Transient EPR studies of ion-paired metalloporphyrin heterodimers

Spin-polarized transient EPR spectra measured at 9 GHz (X-band) and 24 GHz (K-band) are observed for the metalloporphyrin heterodimers formed by MTTAP (metal/free base meso-tetrakis [4-trimethylanilinium] porphyrin, M = Zn, Mg or H-2) and CuTSPP (copper meso-tetrakis[p-sulfonatophenyl]porphyrin). In the temperature range 5-120 K, the EPR transients obtained following pulsed light excitation at 532 nm show a strong temperature dependence. The dimers exhibit two distinct types of EPR signals which may be separated on the basis of their different decay kinetics, The spectrum of one of the signals can be assigned to a triplet state of the dimers, whereas the other spin-polarized spectrum clearly shows involvement of the Cu(II) metal ion. This spectrum is tentatively assigned to the doublet ground state of the CuTSPP moiety in the dimers. It is proposed that the observed spin polarization is generated by a radical triplet pair mechanism (RTPM). In ZnTTAP-CuTSPP and H(2)TTAP-CuTSPP both spectra are observed, whereas only the latter spectrum is obtained in MgTTAP-CuTSPP. In ZnTTAP-CuTSPP, the triplet-state spectrum shows only small but significant differences from that of the isolated ZnTTAP monomer, indicating that the triplet state is localized on the ZnTTAP component of the dimer. The corresponding spectrum in the free-base dimer is considerably narrower than that of the monomer H(2)TTAP which suggests that, in this case, the triplet state is partly delocalized over both halves of the dimer.