Cucurbit[7/8]uril assisted modulations in the photophysical properties of 4',6-diamidino-2-phenylindole dye

The influence of two versatile macrocyclic hosts, cucurbit[7]uril (CB7) and cucurbit[8]uril (CB8), on the photophysical properties of a biologically important dicationic dye, 4 & PRIME;,6-diamidino-2-phenylindole (DAPI), has been investigated following different photophysical studies and quantum chemical calculations. Results indicate that while CB7 with having smaller cavity size forms only 1:1 dye-host inclusion complex with DAPI, the CB8 host with larger cavity size forms both 1:1 and 2:1 dye-to-host inclusion complexes with the dye. Quantum chemical studies indicate that while 1:1 complex is formed by inclusion of either the benzene or indole end of DAPI into a CBn cavity, the 2:1 complex in the DAPI-CB8 system is formed mainly by the inclusion of two benzene ends of two dye molecules into the host cavity, entering through opposite portals of the host. Estimated binding constant values for the dye-host complexes formed in these systems are found to be exceedingly high. Strong ion-dipole interaction of dicationic DAPI with polarizable CBn portals along with usual hydrophobic interaction arising from inclusion of the dye into the host cavity cooperatively provide the extremely high stability for the dye-host complexes formed in these systems. Overall, the results demonstrate that compatibility between the dye structure and the host cavity are mainly responsible for the differential bindings stoichiometries of the dye-host complexes formed in the studied DAPI-CBn systems. Further, these differences in the binding stoichiometries of the complexes are the main reason for the large differences observed in the modulated photophysical properties of the dye in the DAPI-CB7 and DAPI-CB8 systems. Since DAPI is an important fluorescence marker for several biomolecules, present results are having relevance to the applications of DAPI in various biophysical investigations.