Development of a new actinometric method for polychromatic light irradiation. Application to photochromes, drugs and stilbenoids
The photostability data of chemical species is usually described based upon models using equations applicable to standard zero-, first- or second-order kinetics (classical thermal order kinetics) and other such models. These thermal order kinetic models have certain limitations, however, regarding parameter representation of the photoreaction and the reproducibility of associated results. The other models available in the literature use approximation and expansion methods to solve the photochemical rate law but are unfortunately unable to provide robust models to describe the photochemical reaction itself. This research resolves many of the drawbacks typically encountered within the photokinetics domain by improving upon an existing model and designing new models that describe photochemical reactions in solution. These models are the Φ- and n-order kinetics. The former reduces the number of elucidation method steps whilst the latter is designed to describe photochemical reactions under irradiation by polychromatic light for three chosen AB reaction systems. In the development of the above-mentioned methodologies, this research selects species based on their photochemical reaction mechanisms to study their photodegradation, starting from a simple to a complex mechanism. These compounds are: C-DAE for unimolecular AB 〖(1Φ)〗_(ε_(B=0) ), nifedipine and dacarbazine for unimolecular AB〖(1Φ)〗_(ε_(B≠0) ), and the stilbenoids group, axibinib and O-DAE for photoreversible AB (2Φ) reactions. The results obtained in this research show that the Φ-order kinetics approach offers a good description of the photodegradation of these species under continuous monochromatic irradiation. It has been established that both the forward (Φ(A→B)) and reverse (Φ(B→A)) quantum yields increased with increasing irradiation wavelength according to a sigmoid pattern. Additionally, this study presents a methodology to calculate the β factor, which is used to obtain the photoreactivity of the species, in a method that offers a much simpler and faster approach to this determination than traditional methods. The stilbenoids group shows higher photoreactivity than the other species studied according to the β factor ranking scale. Other methods were designed to overcome the drawbacks to describing a photochemical reaction under irradiation by polychromatic light. The validity of the new mathematical models (Φ- and n-order) were verified using simulation studies through numerical integration methods (NIMs) to generate simulated cases for each photochemical reaction system considered. In addition, the Φ- and n-order models were examined and validated using experimental HPLC and spectrophotometry photodegradation data obtained for different species under irradiation by polychromatic light. Results obtained in this study confirm the model’s predictions, as both photoreaction rate constant and initial reaction velocity (υ(0)) were observed to remain constant with variable initial concentrations of the species considered. Furthermore, actinometric methods were developed and applied to study the photochemical reaction of species in solution under irradiation by monochromatic and polychromatic light. The former and latter were developed to determine the light intensity of unknown light sources. Monochromatic light has a much smaller wavelength distribution than polychromatic, which are determined based on the associated β factors. The results show that each method can be used independently to quantify the light intensities from different light sources.
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