*3.2. Spectral Characterizations*

The dendrimer is insoluble in water, but it is very well soluble in organic solvents. The basic spectral characteristics of ACR dendrimer: Absorption (λA) and fluorescence (λF) maxima, Stoke's shift νA−νF), the quantum yield of fluorescence (ΦF) have been investigated in seven organic solvents of different polarity, and the main results are summarized in Table 1.

**Table 1.** Photophysical characteristics of ACR dendrimer in organic solvents of different polarity (see text).


s: Shoulder.

The spectrum of the acridine dendrimer has absorption maxima between 396–412 nm, ascribed to an internal charge transfer (ICT) transition. It emits fluorescence with maxima at 455–480 nm region. In most solvents, the spectra are composed by a single broadband, whereas, in hydrogen bonding solvents (alcohols) a shoulder can be distinguished [26–29]. The results show solvent polarity dependence. Comparing the absorption maxima of the spectra taken in non-polar solvents with those taken polar ones, a positive solvatochromism has been obtained, while the respective maxima in polar environments do not change their position. In the case of fluorescence maxima, it is seen that with increasing the medium polarity, their values decrease and a negative solvatochromism has been found (Figure 1). The lower values of Stokes shift (νA–νF) in a polar medium can be explained by the dipole-dipole interactions and possibility of the formation of hydrogen bonds. That probably stabilizes the acridine molecules in the excited state, and conformational changes are slightly pronounced. This is also confirmed by the results obtained for the quantum fluorescence yield, where the values in polar media are several times higher (Table 1). Similar results have been obtained when acridine functionality has been bonded to a hyperbranched polymer [30].

**Figure 1.** Solvent polarity dependence of absorption and fluorescence maxima: 1-tetrahydrofurane, 2-ethylacetate, 3-chloroform, 4-dichloromethane, 5-acetonitrile, 6-methanol, 7-ethanol.

In order to elucidate the formation of a copper complex, titration of ACR dendrimer with Cu(II) ions has been carried out in acetonitrile solution. Figure 2A shows the decrease of fluorescence intensity with increasing concentration of Cu(II) ions. Figure 2A also shows that Cu(II) forms a complex with a dendrimer molecule at a 1:2 stoichiometry. The possible formation of the coordination of Cu(II) is with the tertiary amino groups in the dendrimer core. IR spectroscopy has been used for the characterization of the isolated solid complex (Figure 2B). The difference is observed in the range 1200–1340 cm−1, where is the absorption of nitrate groups (−NO3).

**Figure 2.** Fluorescence (**A**) and infrared (IR) (**B**) spectral characterization of [Cu2(ACR)(NO3)2].
