Squaraines molecular structures

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... 

Table 9 2PA properties of squaraines and cyanines. The solvent in which the measurement was performed is indicated in parenthesis after the molecular structure... 

The absorption, emission, and redox properties of squaraines make them highly suited for applications as photosensitizers. In view of this, the early studies on squaraines were focused on thin photovoltaic and semiconductor photosensitization properties [1,4,5,91-97], Champ and Shattuck [98] first demonstrated that squaraines could photogenerate electron-hole (e-h) pairs in bilayer xerographic devices. Subsequently, extensive work has been carried out on the xerographic properties of squaraines [2,24,34,47,48,99,100], and these properties have been reviewed recently [11]. In an extensive smdy on the correlation s between cell performance and molecular structure in organic photovoltaic cells, squaraines were found to have much better solar energy conversion efficiencies than a variety of other merocyanine dyes [4,5]. 

In a recent study, Ashwell et al. have observed that centrosymmetric squaraine dyes incorporated into LB monolayers showed second harmonic generation efficiencies, which compare favourably with the highest values hitherto reported for LB monolayers of noncentrosymmetric dyes [126]. Based on comparison of the absorption spectra and SHG characteristics of the LB films of squaraine 45 (Structure 15) as well as the effect of deposition pressure on these properties, it has been suggested that formation of noncentrosymmetric aggregates is responsible for these effects. Nonlinear optical studies have also shown that symmetric squaraines have quite large molecular second hyperpolarizabilities [127-131]. 

Modified anilino squaraines have been widely utilized in surface-enhanced Raman resonance scattering (SERBS) spectroscopy. Their molecular structure displays a quadrupole D-A-D system that is characterized by an electron-deficient cyclobutendione (C Oj) bridge (Figure 5.7). The total structure can be represented in many ways, with the charge homogenously distributed over the entire molecule. Several theoretical studies have been conducted on this class of materials and their unusual nonlinear optical properties [4,81]. 

Figure 5.8 Molecular structures of substituted squaraines mono(dicyanomethylene) squarate 25 frans-bis(dicyanomethylene)squarate 26 c/s-bis(dicyanomethylene)squarate 27 tris(dicyanomethylene)squarate 28 and tetraquis(dicyanomethylene)squarate 29.

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. 

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]. 

In addition to the two crystal structures obtained in this study, six other coordination compounds have been synthesized and characterized [73, 84]. A complete spectroscopic study was performed involving squaraines 1,3- and 1,2-disubstituted with dicyanomethylene groups and four different transition metals cobalt, manganese, zinc, and nickel. The Raman and infrared spectra of all these complexes with both ligands reveal that the ligands exhibit an inverted symmetry element, meaning that the bands which are seen with one technique are not seen with the other, and vice versa. The complexation did not affect the molecular symmetry (in the case of the trans dianion) or the solid-state structure (in the case of the cis dianion). 

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