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™ 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 °C.