Location and freedom of single and double guest in dye-doped polymer nanoparticles

We report on time-resolved fluorescence anisotropy studies of poly(9-vinylcarbazole) (PVK) nanoparticles (NPs) encapsulating Coumarin 153 (C153) and Nile Red (NR). The wobbling-in-a-cone model successfully describes the restricted movements of the encapsulated molecules. For C153-doped PVK NPs, when increasing the C153 content, the diffusional relaxation (τD) times become shorter (τD = 4.6-1.6 ns), following its increased preference for less rigid environments. On the other hand, for NR, τD is affected by the dopant content (τD = 12.2-3.09 ns), thus suggesting a more rigid environment, which is in agreement with its higher ability to interact with the polymer chains. For the two-dye-doped PVK NPs, where the content of one of the trapped guests is kept fixed while varying the concentration of the other one slows down (NR; τR = 0.30-0.37 ns and τD = 3.09-7 ns) or accelerates (C153: τR = 0.14-0.03 ns and τD = 1.57-0.69 ns). This suggests that the guest molecules assume different positions, with C153 being preferentially in the less rigid environment closer to the NP surface, while NR is located in the more rigid ones, closer to the core. We suggest that the dye distribution within PVK NPs is governed by the combination of the Marangoni effect and the consecutive particle swelling. Furthermore, in the two-dye-doped systems, competition for the available less rigid sites is observed upon an increase in the C153 co-dopant content, while in the case of NR as the variable co-dopant this effect is smaller. These findings are relevant for improving our knowledge for a better design of nanophotonic devices based on dye-doped polymer NPs.