Poly(styrene-b-2-(N,N-dimethylamino)ethyl methacrylate) diblock copolymers: Micellization and application in the synthesis of photoluminescent CdS nanoparticles

Fluorescence studies on amphiphilic diblock copolymers of styrene and 2-(N,N-dimethylamino)ethyl methacrylate using 1,8-anilinonaphthalenesulfonate (ANS) as fluorescent probe revealed the formation of stable micelles at extremely low polymer concentrations of ∼0.05%. The micellar microenvironment was characterized by an average polarity of ET(30) = 44–48 kcal mol−1, similar to the moderately polar solvents, and extremely high microviscosity. Increase in hydrophilic: hydrophobic ratio of the copolymers resulted in an increase in the average polarity and decrease in microviscosity. The micelles proved to be excellent hosts for the synthesis and stabilization of photoluminescent CdS nanoparticles with a high degree of quantum confinement and broad photoluminescence, dominated by trap-state emission. Moreover, the size and size-related steady-state optical properties of CdS nanoparticles were significantly dependent on the microenvironment of the host micelle. In contrast, the photoluminescence dynamics of the nanoparticles, involving time-scales from 100 ps to 100 ns, are similar in all cases. Interestingly, the nanoparticles exhibit a large time-dependent Stokes shift, 75% of which is complete within the first ∼100 ps after the excitation. The extremely rapid Stokes shift is attributed to the decay of the initially formed band-edge excitons in a time-scale too fast to be affected by the microenvironment surrounding the particle.