Spectroscopic study on encapsulation of two structurally similar donor–acceptor dyes in model cyclodextrin nanocavity

• Encapsulation of two donor–acceptor dyes with restrictive and unrestrictive C–N bond rotation by β-CD.
• Dye with hindered C–N bond rotation (N2) forms 1:1 complex at lower β-CD concentration.
• N2 forms 1:2 complex at a higher β-CD concentration.
• Dye with rotable C–N bond (DN2) undergoes only 1:1 complexation with β-CD.
• Quantum chemical calculations (PM3) confirm observed modes of encapsulation.

Encapsulation of two structurally similar donor–acceptor dyes in β-cyclodextrin (β-CD) nanocavity has been studied by steady state and time resolved fluorescence spectroscopic procedures. In the dye, (E)-2-((E)-3-(methyl(phenyl)amino)allylidene)-2,3-dihydro-1H-inden-1-one, the rotation about C–N bond is possible, while such a rotation is strategically restricted in the dye (E)-2-((1-methyl-3a,7a-dihydro-1H-indol-3-yl)methylene)-2,3-dihydro-1H-inden-1-one. Results indicate that both the dyes form complexes with β-CD but the stoichiometry and the nature of encapsulation are different. The dye in which the C–N bond rotation is permissible forms a 1:1 complex, with the acceptor centre (carbonyl group), rather than the donor centre (N atom) being encapsulated. The dye where the above mentioned rotation is restricted forms a 1:1 complex at a lower concentration of β-CD, encapsulating the donor centre (indole N). However, at a higher concentration of β-CD, 1:2 complex is formed involving both the acceptor and donor centres. Semi-empirical calculations at the PM3 level provide support to experimental findings. It is evidenced that the molecular structure of the guest is extremely important for encapsulation.

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