Enhanced photochemical degradation of the haematoporphyrin dication irradiated at 560 nm in a lipid-like environment

Abstract Hematoporphyrin dihydrochloride (HpDC) has been irradiated by red light (at λ=560 ± 20 nm) either in binary solvent mixtures modeling a lipid-like environment of dielectric constant 4–7 or in polar solvents. Quantum yields of the photochemical degradation, as well as absorption, fluorescence emission and excitation spectra of HpDC, have been measured. Both Soret and Q(0-0) maxima of HpDC spectra undergo blue shifts 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 acetonitrite 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 on the electrostatic interaction between the porphyrin ring and the environment is not valid, since the electronic transition moments of HpDC in a polar environment are higher than those treasured in a lipid-like environment. In addition, the fluorescence emission maxima of the dication are red shifted in polar solvents; at the same time those of the free base are blue Shifted. The theoretical calculations also show that 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 cavity being 4 A). The quantum yields of the photochemical degradation 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 which was also measured. Non-polar solvents are proposed to provide stabilization of ion-paired structures of the type NH…Cl and an increase in CN bond length which may result in their easier cleavage.