Enhancement of the triplet photosensitizing ability of BODIPY dyes by controlled red-shifted H-aggregate formation using ionic liquids: insights from spectroscopy, microscopy, DFT and biological studies

Abstract

We report an innovative, straightforward, and effective supramolecular method for converting fluorophores into photosensitizers using symmetrical imidazolium ionic liquids (ILs) via the aggregation of BODIPY derivatives. By utilizing “red-shifted H-aggregates” of 2,6-diethyl-4,4-difluoro-1,3,5,7-tetramethyl-8-phenyl-4-bora-3a,4a-diaza-s-indacene (R6-BDP), along with both shorter and longer cationic alkyl chain symmetrical ILs, we enhanced the conversion efficiency of the R6-BDP aggregates from fluorophores to photosensitizers. The optical properties of R6-BDP in both the molecular and aggregated states were investigated through UV-visible, fluorescence and time-resolved fluorescence spectroscopy techniques. Notably, the absorption data indicated the formation of “red-shifted H-aggregates”, which is an unusual phenomenon. The emission intensity of the R6-BDP aggregates decreased by approximately 65-fold compared to their molecular form, suggesting that the aggregation-caused quenching effect (ACQ) occurred in the aggregated state. The average lifetimes of the molecular and aggregate forms were estimated to be 4.52 ns and 1.02 ns, respectively. Photophysical studies suggested that the addition of a shorter alkyl chain IL (C₂C₂im-Br) promotes the formation of R6-BDP aggregates, whereas a longer alkyl chain IL (C₈C₈im-Br) induces their dissociation, returning the aggregates to their molecular form. No significant effect on aggregation was observed with the medium-length alkyl chain IL (C₄C₄im-Br). FESEM (field emission scanning electron microscopy) analysis revealed that in the presence of C₂C₂im-Br, the particle size of the R6-BDP aggregates increased, with a corresponding change in shape from round to rod-like structures. However, the particle size decreased with the addition of C₈C₈im-Br, and the particles appeared circular in shape. Theoretical density functional theory (DFT) studies provided further confirmation of the formation of H-type aggregates for the R6-BDP molecules. Biological studies revealed that the addition of a short alkyl chain IL to the R6-BDP aggregates significantly enhanced singlet oxygen generation, as evidenced by increased green fluorescence in HeLa cells under light irradiation, indicating an improvement in their photodynamic therapy (PDT) performance. The current work offers a novel approach for the research community to realize photodynamic therapy.