Oligothiophene-based photosensitizers with tunable push-pull architectures: design, synthesis and characterization

Donor-acceptor-donor (D-A-D) thiophene-based compounds, characterized by thiophene as a donor unit and benzothiadiazole (Bz) as an acceptor, represent an emerging class of theranostic agents for imaging and photodynamic therapy. Here, we expand this class of molecules by strategically varying the position of the electron-accepting unit within the oligothiophene (OT) backbone structure, realizing a series of different push-pull architectures (A-D, D-A-D, and D-A). This rational design allows for precise modulation of key photophysical parameters, including absorption and emission spectra, molar absorption coefficient, charge separation, and frontier molecular orbitals. Computational predictions indicate that all the designed oligothiophene molecules possess the photophysical properties to be effective photosensitizers (PSs), prompting their chemical synthesis. To enable the use of insoluble oligothiophenes in physiological environments as PSs, an N-succinimidyl (NHS) ester group is introduced in the molecules (NHS-OT), to allow the subsequent conjugation with human serum albumin (HSA). The effective conjugation of the oligothiophenes to the protein (HSA-OT) is confirmed by mass spectrometry and electrophoresis. The HSA-OT bioconjugates are soluble in a physiological environment, exhibit intrinsic bright fluorescence and are non-toxic under dark conditions. Upon irradiation, the bioconjugates efficiently generate reactive oxygen species, following both type I and type II mechanisms, and lead to the complete eradication of cancer cells at much lower concentrations (IC₅₀ = 4.0 nM for the most efficient PS) than common photosensitizers. Real-time fluorescence imaging revealed rapid membrane blebbing when the cells treated with HSA-OT were exposed to light, indicative of necrotic-like immunogenic cell death. Taken together, our findings highlight the power of molecular engineering in optimizing photosensitizer performance and provide proof of concept for the potential of HSA-OT conjugates as a multifunctional theranostic platform for cancer.