Correlating Pyrene Excimer Formation with Polymer Chain Dynamics in Solution. Possibilities and Limitations

Abstract

Four types of pyrene-labeled polystyrene samples (Py−PS) were prepared and the process of excimer formation between the pyrene labels was characterized by steady-state and time-resolved fluorescence to assess the effect the mode of pyrene incorporation into a polymer has on the kinetics of excimer formation. The pyrene label was incorporated into the PS backbone by either (1) reacting 1-pyrenemethoxide with a chloromethylated polystyrene backbone to yield the GrE−PS series, (2) copolymerizing styrene with 4-(1-pyrenyl)methoxymethylstyrene to yield the CoE−PS series, (3) copolymerizing styrene with N-(1-pyrenylmethyl)acrylamide to yield the CoA−PS series, or (4) polymerizing α,ω-dicarboxyl end-capped polystyrenes with l-lysine-1-pyrenemethylamide dihydrochloride to yield the ES−PS series. Steady-state and time-resolved fluorescence experiments demonstrated that the long and flexible linker of GrE−PS and CoE−PS enabled more efficient excimer formation than the short and rigid linker of CoA−PS, and that spacing the pyrene pendants in ES−PS led to a strong reduction in excimer formation. The fluorescence blob model (FBM) was applied to analyze quantitatively the monomer and excimer fluorescence decays of the four Py−PSs. The FBM analysis confirmed that the longer ether linker of GrE−PS and CoE−PS enabled the excited pyrene label to probe a larger volume inside the polymer coil. The level of clustering of the pyrene pendants was found to be minimal for ES−PS, as expected from its structural design. Interestingly, the pyrene pendants were twice more clustered for GrE−PS than for CoE−PS, despite both polymers having an identical chemical structure. The results for the GrE−PS and CoE−PS series suggest that reacting groups distribute themselves differently in a copolymer whether they are incorporated by a grafting onto reaction or copolymerization.

Effect of Side-chain Length on the Side-chain Dynamics of α-Helical Poly(l-glutamic acid) as Probed by a Fluorescence Blob Model

“…Since N blob decreases with increasing pyrene content, as has already been observed for a variety of pyrene-labeled polymers, the value of N blob o is found by extrapolating the N blob trends shown in Figure 5 to zero pyrene content. 24,25,29,35 The so-determined N blob o values are shown in Figure 6 as a function of pyrene lifetime. The N blob o values are clearly larger for PGA-PBA compared to PGA-PMA at all probing times studied and they decrease slightly for both Py-PGA constructs with decreasing probing time.…”

Does Selectivity of Molecular Catalysts Change with Time? Polymerization Imaged by Single‐Molecule Spectroscopy

“…Catalytic polymerization is diverse mechanistically, yields commodity materials (>65 million tonnes of polyolefins annually [7] ), and underpins the discovery of new materials [8, 9] . Catalytically active chain ends bearing single molecular catalysts may experience changes in the local environment due to segmental motion, variation in solvation, and chain growth [10–14] . The distributions (and indeed even the presence and timescales) of variable chemoselectivity within these and other catalytic reaction systems are not easily predicted, and to our knowledge have not been observed in any molecular catalyst system owing to the ensemble averaging inherent to traditional analytical measurements.…”

Excited-State Dynamics of Pyrene Incorporated into Poly(substituted methylene)s: Effects of Dense Packing of Pyrenes on Excimer Formation

“…Another important approach for pyrene-incorporated vinyl polymers is polymerization of (meth)­acrylate monomers with a pyrene-containing ester substituent, where pyrene is connected to the C–C main chain with a linker unit including an ester linkage and a methylene chain with a certain length (i.e., C­(O)­O­(CH 2 ) n ) (Chart b). − Compared to the above-mentioned poly­(vinylpyrene)­s, the repeating unit structure allows pyrenes much greater molecular motion because of the presence of the flexible linker group, thus leading to the ambiguousness in the interpretation of the photophysical phenomena derived from the aligned pyrenes. Accordingly, the number of reports on the photophysical properties of this type of polymers is limited so far.…”

“…Accordingly, the number of reports on the photophysical properties of this type of polymers is limited so far. Instead, the pyrene-containing (meth)­acrylates are frequently utilized as fluorescence probes because of the strong excimer emission, which is quite sensitive to the environment around the pyrenes, and their high copolymerizability with a variety of other acrylic monomers. − …”