Pyrene-coumarin Based Calix Fluorophore Emitting Exciplex

Abstract

The chemistry of calixarenes, cyclic oligomers composed of phenolic and methylene moieties, has been extensively studied in recent years. The preorganized binding sites, easy derivatization and flexible three-dimensional steric structures make them perfect platforms for sensors to generate fluorescent receptors. Fluorescent chemosensors based on calix[4]arene framework and capability of selectively recognizing heavy metal ions and anions have been studied in fields of chemistry and biology. Calix[4]arenes as host molecules for anions or cations have been of particular interest for the following two different reactive sites: (1) phenolic OHs (lower rim) and (2) para positions (upper rim), which can be readily functionalized by various cation-ligating groups such as carboxylic acid, amide, and azacrown ether. In this paper, amide groups as a ligating site are known to capture cations through carbonyl oxygen atoms as well as anions through the hydrogen bonding between anion and the acidic hydrogen atoms on nitrogen atom of coumarin. Most calixarene-based fluorescence sensors have been designed to adopt a photophysical change upon cation or anion binding: PET (photo-induced electron transfer), PCT (photo-induced charge transfer), FRET (fluorescence resonance energy transfer), excimer/exciplex formation and extinction. Although an excimer emission from two facing pyrenes has been widely studied, an exciplex emission between pyrene and coumarin has been rarely observed. Here, we report an exciplex formation between pyrene as an electron donor and coumarin as an acceptor followed by disappearance of the exciplex upon the F binding which executes “Off” sensing. As shown in Scheme 1, reaction of 1 with 7-[N-(2bromoethyl)amino]-4-trifluoromethylcoumarin (4) and K2CO3 as a base in MeCN produces 2 in 75% yield. H NMR spectrum of 2 exhibits two doublets at 3.79 and 3.29 ppm in AB pattern with coupling constant of 13.14 Hz corresponding to protons of the methylene bridge and there is a peak at 31.3 ppm in the C NMR spectrum, suggesting that 2 be in the cone conformation. 2-(4-(Trifluoromethylcoumarin))-N-((1-pyrenyl)methyl)acetamide (3) as a refer-