Squaraines aggregation

The aggregation of 29 and 30 was studied by absorption spectroscopy. By comparison with the absorption spectra of model squaraine aggregates, it was concluded that 29 forms sandwich type dimers, which is essentially controlled by C-0 dipole-dipole interactions. For 30, the aggregate band is red-shifted to that of the monomer, and this has been attributed to an aggregated form involving intermolecular charge-transfer interactions between the electron-... 

Figure 16 Schematics for the molecular arrangements of squaraine aggregates in LB films (a) blue-shifted and (b) J-aggregates of 28 (c) blue-shifted and (d) red-shifted aggregates of 29. (Adapted from Ref. 38.)...

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. 

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... 

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... 

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]. 

Hotchandani, S. Das, S. Thomas, K. G. George, M. V. Kamat, P. V. Interaction of semiconductor colloids with J-aggregates of squaraine dye and its role in sensitizing nanocrystalline semiconductor films, Res. Chem. Intermed. 1994, 20, 927. 

Among the materials for laser printer utilization, we may consider squarylium dyes and trisazo compounds (Fig. 5). Squaraines, for instance, give rise to photoreceptors with superior electrical characteristics, as they can form different crystalline modifications throughout the introduction of suitable impurities at low concentration [42]. This characteristic is due to the ability of squaraines to form aggregates having very broad absorption spectra (400 to 1000 nm). 

Molecular complexes, such as the complex formed between poly(N-vinylcaibazole) and 2,4,7-trinitro-9-fluorenone, and dye-polymer aggregates were widely used as generation materials in many early applications. Since these materials are not infrared sensitive, there has been increasing emphasis on pigments. The more widely studied are various azo, phthalocyanine, squaraine, and peiylene diimide derivatives. A common feature of all of these materials is that they are polymorphic and exist in many different crystal forms. The properties are thus very sensitive to the conditions used in their preparation. Further, the sensitivity of these materials is strongly field dependent as well as dependent on the transport material. For a review of generation materials, see Law (1993). 

Wojtyk, J., McKerrow, A., Kazmaier, R and Buncel, E. (1999). Quantitative investigations of the aggregation behavior of hydrophobic anilino squaraine dyes through UV/vis spectroscopy and dynamic light scattering. Can. J. Chem.—Rev. Can. Chem., 77, 903-12. [88, 205]... 

Photoconducting and other solid state properties of materials are dependent both upon the intrinsic molecular properties of the material as well as the intermo-lecular interactions that occur in the solid state. The sharp and intense bands of squaraines for example become broad and red-shifted in the solid state [2]. Studies on aggregates can help in developing a molecular level understanding of their solid state properties. In view of this, the aggregation behavior of... 

Addition of iodine to acetonitrile solutions of 18 led to an increase in aggregate formation, and this has been attributed to the formation of charge-transfer complexes between iodine and 18. Squaraine dyes such as bis[4-(dimethylamino)phenyl]squaraine 3 have been observed to form charge transfer complexes with iodine [84]. 

The aggregation behavior of the squaraine dyes 26 and 27 (Structure 8) in water in the presence and absence of polyvinylpyrrolidone has been studied [66]. In aqueous solutions, these dyes show two absorption bands with maxima around 595 and 645 nm, which have been attributed to those of the aggregated and monomeric forms, respectively. The aggregated forms were identified as dimers from concentration dependent changes in the absorption spectra. The blue shift in absorption of the aggregated forms indicate the formation of H-type aggregates [82,83], imlike in the case of the bis(hydroxyphenyl)squaraine derivatives, where J-type aggregates were obtained [76]. 

UV-Vis spectroscopic studies showed that the squaraine dye 28 (Structure 9) can exist in the form of two aggregates, formed preferentially in different DMSO-water compositions [74]. In pure DMSO and DMSO-water mixtures containing more than 70% DMSO, 28 exists in the monomeric form that has a sharp absorption band with a maximum around 600 nm. In 70% v/v DMSO-water mixtures, 28 exists in a stable aggregated form that has a blue-shifted absorption band (X = 530 nm, e 80,000 M" cm" ). In DMSO-water mixtures containing 20-50% of DMSO, a second type of aggregate form... 

The aggregate properties of surfactant squaraines 29-31 (Structure 10), which were designed to orient the bricklike squaraine chromophore in three different directions, when organized in monolayers and Langmuir-Blodgett films (LB films), have been reported [38]. 

The observed aggregational behavior has been rationalized in terms of an optimization between hydrophobic interactions (between hydrocarbon chains) and charge-transfer interactions (between the squaraine chromophores). In the... 

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