Unsymmetrical squaraines

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]. 

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... 

Unsymmetrical squaraines (Yanus and Limberg, 1986 Kazmaier et al., 1988a) prepared from different aromatic amines have been compared with the corresponding symmetrical compounds (Kazmaier et al., 1988b). In the same... 

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. 

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]. 

VI. ABSORPTION AND FLUORESCENCE EMISSION OF PSEUDO UNSYMMETRICAL SQUARAINES... 

Many pseudo-unsymmetrical squaraines have been synthesized in the literature for a variety of reasons. Table 7 summarizes a systematic investigation on the effect of structural changes on the absorption, fluorescence emission, and lifetime of these compounds. A bathochromic effect on the absorption is observed for A -alkyl substituted pseudo-unsymmetrical squaraines. The of USql-USq7 are red-shifted relative to Sql. The magnitude of the red-shift is small, however [e.g., the X, of USq7 is 8 nm red-shifted from that of Sql, which is about two times smaller than the red-shift observed from Sq5 to Sql (15 nm)]. We attribute the small red-shift to the smaller number of W-alkyl groups in these unsymmetrical squaraines. The mechanism for the red-shift appears to be identical to that described in Section II.A. 

Similar to symmetrical squaraines, pseudo-unsymmetrical squaraines also exhibit multiple fluorescence emission. Representative emission spectra are given in Figs. 18 and 19. Spectral assignments can be made by comparing the Stokes shifts and the spectral characteristics with those of symmetrical squaraines. The fluorescence emission data, together with the measured lifetimes are included in Table 7. 

Figure 18 Corrected fluorecence emission spectra of pseudo-unsymmetrical squaraines in CHCI3 [cone. -3 x lOr M, (a) USql, (b) USq2, (c) USq7, and (d) USq8],...

Another notable distinction between the absorption of these unsymmetrical squaraines and those of symmetrical and pseudo-unsymmetrical squaraines is the absorption shoulders in the blue edge of the absorption band. As will be shown in Section VII.B, these are vibrational fine structures. In the case of USql3 in CHCI3, these bands are at 580.1 (0,0), 546.3 (0,1), and 517.3 (0,2) (Fig. 20). The fine structures are found to be particularly pronounced when the aniline ring is substituted with a 2-OH group, such as in USql7 (Fig. 20). 

Table 8 Absorption, Fluorescence Emission, and Fluorescence Lifetime Data for Unsymmetrical Squaraines... 

Similar spectral analyses have also been carried out for several more unsymmetrical squaraines. The data consistently suggested that vibronic fine structures are the main contributors to the multiple emissions of these compounds [54]. 

All hydroxy-substituted unsymmetrical squaraines exhibit biexponential decays, one at 2.4 ns and the other at < 1 ns. The fact that all other unsymmetrical squaraines show a monoexponential decay with lifetimes of -2.4 ns suggests that the biexponential may be a conformational effect. For instance, hydroxy-substituted unsymmetrical squaraines may exist in two conformers a planar conformer with a lifetime similar to other unsymmetrical squaraines ( — 2.4 ns), and a nonplanar conformer that is stabilized by the intramolecular H-bonding between the C2 OH group in the aniline ring and the C-0 group... 

Figures 27 and 28 show the fluorescence emission of a number of unsymmetrical squaraines. Multiple emissions similar to that of USql3 are observed, and the spectral data are tabulated in Table 8. Although the low values for USql3-USq26 have been rationalized, within a series of unsymmetrical structures, a systematic variation in < )yr is discernible. The results indicate that substituents, both in the aniline ring and the anisole ring, influence the photophysical behavior of these compounds.

In an attempt to improve the synthetic yield of high-sensitivity squaraines, e.g., Sq-1 and Sq-8, mixtures of squaraines were synthesized by reacting either squaric acid or dialkyl squarate with a mixture of N,N-dimethylanilines [171-173]. The resulting squaraine mixtures show very similar solution and solid state properties as comparable symmetrical squaraines. More importantly, they also exhibit acceptable xerographic sensitivity. Structural analyses indicated that each of these mixtures consists of two symmetrical squaraines and one unsymmetrical squaraine. The observation of favorable photoconductive properties from these compositions was initially a surprise because it is generally... 

To date, two classes of unsymmetrical squaraines have been synthesized. For simplicity, we designate the unsymmetrical squaraines with two different aniline rings as pseudo unsymmetrical squaraines and those with one aniline ring and one anisole ring as true unsymmetrical squaraines. Pseudo unsymmetrical squaraines were synthesized by condensing a -(/ -A,A-dialkylaminophenyl)-2-hydroxycyclobutene-3,4-dione derivative witii an N,A -dialkylaniline derivative (Scheme 10.3) [174], Kazmaier et al. [175] reported that the key precursor for the synthesis, l-(p-dimethy-laminopheny l)-2-hy droxycy clobutene-3,4-dione (II),... 

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