3.2. Electronic spectra The absorption spectra in the UV–vis region were recorded using a Varian CARY 50 UV/vis spectrophotometer operating at room temperature. The electronic spectra were recorded in freshly prepared acetonitrile solutions (0.0985 mM and 1.04 mM) due to the poor solubility in water and the higher stability of Au-azpy in acetonitrile. The spectrum is characterized by intense peaks in the region that comprises 200–550 nm. The bands appearing at 225 (logeM = 4.782) and 335 (log eM = 4.386) are considered mainly as intraligand charge-transfer transitions, as they have high molar extinct coefficients and could be observed in the free ligand [32] as well (Fig. 3). In the particular case of the absorption peak at 225 nm a higher LMCT nature is observed due to the extremely high molar extinction coefficient, which is comparable to the one present in the [AuCl4] anion (230 nm, logeM = 4.5359). The energy of the p ! p* transition in a free azpy (at 315 nm) is lower for Au-azpy (at 335 nm), which is consistent with