Selective inclusion of electron-donating molecules into porphyrin nanochannels derived from the self-assembly of saddle-distorted, protonated porphyrins and photoinduced electron transfer from guest molecules to porphyrin dications

A doubly protonated hydrochloride salt of a saddle-distorted do-decaphenylporphyrin (H₂DPP), [H₄DPPP]Cl₂, forms a porphyrin nano-channel (PNC). X-ray crystallography was used to determine the structure of the molecule, which revealed the inclusion of guest molecules within the PNC. Electron-donating molecules, such as p-hydroquinone and p-xylene, were selectively included within the PNC in sharp contrast to electron acceptors, such as the corresponding quinones, which were not encapsulated. This result indicates that the PNC can recognize the electronic character and steric hindrance of the guest molecules during the course of inclusion. ESR measurements (photoirradiation at λ> 340 nm at room temperature) of the PNC that contains p-hydroquinone, catechol, and tetrafluorohydroquinone guest molecules gave well-resolved signals, which were assigned to cation radicals formed without deprotonation based on results from computer simulations of the ESR spectra and density functional theory (DFT) calculations. The radicals are derived from photoinduced electron transfer from the guest molecules to the singlet state of H₄DPP²⁺. Transient absorption spectroscopy by femtosecond laser flash photolysis allowed us to observe the formation of ¹(H₄DPP²⁺) *, which is converted to H₄DPP^+. by electron transfer from the guest molecules to ¹(H₄DPP ²⁺)*, followed by fast disproportionation of H ₄DPP^+., and charge recombination to give diamagnetic species and the triplet excited state ³(H₄DPP ²⁺)*, respectively.