## 4 Results and Discussion
### Static Fluorescence in Binary and Ternary Systems
#### 4.2.2. Ternary Systems  
Figure 9 shows \(\Delta\)(%) as a function of \(R\) for all alkan-lols investigated. Two important aspects emerge from this figure. First, for all the experimental data of the mixtures, \(\Delta\)(%) > 0, namely in the ternary system, the emission wavelength is always longer. Second, the parameter \(\Delta\)(%) exhibits a conspicuous maximum. For methanol and ethanol, the maximum falls in the range \(R>1\), while for other alkanols, the maximum is in \(R<1\) and decreases as \(n\) increases along the alkanol chain.  
It is well established that longer wavelengths are recorded when the excited state is more polar as compared to the ground state, so that a polar solvent stabilizes the excited state more than the ground state. The shorter wavelengths are produced when the ground state is more polar than the excited state. The presence of a maximum in the plot of Figure 9 shows that the ground state polarity in NR molecules is heavily dependent on the molecular ratio \(N_{W}/N_{A}\). Since the polarity of the solvent increases with R, we deduce that the hydrophobicity of the medium also has a decisive contribution.  
To further analyze the influence of the solvent on the emission wavelength shift, the \(\lambda_{max}^{em}\) values displayed in Figure 7 were extrapolated to water infinite dilution and compared with the corresponding values measured in the binary systems in the complete absence of water. Results are shown in Figure 10 as a function of \(n\) on a logarithmic scale; the same plot also shows \(\lambda_{max}^{em}\) values measured in the ternary systems with 4 and 8% (\(v/v\)) of water. It is immediately noted that the extrapolated values (black curve) and the values measured in the binary system (red curve) are practically coincident within the experimental error. This experimental observation ensures that the NR molecules exposed to water do not undergo irreversible conformational changes. In other words, alkanol and water can be considered a \"pure solvent\" whose molecular characteristics are composition averages. As one can see in Figure 10, the reduction in \(\lambda_{max}^{em}\) is drastically reduced, with the same \(n\), if measured in  
Figure 8: Characteristic parameters of fluorescence emission (Equation (28)) as a function of the number of carbon atoms.  
the presence of 4 or 8% (\(v/v\)) of water. On the other hand, it is well known that when the fluorescent is bound to the \(\pi\) electron system through an H-bond, the fluorescence emission is quenched or enhanced, depending on whether the H-bond can conjugate directly with the \(\pi\) electron system or not [35]. Accordingly, reduction of \(\lambda^{em}_{max}\) ongoing from a non-polar solvent to a polar solvent implies a partial fixation of the non-bonding electron by the intermolecular H-bonding.  
Figure 10: Fluorescence emission values obtained by dissolving NR in alkanols at different carbon numbers H\(-\)(CH\({}_{2}\))\({}_{n}-\)OH (\(n=1\)–\(8\)) compared with the same samples in the presence of 4% and 8% water.  
Figure 9: Average percent increase in the red shift of NR fluorescence in the ternary systems NR–\(n\)–alkanol–water, as a function of the parameter \(R\). The number on the curves indicates the number of carbon atoms in the alkanol molecule.