Fluorescence enhancement of Di-p-tolyl viologen by complexation in cucurbit[7]uril

A viologen derivative, 1,1′-di-p-tolyl-(4,4′-bipyridine)-1, 1′-diium dichloride (DTV ²⁺), was studied in solution and encapsulated in cucurbit[7]uril (CB7), a macrocyclic host. Upon encapsulation, DTV ²⁺ exhibited dramatically enhanced fluorescence. Aqueous solutions of DTV ²⁺ were weakly fluorescent (Φ = 0.01, τ < 20 ps), whereas the emission of the DTV ²⁺@2CB7 complex was enhanced by 1 order of magnitude (Φ = 0.12, τ = 0.7 ns) and blue-shifted by 35 nm. Similar properties were observed in the presence of NaCl. DTV ²⁺ in a poly(methyl methacrylate) matrix was fluorescent with a spectrum similar to that observed for the complex in solution. ¹H NMR and UV-vis titrations indicated that the DTV ²⁺@2CB7 complex is formed in aqueous solutions with complexation constants K ₁ = (1.2 ± 0.3) × 10 ⁴ M ^-1 and K ₂= (1.0 ± 0.4) × 10 ⁴ M ^-1 in water. Density functional theory and configuration interaction singles calculations suggested that the hindrance of the rotational relaxation of the S ₁ state of DTV ²⁺ caused by encapsulation within the host or a polymer matrix plays a key role in the observed emission enhancement. The absorption and emission spectra of DTV ²⁺@2CB7 in water exhibited a large Stokes shift (ΔSt ∼ 9000 cm ^-1) and no fine structure. DTV ²⁺ is a good electron acceptor [E°(DTV ²⁺/DTV ^•+) = -0.30 V vs Ag/AgCl] and a strong photooxidant [E°(DTV ²⁺/DTV ^•+) = 0.09 V vs NHE]).