The Effect of Terminal Alkane Chains of Terphenyl Hemicages on the Binding Affinity and Solid-State Fluorescence of Perylene Diimide Dye.

Perylene diimide (PDI) is a class of dyes with excellent monomeric fluorescence properties. However, the severe self aggregation of PDI in the solid state leads to significant fluorescence quenching. Here we report the significant enhancement of solid-state fluorescence of PDI-C8 achieved through modification of the terphenyl hemicage. Five new terphenyl hemicages (5a-e) were synthesized by introducing alkane chains with different lengths and branching structures at the hemicage termini. The binding affinity between 5a-e and PDI-C8 was studied through UV-vis titration and nonlinear fitting analysis. It was found that the length of the alkane chain had a negligible effect on binding affinity, while branched alkane chains significantly reduced the binding affinity. Solid-state UV-illuminated images and fluorescence quantum yield studies demonstrated that these hemicages can significantly enhance the solid-state fluorescence of PDI-C8, and the trend of increase is consistent with the trend of binding affinity. In the case of 5a-c, the solid-state fluorescence quantum yield of PDI-C8 increased from 2.7% to 55.1%, 56.5% and 50%, respectively. In addition, the solid-state fluorescence lifetime of PDI-C8 was also significantly prolonged, from ~ 1 ns to ~ 18 ns in the case of 5a-c, which may contribute to the application on thermally activated delayed fluorescence (TADF) materials. Therefore, this study provides a good example for enhancing the solid-state fluorescence of PDI dyes through the design and variation of terminal functional groups.