Tuning the singlet-triplet energy gap: a unique approach to efficient photosensitizers with aggregation-induced emission (AIE) characteristics

Abstract

The efficiency of the intersystem crossing process can be improved by reducing the energy gap between the singlet and triplet excited states (Δ<i>E</i>_ST), which offers the opportunity to improve the yield of the triplet excited state. Herein, we demonstrate that modulation of the excited states is also an effective strategy to regulate the singlet oxygen generation of photosensitizers. Based on our previous studies that photosensitizers with aggregation-induced emission characteristics (AIE) showed enhanced fluorescence and efficient singlet oxygen production in the aggregated state, a series of AIE fluorogens such as <b>TPDC</b>, <b>TPPDC</b> and <b>PPDC</b> were synthesized, which showed Δ<i>E</i>_ST values of 0.48, 0.35 and 0.27 eV, respectively. A detailed study revealed that <b>PPDC</b> exhibited the highest singlet oxygen efficiency (0.89) as nanoaggregates, while <b>TPDC</b> exhibited the lowest efficiency (0.28), inversely correlated with their Δ<i>E</i>_ST values. Due to their similar optical properties, <b>TPDC</b> and <b>PPDC</b> were further encapsulated into nanoparticles (NPs). Subsequent surface modification with cell penetrating peptide (TAT) yielded <b>TAT-TPDC NPs</b> and <b>TAT-PPDC NPs</b>. As a result of the stronger singlet oxygen generation, <b>TAT-PPDC NPs</b> showed enhanced cancer cell ablation as compared to <b>TAT-TPDC NPs</b>. Fine-tuning of the singlet-triplet energy gap is thus proven to be an effective new strategy to generate efficient photosensitizers for photodynamic therapy.