Phenothiazine-Bridged Cyclic Porphyrin Dimers as High-Affinity Hosts for Fullerenes and Linear Array of C60 in Self-Assembled Porphyrin Nanotube

Free-bases and a nickel­(II) complex of phenothiazine-bridged cyclic porphyrin dimers bearing self-assembling 4-pyridyl groups (M2-Ptz-CPDPy(OCn); M = H2 or Ni, OCn = OC6 or OC3) at opposite meso-positions have been prepared as host molecules for fullerenes. The free-base dimer (H4-Ptz-CPDPy(OC6)) includes fullerenes with remarkably high association constants such as 3.9 ± 0.7 × 106 M–1 for C60 and 7.4 ± 0.8 × 107 M–1 for C70 in toluene. This C60 affinity is the highest value ever among reported receptors composed of free-base porphyrins. The nickel dimer (Ni2-Ptz-CPDPy(OC6)) also shows high affinities for C60 (1.3 ± 0.2 × 106 M–1) and C70 (over 107 M–1). In the crystal structure of the inclusion complex of C60 within H4-Ptz-CPDpy(OC3), the C60 molecule is located just above the centers of the porphyrins. The two porphyrin planes are almost parallel to each other and the center-to-center distance (12.454 Å) is close to the optimal separation (∼12.5 Å) for C60 inclusion. The cyclic porphyrin dimer forms a nanotube through its self-assembly induced by C–H···N hydrogen bonds between porphyrin β-CH groups and pyridyl nitrogens as well as π–π interactions of the pyridyl groups. The C60 molecules are linearly arranged in the inner channel of this nanotube.