Magnetic resonance studies and molecular orbital calculations on the doublet and triplet states of bacteriopurpurin: a potential second-generation photosensitizer for photodynamic therapy

Bacteriopurpurin is a macrocyclic compound at the saturation level of bacteriochlorins that was specifically synthesized as a potential photosensitizer in photodynamic therapy. It exhibits a long-wavelength Q y absorptionband at 843 nm, which is a necessary prerequisite for a deep tissue penetration. Magnetic resonance spectroscopic properties of bacteriopurpurin have been examined by fluorescence-detected magnetic resonance (FDMR) and electron paramagnetic resonance (EPR) in conjunction with molecular orbital calculations. The electronic structure of the radical cation state obtained by oxidation of bacteriopurpurin with iodine is mapped by the determination of isotropic proton hyperfine coupling constants from electron-nuclear double-resonance spectroscopy at 298 K. A tentative assignment of the couplings to the various proton positions in the molecule is achieved by comparison of the experimental values with simulated ones based on semiempirical intermediate neglect of differential overlap calculations and density functional theory. The triplet-state electronic structure of the ground-state trans-configured bacteriopurpurin is characterized by the determination of the zero-field splitting parameters which represent sensitive probes for the geometric structure of the molecule. The ‖D‖ and ‖E‖ values obtained from time-resolved EPR (24.4 and 5.4 mT) are in favorable agreement with the respective values obtained from FDMR (24.0 and 5.3 mT). By both techniques, the presence of additional photoexcited triplet states has been confirmed. It is assumed that they arise from higher-energy tautomers of bacteriopurpurin. Their occurrence is discussed in terms of a photoinitiated intramolecular one-proton transfer from either ring II or ring IV of the trans-configured ground state to ring I or ring III to form one of the cis-configured bacteriopurpurins.