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%.