Substituent Effect on the Photoreduction Kinetics of Benzophenone

Demeter, Attila and Horváth, Klaudia and Böőr, Katalin and Molnár, Laura and Soós, Tibor and Lendvay, György (2013) Substituent Effect on the Photoreduction Kinetics of Benzophenone. JOURNAL OF PHYSICAL CHEMISTRY A, 117 (40). pp. 10196-10210. ISSN 1089-5639

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The kinetics of the photoreduction of four benzophenone derivatives by isopropyl alcohol was examined in acetonitrile, namely tetra-meta-trifluoromethyl-, di-para-trifluoromethyl-, di-para-methoxy benzophenone and for comparison the unsubstituted molecule itself. The basic spectroscopic (absorption and phosphorescence spectra) and photophysical (quantum yields, excited state energies) properties were established, and the key kinetic parameters were determined by the laser flash photolysis transient absorption technique. The rate coefficients of both the primary and secondary photoreduction reaction show remarkable dependence on ring substitution. This substantial effect is caused by the considerable change in the activation energy of the corresponding process. The experimental results as well as DFT quantum chemical calculations clearly indicate that these benzophenone derivatives all react as n-p* excited ketones, and the rate as well as the activation energy of the reduction steps change parallel with the reaction enthalpies, the determining factor being the stability of the forming aromatic ketyl radicals. The secondary photoreduction of benzophenones by the aliphatic ketyl radical formed in the primary step occurs via a hydrogen bonded complex. The binding energy of the hydrogen bonded complex between the aliphatic ketyl radical reactant and a solvent molecule is a critical parameter influencing the observable rate of the secondary photoreduction.

Item Type: Article
Subjects: Q Science / természettudomány > QD Chemistry / kémia > QD02 Physical chemistry / fizikai kémia
Depositing User: Dr. Demeter Attila
Date Deposited: 02 Dec 2013 10:52
Last Modified: 31 Dec 2014 01:15

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