Intramolecular charge transfer character in tetrathiafulvalene-annulated porphyrinoids: effects of core modification and protonation

The synthesis, characterization, and photophysical and electrochemical properties of a novel tetrathiafulvalene (TTF)-annulated core-modified porphyrin (1) and its expanded rubyrin analogue (2) are described. The sulfur core modifications in 1 and 2 allow a feasible intramolecular charge transfer from the TTF fragments to the central conjugated core as inferred from comparative spectroscopic and electrochemical measurements. DFT calculations also support the intramolecular charge transfer nature of 1 and 2 upon excitation. Further the electronic perturbation of the TTF-annulated porphyrins was achieved by protonation, giving rise to a drastic change in the optical features with an extremely low energy band in the NIR region. The pronounced electron accepting ability of the macrocyclic core of the dicationic species (H212+ and H222+) resulted in the thermally excited electron transfer occurring at room temperature as elucidated by EPR spectroscopy.