Squaraine dye

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]. 

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. 

It has been recently demonstrated that squaraine dye 3, first synthesized by Treibs and Jacob [40], acts as a dual mode recognition system for serum albumin... 

Dialkylanthracene-containing squaraine dyes 17 show intense absorption and emission in the NIR region (720-810 nm) [74]. They are compatible with aqueous environments and show substantial enhancement of quantum yields and fluorescence lifetimes in hydrophobic and micellar media, suggesting that these dyes can be potentially useful as fluorescent probes in biological applications, e.g., for imaging of hydrophobic domains such as cell membranes. 

A Ca2+-ion selective rigid -flexible - rigid type bichromophoric sensors based on the conformation liable bis-squaraine dyes 27 works on the principle of Cation-steered folding, which leads to dramatic perturbations in the optical properties as a result of exciton interactions [87],... 

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. 

Table 2 Spectral properties of squaraine dyes in absence and presence of 6 mg/mL BS A and when covalently bound to BSA (phosphate buffer, pH 7.4) [18]...

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. 

The dicyanomethylene-squaraine dye 41e was found to be highly sensitive to trace protein-lipid interactions [109]. Lysozyme association with the lipid bilayer leads to a noticeable decrease in the fluorescence intensity of 41e. In a separate... 

The photostability of ring-substituted squaraines, especially dicyanomethylene-and f/no-squaraines is, in general, higher compared to oxo-squaraines and cyanines (Fig. 2). Increasing of the number of sulfo groups further improves the photostability of these dyes the photostability for hydrophilic dyes increases in the order 10b < 13b < Cy5 < 41h < 41i 41j < 41f < 41g [18]. Thio-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. 

The commercially available dicyanomethylene squaraine dye Seta-670-mono-NHS showed extremely low blinking effects and good photostability when used in single-molecule studies of multiple-fluorophore labeled antibodies [113]. Seta-670-mono-NHS and Seta-635-NH-mono-NHS were covalently labeled to antibodies and used in a surface-enhanced immunoassay [114]. From the fluorescence intensity and lifetime changes determined for a surface that had been coated with silver nanoparticles, both labeled compounds exhibited a 15- to 20-fold... 

While open-chain cyanines such as Cy5, Cy5.5, or Alexa 647 typically exhibit small fluorescence lifetime changes upon binding to biomolecules, squaraine dyes, in general, demonstrate pronounced lifetime changes upon binding to large-molecular-weight analytes [17, 18], which can be used for FLT-based assays. 

Cyanine and squaraine dyes with hydrogen substituents on the indolenine-nitrogen in one or both of the heterocyclic end-groups, the so-called norcyanines and norsquaraines, are useful as fluorescent pH-indicators due to the reversible equilibrium between their protonated and deprotonated forms ... 

Due to their low sensitivity toward the environment, cyanine dyes are perfect candidates as fluorescent labels. Squaraine dyes on the other hand display a highly environment-sensitive response and are therefore not only useful as fluorescent probes and labels but also, in particular, well-suited for lifetime-based applications. 

Tatarets AL, Fedyunyayeva IA, Dyubko TS, Povrozin YA, Doroshenko AO, Terpetschnig EA, Patsenker LD (2006) Ring-substituted squaraine dyes as probes and labels for fluorescence assays. Anal Chim Acta 570 214-223... 

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

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

Chen H, Farahat MS, Law KY, Whitten DG (1996) Aggregation of surfactant squaraine dyes in aqueous solution and microheterogeneous media correlation of aggregation behavior with molecular structure. J Am Chem Soc 118 2584-2594... 

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... 

Jisha VS, Arun KT, Hariharan M, Ramaiah D (2006) Site-selective binding and dual mode recognition of serum albumin by a squaraine dye. J Am Chem Soc 128 6024—6025... 

Binda M, Agostinelli T, Caironi M, Natali D, Sampietro M, Beverina L, Ruffo R, Silvestri F (2009) Fast and air stable near-infrared organic detector based on squaraine dyes. Org Electron 10 1314-1319... 

Ertekin K, Tepe M, Yenigii B, Akkaya EU, Henden E (2002) Fiber optic sodium and potassium sensing by using a newly synthesized squaraine dye in PVC matrix. Talanta 58 719-727... 

Chithra P, Varghese R, Divya KP, Ajayaghosh A (2008) Solvent-induced aggregation and cation-controlled self-assembly of tripodal squaraine dyes optical, chiroptical and morphological properties. Chem Asian J 3 1365-1373... 

Sreejith S, Divya KP, Ajayaghosh A (2008) A near-infrared squaraine dye as a latent ratiometric fluorophore for the detection of aminothiol content in blood plasma. Angew Chem 120 8001-8005... 

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... 

Pham W, Weissleder R, Tung CH (2003) A practical approach for the preparation of monofunctional azulenyl squaraine dye. Tetrahedron Lett 44 3975-3978... 

Griffiths J, Park S (2002) Facile preparative redox chemistry of bis(4-dialkylaminophenyl) squaraine dyes. Tetrahedron Lett 43 7669-7671... 

Nizomov N, Ismailov ZF, Nizamov SN, Salakhitdinova MK, Tatarets AL, Patsenker LD, Khodjayev G (2006) Spectral-luminescent study of interaction of squaraine dyes with biological substances. J Mol Struct 788 36-42... 

Matveeva EG, Terpetschnig EA, Stevens M, Patsenker L, Kolosova OS, Gryczynski Z, Gryczynski I (2009) Near-infrared squaraine dyes for fluorescence enhanced surface assay. Dyes Pigm 80 41 16... 

The first set of dyes, so called visible set , is presented by polymethine dye PD 2630, squaraine dye SD 2243, and tetraone dye TD 2765, all with benzo[e]indolium terminal groups. The second set of dyes, so called NIR set , is presented by polymethine dye PD 2658, squaraine dye SD 2878, and tetraone dye TD 2824, all with 5-butyl-7,8-dihydrobenzo rJ furo[2,3-/]indolium terminal groups. A distinguishing feature seen from this figure is a remarkably large, 300 nm, red shift of the absorption bands for PD 2658 and SD 2878 as compared to PD 2630 and SD 2243. The absorption spectrum of TD 2824 is red-shifted by 200 nm as compared to TD 2765. Thus, the effect of the 5-butyl-7,8-dihydrobenzo[coT]furo[2, 3-/]indo-lium terminal groups is equivalent to the extension of the chain to three vinylene groups. 

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