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

The deactivation of the excited S-1 state of 5,10,15,20-tetrakis(4-N-methylpyridvl)porphyrin (H(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-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, 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-1 state is caused by the annihilation of excitation in porphyrin aggregates.