One‐Dimensional, Cofacial Porphyrin Polymers Formed by Self‐Assembly of meso‐Tetrakis(ERE Donor) Zinc(II) Porphyrins

Abstract

A series of meso-tetrakis-(ERE donor) zinc(II) porphyrins nZn (ERE donor = 4-R-3,5-bis[(E)-methyl]phenyl; 1Zn: E = NMe2, R = Br; 2Zn: E = NMe2, R = H; 3Zn: E = OMe, R = Br; 4Zn: E = OMe, R = H) have been synthesized in excellent yields. As a result of the combination of a Lewis acidic site and eight Lewis basic sites within one molecule, monomeric molecules of nZn self-assemble to form one-dimensional porphyrin polymers [nZn](infinity) in the solid state, as confirmed for 1Zn and 3Zn by X-ray crystallography. The coordination environment around the zinc(II) ions in these polymers is octahedral. They are ligated by four equatorial nitrogen atoms of the porphyrin and two apical E atoms (E = N, O) provided by the EBrE donor groups of adjacent nZn molecules. Complexes 2Zn and 4Zn did not form single crystals, but solid-state UV/Vis analysis points to the formation of similar structures. Solution UV/Vis and 1H NMR spectroscopy indicated that interactions between 1Zn and 2Zn monomers in the polymers are stronger than between 3Zn and 4Zn monomers. Interestingly, they also revealed that the presence of a neighboring bromine atom in the EBrE donor groups has a considerable influence on the coordination properties of the benzylic N or O atoms. The zinc(II) ions of the porphyrins most likely adopt only hexacoordination in the solid state, owing to the unique predisposition of Lewis acidic and basic sites in the nZn molecules. Several parameters of the aggregates, for example, the interplanar separation between porphyrins and the zinc-zinc distances, change as a function of the coordinating E groups. The high degree of modularity in their synthesis makes these zinc(II) porphyrins an interesting new entry in noncovalent multiporphyrin assemblies.

Efficient Exciton Transport in Layers of Self-Assembled Porphyrin Derivatives

“…and electron-accepting groups, comparable self-assembling behavior is expected, as also proposed earlier 46 and validated below. ZnOPBr is not included in the current study on exciton transfer, since the heavy Br atom leads to significant intersystem crossing from singlet to triplet excited states, 47 which will be the topic of another study.…”

“…The redshift in absorption and increase of the intensity of the lowest Q-band are less pronounced for ZnOP than those for ZnNP, which might originate from the stronger coordinative bond in the case of the latter. 46,59 The Soret absorption band of the ZnOP film shows a more pronounced split than that for ZnNP. This is attributed to a stronger excitonic coupling between selfassembled ZnOP molecules in a stack, possibly caused by a smaller center-to-center distance between adjacent molecules 46 leading to a larger excitonic splitting.…”

“…Synthesis of Porphyrin Derivatives. The porphyrin derivatives ZnOP, ZnNP, and ZnBuP were synthesized as described in ref 46.…”

“…A close analogue of ZnOP having Br atoms at the para-positions of the phenyl groups (ZnOPBr) has recently been observed by single-crystal X-ray crystallography to self-assemble as shown in Figure 1B. 46…”

62 Porphyrin-Based Tectons in Molecular Tectonics

“…The same type of architecture has been also observed for M(PE 2 R) 4 Po porphyrins bearing at both meta positions of the bulky aryl group coordinating sites. 51 In that case, the Zn(II) center is again hexacoordinated and both axial positions are occupied by benzylic E ligands (E = -CH 2 -NMe 2 or -CH 2 -OMe). As stated above, among the four aryl groups present at the meso positions of the porphyrin, only two in the trans configuration participate in the formation of the network through binding to adjacent Zn atoms ( Figure 6).…”