No abstract

The metal complexes of ligands I, a pyrrol-2-yl Schiff base or pyrrol-2-ylmethyleneamine, have been known for a long time. [1][2][3] Macrocycles containing pyrrol-2-ylmethyleneamine units, such as texaphyrins and expanded porphyrins, [4] have been extensively investigated. Linear-spacer-bridged bis(pyrrol-2-ylmethyleneamine)s and their complexes with metal ions were recently reported, [5][6][7][8][9] which showed that both the preparation of the ligands and the complexes were highly efficient, and the metal complexes formed by pyrrol-2-ylmethyleneamines possessed good solubility in common solvents. These facts encouraged us to explore their use as a building block for supramolecular self-assembly. In this paper we report the neutral doublestranded helicates, trimeric triangles, and tetrameric squares formed through the self-assembly of spacer-bridged bis(pyrrol-2-ylmethyleneamine) ligands with Zn II ions. Ligands 2-5 were prepared in high yields (Scheme 1) by the condensation of diamines with 4-ethyl-5-formyl-3-methyl-1H-pyrrole-2-carboxylic acid ethyl ester (1) in ethanol. The ligands 2-5 were each reacted with zinc(ii) acetate dihydrate in THF (Scheme 2), to give the complexes 6-9, respectively, in excellent yields. The complexes, isolated as red (6 and 8) or yellow (7 and 9) powders, are soluble in most common organic solvents. Crystals were grown by slowly evaporating their solutions in THF/EtOH (6 and 7), THF/MeOH (8), or THF/ di(ethylene glycol) (9). All of the crystals are stable in the dark under a nitrogen atmosphere, but decomposed quickly in the air.Analysis of the complexes, primarily by MALDI-TOF mass spectrometry, showed that the molecular masses of 6 and 7 were 1105 and 1284, respectively, which corresponds to dimeric complexes with two ligands and two zinc centers. However, the molecular masses of 8 and 9 were found to be 2209 and 1675, which corresponds to a tetrameric complex (four ligands and Zn centers) and trimeric complex (three ligands and Zn centers), respectively.The 1 H NMR spectra of the zinc complexes 6-9 in CDCl 3 show a single set of proton resonances that could be fully assigned, which suggests that they exist as single species in solution. In all cases, the disappearance of the NH protons from the free ligands was observed, which is in agreement Scheme 1. Synthesis and structure of ligands 2-5.Scheme 2. Schematic representation of complexes 6-9.

A series of flexible bis(9-anthryldiamine) ligands (L1-L3) linked with alkyl spacers of different chain length was synthesized and characterized, in order to investigate the coordination behavior of these diamine ligands with metal ions (Zn2+, etc.) based on fluorescence measurements. The results showed that, in the case of anthryldiamine ligands bearing two- or four-carbon links, the zinc ion induced a chelation-enhanced fluorescence (CHEF) effect in aqueous media, while a trace amount of water could selectively quench the blue emission of the Zn(II) complex with a three-carbon-linked ligand (1). Meanwhile, the introduction of more water (concentration >11 %) resulted in the formation of a new green luminescent species; the luminescence intensity was enhanced stepwise to a maximum with addition of approximately 30 % water in THF solution. The peak position (centered at approximately 500 nm) and the lifetime measurement (tau=19.59 ns) indicated that the green luminescence was attributable to a novel edge-to-face dimeric conformation ("T-shaped" conformation) of anthracene, and not to the more common face-to-face dimeric conformation. Accordingly, 1H NMR spectroscopic studies in nonaqueous or aqueous solution confirmed this T-shaped conformation, which is consistent with the results of single-crystal X-ray structure analysis and solid-state photoluminescence studies.