## 3 Results and Discussion
However, it was found that as the content of VK units increased in the copolymer, the luminescence intensity \(I_{544}\) in the \({}^{5}\)D\({}_{4}\)--\({}^{7}\)F\({}_{5}\) transition peak for Tb\({}^{3+}\) passed through a maximum at approximately 16 mol. % of VK units (Figure 8, curve 3). A further increase in the content of VK units led to a significant decrease in the luminescence intensity of \(I_{544}\), while the efficiency of the excimer formation according to the ratio of %\(I_{\text{E}^{*}}\)/%\(I_{\text{M}}\) increased by approximately 2.7 times (Figure 8, cf. curve 3 and 6; Table S1 in ESM). The results presented above are related to the sensitizing effect of VK units on Tb\({}^{3+}\) luminescence, i.e., to the so-called molecular antenna effect. The point is that the efficiency of antenna effect for Tb\({}^{3+}\) decreased with an increase in the probability of excimer formation characterized by %\(I_{\text{E}^{*}}\)/%\(I_{\text{M}}\): the energy of electronic excitation of VK units was not spent on the photoluminescence of Tb\({}^{3+}\) ions, but fell into excimer traps. It should be noted that the value of %\(I_{\rm Tb(III)}\)/%\(I_{\rm M}\) symbatically changed with the value of \(I_{\rm 544}\). The latter fact allowed us to attribute this ratio as a value characterizing the enhancement of the electronic excitation energy transfer from monomeric VK units to lanthanide ions--Tb\({}^{3+}\) luminescence EET rate (Figure 8, cf. curve 3 and 4). As a result, it appears that the energy transfer to Tb\({}^{3+}\) was most effective at 16 mol. % of the VK units. It is interesting to note that the appearance of the maximum in the curve for \(I_{\rm 544}\) and its absence in the curve for %\(I_{\rm Tb(III)}\) was related to the fact that the antenna effect first increased with increasing the VK content and the constant Tb\({}^{3+}\) content (Figure 8, curve 2 and 3) at a low content of VK units in the copolymer. At this stage, the excimer formation efficiency was low: the %\(I_{\rm E^{*}}\)/%\(I_{\rm M}\) value was only 2.2, while at its maximum it was twice as high (Table S1 in ESM). In other words, at this stage, the VK units were more likely to find lanthanide ions and transfer electron excitation energy to them than to find a similar unit removed along the chain or due to an intermacromolecular interaction to form an excimer [VK\(\cdots\)VK]\({}^{*}\) in the excited state. However, the contribution of %\(I_{\rm Tb(III)}\) decreased steadily depending on the content of VK units, which suggests, together with the above, that there were Tb(III) ions both bound and unbound to VK units in the system. When near each Tb\({}^{3+}\) (ca. 6 nm) there was a VK unit, and \"free\" Tb\({}^{3+}\) practically did not remain in the system, we observed a transition to a decrease of \(I_{\rm 544}\) value. The value of %\(I_{\rm E^{*}}\)/%\(I_{\rm M}\), which reflects the efficiency of the electron excitation energy transfer from VK units to Tb\({}^{3+}\), just reached its maximum at this content of VK units. With a further increase in the content of VK units in the system, all the excitation energy was spent on excimer formation. This result was in good agreement with previously obtained data [36] on a similar effect of 12-21 mol. % of VK units in copolymers of \(N\)-vinylpyrrolidone and \(N\)-vinylcaprolactone, which manifested itself in an increase in the luminescence intensity of terbium ions by almost two orders of magnitude compared with its luminescence intensity in the presence of copolymers with a higher content of \(N\)-vinylcarbazole units. This effect was due to the competition of two photophysical processes (the formation of excimers and the transfer of electronic excitation energy) proceeding in opposite directions.