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.