Ionic-liquid-assisted aggregation modulation of BODIPY nanoparticles for tunable photodynamic therapy performance

Development of heavy atom free triplet photosensitizers is essential for effective photodynamic therapy of cancer with low dark toxicity. Several design strategies are available based on the structural modifications, which are laborious, time consuming and not cost effective. Here, we developed a new strategy to enhance the triplet conversion of BODIPY dyes via ionic liquid (IL) assisted nano-formulations. For this, two structurally related BODIPY derivatives, namely BDP and styryl-BDP, are synthesized, where the latter dye shows red-shifted absorption compared to BDP, which is suitable for PDT applications. A short-alkyl-chain symmetric ionic liquid, 1,3-diethylimidazolium bromide (C₂C₂im-Br), was employed as a nano-engineering additive to regulate aggregation behavior and excited-state dynamics. Both dyes were formulated into nanoparticles in the presence and absence of the IL to elucidate the combined effects of molecular π-conjugation and ionic-liquid-induced aggregation. Spectroscopic and microscopic studies reveal that IL incorporation induces pronounced aggregation and morphological changes, leading to modified absorption and emission characteristics. Both dyes have low triplet generation capabilities, which are enhanced in the aggregated state in the presence of IL. Notably, styryl-BDP exhibits enhanced fluorescence in the molecular state; however, aggregation and ionic-liquid incorporation suppress its fluorescence more effectively than in BDP, favoring triplet-state formation over radiative decay. DPBF assays confirm enhanced singlet oxygen generation for ionic-liquid-modified nanoparticles, with the styryl-BDP system exhibiting higher intrinsic reactive oxygen species (ROS) production than the parent BDP. Consistently, intracellular ROS imaging in HeLa cells under LED irradiation reveals stronger light-triggered oxidative stress, which is further amplified in the presence of the IL. Density functional theory (DFT) and time-dependent DFT calculations support these observations by showing reduced HOMO-LUMO and singlet-triplet energy gaps upon styryl substitution, favoring triplet-state formation. In vitro cytotoxicity studies show that the photosensitizer exhibits enhanced cytotoxicity in HeLa cells upon light irradiation, while showing low toxicity in the dark and minimal effects on normal cells. Overall, this work provides molecular and nanoscale-level insights into how ionic-liquid-assisted aggregation and π-conjugation engineering can be combined to tune singlet oxygen generation and photodynamic performance, offering a rational strategy for optimizing BODIPY-based nanoparticle photosensitizers.