Electron Transfer in the Supramolecular Donor−Acceptor Dyad of Zinc Hemiporphycene

Abstract

In the present study, photoinduced electron transfer (ET) processes of supramolecular donor-acceptor dyads of Zn 2,3,7,8,11,12,17,18-octaethylhemiporphycene (ZnHPc) and axial ligands were investigated by using various spectroscopic methods. The formation of 1:1 dyads was confirmed by absorption spectral change during titration of axial ligand. The association constants were determined from the spectral change. Quenching of the fluorescence intensity was observed when electron acceptor ability of the axial ligand increased. The driving forces for ET were estimated based on the estimated redox potentials and structural parameters. It became clear that various ZnHPc dyads showed ET because of slightly higher donor-ability and larger excitation energy of ZnHPc when compared to the corresponding dyads of Zn porphycene (ZnPcn). The transient absorption spectra during the sub-picosecond laser flash photolysis showed the formation of charge separated state, that is, radical cation of ZnHPc and radical anion of axial ligand, from the singlet excited ZnHPc. The observed ET rates were compared with the previously reported values for Zn porphyrins and ZnPcn. The ET rates of ZnHPc were located between those observed with porphyrins and ZnPcn supramolecular dyads, even when the -DeltaG values were similar to each other. This observation was explained on the basis of the variation in reorganization energy and electronic coupling (V) values. Furthermore, distribution of HOMO electron density gave a plausible explanation for the variation in V values of these dyads.

Photoinduced Electron Transfer of Porphyrin Isomers: Impact of Molecular Structures on Electron‐Transfer Dynamics

“…With an increase in the delay time, absorption bands at 720 and 890 nm became apparent, which can be attributed to the radical anion of PI and the radical cation of ZnOEPcn, respectively, which indicates the intramolecular charge separation (CS) from the singlet excited ZnOEPcn. Similar CS and CR processes were confirmed with other dyads …”

“…The absorption spectra of radical cations of Zn II ‐incorporated OEPcn, OHPcn, and OECn were reported by our groups employing γ‐ray radiolysis of the 77 K n ‐butylchloride matrix . The radical cations of ZnOEPcn, ZnOHPc, and ZnOEcn showed clear peaks at 860, 756, and 705 nm, respectively (Figure ).…”

“…To clarify the characteristics of the porphyrin isomers in the ET processes, parameters, such as the reorganization energy and electronic coupling, have to be compared. For this purpose, we systematically investigated the ET processes in the donor–acceptor supramolecular dyads, in which isomers of ZnOEP, that is, ZnOEPcn, ZnOEHPcn, and ZnOECn, were used as the donor . As an electron acceptor, derivatives of pyromellitic imide and phthalimide bearing a pyridine ring for ligation to Zn ion were employed (Scheme ).…”

“…Data is taken from Ref. . Reprinted with permission from American Chemical Society (Copyright (2009, 2010)), Elsevier (Copyright (2011)), and the PCCP Owner Society.…”

Photoinduced Electron- and Energy-Transfer Processes in Supramolecules using Imide Compounds

“…For the detailed understanding of the observed tendency of electron transfer rates of the supramolecular dyads of ZnOEP, ZnHPc, ZnCn, and ZnPcn, MO and molecular structures were investigated by means of MO calculations, because these isomers have same MO pattern due to the same substitution pattern. [45][46][47] It has been pointed out that electron transfer rates from the excited porphyrinoids relate to HOMO electronic densities. 37,[51][52] The largest HOMO electron density was found at the α-carbon of pyrrole ring of each porphyrinoids of the present dyads.…”