Encapsulation of 2-(4′-N,N-dimethylamino)phenylimidazo[4,5-b]pyridine in β-cyclodextrin: effect on H-bond-induced intramolecular charge transfer emission

Abstract

The effect of β-cyclodextrin (β-CD) inclusion complex formation on the hydrogen bond-induced intramolecular charge transfer (ICT) of 2-(4′-N,N-dimethylamino)phenylimidazo[4,5-b]pyridine (DMAPIP-b) has been examined by fluorescence excitation, emission and time-resolved fluorescence techniques. The study reveals that DMAPIP-b forms 1 : 1 inclusion complex with β-CD. The host–guest complex is formed by partial inclusion of DMAPIP-b, i.e. only the dimethylaminophenyl ring is encapsulated inside the core of the β-CD nanocavity. The imidazopyridine ring of the guest molecule resides outside CD cavity and forms H-bonds with the water molecules that are present near the rim and in the bulk phase. ¹H NMR studies are used to confirm the inclusion complex. The H-bond of water with the pyridine nitrogen ensures the formation of the ICT state and both normal and ICT emissions are enhanced inside the β-CD cavity. Fluorescence lifetime measurements suggest that the formation of the ICT state from the locally excited state is irreversible. Dual emission is observed in the presence of β-CD at pH ∼ 3.5, due to emission from monocations formed by the protonation of pyridine nitrogen (MC1) and imidazole nitrogen (MC2).