A viologen derivative, 1,1'-di-p-tolyl-(4,4'-bipyridine)-1,1'-diium dichloride (DTV2+), was studied in solution and encapsulated in cucurbit[7]uril (CB7), a macrocyclic host. Upon encapsulation, DTV2+ exhibited dramatically enhanced fluorescence. Aqueous solutions of DTV2+ were weakly fluorescent (Phi = 0.01, tau < 20 ps), whereas the emission of the DTV2+@2CB7 complex was enhanced by 1 order of magnitude (Phi = 0.12, tau = 0.7 ns) and blue-shifted by 35 nm. Similar properties were observed in the presence of NaCl. DTV2+ in a poly(methyl methacrylate) matrix was fluorescent with a spectrum similar to that observed for the complex in solution. H-1 NMR and UV-vis titrations indicated that the DTV2+@2CB7 complex is formed in aqueous solutions with complexation constants K-1 = (1.2 +/- 0.3) X 10(4) M-1 and K-2 = (1.0 +/- 0.4) X 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, state of DTV2+ 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 DTV2+@2CB7 in water exhibited a large Stokes shift (Delta St similar to 9000 cm(-1)) and no fine structure. DTV2+ is a good electron acceptor [E degrees(DTV2+/DTV center dot+) = -0.30 V vs Ag/AgCl] and a strong photooxidant [E degrees(DTV*(2+)/DTV center dot+) = 0.09 V vs NHE]).