4.5.1 Single photon excitation donor type
In the past few years, a large number of fluorescent materials have been developed as donors to transfer energy to PS via photonic excitation of the donors to enhance the production of1O2. Due to the good light trapping ability of conjugated polymers, the transfer of excitation energy to the acceptor PS drug along its backbone can lead to a high amplification of the PS signal(Li et al. 2016; Liu et al. 2015; Shen et al. 2011; Yuan et al. 2014). On this basis, Chang and co-workers developed an efficient polymer-dot (Pdots) photosensitizer(Chang et al. 2016) by covalently doping porphyrins on the polymer backbone, in which the PS drug of tetraphenylporphyrin is covalently bound into the p-conjugated backbone of [(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(1,4-benzo-2,1,3-thiadiazole)] (PFBT). The resulting Pdots have excellent stability and solve the problem of photosensitizer leaching encountered in the photosensitizer-doped Pdots. Compared with the pure PFBT Pdots, the fluorescent PFBT-TPPx Pdots were significantly quenched due to the introduction of TPP, and the light-trapping polymer backbone mainly transferred the excitation energy to the TPP unit. At the same time, the Pdots exhibited excellent performance including high1O2 quantum yield (35%) and low dark toxicity. The cytotoxic effects and photodynamic effects of Pdots on MCF-7 cells were determined by colorimetric assay with tetramethylazole salts. The results showed that Pdots could efficiently kill cancer cells and produce large amounts of 1O2. The therapeutic effects of PFBT-TPP Pdots were further investigated in tumor-bearing mice. The results showed that in some cases, PFBT-TPP Pdots significantly inhibited or eradicated the transplanted tumors.
In addition, Zhou and co-workers designed and synthesized a poly (metallocene) hyperbranched conjugated polyelectrolyte containing a platinum (II) porphyrin complex, which was used to efficiently generate1O2 for PDT(Zhou et al. 2016). Based on the overlap between the luminescence band of Pdots at 420 nm and the Soret and Q absorption bands, a Förster radius of 6.6 nm was calculated, indicating that the FRET from poly(furan) to platinum (II) porphyrins is effective. The FRET process was further investigated and determined to exist in Pdots. Furthermore, the good ability of Pdots to kill cancer cells was confirmed by tetramethylazole salt colorimetry, flow cytometry analysis and real-time fluorescence imaging of photogenic cell death in situ, which was attributed to its high1O2 quantum yield (80%) by the introduction of oxygen-sensitive phosphorescent platinum (II) porphyrin complexes.
In recent years, water-soluble conjugated oligomers, including oligo-(phenylene vinylene) (OPV), oligo- (phenylene ethynylene), and oligo-(thiophene ethynylene), have attracted a lot of extensive attention due to their good molecular structures and tunable optical properties(Wang et al. 2019). In 2019, Zhao and co-workers designed and synthesized a novel donor-acceptor porphyrin PS in the form of covalent bonds through condensation reactions with cationically conjugated oligo-(thiophene ethynylene) as the donor and 5,10,15,20-4 (4-aminophenyl) porphyrins (TPP) as the acceptor(Zhao et al. 2019). The positively charged OPV plays the role of an ”antenna” by its excellent light trapping ability. Under white light irradiation, the OPV overlaps strongly with the porphyrins spectrally at very short distances, and good FRET (99%) occurs, which greatly improves the1O2 yield of the porphyrins. Under white light irradiation at 5 mW/cm2, the energy transfer from the two-armed OPV to the TPP core increased the1O2 yield of OPV-modified porphyrins by about 54-fold. On this basis, the toxic effects of OPV-C3-TPP, OPV-C6-TPP and TPP on MCF-7 cells were studied by colorimetric method using tetramethylazole salt. Under light irradiation conditions, the cell viability of OPV-modified porphyrins decreased with increasing concentration after incubation with cells. At a concentration of 5 μM, cell mortality reached more than 98%.