Triplet-triplet annihilation upconversion in polymeric systems and model membranes

Triplet-triplet annihilation upconversion (TTAUC) is gaining prominence in the field of optical spectroscopy and holds the potential to revolutionize various emerging technologies such as solar cells, bioimaging and light-activated drug release. This is attributed to its capability of converting long-wavelength photons to higher energy photons, even at low excitation power densities (~5 mW cm-2). The low excitation power density can activate the drug release without damaging the tissue by excitation radiation. The implementation of TTAUC in light-activated drug release necessitates the transfer of the upconversion system comprising sensitizer and annihilator, from solution to solid phase. However, TTAUC is most efficient in solutions and needs inert conditions. The challenge lies in the fact that diffusion plays a crucial role in molecular processes for triplet-triplet energy transfer (TTET) and triplet-triplet annihilation (TTA). In this work, liposomes with sensitizer, annihilator and drug molecules have been synthesized. Time-resolved studies of sensitizer and annihilator in lipid bilayer membranes reveal the triplet lifetime properties, the triplet lifetime of sensitizer and annihilator molecules play a vital role in determining the efficiency of energy transfer processes. The study reveals sensitizer and annihilator molecules localize within the lipid bilayer. Therefore, increasing the local concentration leads to the self-quenching of sensitizer triplets, but localization enhances the energy transfer rates. The observations made using time-resolved study of TTAUC in LUVs indicate despite the higher viscosity of the medium, energy transfer is faster in the lipid bilayer system. Long triplet lifetimes of the sensitizer and annihilators are advantageous in getting high upconversion yield, this study will be crucial in designing efficient light-activated drug delivery systems using liposomes