Photophysical and Quantum Chemical Studies on the Interactions of Oxazine-1 Dye with Cucurbituril Macrocycles

Supramolecular host–guest interaction of cationic oxazine-1 (OX1) dye with two cucurbit[n]uril (CBn) hosts, namely, CB7 and CB8, has been investigated using photophysical and quantum chemical studies. Both CB7 and CB8 display much stronger binding affinities for OX1 dye compared to conventional cyclodextrin (CD) hosts, which arises due to strong ion–dipole interaction in stabilizing the dye–host inclusion complexes in the present systems. From photophysical studies supported by ¹H NMR results and quantum chemical calculations, it is inferred that 1:1 inclusion complexes are mainly formed in the present systems, though a small percentage of 1:2 (dye·host₂) complexes are also indicated from time-resolved (TR) fluorescence studies. Longer rotational relaxation times for dye-CBn systems compared to the free dye as estimated from TR anisotropy studies support the inclusion complex formation in the present systems. The binding constant value is estimated to be significantly higher for the OX1-CB7 system than the OX1-CB8 system, and these results are in accordance with compatible portal diameter of CB7 cavity compared to the much larger portal diameter of CB8 cavity relative to the width of the OX1 molecule. Accordingly, CB7 cavity renders a relatively stronger binding than the CB8 cavity for an axially incorporated OX1 dye into the host cavity. Results from the quantum chemical calculations are overall supportive to the inferences drawn from photophysical measurements. Observed results clearly suggest that the dimensions of the CBn cavities play an important role in determining the interaction strength and stoichiometry of the host–guest complexes formed and thus bring out significant changes in the photophysical properties of the bound dye. The host-assisted modulation in the photophysical properties of the dye, as observed in the present study, has a direct relevance to applications like aqueous dye lasers, sensors, fluorescence assays, and so on.