Squarylium, structure

Nakazumi H, Natsukawa K, Nakai K, Isagawa K (1994) Synthesis and structure of new cationic squarylium dyes. Angew Chem Int Ed Engl 33 1001-1003... 

The squarylium (4.6) and croconium (4.7) dyes are closely related structurally to cyanines but are in fact donor-acceptor molecules and consequently the design principles of near-IR absorbers based on these chromophores are different. The synthesis of these chromophores is achieved easily by reacting either squaric acid, or preferably an alkylated derivative, e.g. di-n-butyl squarate, or croconic acid with electron-donor molecnles. The croconium dyes absorb at significantly longer wavelengths than the sqnarylinms as shown in Figure 4.2. 

Wide variation of the donor gronps on the squarylium dyes of structure type (4.6) only gave a maximnm wavelength of absorption of 708 mn, and it was necessary to nse the strongly electron-donating dihydroperimidine ring system to achieve absorption fully into the near-IR, as exemplified by (4.8), 800-810 nm, 150 000. °... 

Somewhat related to the (cationic) cyanines are the squarylium dyes which are overall charge neutral species derived from squaric acid. They are easily prepared, have high molar absorptivities (>100,000), but typically are unstable to hydrolysis in dipolar aprotic solvents. They are characterized by a sharp strong absorption which lies at wavelengths longer than 640 nm, with no other identifiable electronic transitions in the visible (Figure 2). The vibronic structure of these dyes may show only one shoulder corresponding to a reasonable value for a C-C or C-0 stretch. We were able to systematically vary the structure of a series of squarylium dyes in order to test assumptions about the structure-property relationship for minimization of the residual absorption. 

Figure 2. Structure and spectral properties of squarylium dyes synthesized in this work.

In this paper, I propose a promising new electron acceptor of cyclobutenedione for nonlinear optical materials to prevent centrosymmetric crystal structures by the introduction of chirality and hydrogen bonding property into the acceptor itself. Compared with electron donative groups, electron acceptor is not yet well studied for nonlinear optical materials. The most commonly used electron acceptor is nitro (NO2) group. Therefore, we evaluated the possibility of cyclobutenedione as a new electron acceptor for nonlinear optical materials. One of the most simple cyclobutenediones is squaric acid. Squaric acid is known to be soluble in water and show very strong acidityQ2), as squarylium anion formed in water has a stable 2n delocalized electron system as shown below. 

Squaraines are derivatives of 3,4-dihydroxycyclobut-3-ene-l,2-dione, or squaric acid, so called because the central structural motif consists of four carbon atoms linked to form a square [8], Numerous squarylium dyes, such as those in Fig. 6.8,... 

Squaraines are 1,3-disubstituted products synthesized by condensing 1 equivalent of squaric acid with 2 equivalents of N,A/-dialkylanilines [132]. Because of the unique electronic structure, they have been named as cyclobutenediylium dyes [132], cyclotrimethine dyes [133], 3-oxo-l-cyclobutenolates [134], cyclobutene-diylic dyes [135], squarylium dyes [136], etc. in the past. In 1980, Schmidt proposed the name squaraine for this class of compound [137]. We find the Schmidt nomenclature system very systematic and compounds with a variety of substitutions can be named unambiguously. Squaraines were initially used as sensitizers in single layer ZnO photoreceptors [138]. Subsequent... 

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