Fluorescence Sensing of Anions via Intramolecular Excimer Formation in a Pyrophosphate-Induced Self-Assembly of a Pyrene-Functionalized Guanidinium Receptor

Abstract

In mimicry of biological systems such as DNA, the fabrication of molecular assemblies and supramolecular arrays is one of the current research topics in supramolecular chemistry. 1 A wide variety of abiotic self-assembling systems such as catenanes and double helices have been described. 1 Additionally, artificial selfassembled receptors, where their substrate binding sites or cavities are organized by metal templation 2 or by self-association of monomeric ligands, 3 have been proposed to bind target substrates. The self-assembly of a receptor monolayer with a biomolecule such as ATP at the air-water interface has also been reported. 4 As a novel example of self-assembly for optical ion sensing, a K + ion-induced self-assembly has been described which consists of pyrene-tethered benzo-15-crown-5, γ-cyclodextrin, and K + ion. 5 This ternary complex, formed in water with high selectivity and sensitivity for K + , can be probed by pyrene dimer emission. 5 Thus, it is expected that the self-assembly of a chromophore-tethered receptor 6 by specific substrate binding offers a novel approach to the sensing of substrates in solution.Here we report a particularly simple self-assembling system for sensing of anions, with a pyrene-functionalized monoguanidinium receptor 1. The assembled system was quantitatively analyzed by means of 1 H NMR and fluorescence measurements. Although little attention has been paid to the structural analysis of such molecular assemblies, 7 determination of stoichiometry and structure is essential for understanding the photophysical origin of fluorescence and for making a rational design of a sensing system to distinguish structurally related substrates. The receptor 1 was found to self-assemble to form a 2:1 (host:guest) complex with high selectivity for biologically relevant pyrophosphate 8 (P 2 O 7 4-, PPi) in MeOH. From the complexation-induced changes in chemical shifts of the pyrenyl protons of 1, it is concluded that a sandwich-like ground-state pyrene dimer is present in the self-assembly. Formation of the self-assembly results in a remarkable change in the ratio of emission intensities of excimer to monomer due to the pyrenyl moiety of 1. It should be noted that only when the self-assembly is formed does 1 show a change in fluorescence spectrum. The present system, therefore, shows high selectivity for PPi that can promote formation of the self-assembly. In addition, calibration via ratiometry becomes possible by exploiting both monomer and excimer emissions. For anionic species, such emission ratio sensing has been known in only a few instances, 8b,9a despite its importance from a practical viewpoint.1 was synthesized by reacting 1-pyrenemethylamine hydrochloride with 3,5-dimethylpyrazole-1-carboxamidine nitrate in THF. Recrystallization from THF/MeOH gave 1 as nitrate salt. 10 The effect of anions on the fluorescence spectrum of 1 (8.0 × 10 -4 M) was examined in MeOH, 11 and the results are shown in Figure 1. Spectrum a was measured in the absence of anions, where 1 showed a structur...

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