Squaraine dyes have the potential for application in organic photovoltaics, sensors and biomedical applications. Many squaraine derivatives have sharp UV/Vis absorption bands and high fluorescence quantum yields which is beneficial for biomedical applications. Notably, the spectral features of most squaraine dyes depend on the solvent polarity. Furthermore, spectral changes are also obtained upon aggregation which is impacted by an interplay of long- and short-range coupling between the chromophores. These changes can be very significant and are sensitive to structural rearrangements. Hence, it is crucial to establish structure-property relationships for the dye aggregates in order to design materials with tailored spectral properties.

We have synthesized a water-soluble squaraine dye and investigated the UV/Vis absorption properties in various solvents. The employed water-solubilizing group enabled us to perform UV/Vis studies in non-polar and polar solvents including water. Interestingly, the monomer absorption spectra differ due to the different solvent polarity which can be attributed to changes of the optimum geometry in the ground and excited state of the chromophore. We will also present our results obtained from UV/Vis aggregation studies in different solvents which reveal spectral changes for the aggregate spectra in comparison to the monomer.