Intramolecular Electron Transfer in Donor-Acceptor Systems. Porphyrins Bearing Trinitroaryl Acceptor Group

Article abstract of DOI:10.1021/j100270a022

Porphyrins bearing picryl group in ortho, meta, and para positions of one of the mesoaryl groups of tetraphenylporphyrin (TPP) have been synthesized in the free-base form.The metal derivatives of the free-base picrylporphyrins (PPc) have been prepared.The broadened Soret absorption, the decreased optical absorbance values of Q bands, and the reduced singlet emission quantun yields of PPc indicate the existence of intramolecular interaction.The extent of this interaction is found to be greater than those observed for the intermolecular systems and varies with the position at which the picryl moiety is attached to the porphyrin as ortho > meta > para.The energies of the redox states, E(CT) of P⁺Pc^-, calculated from the electrochemical redox potentials depend on the nature of the metal ion as free-base PPc > CuPPc > ZnPPc.The nature of intramolecular interaction between the picryl moiety and porphyrin unit is essentially ?-? (CT).Conclusive evidence for light-induced electron transfer in ZnPPc is presented from EPR studies.The decay profiles of the EPR signals vary with the position of the picryl moiety as ortho < para < meta.Computer simulation of structures substantiated by the 1H NMR results point out the restricted conformational freedom of the picryl moiety in ZnPPc.Arguments based on symmetry considerations of HOMO of the excited singlet state PPc and LUMO of picryl group indicate plane-to-plane orientation of the donor and acceptor in the picrylporphyrins.