Effect of peripheral substitution on electron and energy transfer in electrostatically bound zinc-magnesium porphyrin dimers

We report on intramolecular electron and/or energy transfer (IET and IEnT), in two photoexcited electrostatically bound dimers: (1) zinc(II) tetrakis(4-sulfonatophenyl)porphyrin and magnesium(II) tetrakis(4-N,N,N-trimethylanilinium)porphyrin, [ZnTSPP-MgTTAP]; and (2) zinc(II) tetrakis(4-N,N,N-trimethylanilinium)porphyrin and magnesium(II) tetrakis(4-sulfonatophenyl)porphyrin, [ZnTTAP-MgTSPP]. The photochemical properties of both dimers, dissolved in dimethylsulfoxide-glycerol (DMSO-Glyc, 140 K), were investigated by continuous wave time-resolved electron paramagnetic resonance (CW-TREPR) spectroscopy. It is demonstrated that by changing the peripheral substitution of the particular metalloporphyrin in the dimer, an IET route, parallel to the singlet IEnT, can be triggered. Thus, singlet IEnT from the Zn to the Mg constituent is the only process in [ZnTSPP-MgTTAP], while IET to produce a charge-separated state, [ZnTTAP(.+)-MgTSPP.-] accompanies the IEnT in [ZnTTAP-MgTSPP].