Cucurbit[8]uril Mediated Donor–Acceptor Ternary Complexes: A Model System for Studying Charge-Transfer Interactions

Abstract

A supramolecular self-assembly approach is described which allows for the convenient preparation of a wide range of charge-transfer (CT) donor-acceptor complexes in aqueous solutions. When one equiv of the macrocyclic host cucurbit[8]uril (CB[8]) is added to an aqueous donor and acceptor solution, a heteroternary complex forms inside the host's cavity with a well-defined face-to-face π-π-stacking geometry of the donor and acceptor. This heteroternary, CB[8]-mediated complex offers the opportunity to study the CT phenomena at low concentrations and free from complications arising from any donor-donor and acceptor-acceptor interactions as a result of the large binding affinities and the very high selectivity over the formation of these homoternary complexes. Thus, this supramolocular self-assembly strategy is a practical donor-acceptor mix-and-match approach with synthetic advantages over much more cumbersome tethering schemes. While the characteristic UV/vis features of a few CB[8] ternary systems had been described as a CT band, we present for the first time systematic evidence for the existence of CT interactions between several donor-acceptor pairs that are mediated by the host CB[8]. Correlation of the experimentally obtained CT λ(max) to computed HOMO-LUMO energies demonstrated that the CT process in the host's cavity can be described by the Mulliken model. Furthermore, the literature claim of a "CT driving force" for the formation of CB[8] ternary complexes was scrutinized and evaluated by calorimetric (ITC) and ESI-MS measurements. The findings indicated that neither in the aqueous medium nor in the "gas-phase" is CT of energetic relevance to the Gibbs free binding energy. In contrast, electrostatic considerations combined with solvation effects are much better suited to rationalize the observed trends in binding affinities. Additionally, the CT λ(max) was found to be much more red-shifted (≥75 nm) inside the CB[8] cavity than in any polar organic solvents or water, indicating a significant stabilization of the CT excited state within the host cavity, further demonstrating the unique electrostatic, polar properties of the host cavity.

Effects of sequence context on the binding of tryptophan-containing peptides by the cucurbit[8]uril–methyl viologen complex

“…Compared to other cucurbit[n]urils, Q8 has been studied extensively for its ability to bind two guests simultaneously in aqueous solution (20)(21)(22)(23)(24)(25)(26)(27)(28)(29). Much of our work with Q8 has focused on the molecular recognition of peptides (30)(31)(32)(33)(34)(35)(36).…”

Sequential inclusion of two berberine cations in cucurbit[8]uril cavity: kinetic and thermodynamic studies

“…CB8 has a rather uniform negative electrostatic potential inside the cavity and at the portal area, 18 which probably contributes to the screening of the electrostatic repulsion between the two positively charged guests.…”

“…17 Biedermann and Scherman demonstrated that the CT excited state is much more efficiently stabilized by the uniform negative electrostatic potential of the CB8 interior than in any polar solvents. 18 They provided evidence for the CB8-promoted CT interaction between guests, and concluded that CT is not the dominant factor in the stabilization of the ternary complexes. The coinclusion of electron-deficient and electron-rich moieties facilitated the creation of host-stabilized supramolecular polymers, 19,20 polyrotaxanes, 21 molecular necklaces, 22 vesicles, 23 and rotaxane dendrimers.…”

“…Biedermann and Scherman have established that charge transfer interaction between guests of electron donor and acceptor characters is not the primary driving force of ternary complex formation with CB8. 18 They developed a comprehensive model, which emphasized the pivotal role of the exclusion of highenergy water from CBn cavity in the stabilization of inclusion complexes. 54,57 The concept, which explains the trends in the thermodynamic parameters of many CBn complexes, 54,57 is also valid for the homoternary 2B + @CB8 associate.…”

Stimuli‐Responsive Cucurbit[n]uril‐Mediated Host‐Guest Complexes on Surfaces

“…These portals narrow the cavity entrance compared to the inner diameter of the cavity (Table 1). [7,9] Electrostatic potential maps show a negative potential at the CB[n] portals, [5,10] which drives the molecular recognition properties by forming charge-dipole and hydrogen bonding interactions with guests. [5,10,11] Much research on CB[n] macrocycles focusses on understanding CB[n]-guest complexation and modulation of the CB[n]-guest binding affinity by external stimuli.…”