Squaraine molecules

Shown in Fig. 10 are examples of fitting Z-scan results for a squaraine molecule where triplet states are significantly populated during picosecond excitation. In this case, femtosecond excitation was needed to determine the singlet parameters, shown in Fig. 10a. Picosecond excitation was required to model the full five-level system, shown in Fig. 10b [73], to determine the triplet parameters. To account for the triplet states, it is necessary to consider at a minimum the 5-level system shown Fig. 2 (three singlet and two triplet states). Typically, PDs do not populate triplet states during picosecond excitation [74, 75],... 

Fig. 10 Example of fitting Z-scan data for a squaraine molecule for (a) a 3-level model (singlet states only) with femtosecond excitation and (b) a 5-level model (both singlet and triplet states) with picoseconds excitation. The schematic is shown in Fig. 2a and the propagation and rate (9) are used for modeling...

Fu J, Padilha LA, Hagan DJ, Van Stryland EW, Przhonska OV, Bondar MV, Slominsky YL, Kachkovski AD (2007) Experimental and theoretical approaches to understanding two-photon absorption spectra in polymethine and squaraine molecules. J Opt Soc Am B 24 67-76... 

C. A. T. Laia and S. M. B. Costa, Phys. Chem. Chem. Phys., 1, 4409 (1999). Probing the Interface Polarity of AOT Reversed Micelles Using Centro-Symmetrical Squaraine Molecules. 

The commercially available dicyanomethylene squaraine dye Seta-670-mono-NHS showed extremely low blinking effects and good photostability when used in single-molecule studies of multiple-fluorophore labeled antibodies [113]. Seta-670-mono-NHS and Seta-635-NH-mono-NHS were covalently labeled to antibodies and used in a surface-enhanced immunoassay [114]. From the fluorescence intensity and lifetime changes determined for a surface that had been coated with silver nanoparticles, both labeled compounds exhibited a 15- to 20-fold... 

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... 

The combination of dyes with microporous materials opens-up a way to develop selective chemosensors microporous zeolites with an anchored squaraine 27 (Fig. 13) and some other types of dyes can be used as chemosensors for the chromogenic discrimination of amines [75], These dye-zeolite hosts are expected to be promising sensor materials allowing the visible discrimination of selected target guests by size and/or polarity within families or closely related molecules. It was found that the response of the solid to amines was basically governed by the three-dimensional architecture of the solid material. 

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. 

On the contrary, D-jt-A-jt-D molecules are generally known to provide high (7TPA values from the lowest two-photon excited states with a weaker detuning energy term this has been observed recently in conjugated porphyrins [93], in extended phenylenevinylene oligomers for which ajpA values up to 5300 GM have been obtained [94], and in squaraines [95]. 



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