Symmetrical squaraines, absorption

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]. 

Table 1 Absorption Spectral Data of Symmetrical Squaraines in CHjClj...

V. EFFECTS OF SOLVENT AND TEMPERATURE ON THE ABSORPTION AND FLUORESCENCE EMISSION OF SYMMETRICAL SQUARAINES... 

Table 9 Absorption and Fluorescence Emission Spectral Data of Miscellaneous Symmetrical Squaraines... 

In xerographic applications, the major technology shortfalls for symmetrical squaraines have been the low synthetic yield, high dark-decay and poor spectral response in the visible region, e.g., < 600 nm. Law and Bailey reported efforts to address these issues via unsymmetrical squaraines, such as USq-5 to -15 in Table 10.5 [178]. The precursors for the synthesis, III-V (structures in Scheme 10.3), were synthesized by a [2 + 2] cycloaddition reaction between p-methoxy-phenylketene, or 3,4-dimethoxyphenylketene or 3,4,5-trimethoxyphenylketene with tetraethoxyethylene, followed by a hydrolysis reaction. Unsymmetrical squaraines can then be prepared by condensing III or IV or V with an aniline derivative. The synthetic and absorption spectral data for the synthesized unsymmetrical squaraines are tabulated in Table 10.5. Studies of... 

Symmetrical and unsymmetrical quinaldine-based squaraines 14 linked to cellular recognition elements that exhibit near-infrared absorption (>740 nm) could have potential biological and photodynamic therapeutical applications [68]. 

A series of symmetrical and unsymmetrical, hydrophobic and hydrophilic squaraine probes such as 41 (Table 1) with substituted squaraine ring oxygen was developed and compared to conventional oxo-squaraines 10a,b and 13b [18, 50, 98, 107]. The substituent on the squaraine ring have a strong influence on the spectral properties. Substitution of the squaraine oxygen by S, C(CN)2> C(CN)COOR, N(CN), N(OH), C(CN)[PO(OEt)2], indanedione, barbituric, and thiobarbituric acid causes red-shifted absorption and emission spectra [50]. 

In order to determine the structural factors maximizing 2PA cross section values, we analyze (8) from Sect. 1.2.1. For all cyanine-like molecules, symmetrical and asymmetrical, several distinct 2PA bands can be measured. First, the less intensive 2PA band is always connected with two-photon excitation into the main absorption band. The character of this 2PA band involves at least two dipole moments, /

symmetry forbidden for centro-symmetrical molecules, such as squaraines with C, symmetry due to A/t = 0, and only slightly allowed for polymethine dyes with C2V symmetry (A/t is small and oriented nearly perpendicular to /t01). It is important to note that a change in the permanent dipole moment under two-photon excitation into the linear absorption peak, even for asymmetrical D-a-A molecules, typically does not lead to the appearance of a 2PA band. 2PA bands under the main absorption peak are typically observed only for strongly asymmetrical molecules, for example, Styryl 1 [83], whose S0 —> Si transitions are considerably different from the corresponding transitions in symmetrical dyes and represent much broader, less intense, and blue-shifted bands. Thus, for typical cyanine-like molecules, both symmetrical and asymmetrical, with strong and relatively narrow, S (I > S) transitions, we observe... 

For the symmetrical bis[4-(dimethylamino)phenyl]squaraine 3 and its derivatives, complexation with alcohol solvents was shown to bring about bathochromic shifts in their absorption spectra [58]. For the bis(benzo-thiazolylidene)squaraine, such as 5 (Scheme 2), however, complexation with alcohol solvents brought about marked hypsochromic shifts in its absorption and emission maxima [62] (Figs. lA and B). 

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). 



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