Dendrimer–Metallomacrocycle Composites: Nanofiber Formation by Multi‐Ion Pairing

Abstract

Adv. Mater. 2008, 20, 1381–1385 2008 WILEY-VCH Verlag G Non-covalent self-assembly via hydrogen bonding, van der Waals, and coordinative or ionic interactions have found widespread application enroute to supramolecular assemblies. Examples of ion-based materials include a molecular capsule made by two oppositely charged calixarenes, pseudorotaxanes accessed by ion-pair templation, polyelectrolyte-amphiphile monolayers, and ion-pair interactions between biomolecular recognition sites and their guests. Although these non-covalent associations are usually weaker and reversible when compared to most covalent interactions, ionically driven attractions exhibiting multiple interactions per specie can result in very strong binding energies. Incorporation of these interactions has also facilitated the construction of materials and devices for applications in photonics, electronics, conducting polymers, and polyelectrolytes. Herein, we report the ion-promoted, automorphogenic, and stoichiometric self-assembly of nanoscale composite fibers using a structurally rigid hexameric macrocycle [(112þ)(PF 6 )12] possessing photonic and electron storage potential and a dodeca carboxylate-terminated, 1 generation 1! 3 C-branched dendrimer [(2 )(Na)12] (Scheme 1). For comparison, the structurally related 3rd generation dendrimer [(3 ) (Na)108; Scheme 2], was treated with [(1 12þ)(PF 6 )12] to afford a neutral, sphere-like motif [(1)9(3)]m. These polyanionic dense-packed counterions led to ion pair superstructures in which the randomness of singly-charged counterions was eliminated. Numerous examples of these Ru-based cyclized metallomers have been reported. In this instance, meta-substituted bis(terpyridinyl)arene building blocks constructed with either hydrocarbon aliphatic chains or PEG-moieties were incorporated to increase the solubility of these selfassembled metallohexamers in organic or aqueous solutions, respectively. This solubility enhancement resulted in the increase of the yields of the macrocyclization products. Treatment of 3,5-[bis(terpyridinyl)]phenol with n-bromohexane using standard alkylation conditions (K2CO3, DMF) afforded a white microcrystalline ditopic ligand, which upon treatment with 1 equiv. of [Ru(Cl)2(DMSO)4], [36] gave (40–45%) of the desired hexaruthenium macrocycle [(112þ)(Cl )12]. Following counterion exchange by treatment with NH4PF6, the structure of [(112þ)(PF 6 )12] was confirmed ( H NMR) by observation of the characteristic triplet absorption at 4.50 ppm (ArCH2OCH2) and singlet at 9.28 ppm indicative of the 3 0,50-tpy protons. Its MALDI-TOF MS [m/z 6046.89 (Mþ 1PF 6 )] further confirmed the structure. Complex [(112þ)(PF 6 )12] exhibited significantly enhanced solubility in common organic solvents such as MeCN, acetone, DMSO, and CHCl3 in contrast to its corresponding methyl-substituted hexameric homolog. Pertinent dimensions of [(112þ)(PF 6 )12], based on molecular modeling studies, include a 17.5 A internal diameter void region and a 37.5 A overall diameter of the terpyridine lattice. The G1 dendrimer [(2 )(Na)12] was prepared by hydrolysis of the corresponding 12-tert-butyl ester with formic acid and was previously reported to possess a hydrodynamic diameter of 23.6 A at basic pH, as determined by 2D diffusion ordered spectroscopy (DOSY) NMR experiments. Notably, this diameter is larger than the internal void region diameter (17.5 A) of [(112þ)(PF 6 )12], which suggests that an ordered molecular packing would be predicated on the symmetrical association of the dendrimer smoothly fitting above and not entirely into the central cavity of the hexamer. The polycarboxylate [(2 )(Na)12] (2.2 mg, 1.4 mM) in MeOH and water was carefully layered on top of a deep red MeCN (10 ml) solution of [(112þ)(PF 6 )12] (8.5 mg, 1.4 mM) and allowed to set undisturbed at 25 8C for two weeks (Fig. 1) in a sealed vial. After several hours, the formation of a cotton-like fiber was observed, and after 12 days, the solution became completely colorless, indicative of a self-assembly process. The resultant red fibers were filtered and washed sequentially with MeCN (3 5 ml) then water (5 5 ml) to remove traces of starting materials, if present, and any inorganic salts. In comparison, mixing 12 equiv. of NaOAc and 1 equiv. of [(112þ)(PF 6 )12] under the identical conditions resulted in no