Charge transfer interaction of 8-hydroxyquinoline with DDQ: Spectrophotometric, thermodynamic and molecular modeling studies

The charge transfer complex of the donor, 8-hydroxyquinoline (8HQ) with the pi-acceptor 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) in acetonitrile medium has been studied spectrophotometrically at different temperatures. The 8HQ reacts instantaneously with DDQ to form reddish brown charge transfer complex. Absorption spectra of the complex shows multi-charge transfer bands at 456, 548 and 588 nm. The high values of the formation constant and molecular extinction coefficient, calculated from the Benesi-Hildebrand equation, show the high stability of the CT complex. Enthalpy and entropy of CT complex formation have been estimated by using van't Hoff equation where the more negative value of Delta H degrees confirms strong bonding between donor and acceptor. The signs of Delta H degrees, Delta S degrees and Delta G degrees reveal that the CT complex formation process is exothermic and a spontaneous process. The experimental studies are complemented by quantum chemical calculations at DFT (B3LYP/6-31G) level of theory, including bond lengths, bond angles, Mulliken electron charge on atoms and molecular electrostatic potential maps which are helpful in assigning the CT route. In addition, GAMESS interface computations are performed to characterize the highest occupied molecular orbital and lowest unoccupied molecular orbital surfaces of the complex. A good consistency was found between experimental and theoretical results.