Thermodynamic Insights on Isodesmic Perylene Bisimide Dye Aggregation From Concentration-, Solvent- and Temperature-Dependent Studies

Solvent- and temperature-dependent self-assembly studies are often applied as alternatives to concentration-dependent analysis of supramolecular polymerization. However, spectral changes due to (de)aggregation are overlaid by solvatochromic or thermochromic effects, respectively, that can have significant impacts on the fit analysis. Here, a non-aggregating N,N'-2,6-diisopropylphenyl-substituted perylene bisimide (PBI) was studied in eleven solvents from 283 to 363 K, to acquire information on the impact of solvent and temperature on absorption properties. Subsequently, isodesmic aggregation of a PBI chromophore was studied in different solvent mixtures of toluene and 1,1,2,2-tetrachloroethane, CHCl₃, or 1,2-dichlorobenzene, revealing that even small solvatochromic effects can significantly falsify data analysis, and derived thermodynamic parameters highly depend on the wavelengths chosen for data analysis. This wavelength-dependency and deviation of fit results from concentration-dependent reference values was also found in the temperature-dependent study due to varying degrees of thermochromism at different wavelengths. Still, if evaluated carefully by proper selection of suitable wavelengths, both methods can give reasonable results. The present example serves to illustrate the accuracy one can expect from the different methods and the reliability of thermodynamic parameters from concentration-, solvent-, and temperature-dependent UV-vis studies for one of the most studied dyes in the field of self-assemblies and supramolecular polymers.