Cavity size dependent stoichiometry of probe-cyclodextrin complexation: Experimental and molecular docking demonstration

Host-guest interaction of a newly synthesized intramolecular charge transfer (ICT) probe, namely (E)-4-(4-(diethylamino)benzylideneamino)-2-(trifluoromethyl)benzonitrile (DBTFB), with supramolecular assemblies like alpha- and beta-cyclodextrin (CD) has been investigated exploiting various spectroscopic techniques. Steady state fluorescence studies reveal that depending on the different cavity sizes of the hosts, probe-CD inclusion complexes of different stoichiometries are formed. The stoichiometries and association constants of these complexes have been determined exploiting Benesi-Hildebrand equation and the stoichiometries are found to be 1:1 and 1:2 for probe alpha-CD and probe-beta-CD inclusion complexes respectively. Relatively higher steady state fluorescence anisotropy value of the probe in beta-CD compared to that in alpha-CD substantiates the stoichiometrically different probe-CD interactions. Time resolved fluorescence study further corroborates the differential stoichiometry in the two cases. Hydrodynamic diameters of CD-encapsulated probe as obtained from dynamic light scattering (DLS) experiments demonstrate the cyclodextrin dependent stoichiometries. Molecular docking has been exploited to get a qualitative molecular based picture of the probe-CD complexations in the two cases.