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. (1)H 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]).