Deactivation of the S1 state of a water-soluble porphyrin cation in a complex with DNA studied by the method of picosecond absorption spectroscopy

The deactivation of the excited S₁ state of 5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrin (H₂TMPyP4) in a complex with DNA is studied by the methods of kinetic absorption spectroscopy for different concentration ratios r of porphyrin and DNA base pairs. It is found that, when the DNA bound porphyrin is assumed to be in a monomer (non-aggregate) state (r = 1:30), the S₁ state of porphyrin excited by high-power picosecond pulses is rapidly deactivated, which is manifested as an additional decay component with the decay time 200–250 ps. This effect is interpreted as singlet–singlet annihilation of excitation indicating the presence of sites on the DNA surface with a higher local concentration of porphyrin molecules. The decrease in the relative content of DNA in solution down to r = 1:3 further accelerates the deactivation of the S₁ state of porphyrin and causes the hypsochromic shift of its absorption spectrum due to the formation of stack-like porphyrin aggregates along the DNA surface. It is assumed that the additional component with the decay time 44 ± 7 ps observed in the decay curve of the S₁ state is caused by the annihilation of excitation in porphyrin aggregates.