Pseudo-unsymmetrical squaraines

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). 

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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