Modulation of energy transfer into sequential electron transfer upon axial coordination of tetrathiafulvalene in an aluminum(III) porphyrin-free-base porphyrin dyad

Axially assembled aluminum(III) porphyrin based dyads and triads have been constructed to investigate the factors that govern the energy and electron transfer processes in a perpendicular direction to the porphyrin plane. In the aluminum(III) porphyrin-free-base porphyrin (AlPor-Ph-H2Por) dyad, the AlPor occupies the basal plane, while the free-base porphyrin (H2Por) with electron withdrawing groups resides in the axial position through a benzoate spacer. The NMR, UV-visible absorption, and steady-state fluorescence studies confirm that the coordination of pyridine appended tetrathiafulvalene (TTF) derivative (TTF-py or TTF-Ph-py) to the dyad in noncoordinating solvents afford vertically arranged supramolecular self-assembled triads (TTF-py→AlPor-Ph-H2Por and TTF-Ph-py→AlPor-Ph-H2Por). Time-resolved studies revealed that the AlPor in dyad and triads undergoes photoinduced energy and/or electron transfer processes. Interestingly, the energy and electron donating/accepting nature of AlPor can be modulated by changing the solvent polarity or by stimulating a new competing process using a TTF molecule. In modest polar solvents (dichloromethane and o-dichlorobenzene), excitation of AlPor leads singlet-singlet energy transfer from the excited singlet state of AlPor (1AlPor*) to H2Por with a moderate rate constant (kEnT) of 1.78 × 108 s-1. In contrast, excitation of AlPor in the triad results in ultrafast electron transfer from TTF to 1AlPor* with a rate constant (kET) of 8.33 × 109-1.25 × 1010 s-1, which outcompetes the energy transfer from 1AlPor* to H2Por and yields the primary radical pair TTF+•-AlPor-•-H2Por. A subsequent electron shift to H2Por generates a spatially well-separated TTF+•-AlPor-H2Por-• radical pair.