Singlet oxygen lifetime dependence on photosensitizer concentration in lipid films

It was shown that lipids substantially influence singlet oxygen lifetime. Question arises whether photosensitizers triplet states and excitation energy transfer to oxygen are also affected by lipids. In this contribution, the influence of lipids on excitation energy transfer from lipophilic photosensitizer tetraphenylporphyrin (TPP) to oxygen was investigated in bulk lipids and dry lipid films.Two components of TPP triplets decays were identified: quenching by oxygen which does not depend on TPP concentration and triplet-triplet annihilation. Rather long lifetimes of the TPP triplets around are due to low solubility and diffusion coefficient of oxygen in the lipid. They are also reflected in low signal of singlet oxygen luminescence. Kinetics of the singlet oxygen luminescence follow convolutions of two exponential decays: rise-time independent on concentration and well corresponding to the short component of TPP triplet decay and decay time decreasing from 14 to with increasing TPP concentration due to quenching of singlet oxygen by TPP.