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
n
A, where n = 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
n
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 (K) describing the foldamer dimerization,
whose value equaled 1.0 (±0.2) × 106 M–1 for the three longer OPV-Q
n
A foldamers,
but was about 10 times smaller for the shortest one (n = 4). Association of OPV-Q8A and Q16A yielded
a complex with a ⟨ϕ⟩ matching that of OPV-Q24A, and K 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.