Squaraines complexation with solvents

Three emission bands (a, p and y in the order of decreasing energy) are observed in CH2CI2 solution and are found to be the emission from the excited state of 1, from the excited state of a solute-solvent complex and from a relaxed twisted excited state of the solute-solvent complex, respectively. Model compound studies show that squaraine forms strong solute-solvent complexes with alcoholic solvent molecules. Analogous complexation process between 1 and the OH groups in PVF is also shown to occur. A model for the stabilization of particles of 1 in polymer solution is put forward where we propose that the stabilization mechanism is a steric effect achieved by adsorption of PVF macromolecules onto particles of 1 via the formation of the PVF 1 complex. 

Here we report preliminary results on the multiple fluorescence emission of 1 and 2. From structure-property relationships, solvent effect and temperature effect studies, we are able to show that the multiple emission is from the emission of free squaraine in solution, the emission of the solute-solvent complex and the emission of a twisted relaxed excited state. Further solvent effect study using 2 as a model shows that squaraine forms strong solute-solvent complexes with alcoholic solvent molecules. Analogous complex-ation process is also detected between 1 and the hydroxy groups on the macromolecular chains of poly(vinyl formal). The Important role of this complexation process on the stabilization mechanism of particles of 1 in polymer solution is discussed. 

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). 

Probable causes for the multiple emission are (a) vibronic fine structure of the squaraine emission, (b) emission from a relaxed excited state, and (c) emission from an exciplex (with solvent) or the excited state of the squaraine-solvent complex. To differentiate these possibilities, the effects of solvent, temperature, and structural changes on the multiple emission have been studied. Sq4 was chosen as a model for these investigations because of its high solubility in... 

The solute-solvent complex model proposed above is further supported by results obtained from mixed-solvent experiments. In diethyl ether, Sq4 exists primarily as free squaraine (Fig. 3a). The addition of a complexing solvent should drive the equilibrium for complexation. As a result, both and Xp should shift to the red and the intensity of the p-emission should increase. To eliminate any complication due to the increase in dielectric constant of the mixed solvent during experimentation, the mixed-solvent experiment was first performed in a ternary system consisting of ether (e = 4.43), chloroform (e = 4.7), and n-hexane (e = 1.9). The addition of n-hexane in the mixture is to keep the dielectric constant constant as the concentration of chloroform increases. The absorption and fluorescence spectral results are summarized in Figs. 4a and 4b, respectively. The data showed that nd Xp shift to the red and the intensity of the P-emission increases as [CHCI3] increases at [CHCl3]<1.12 M. Simultaneously, an isosbestic point at -625 nm and isoemissive point -662 nm are observed in the absorption and fluorescence spectra, respectively. The observation of an isosbestic point in the absorption spectra and an isoemissive point in the emission spectra provide positive evidence that (1) Sq4 forms a complex with chloroform in the ethereal solutions,... 

Similar to other triptycene-derived macrocyclic arenes, triptycene-derived tetralactam macrocycles also had fixed conformations with large electron-rich cavities, which made them promising candidates as the host for some electron-deficient guests with comparatively large sizes. Squaraines [26] were a family of fluorescent dyes with specific near-IR photophysical properties, which had wide potential applications. However, their instability limited the utilization of them, and thus improving their chemical stability and the photophysical properties were the key to applications of squaraines [27]. Consequently, we [25, 28] found that macrocycles 35a-b could form a new kind of stable pseudorotaxane-type complexes with the squaraine in both solution and solid state. We further studied the chemical stability of squaraine in these complexes, and found that free guest 35b underwent hydrolytic decomposition to turn colorless in polar THF-water solvent in 4 days, but for squaraine 36b (Fig. 18.15) in complexation with 35a-b, its blue colors could be retained for several weeks. This observation revealed that the formation of complexes could efficiently protect the squaraine dyes from polar solvents. 

An admixture of 16b and 17b self-assembles quantitatively at millimolar concentration in chloroform solution to produce an inclusion complex whose absorption and emission maxima are red-shifted by 40 nm [56]. Clicking both ends of this pseudorotaxane with two molar equivalents of stopper S2 produces the squaraine rotaxane 16b D 19b in near-quantitative yield [58]. Pseudorotaxane 16b D 17b partially dissociates at micromolar concentrations in chloroform and produces two emission peaks, one at 638 nm which corresponds to free squaraine 17b and one at 694 nm, corresponding to the pseudorotaxane. In contrast, the squaraine rotaxane 16b D 19b does not dissociate under these conditions or in more polar solvents such as pure methanol. 

Xp=66A nm. By comparison with the room-temperature spectrum, one can assign the emission band at 660 nm to the emission from the solute-solvent complex and the emission band at 650 nm to the emission from the excited squaraine itself. 

Figure 6 shows the fluorescence excitation and emission spectra of Sq2-Sq5 in CH2CI2. In each case, the excitation spectrum was found to be identical to the absorption spectrum and is independent of the monitoring wavelength. The spectral results are summarized in Table 3. Although the effect of chain length on Xp may be small, it has a profound effect on the composition of the emission band. For example, for N = CH3, the intensities of the a- and p-bands are about the same (Fig. 1). As the chain length is increased, the intensity of the a-band decreases whereas the opposite is observed for the p-band (Fig. 6). The gradual dominance of the P-emission indicates that the equilibrium constant for the solute-solvent complex increases as the chain length increases. This is actually consistent with the solute-solvent complex model discussed above. Namely as the CT D-A-D state of squaraine is stabilized (by the electron-releasing N-alkyl group), the tendency for complexation increases [6].

The fluorescence emission spectra of Sq7 to Sq9 are given in Figs. 8a-8c. Three emission bands are observed for Sq7, and the emission becomes dominated by the p- and -emission in Sq8 and Sq9. This trend is identical to that seen in Sql-Sq5. The gradual dominance of the p-emission in the emission spectra along with a small red-shift on Xp as the chain length of the A -alkyl group increases suggest that as the D-A-D CT character in squaraine is enhanced, more solute-solvent complexes are formed. 

A number of attempts were made to correlate the solvent effects with different solvent parameters, such as the dielectric constant Ej. [46], Z [47], 6 [48], Py [49], n [50], and so forth. The relationships between and these solvent parameters are quite scattered except n. The plot of of Sq4 as a function of solvent parameter n is given in Fig. 10. Along with the red-shift on a systematic and gradual change in the composition of the multiple emission band is observed (see insets in Fig. 10). Sq4 exhibits primarily a-emission in diethyl ether. As the solvent polarity increases, the intensity of the P-emission increases. The p-emission eventually dominates the fluorescence. Because the P-emission is the emission from the solute-solvent complex, the overall spectral results suggest that the solvent effect on may be due to the shift in equilibrium for the complex formation as n increases. For solvents with 7t ranging from 0.273 to 0.567, both a- and P-emission bands are discernible simultaneously. Assuming that the spectral bandwidths of these two bands are similar and that they are not sensitive to solvent. Law [30] has deconvoluted the contribution of the a- and P-bands in the multiple emissions. The relative intensity of these two bands can then be used to estimate the relative concentrations of the free squaraine and the complex. From the ratio of the a- and P-emissions and the molar concentration of the solvent, the equilibrium constants (K in these solvents are calculated. A plot of versus n is depicted in Fig. 11, and a linear plot is obtained. The result simply indicates that the equilibrium constant for solute-solvent complexation increases as n increases. 

According to Scheme 2, Law [30] suggested that three different excited states of Sq4 in toluene were detected. Because the decay at 645 nm is primarily from the free squaraine, the 2.4-ns decay was assigned to the excited Sq4, the 3.5-ns decay to the excited solute-solvent complex, and the 2.7-ns decay to the relaxed excited state. The fact that the solute-solvent complex has a longer lifetime is consistent with the geometry of the complex, which is shown to be rigidized upon complexation due to n-rt interaction (Scheme 4b). The three lifetimes recorded for Sq4 in toluene provide kinetic evidence for the existence of three different excited states of squaraine in solution. 

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