Highly Fluorescent Molecularly Insulated Perylene Diimides: Effect of Concentration on Photophysical Properties

A series of four perylene diimide (PDI) chromophores were prepared with increasing steric bulk on the imide substituents with the aim of retarding the effect of concentration quenching on photoluminescence, commonly observed with these dyes. Spectroscopic investigations of the compounds in dilute solution confirmed that the photophysical properties of the PDI core chromophore were not perturbed by the bulky substituents. Solid film samples containing the PDI compounds at various concentrations dispersed in a poly­(methyl methacrylate) (PMMA) matrix were examined and compared to amorphous neat films as well as crystalline samples. The PDI compounds containing di-tert-butylphenyl (bPDI-3) and trityl (bPDI-4) substituents showed near unity photoluminescence quantum yield (PLQY) up to 20 mM in PMMA compared to 10% PLQY for the reference compound (bPDI-1) without molecular insulation. Surprisingly, high concentrations (>40 mM) of a phenyl-substituted PDI compound (bPDI-2) with moderate molecular insulation formed emissive aggregates that showed a higher PLQY compared to the PDI derivatives with greater steric bulk. By examining the molecular structure and solid state packing in conjunction with a series of photophysical measurements, new insights into designing highly fluorescent dyes, particularly in the solid state, were obtained. The trityl-substituted PDI compound (bPDI-4) was used in a luminescent solar concentrator with optical quantum efficiency of 54%, flux gain of 6.4, and geometric gain of 45.