Improving the Quantum Yields of Perylene Diimide Aggregates by Increasing Molecular Hydrophobicity in Polar Media.

Here we report the quantum yield of four aggregated perylene diimide (PDI) species that vary by the length of the branched side chains attached at the N,N' imide positions. The PDI molecules were dissolved in binary water:methanol solvents as a means to vary the solvent polarity and control the degree of aggregation in solution. By performing spectroscopy, kinetics, and light scattering experiments, the nature of the molecular interactions in the solutions was determined. The maximum quantum yield of the aggregated molecules increased from 0.04 for the shortest chain molecule (B2) to 0.20 for the largest chain molecule (B13). The higher quantum yield of B13 compared with B2 correlates well with an increase in the fluorescence lifetime. The monomer emission lifetime was 4.8 ns whereas a lifetime as high as 21.2 ns was measured for the B13 aggregate fluorescence. A shorter sub-nanosecond lifetime was also measured for suspended colloids in B5, B9, and B13. The enhanced quantum yield is attributed to an increase of disorder in the B13 aggregates. As the polarity of the solution increases, the hydrophobic effect further enhances the disorder, and, therefore, the quantum yields in these particles.

Enhanced Davydov Splitting in Crystals of a Perylene Diimide Derivative.

We report the polarized absorption spectra of high-quality, thin crystals of a perylene diimide (PDI) species with branched side chains (B2). The absorption spectrum shows exemplary polarization-dependent H-like and J-like aggregate behavior upon orthogonal excitation, with a sizable Davydov splitting (DS) of 1230 cm-1 and peak to peak splitting of 3040 cm-1. The experimental results are compared to theoretical calculations with remarkable agreement. The theoretical analysis of the polarized absorption spectra shows evidence of a high degree of intermolecular charge transfer, which, along with Coulombic coupling, conspires to create the unprecedented DS for this family of dye molecules. The large polarization dependence of the electronic spectra is attributed to the unique twisted crystal structure, in which a substantial rotational displacement exists between neighboring chromophores within a π-stack. These results highlight the strong sensitivity of the Davydov splitting to intermolecular geometry in PDI systems.