Photoinduced electron transfer in supramolecular complexes of a π-extended viologen with porphyrin monomer and dimer

A π-extended viologen has been synthesized, forming supramolecular complexes with a freebase tetraphenylporphyrin (H ₂TPP) and the cofacial porphyrin dimer with an anthracene spacer [H ₄(DPA)] through π-π interaction in benzonitrile (PhCN). Formation of the H ₂TPP-BHV ²⁺ supramolecular complex was probed by UV-vis and fluorescence spectra. The fluorescence of H ₂TPP was strongly quenched by electron transfer from the singlet excited state ( ¹H ₂TPP ^*) to BHV ²⁺ in the supramolecular complex. The transient absorption spectrum of the charge-separated (CS) state (H ₂TPP ^•+ and BHV ^•+) was successfully detected by the laser flash photolysis measurements of the H ₂TPP- BHV ²⁺ supramolecular complex in PhCN. The lifetime of the CS state of the supramolecular complex was determined as 0.37 ms in PhCN at 298 K. The H ₄(DPA)-BHV ²⁺ supramolecular complex was formed with a larger formation constant as compared to the H ₂TPP-BHV ²⁺ supramolecular complex because of the stronger π-π interaction of BHV ²⁺ with the cofacial porphyrin dimer and the lifetime of the CS state was determined to be 0.59 ms in PhCN at 298 K.