## 4 Discussion  
In zirconium phthalocyanine complexes, chosen in the current study, the coordination number of metal (up to eight) is high enough for axial ligand substitution and extracoordination of one or two solvent molecules. Therefore, relatively high solubility of chosen Pcs complexes in water or DMSO can be expected without additional lateral substitution with hydrophilic moieties, as proposed for most known Pc-based PDT agents (e.g., tetrasulfo- or tetracarboxphthalocyanines of Al or Zn). At the same time, ligands can assure covalent bonding of the complex molecule to the surface of graphite oxide through functional oxygen groups. Phthalocyanine complexes tend to aggregate (dimerize), which greatly deteriorates their optical properties and significantly reduces their performance as photosensizers. To avoid this effect on the surface of graphite oxide, it is important to obtain covalent bond between the complex and the GO through the formation of amide bonds of the ligand's terminal amino groups with the carboxyl groups of the carrier. However, lability of the system may cause the phthalocyanine ring itself to be attached to the carrier or other phthalocyanine rings. Therefore, two ligands with different chain lengths were used to tailor effectiveness of the interaction between the complex and the carrier. Carboxyl groups are unevenly dispersed over the surface of the carrier. With longer linear aliphatic ligands, the likelihood  
Figure 4: (**a**) Photoluminescence excitation spectra of bis(4-aminobutyrao)ZrPc before and after immobilization on GO (\(\lambda_{\text{em}}\) = 700 nm); (**b**) photoluminescence spectra of all investigated samples—two ZrPcs and their composites with GO (\(\lambda_{\text{exc}}\) = 620 nm). that both ligands will be bounded by the carboxyl groups of the carrier increases. Such binding will be strong and hard to undergo hydrolysis. In the case of two short aliphatic ligands, only one of them will be bounded to the carboxyl group, but this connecting bridge should also prevent direct ring interaction with the carrier or other phthalocyanine molecules. Such binding will be less stable, and the phthalocyanine complex might be more easily discharged from the carrier. In both cases, no conformational changes resulting in modification of optical properties were observed. Among the two studied phthalocyanine complexes, the one with 4-aminobutyric ligand has higher absorption coefficient, and absorption bands with higher intensity were thus observed for both **bis(GABA)ZrPc** and **bis(GABA)ZrPc/GO** in comparison to **bis(UDA)ZrPc** samples. Thanks to the high absorption coefficient, the absorption spectra confirmed the presence of phthalocyanine complexes on the GO flakes (Figure 2). On the contrary, axially substituted ZrPcs bands were not registered in the Raman spectra of the composites (Figure 3b), indicating low content of complexes in these samples. The photoluminescence excitation spectra (Figure 4a) confirmed that the wavelengths of the absorption bands can excite the ZrPc molecules, causing light emission. The same spectrum shape but with lower intensity was recorded for the composites, confirming the presence of ZrPc derivatives on the carbon structures. The photoluminescence could be observed using excitation in both the UV-blue and red-NIR ranges. In the experiment, wavelength of 620 nm was used to stimulate and measure the emission of ZrPcs solutions and composite suspensions (Figure 4b). The intensity of the photoluminescence from the composites was much lower in comparison to ZrPc solutions but still clearly observed in the range of 650-840 nm. The photosensitizing potential of the studied materials was verified through the test of singlet oxygen generation by an indirect method. Yellow DPBF indicator showed significant bleaching when all samples were irradiated with the lamp. Time needed to degrade dye molecules was below 5 min (slightly longer in the case of bis(UDA)ZrPc/GO).