Photoinduced energy and electron transfer in a cofacial aluminum(III) porphyrin – Phosphorus(V) porphyrin heterodimer

Abstract

A cofacial ‘aluminum(III) porphyrin – phosphorus(V) porphyrin’ (AlPor-Ph-PPor.PF6) heterodimer has been synthesized to investigate the factors that govern the energy and electron transfer processes in a direction perpendicular to the porphyrin plane. The AlPor and PPor+ are bound in the cofacial arrangement through a benzoate spacer. The electron-rich aluminum(III) porphyrin acts as an electron and energy donor, whereas the electron-poor phosphorus(V) porphyrin performs the role of electron and energy acceptor. Steady-state absorption studies and DFT calculations indicate that the energy states and orbitals of the two porphyrins are not significantly perturbed in the heterodimer compared to the corresponding monomers. However, fluorescence, femtosecond transient absorption and time-resolved electron paramagnetic resonance studies reveal that the excited state behavior of the heterodimer is very different from that of its constituent monomeric porphyrins. Excitation of the AlPor unit in the heterodimer results in both energy and electron transfer from the 1AlPor* to the PPor+ unit. The measured rate constants for energy and electron transfer in o-dichlorobenzene were found to be 2.9 × 109 s–1 and 5 × 109 s–1, respectively. The relative yield of the energy and electron processes can be modulated by changing the solvent polarity with energy transfer being slightly favored in less polar solvents. In contrast, excitation of the PPor+ unit results exclusively in electron transfer from the AlPor unit to 1(PPor+)*. Regardless of the excitation wavelength, the resulting charge-separated state eventually decays to the triplet state of the PPor+ unit. Finally, the photophysical properties of the investigated heterodimer, AlPor-Ph-PPor.PF6 are compared with a previously reported μ-oxo-heterodimer, AlPor-O-PPor.PF6, in which the distance between the AlPor and PPor+ is much shorter. In AlPor-O-PPor.PF6 the strong exciton coupling between AlPor and PPor+ leads to a charge transfer (CT) absorption, while in AlPor-Ph-PPor.PF6 neither of these features is observed because of the larger distance between the porphyrins.