Modulation of Energy Transfer into Sequential Electron Transfer upon Axial Coordination of Tetrathiafulvalene in an Aluminum(III) Porphyrin–Free-Base Porphyrin Dyad

• Published in: Inorganic chemistry 2015-09, Vol.54 (17), p.8482-8494
• Main Authors: Poddutoori, Prashanth K, Bregles, Lucas P, Lim, Gary N, Boland, Patricia, Kerr, Russ G, D’Souza, Francis
• Format: Article
• Language: English
• Subjects: Aluminum - chemistry; Electron transfer; Electron Transport; Energy Transfer; Excitation; Heterocyclic Compounds - chemistry; Metalloporphyrins - chemical synthesis; Metalloporphyrins - chemistry; Molecular Structure; Porphyrins; Pyridines; Radicals; Rate constants; Solvents
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.5b01190

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 (k EnT) 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 (k ET) 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.