Probing the Closed Association of Oligoquinoline Foldamers by Time-Resolved Fluorescence Anisotropy

The metal-mediated dimerization of oligoquinoline foldamers terminated at one end with an oligo(phenylenevinylene) and at the other with a carboxylic acid (OPV-Q_<i>n</i>A, where <i>n</i> = 4, 8, 17, and 33), and the complexation of OPV-Q₈A and Q₁₆A was promoted in chloroform by the addition of a concentrated 16 M aqueous sodium hydroxide solution. UV-vis absorption and time-resolved fluorescence anisotropy (TRFA) experiments were conducted to determine, respectively, the concentration and the average rotational time ⟨ϕ⟩ of the mixture of unassociated and associated foldamers across a range of foldamer concentrations spanning 5 orders of magnitude. Plots of ⟨ϕ⟩ as a function of acid group concentration revealed that ⟨ϕ⟩ increased with increasing foldamer concentration only when the foldamer solution in chloroform was vigorously mixed with the 16 M sodium hydroxide aqueous solution. Furthermore, all plots showed that ⟨ϕ⟩ reached a plateau at high foldamer concentration. The increase in ⟨ϕ⟩ reflected the association of foldamers into larger objects through metal ion coordination with the carboxylate anions generated by deprotonation of the carboxylic acid of OPV-Q_<i>n</i>A with NaOH, while the plateau obtained at high foldamer concentration indicated that these interactions led to the dimerization of the foldamers via a closed association mechanism. Analysis of the ⟨ϕ⟩ trends yielded the equilibrium constants (<i>K</i>) describing the foldamer dimerization, whose value equaled 1.0 (±0.2) × 10⁶ M^-1 for the three longer OPV-Q_<i>n</i>A foldamers, but was about 10 times smaller for the shortest one (<i>n</i> = 4). Association of OPV-Q₈A and Q₁₆A yielded a complex with a ⟨ϕ⟩ matching that of OPV-Q₂₄A, and <i>K</i> for this complexation was similar to that for dimerization. These experiments illustrate the robust nature of TRFA as an experimental method to probe the size of rigid, self-assembled foldamers in solution.