Synthesis and Linear and Nonlinear Photophysical Characterization of Two Symmetrical Pyrene-Terminated Squaraine Derivatives

Two indole-based squaraine dyes, bearing two pyrenyl groups through vinyl or ethynyl linkers, were synthesized with the aim of enhancing the intramolecular charge transfer interaction in addition to improving their optical properties. The absorption and emission properties of these derivatives were determined to gain insight into the intensity of this type of interaction, their aggregation behavior, and for comparison with results obtained through quantum chemical calculations. Both compounds exhibited high photochemical stability, high molar absorptivity, large fluorescence quantum yields, and relatively low tendency of forming excimers in several solvents. Nonlinear spectroscopic studies revealed two-photon absorption (2PA) cross section maxima greater than 10 000 GM (1 GM = 1 × 10^-50 cm⁴ s/photon), which are higher values relative to the indole-based squaraine core. Experimental results were compared with time-dependent DFT calculations. These observations contribute to the study of intramolecular charge transfer interaction, and its tailoring for the improvement of linear and nonlinear optical properties as well as suggests a paradigm in the construct of highly absorbing organic molecules containing pyrenyl groups for the development of new photonics materials.