Spin-Selective Electron Transfer and Charge Recombination in Self-Assembled Porphyrin Naphthalenediimide Dyads

The spin selectivity of electron transfer in a series of metalloporphyrin pyridyl-linked naphthalenediimides (MTPP-Pyr(CH ₂) _nNDI, where M = Zn, n = 2, 4, 7, and M = Al(OCOPh), n = 7) is studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy in the nematic liquid crystal 4-cyano-4′-pentylbiphenyl (5CB). Following pulsed laser excitation, all of the complexes show a narrow antiphase doublet that is assigned to the triplet state of the radical pair MTPP ^•+NDI ^•-. Initially, the antiphase doublet has an emission/absorption (E/A) polarization pattern characteristic of singlet electron transfer. At later times the polarization inverts to an A/E pattern. The intensity of the late signal depends very strongly on the nature of the metal in the porphyrin. A qualitative model that rationalizes this result is presented. It is proposed that both singlet and triplet electron transfer occur in the dyads and that the differences in the intensity of the polarization are the result of differences in the spin selectivity of intersystem crossing for the different metals. The consequences of this model for magnetic field effects in such systems are briefly discussed.