Encapsulation of electron donor-acceptor dyads in β-cyclodextrin cavity: unusual planarization and enhancement in rate of electron-transfer reaction

Abstract

Interaction of β-cyclodextrin (β-CD) with a few novel electron donor acceptor dyads 1a-c and 2a-c, having aryl and flexible methylene spacer groups, has been investigated through photophysical, chiroptical, electrochemical, NMR, and microscopic techniques. Dyads 1a and 1c, with p-tolyl and biphenyl spacer groups, respectively, exhibited significantly decreased fluorescence quantum yields and lifetimes in the presence of β-CD, while negligible changes were observed for dyad 1b with an o-tolyl spacer. In contrast, spacer-length-dependent significant enhancement in fluorescence quantum yields and lifetimes was observed for dyads 2a-c, with flexible polymethylene (n = 1, 3, 11) spacer groups. Association constants of β-CD encapsulated complexes have been determined and the contrast behavior observed in these systems is explained through an electron transfer (k_ET) mechanism based on calculated favorable change in free energy (ΔG_ET = −1.27 eV) and the redox species characterized through laser flash photolysis studies. Rates of k_ET have been estimated and are found to increase ca. 2-fold in the case of dyads 1a and 1c when encapsulated in β-CD, while significantly decreased k_ET values were observed for the dyads 2a-c with flexible spacer (ca. 9-fold for 2c). As characterized through cyclic voltammetry, 2D NMR [correlated (COSY) and nuclear Overhauser enhancement (NOESY) spectroscopy], and laser flash photolysis studies, theβ-CD encapsulation of dyads with aliphatic spacer groups leads to the conformational unfolding of a sandwich type of structure, whereas dyads with rigid aryl spacer groups undergo unusual planarization as compared to the uncomplexed dyads, resulting in enhanced electron-transfer reaction between the donor and acceptor moieties.