Substituent Effect on the Photoreduction Kinetics of Benzophenone

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• 作者：Attila Demeter ; Klaudia Horv谩th ; Katalin B枚艖r ; Laura Moln谩r ; Tibor So贸s ; Gy枚rgy Lendvay
• 刊名：Journal of Physical Chemistry A
• 出版年：2013
• 出版时间：October 10, 2013
• 年：2013
• 卷：117
• 期：40
• 页码：10196-10210
• 全文大小：559K
• 年卷期：v.117,no.40(October 10, 2013)
• ISSN：1520-5215

文摘

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 and 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鈥撓€* 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.