Pseudo-unsymmetrical squaraines fluorescence

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 19 Corrected fluorescence emission spectra of pseudo-unsymmetrical squaraines in CHCI3 [cone. -3 X 1()- M (a) USq9, (b) USqlO, (c) USqll, and (d) USql2].

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