarbonyl dichloride and 2,7-diaminotriptycene in dry THF with Et3N. Macrocycles 24a and 24b readily form... 

An overview of the synthesis, structure, photophysical properties, and applications of squaraine rotaxanes as fluorescent imaging probes and chemosensors is provided in a recent review [67]. Although a variety of squaraine dyes form rotaxanes with the molecular cage 25 or with a tetralactam macrocyclic system introduced by Leigh and co-workers [16, 17], there is no evidence in the literature that conventional cyanine dyes can be embedded in these macrocycles. 

Gassensmith JJ, Barr L, Baumes JM, Paek A, Nguyen A, Smith BD (2008) Synthesis and photophysical investigation of squaraine rotaxanes by clicked capping . Org Lett 10 3343-3346... 

Xue M, Chen CF (2008) Triptycene-based tetralactam macrocycles synthesis, structure and complexation with squaraine. Chem Commun 46 6128-6130... 

Scheme 5.14 Modular one-pot, three-component synthesis of poly(squaraines) from terminal a,co-diamines.

The azulenyl furanone 60 was a key intermediate in the synthesis of the monofunctional azulenyl squaraine dye NIRQ700 61 (NIRQ = near-infrared quencher) <2003TL3975>. The resulting nonfluorescent squaraine dye absorbed in the 600-700 nm range and can potentially be used to quench a number of available NIR fluorochromes in order to extend the spectrum of biological quenching assays. 

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

Squaric acid undergoes condensation reactions with a variety of nucleophiles to form 1,3-disubstituted products possessing intense absorption in the visible and near-infrared region [7-11]. Schmidt [12] has proposed the widely accepted name squaraine for this class of dyes which was first reported by Treibs and Jacobs [7]. A major part of the earlier work on the synthesis of squaraines was carried out by the former group [7,13] and by Sprenger and Ziegenbein [14-16]. Later, Law and coworkers have contributed extensively to the synthesis of unsymmetric squaraines [17-20]. More recently, Nakazumi et al. [21] have reported a new class of cationic squaraines. 

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. 

Of the several strategies reported in the literature for the synthesis of unsymmetrical squaraines, [17-20,27-34] two methods are discussed here. The first method, which was developed by Bellus [33] and improved upon by Law [17-20], involves the use of a cycloaddition reaction of an appropriate ketene with tetraethoxyethylene to yield 8 and 9 (Strucmre 2). 

The reaction sequence for the synthesis of l-(/7-methoxyphenyl)-2-hydroxycyclobutene-3,4-dione, 8 [17,19,20] is illustrated in Scheme 3. The second strategy, involving the steps outlined in Scheme 4, was developed by West and coworkers [27] and by Law and Bailey [20]. The monoadducts 8 and 9 can react with various nucleophiles to yield unsymmetrical squaraines [11,17-20,35-39]. 

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