Light-Induced Spin Polarization in Porphyrin-Based Donor-Acceptor Dyads and Triads

The light-induced spin polarization generated by sequential electron transfer in an axially bound triad based on Al(III) porphyrin (AlPor) is discussed. In the triad, TTF-Ph-py→AlPor-Ph-NDI, the electron acceptor naphthalene diimide (NDI) is attached covalently to the Al(III) center, while the donor tetrathiafulvalene (TTF) coordinates to Al(III) via an appended pyridine (py) on the opposite face of the porphyrin ring. Excitation of the porphyrin at room temperature in solution leads to charge separation between the donor and acceptor. In the liquid crystalline solvent 5CB, a spin-polarized transient electron paramagnetic resonance spectrum of a weakly coupled radical pair is observed and is assigned to the state TTF^·+NDI^·-. In the absence of the donor, a spectrum of the triplet state of the strongly coupled radical pair AlPor^·+NDI^·-is obtained. The analysis of the spectra is described using a model developed by Kandrashkin et al. (Appl Magn Reson 15: 417-447, 1998). It is shown that in the triad the spectrum of TTF^·+NDI^·- shows evidence of the singlet-triplet mixing in the precursor AlPor^·+NDI^·-. At later time, singlet recombination leads to inversion of the spectrum, from which the singlet back reaction lifetime is estimated as 350 ns. The decay of the inverted spectrum yields a lifetime of 8. 3 μs for the triplet back reaction lifetime.