## 4 Discussion
It reacts with CV molecules, making it fragment into colorless species [92], and it finally produces simple inorganic molecules [93]. Processes of N-demethylation and oxidative degradation are placed among the first steps of CV photodegradation [93; 94]. The monitored decolorization of MB and CV aqueous solutions is related to the first step of a particular dye's photodecomposition. This procedure is commonly used in the photoactivity tests [5; 86; 87; 88; 89; 90; 91; 92; 93; 94] because of its simplicity (spectrophotometric studies, the zero-order or first-order kinetic equations) and sensitivity (e.g., significant changes of absorption). Among the tested dyes, MB solutions showed the highest photocatalytic degradation efficiency, as well as MB's susceptibility to light-induced breakdown under both UV and visible light irradiation [95]. The highest rate constant for the photoinduced decolorization process was observed in UV-irradiated PMMA + (**2**) samples, where the per cent of MB decolorization increased from 33.6% to 63.6% as the complex concentration dose rose from 2 to 20 wt.% (Figure 7, Table S4). The tested composites can be classified by their activity profile as follows: (i) composites mainly active in the UV range, enriched with complexes (**1**) and (**2**); (ii) composites active under visible light, containing complexes (**4**) and (**5**); and (iii) a composite showing activity in both UV and visible light ranges, incorporating complex (**3**). In the systems evaluated, oxo-Ti(IV) complexes demonstrated the greatest photocatalytic activity. Findings from previous structural studies of complexes **(1)-(3)**[31; 32] and their physicochemical properties suggest that the stabilizing carboxylate ligand type is the primary factor impacting their photocatalytic activity. Complexes **(1)** and **(2)**, stabilized by \(\alpha\)-hydroxycarboxylic ligands (mandelate or 9-hydroxy-9-fluorencearboxylate), exhibited relatively large HOMO-LUMO energy gaps of 3.12 eV and 2.96 eV, respectively, accounting for their strong UV activity, which is confirmed by the rate constants of the photoinduced decolorization of MB and CV solutions. In contrast, complex **(3)**, stabilized by 9-hydroxy-9-fluorencearboxylate and propionate ligands, has a reduced \(E_{\rm p}\) value of 2.39 eV, resulting in lower UV activity but increased visible light activity. The PMMA + **(3)** composite (2 wt.%) showed superior activity in the visible light range compared to the UV range. It slightly decreased with the complex concentration increase (Figure 6). Ruthenium(III) complexes **(4)** and **(5)** exhibited generally lower photocatalytic activity compared to the Ti(IV)-based complexes **(1)-(3)**. They were primarily active under visible light, likely due to their HOMO-LUMO energy gap values of 2.75 and 2.78 eV for **(4)** and **(5)**, respectively. The reduced activity of the Ru(III) complexes may be attributed to their photocatalytic mechanism, which involves the reduction of Ru(III) to Ru(II). In aqueous solutions, this reduction produces H\({}_{2}\)O\({}_{2}\) molecules that subsequently form hydroxyl radicals [96; 97]. The lower reactivity of Ru(III) complexes suggests that they generate fewer hydroxyl radicals compared to the number of reactive oxygen species (ROS) produced by Ti(IV)-oxo complexes stabilized with \(\alpha\)-hydroxycarboxylate ligands. These results indicate that, in this case, the type of ligand also affects the activity differences observed between complexes **(4)** and **(5)**. The photocatalytic mechanism involves the excitation of electrons from the valence band to the conduction band upon irradiation with light of an appropriate wavelength, leading to the formation of electron-hole pairs. These species interact with oxygen or water, generating reactive oxygen species (ROS), which drive photocatalytic reactions [84]. In metal complexes, electron transfer occurs between the HOMO (ligand) and LUMO (metal center, Ti or Ru) [29].