Ground state absorption and steady-state and time-resolved fluorescence measurements have been carried out to understand the host-guest interactions of p-diethylaminobenzonitrile (DEABN) and p-dimethylaminobenzonitrile (DMABN) dyes with α-cyclodextrin (α-CD) and β-cyclodextrin (β-CD) hosts. DEABN and DMABN dyes show both locally excited (LE) state and intramolecular charge transfer (ICT) state emissions in solution. The LE and ICT emissions of the dyes are seen to get modulated in the presence of α-CD and β-CD hosts. The results indicate that the dyes form 1:1 inclusion complexes with both the hosts. Comparing the binding constants and the fluorescence characteristics of different dye-CD systems it is inferred that DEABN adopts a completely different orientation on complexation with α-CD than in the other cases of dye·CD systems. It is indicated that while in all other cases of dye-CD systems the N,N-dialkyl group of the dyes enters the host cavity leaving the C≡N group projected out into the water phase, the DEABN dye enters the α-CD cavity (smallest CD) with its C≡N group entering the host cavity. The differences in the orientation of the dye in the host cavities is understood to be determined by the requirement of maximum van der Waals contact of the encapsulated dye with the host cavity for maximum stability of the complex and the relative sizes of the substituents of the dye compared to the host cavities. From the observation that the binding constants for the present dye-CD systems are not that significantly high, it is inferred that the hydrophobic interaction mainly govern the inclusion complex formation in the present systems.
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