Photochemical processes in aqueous benzoquinone and anthraquinone solutions studied by spectrophotometry

In this paper, we focus on the experimental and theoretical limitations and advantages of using spectrophotometric technique to study a homogeneous, liquid-phase photochemical reaction. 1,4-Benzoquinone and anthraquinone derivatives dissolved in water were examined in a PhotoCube (TM) photoreactor under illumination with 365, 395, 457, 500, 523, 595, 623 nm, or white light. Derivatives with different electron-withdrawing and electron-donating substituents were studied. The photosensitivity of 1,4-benzoquinone solutions is known. However, a different kind of product was formed during storage in the dark than under illumination. Using classical matrix rank analysis and the M3 method (which uses the steps of Gauss-Jordan elimination), the number of light-absorbing particles in each photochemical reaction was determined. In general, one of the colored particles is the quinone, and the other is a constant ratio mixture of hydroquinone and hydroxyquinone formed as a product, which are weak acids, so their protonation/deprotonation also occurs as the pH of the solution changes, thereby increasing the number of colored particles. This effect was previously avoided by using buffers or pH-stat technique. In the case of the 2,6-dichloro-1,4-benzoquinone derivative, photochemical kinetic measurements at various temperatures were carried out in a diode array spectrophotometer with polychromatic light source. The reaction was only slightly accelerated by increasing the temperature from room temperature to 45 degrees C.