Cyclodextrin-assisted modulation of the photophysical properties and acidity constant of pyrene-armed calix[4]arene

Abstract

The supramolecular interaction of cyclodextrin homologues with a pyrene-armed calixarene has been investigated. p-tert-Butylcalix[4]arene-di-4-(pyren-1-yl)butanoic acid (PCX4) has been synthesized and characterized by ¹H NMR, FT-IR and mass spectrometry. The modulation in the photophysical properties, especially the monomer and excimer emission behaviour of the pyrene group in PCX4 with β-cyclodextrin (β-CD) and γ-cyclodextrin (γ-CD), has been explored in aqueous solutions by using optical absorption, steady-state and time-resolved fluorescence measurements. Steady-state fluorescence studies illustrate that upon gradual addition of γ-CD to the PCX4 solution, initially there is a decrease in the monomer emission (at 378, 398 and 420 nm) along with the appearance of weak excimer emission at 482 nm. Beyond a particular concentration of γ-CD (>1.2 mM), PCX4 shows strong excimer emission along with an increase in the monomer emission, indicating two types of interaction modes between PCX4 and γ-CD. Fluorescence anisotropy measurements confirm intramolecular excimer formation with a lower concentration of γ-CD, whereas it predominantly becomes an intermolecular excimer beyond 1.2 mM of γ-CD. On the other hand, the monomer emission of the pyrene group of PCX4 increases in the presence of β-cyclodextrin having lower cavity sizes due to the formation of a 1 : 1 complex. For the first time, the intrinsic fluorescence from the pyrene moiety of PCX4 is employed to determine the pK_a value of PCX4 and it is ∼6.1. The cyclodextrin-encapsulated PCX4 brings a 1.2 unit downward pK_a shift. Along with the pH-responsive interconversion of monomer to excimer emission of γ-CD complexed PCX4, this system can find applications in ratiometric sensing and optical supramolecular pH-sensors.