Deactivation of the S{sub 1} 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 S{sub 1} state of 5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrin (H{sub 2}TMPyP4) 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 S{sub 1} 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 S{sub 1} 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 S{sub 1} state is caused by the annihilation of excitation in porphyrin aggregates. (laser applications and other topics in quantum electronics)