## 3 Results
### Characterization of PMMA + Complex Composites  
The obtained compounds and their composites were analyzed using solid-state UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS). The HOMO-LUMO energy gap of powders (**1**)-(**5**) was determined using the Kubelka-Munk (K-M) function plotted against light energy (i.e., K = f(hv), where K = (1\(-\)R)\({}^{2}\)/2R and R is the reflection). In the case of TOCs, the HOMO-LUMO gap is 3.12 eV, 2.96 eV, and 2.39 eV for (**1**) (Figure 1a), (**2**) [31], and (**3**) [31], respectively. In turn, for ruthenium complexes, it is 2.75 eV and 2.78 eV for (**4**) and (**5**), respectively (Figure 1b). Analysis of spectral data revealed that \(\lambda_{\max}\) was 308 nm (**1**) (Figure 1c), 306 nm (**2**) [31], 330 nm, 395 nm (**3**) [31], and 334 nm (**4**), (**5**) (Figure 1d). The maximum absorption was also determined for the composite materials. Figure 1: The results of studies on micro-grains (**1**), (**4**), and (**5**), carried out using UV-Vis diffuse reflectance spectroscopy (DRS) to investigate their optical properties (**a**,**b**), are presented together with the determination of the HOMO-LUMO gap for these compounds, derived from Kubelka–Munk function plots versus light energy (**c**,**d**). Samples containing complex (**I**) (2% and 5% by weight) exhibited two maxima at 236 nm and 281 nm (Figure 2a), corresponding to the absorption maxima of PMMA. The PMMA + (1) 20 wt.% composite showed a broad absorption range (200-450 nm) with a low-intensity band peaking at approximately 330 nm. In the case of the PMMA + (**2**) series, the absorption edge is slightly shifted toward the visible light range (400 nm for PMMA + (**2**) 2% and 5%, and 480 nm for PMMA + (**2**) 10% and 20%) (Figure 2b). However, for the PMMA + (**3**) composites, the absorption edge is positioned at 480 nm for PMMA + (**3**) 2% and 5% samples and 500 nm for PMMA + (**3**) 10% and 20%. The absorption maximum remains in the ultraviolet range (Figure 2c). Composite materials made of PMMA and ruthenium complexes are characterized by an absorption edge in the 400-450 nm range. Almost all samples have two maximum absorptions in the 238-336 nm range (\(\lambda\) = 236-281 nm PMMA range). The exception is the PMMA + (**5**) 20 wt.% sample, where the absorption edge is located around 550 nm and one of the maximum absorptions was found at 420 nm (Figure 2d,e). Composite materials based on poly(methyl methacrylate) containing 2%, 5%, 10% and 20% of complexes (**1**)-(**5**) were spectrally characterized to confirm the presence of the complex and that the structure of the compound does not change after introduction into the polymer matrix (Figure 3). Previously, our research exhibited that the structures of titanium-oxo complexes (**1**)-(**3**) are stable in contact with water. IR spectra and XRD diffractograms did not register significant differences [31; 32]. Figure 2: Solid-state UV-Vis-diffuse reflectance spectra (DRS) of the composites PMMA + (**1**)–(**5**) (**a**–**e**). The analysis of scanning electron microscope (SEM) images provided insights into the dispersion of complexes **(1)**-**(5)** within the polymer matrix (Figures S1 and S2). The PMMA + **(1)** and PMMA + **(2)** composite samples are comparable, containing grains with diameters between 60 and 200 nm. Their high surface energy promotes aggregation, forming 30 to 250 \(\upmu\)m grains. The packing density of these aggregates increases with the wt.% concentration of the complexes in the PMMA matrix. In the PMMA + **(3)** sample, densely packed crystalline grains are observed, with sizes dependent on the complex concentration in the matrix. At lower concentrations (2 wt.% and 5 wt.%), the sample primarily comprises grains with diameters between 145 and 355 nm and occasional larger grains measuring 200 to 500 \(\upmu\)m. At higher concentrations (10 and 20 wt.%), the samples contain microcrystals of the oxo complex ranging from 900 nm to 5 \(\upmu\)m in size (Figure S1).