ENHANCED PHOTOCHEMICAL DEGRADATION OF THE HEMATOPORPHYRIN DICATION IRRADIATED AT 560 NM IN A LIPID-LIKE ENVIRONMENT

Haematoporphyrin dihydrochloride (HpDC) has been irradiated by red lig ht (at lambda=560+/-20 nm) either in binary solvent mixtures modelling a lipid-like environment of dielectric constant 4-7 or in polar solve nts. Quantum yields of the photochemical degradation, as well as absor ption, fluorescence emission and excitation spectra of HpDC, have been measured. Both Soret and Q(0-0) maxima of HpDC spectra undergo blue s hifts in polar environments of dielectric constant values more than 12 . The energy of the Q(S-1) state of the dicationic form of Hp is 47.8 kcal mol(-1) in non-polar solvents and 48.2 kcal mol(-1) in acetonitri le and these are higher than the values for Q(S-1) for Hp free base in the same solvents. The results also indicated that model based only o n the electrostatic interaction between the porphyrin ring and the env ironment is not valid, since the electronic transition moments of HpDC in a polar environment are higher than those measured in a lipid-like environment. In addition, the fluorescence emission maxima of the dic ation are red shifted in polar solvents; at the same time those of the free base are blue shifted. The theoretical calculations also show th at the dipole moment of HpDC is zero in the ground state and about 3.5 D in the excited state (with the effective radius of the Onzager cavi ty being 4 Angstrom). The quantum yields of the photochemical degradat ion are maximal in a lipid-like environment. The ratio of the quantum yields measured for the degradation in the Soret and in the Q bands is 1 +/- 0.17. In a polar environment photochemical deprotonation occurs resulting in the formation of Hp monocation, the fluorescence of whic h was also measured. Non-polar solvents are proposed to provide stabil ization of ion-paired structures of the type NH ... Cl- and an increas e in C-N bond length which may result in their easier cleavage.