Electron Transfer from the S1 and S2 States of Pentacoordinated Tetrapyrrole Macrocycles to Pyromellitic Diimide as an Axial Ligand

Abstract

Electron transfer (ET) reactions from the S(1) and S(2) states of some porphyrins and phthalocyanines to the axial ligand have been investigated by means of femtosecond laser flash photolysis. As the axial ligand, which acts as an acceptor, we synthesized an asymmetric pyromellitic diimide (PI) compound that has an alkyl chain and a pyridine ring on N and N' atoms, respectively. The pyridine ring of PI can coordinate to Zn of tetrapyrrole macrocycles. The coordination was confirmed by UV-vis and (1)H NMR spectra. ET from the S(1) state of Zn tetraphenylporphyrin (ZnTPP), Zn octaethylporphyrin (ZnOEP), Zn phthalocyanine (ZnPc), and Zn naphthalocyanine (ZnNc) to PI was confirmed with transient absorption spectroscopy by observing PI*-. ET from the S1 state occurred at the rate constant of (8.6 ps)(-1) - (78 ps)(-1), and the yield was almost unity. Furthermore, ET from the S(2) state of ZnTPP and ZnPc to PI was confirmed. ET from the S(2) state of ZnPc was observed for the first time. The ET rate from the S(2) state was faster than that from the S(1) state. In the case of ZnOEP-PI and ZnNc-PI complexes, ET from the S(2) state was not observed.