Characterization of the Local Volume Probed by the Side-Chain Ends of Poly(oligo(ethylene glycol) 1-Pyrenemethyl ether methacrylate) Bottle Brushes in Solution Using Pyrene Excimer Fluorescence

Four poly(oligo(ethylene glycol) 1-pyrenemethyl ether methacrylate) [P(PyEG(y)MA), with y = 3, 5, 8, and 12] samples were synthesized, where each side chain was terminated with a 1-pyrenemethoxy derivative. The efficiency of excimer formation between an excited-state pyrene and a ground-state pyrene was used to assess the conformation of the PyEG y side chains in these polymeric bottle brushes by conducting time- resolved fluorescence measurements in tetrahydrofuran (THF), N,N- dimethylformamide (DMF), dioxane, and dimethylsulfoxide (DMSO). These experiments took advantage of the dependency of the average rate constant (k) of pyrene excimer formation (PEF) on the local pyrene concentration [Py](loc) experienced by an excited pyrene bound to the P(PyEG(y)MA) samples. [Py](loc), could be estimated theoretically by assuming that the EG(y) side chains adopted a Gaussian conformation. Linear plots of < k >/f(diff) as a function of [Py](loc), where f(d)(iff) is the molar fraction of pyrenyl labels forming excimers by diffusion and was introduced to account for residual pyrene aggregation, were obtained in all four solvents, with slopes that depended on solvent viscosity and the probability of PEF upon the diffusive encounter between two pyrenyl labels. Solvent-induced differences in PEF efficiency could be accounted for by determining the bimolecular rate constant k(diff)[inter] for intermolecular PEF with the model compound 1-pyrenemethyl diethylene glycol methyl ether in the four solvents. Except for DMSO, which was too polar and led to the segregation of the pyrenyl labels close to the polymethacrylate backbone, the data obtained for all P(PyEG(y)MA) samples in THF, DMF, and dioxane collapsed on a master line, similar to the one obtained earlier with a series of pyrene-labeled dendrons, when plotting < k >/(f(diff) X k(diff)[inter])-vs-[Py](loc), The < k >/(f(diff) X k(diff) [inter])-vs-[Py](loc) master line confirmed the direct relationship that exists between the PEF efficiency and [Py](loc), a relationship that could be used to probe the conformation of highly branched macromolecules in solution.

Automated summary

Four polymeric bottle brushes with PyEG y side chains were synthesized and their conformation was evaluated through fluorescence measurements in different solvents. Solvent-induced differences in excimer formation efficiency were observed, leading to a direct relationship between the efficiency and local pyrene concentration, which can be used to probe the conformation of highly branched macromolecules in solution.