THE TWISTED-INTRAMOLECULAR-CHARGE-TRANSFER-STATE-FORMING COMPOUND AS A GUEST FOR CYCLODEXTRINS

Spectral luminescence behaviour of N-methyl-p-(p-dimethylaminophenyl)-pyridinium (APP) iodide in aqueous alpha- and beta-cyclodextrin (CD) solutions has been studied. The inclusion complex formation of most probably 1:1 constitution was established with binding constant K of about 400 M-1 for beta-CD. The inclusion of APP is far less favourable for alpha-CD (K=7 M-1) than for beta-CD. The data obtained confirm the suggestion that the APP cation is able to form the twisted intramolecular charge transfer (TICT) state upon excitation. The inclusion host-guest complex formation of APP with alpha- and beta-CD is inferred to cause the non-TICT fluorescence quantum yield enhancement by factors of 1.3 and 3.33 respectively, and to hamper the TICT state formation mainly through spatial restriction experienced by the guest molecule within the CD cavity. The ability of the CD complexation to assist in indentifying the guest's excited state nature has thus been demonstrated.