## 4 Discussion
Basing on the obtained results, the next stage of our study focused on the use of methylene blue (MB) and crystal violet (CV) solutions. To evaluate the photocatalytic activity of the studied complexes (1)-(5), it was crucial to determine their maximum absorption within the composite materials as well as their HOMO-LUMO gap values. These parameters can be tailored to achieve desired properties, for instance, through structural modifications of the complexes, such as functionalizing the stabilizing ligands. [PERSON] et al. demonstrated that introducing salicylate ligands can reduce the Ep values of complexes from 2.24 to 1.88 eV [80], while chromophore ligands broaden this range from 3.41 to 1.98 eV [81]. For ruthenium compounds, the existing literature suggests that modifications to the HOMO-LUMO gap are primarily focused on Ru(II) complexes, particularly those containing bipyridine ligands. Adjusting the position of the bipyridine substituent or altering the halide coordination can result in either an increase or decrease in the HOMO-LUMO gap [82,83]. Similar trends are expected to apply to Ru(III) complexes; however, these compounds have been studied less extensively. Among the tested Ru(III) compounds, complexes **(4)** and **(5)** exhibit comparable Ep values of 2.75 and 2.78 eV, respectively, placingthem between studied complexes (**2**) (Ep = 2.96 eV) and (**3**) (Ep = 2.39 eV). This suggests that these complexes are also likely to exhibit photocatalytic activity [84; 85]. The photocatalytic activity of the studied composites containing 2, 5, 10, and 20 wt.% of complexes (**1**)-(**5**) was assessed by analyzing the decolorization of MB solution samples irradiated with UV and visible (Vis) light, following the ISO 10678 standards [86]. Additionally, tests in UV was carried out using CV solutions. The performed tests confirmed that some percentage of each studied dye is adsorbed on the composite surface (Table S3). To maintain the same concentration condition in each kinetic run, during photodegradation tests, the \"old\" dye solution was replaced after the adsorption process with a fresh one. Thus, the starting dye concentration during each kinetic run was the same. This is important considering the equation used for calculation of the observed rate constants (Equation (6)), where \(k_{\text{obs}}\) is defined as  
\[k_{\text{obs}}=\frac{100\%k}{c_{0}} \tag{7}\]  
As a measure of a dye concentration, the percentage of dye decolorization was used. Because both of these parameters are inversely proportional to each other, the observed linear dependencies for decolorization vs. time increase, although the dye undergoes decomposition. Application of the change in the solution color during the photodegradation of MB and CV is based on the mechanism of their photodecomposition, studied earlier [5; 87; 88]. Paths of MB photodegradation have been studied for years [5; 87; 88; 89]. The process can be studied spectrophotometrically because a blue, oxidized form of MB in a reaction with reactive oxygen species, mainly \({}^{\bullet}\)OH, generated on a photocatalyst surface forms colorless photodegradation intermediates [87]. The decolorization is caused by breaking of the MB central aromatic ring [5]. The further decomposition of the intermediates continues until CO\({}_{2}\), H\({}_{2}\)O, and other inorganic ions are formed as final photodegradation products [5; 87]. Moreover, spectrophotometric MB photodegradation is a kind of reference process, being involved in the norm ISO 10678:2024 (formerly ISO 10678: 2010) [86; 90]. Photodegradation of CV proceeds similarly. A free electron generated during composite irradiation reacts with water molecules on the composite surface, producing different ROS. This process, i.e., the water splitting reaction, in fact, is crucial for the whole photodegradation process here and in other, similar photodegradation reactions [91]. One of the most common ROS in aqueous systems is hydroxyl radical [87; 91; 92].