Spectroscopic Studies on Charge-Transfer Complexation of Iodine with Dibenzo-15-crown-5 and Benzo-12-crown-4 in Chloroform, Dichloromethane and 1,2-Dichloroethane

The formation of charge-transfer complexation between dibenzo-15-crown-5 (DB15C5) and benzo-12-crown-4 (B12C4) (Donor) and iodine is investigated spectrophotometrically in three chlorinated solvents,chloroform, dichloromethane (DCM) and 1,2-dichloroethane (DCE) solution at 25°C. The change in polarityof the solvent also doesn’t affect the stoichiometry of the complexes. Values of formation constants reflectthe order of ionization potentials of the donors. The observed time dependence of the charge-transfer bandand subsequent formation of I3- ion are related to the slow formation of the initially formed 1:1 Donor.I2outer complex to an inner electron donor-acceptor (EDAr) complex, followed by fast reaction of the innercomplex with iodine to form a triiodide ion, as follows:Donor + I2 → Donor. I2 (outer complex), fastDonor.I2 (outer complex) → (Donor. I+)I- (inner complex), slow(Donor . I+)I- (inner complex) + I2 → (Donor .I+)I3-, fastThe pseudo-first-order rate constants for the transformation process were evaluated in different solventsystems. The stability constants of the resulting EDAr complexes were also evaluated and the solvent effecton their stability are discussed. The resulting complexes were isolated and characterized by FTIR and 1HNMRspectroscopy.