Design of Porphyrin Nanoclusters toward Discovery of Novel Properties and Functions

Abstract

Abstract We have designed a variety of functional metalloporphyrin nanoclusters, connected via covalent or noncovalent linkages, since a discovery of fused bismetalloporphyrins [Ar3P]2(II) and [Ar3P]2(III) (Schemes 1–10 include symbols of the compounds), which have a strong π-electronic interaction over the bisporphyrin array. Among them, the following three topics are described in this review article. (1) Fused porphyrin nanoclusters: Directly fused oligoporphyrins, obtained through oxidative fused reactions of metalloporphyrins, show extremely strong π-electronic communications among the multiporphyrin components. (2) Self-assembled porphyrin nanoclusters: A zinc complex of meso–meso linked bisporphyrin [PyP]2(I) bearing 4-pyridyl substituents, which allow Zn–N coordination interaction, forms a homochiral box-shaped cyclic tetramer {[PyP]2(I)}4. Its rotamers upon insertion of an oligoalkynylene bridge between the two porphyrin components [PyP]2(≡)n (n = 1, 2, and 4) result in “conformational solvatochromism in nonpolar solvents” in n = 2 and “chiroptical sensing of asymmetric hydrocarbons” in n = 4, due to molecular recognition of solvents at the nanoscale cavity of box-shaped architecture. While a zinc porphyrin bearing a 3-pyridyl substituent (3-Py)P forms a cyclic tetramer with considerable structural distortion, its π-extended derivative (3-Py)P(≡)1 bearing a trimethylsilylethynyl group shows “supramolecular thermochromism” with a vivid color change, due to a temperature-dependent self-assembling event. A supramolecular polymer of J-aggregated zinc porphyrin dendrimer DP can visualize chiroptically a macroscopic chirality of vortex flows, generated by mechanical rotary stirring of a fluid. (3) Inorganic–organic nanocomposites: A giant ring-shaped polyoxomolybdate cluster MC having a large cavity with a diameter of approximately 2.3 nm accommodates up to three molecules of aminophenyl-substituted metalloporphyrins TAP via hydrogen-bonding interactions, resulting in formation of discrete inorganic/organic inclusion complexes. Template-assisted cofacial assembly of MC by an oligomeric p-phenylenebutadiynylene rigid rod molecule bearing ammonium ion pendants PBn allows formation of an inorganic/organic one-dimensional (1D) nano-object, having a polypseudorotaxane structure.