Axially Linked Triphyrin(2.1.1)-Metalloporphyrin Dyads: Interplay between Aromatic−Aromatic and Antiaromatic−Aromatic Systems

Two aromatic-aromatic axially bonded dyads and two rare antiaromatic-aromatic axially bonded dyads containing the triphyrin(2.1.1)/P(O) complex of triphyrin(2.1.1) and metalloporphyrin (ZnP/Ru(CO)P) were synthesized via M^II-pyridyl "N" interaction. The dyads are stable and thoroughly characterized by NMR spectroscopy and X-ray crystallography for one of the dyads. The X-ray structure analysis of the dyads showed that the β-pyridyl triphyrin unit was oriented almost perpendicular to Zn(II) of the ZnP unit via a noncovalent interaction. NMR studies revealed pronounced upfield shifts of the pyridyl protons due to the diatropic ring current of the metalloporphyrin, with the largest shift observed for the 2,6-pyridyl protons upon coordination to Ru(CO)P in the triphyrin-Ru(CO)P and P(O)triphyrin-Ru(CO)P dyads. Absorption and electrochemical studies revealed that the components in the dyads interact weakly and retain their individual characteristic features. The antiaromatic-aromatic dyads additionally exhibited a characteristic broad band in the region of 630-1000 nm and showed a slightly lower binding constant than the aromatic-aromatic dyads. The steady-state fluorescence studies on the aromatic-aromatic triphyrin(2.1.1)-ZnP dyad indicated a possibility of energy transfer in the singlet state from the axial triphyrin unit to the ZnP unit, whereas the antiaromatic-aromatic dyads were nonfluorescent. DFT and TD-DFT studies were in agreement with the experimental observations.