Quenching of the Perylene Fluorophore by Stable Nitroxide Radical

Oct 20, 2014 - In the case of the model compound, 4-oxo-TEMPO, quenching of the perylene excited state was found to be dominated by a dynamic ...
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Quenching of the Perylene Fluorophore by Stable Nitroxide RadicalContaining Macromolecules Barbara K. Hughes, Wade A. Braunecker, Andrew J. Ferguson, Travis W. Kemper, Ross E. Larsen, and Thomas Gennett* Chemical and Materials Science Center, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States S Supporting Information *

ABSTRACT: Stable nitroxide radical bearing organic polymer materials are attracting much attention for their application as next generation energy storage materials. A greater understanding of the inherent charge transfer mechanisms in such systems will ultimately be paramount to further advancements in the understanding of both intrafilm and interfacial ion- and electron-transfer reactions. This work is focused on advancing the fundamental understanding of these dynamic charge transfer properties by exploiting the fact that these species are efficient fluorescence quenchers. We systematically incorporated fluorescent perylene dyes into solutions containing the 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) radical and controlled their interaction by binding the TEMPO moiety into macromolecules with varying morphologies (e.g., chain length, density of radical pendant groups). In the case of the model compound, 4-oxo-TEMPO, quenching of the perylene excited state was found to be dominated by a dynamic (collisional) process, with a contribution from an apparent static process that is described by an ∼2 nm quenching sphere of action. When we incorporated the TEMPO unit into a macromolecule, the quenching behavior was altered significantly. The results can be described by using two models: (A) a collisional quenching process that becomes less efficient, presumably due to a reduction in the diffusion constant of the quenching entity, with a quenching sphere of action similar to 4-oxo-TEMPO or (B) a collisional quenching process that becomes more efficient as the radius of interaction grows larger with increasing oligomer length. This is the first study that definitively illustrates that fluorophore quenching by a polymer system cannot be explained using merely a classical Stern− Volmer approach but rather necessitates a more complex model.

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(PTMA) was the first to be exploited for application in ORBs as an active cathode material.18 Interestingly, even as these materials have emerged as new and exciting materials for integration into advanced energy storage applications, very little is known about the fundamental electronic processes that govern their redox performance.19,20 Ultimately, this work is focused on the critical task of evaluating and understanding charge-transfer processes through spectroelectrochemical studies that exploit the known fluorophore quenching properties of the nitroxide radical. Investigations of interactions between stable radicals and organic fluorophores have been performed in the past, in which indirect methods have been employed to probe the effective excited-state quenching of organic dyes by a stable radical. These studies have employed radical species in either their free “monomer” form or as a part of rigid/flexible molecules containing both fluorophore and radical moieties.6,21−24 However, these past studies do not attempt to describe how a charge transfer

INTRODUCTION Stable organic radicals have been extensively studied in a multitude of research areas ranging from biological labeling1,2 to organic synthesis3−5 to photochemical applications,6,7 for both their desirable properties as a paramagnetic material as well as for their unique redox properties.5,8−10 Furthermore, stable molecular radicals are well-known quenchers of fluorescent molecules, and this photochemical interaction plays an essential role in a number of photosystems.11−13 More recently, stable organic radicals, such as those based on the 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) moiety, have come under investigation for their potentially advantageous redox properties in the area of energy storage [i.e., organic radical battery (ORB) research].10,14−17 Nitroxide radicals are known to undergo reversible oxidation and reduction reactions, and their reduction potentials can be tuned over a wide range with various substituents and functional groups. In energy storage applications, the free radical-containing units were incorporated into macromolecules to prevent their dissolution into the battery electrolyte. Of the many stable radical polymers to be prepared, poly(4methacryloyloxy-2,2,6,6-tetramethylpiperidine-N-oxyl) © 2014 American Chemical Society

Received: June 23, 2014 Revised: September 12, 2014 Published: October 20, 2014 12541

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Synthesis. Poly(−4-methacryloyloxy-2,2,6,6-tetramethylpiperidin-N-oxyl). 4-Methacryloyloxy-2,2,6,6-tetramethylpiperidine-N-oxyl and its corresponding polymer (PTMA-100) were synthesized according to a literature procedure.16 PTMA-20 (Mn = 24.0 kg/mol) and PTMA-60 (Mn = 13.7 kg/mol) were synthesized using the same general procedure but with appropriate molar ratios of TEMPO methacrylate and methyl methacrylate in the monomer feed. We have observed good agreement between the lithium capacity of the cross-linked nitroxide-radical polymers as calculated based on reagent stoichiometry, 111 mAhr/g, to the experimentally observed, 103 mAhr/g. Note, large batch-to-batch variations in Mn were typically observed with this procedure that varied between 5 and 20 kG/mol. Polymer Molecular Weight Determination. Polymer samples were dissolved in HPLC-grade chloroform (∼2 mg/ mL) and filtered through a 0.45 μm PVDF filter. A 4 mL sample was then injected into two preparatory PL-Gel 300 × 25 mm (10 μm) mixed D GPC columns connected in series. An Agilent 1200 series autosampler, inline degasser, and diode array detector were employed. The column and detector temperatures were 25 °C. HPLC-grade chloroform was used as eluent (10 mL/min). Linear polystyrene standards were used for calibration. A PTMA-100 sample with Mn = 4800 g/mol and PDI = 1.7 was employed to obtain the 14, 25, and 49PTMA samples through fractionation on a preparatory scale. UV−vis Absorbance. Absorbance spectra were taken to determine the concentration of organic dye (peryelene) prior to the addition of the nitroxide radical quencher. Absorbance measurements were taken on a Shimadzu 3600 UV−vis/NIR spectrophotometer. Steady-State Photoluminescence (PL). Steady-state PL spectra were measured on a Horiba Jobin-Yvon Model FL1039/40 Fluorolog III, equipped with an iHR320 monochromator and liquid nitrogen cooled G35 CCD detector. Samples were excited at 420 nm and corrected for the spectral response of the detection system. Photoluminescence Decays. PL decays were measured using the time-correlated single-photon counting (TCSPC) technique.26 Excitation at 420 nm was afforded by a supercontinuum fiber laser (Fianium, SC-450-PP) operating at a repetition rate of 10 MHz. Emission at 467 nm was collected by a short focal length Czerny-Turner spectrometer (Horiba Jobin-Yvon, MicroHR) and a single-photon detection module (Horiba Jobin-Yvon, TBX 850) and processed using a single-photon counting electronics module (Becker & Hickl, SPC-130). For each sample, as a function of quencher concentration, the PL decays were measured for an equivalent number of absorbed photons, allowing comparisons of the absolute PL intensity as well as the decay lifetime. The instrument response function was measured by collection of the scattering of the excitation at 420 nm, by a colloidal silica (Ludox) solution. The PL decays were analyzed using an established nonlinear least-squares iterative reconvolution procedure,27 in which the finite width of the instrument response function was effectively deconvoluted from the measured data to give an overall temporal response of approximately 20 ps. Data were fitted using a single exponential decay function and the quality of fit judged using stringent statistical procedures.26 Analysis of Photoluminescence Quenching Data:28 The physical constants that describe the steady-state and timeresolved PL quenching data are obtained using Stern−Volmer

process in the energy storage materials may occur. Therefore, this study is the first to establish an unequivocal, deeper understanding of the fluorophore-radical polymer quenching interaction. This work provides irrefutable evidence that fluorophore quenching by a polymer system cannot be explained using a classical Stern−Volmer approach but rather necessitates a more complex model of macromolecular quenching. In this study, we designed a series of steady-state and timeresolved photoluminescence (PL) experiments to explore the interplay between collisional and static quenching processes. Specifically, we focused our work to probe PL quenching of the perylene fluorophore by TEMPO-containing macromolecules with varying lengths and compositions. We compare the results for the TEMPO model compound 4-oxo-2,2,6,6-tetramethylpiperidine-N-oxyl (4-oxo-TEMPO) with those for noncrosslinked PTMA oligomers (i) of various lengths and (ii) with varying densities of the TEMPO radical moieties along the chain (i.e., with some TEMPO-methacrylate units replaced by methyl methacrylate), as shown in Figure 1.

Figure 1. Model compound (4-oxo-TEMPO) and polymer structures (PTMA and PTMA-X) investigated for their ability to quench perylene dye.

We found that the quenching behavior was altered significantly as we moved from the monomeric to higher molecular weight oligomeric species, manifested as a relative change in the contribution of static and dynamic quenching processes. Our results imply that polymer shape and folding have significant impact on the quenching process as well as on the formation of pseudo complexes between the polymer and the organic dye. The results reported here are also supported by our recent theoretical investigation, which predicts how the nitroxyl radical systems may orient themselves for maximum electron hopping efficiency during redox processes.25

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EXPERIMENTAL METHODS Materials. All reagents employed in this study were obtained from commercial sources at the highest available purity and used without further purification, unless otherwise noted. All reactions were performed under dry N2. THF was purified by passing through alumina in an MBraun solvent purification system. Prior to polymerization, the monomer, initiator, and catalyst solutions were all dried separately over molecular sieves. Column chromatography was performed with Fluka Silica Gel 60 (220−440 mesh). All samples for spectroscopic measurements were prepared in degassed, anhydrous acetonitrile or dichloromethane. All solutions were prepared under air-free conditions in a glovebox and remained sealed for the duration of the measurements. 12542

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Figure 2. (a) Steady-state PL spectra and (b) PL decays for the perylene fluorophore (4.7 mM in acetonitrile) and (c) Stern−Volmer plots of F0/F (blue) and τ0/τ (red) as a function of 4-oxo-TEMPO concentration.

Careful consideration of how to best apply Stern−Volmer treatment of this oligomer size-dependent data reveals two scenarios for determining quencher concentrations. In the “monomer view”, quencher concentrations can be determined purely by the number of free-radical groups present in solution, essentially treating the polymer samples to be free PTMA monomers. Alternatively, in the “oligomer view” each oligomer chain is viewed as a separate quenching entity. As the data below illustrates, the actual scenario lies between these two extremes. Molecular Dynamics Simulations.25,29 The molecular dynamics simulations were performed using the AMBER forcefield and electrostatic potential fit based charges, and MD simulations were run with GROMACS.25 Simulations were conducted with a time step of 0.5 fs using the leapfrog integration scheme. A particle mesh Ewald summation was used to describe Coulombic interactions with a cutoff of 15 Å, and the same cutoff was also applied to the Lennard-Jones interactions. The temperature was controlled with a NoseHoover thermostat, and the pressure was controlled using a Parrinello−Rahman barostat scheme, both with time constants of 0.5 ps, under the NPT thermodynamic ensemble. Initial low-density structures were equilibrated, by heating and cooling under the NPT thermodynamic ensemble. PTMA chains of 12, 24, and 48 monomers in length were placed in cubic simulation cells with box lengths of 8.0, 10.0, and 14.0 nm, respectively, to eliminate self-interactions based on the lengths of each oligomer. Acetonitrile molecules were then added to the box with increasing lengths have 3092, 7368, and 7983 molecules, respectively. Molecular dynamics simulations were then conducted at NVT for 100 ps at 300 K then NPT for 15 ns at 300 K to reach equilibration. The systems were then heated up to 600 K at 300 K/ns, held at 600 K for 1 ns, and then cooled back to 300 K at the same rate. Subsequently, another 15 ns equilibration was done, and data was taken during the last 10 ns. The radius of gyration of the quenching sphere is defined as the normal radius of gyration rgy with an additional length added to account for the quenching sphere. In that

analysis, which, in the case of a purely dynamic (collisional) quenching process, is given by F0 τ = 0 = 1 + kqτ0[Q ] = 1 + KD[Q ] F τ

(1)

where F0 and τ0 are the PL intensity and lifetime of perylene in the absence of quencher, respectively, [Q] is the quencher concentration, kq is the bimolecular quenching rate constant, and KD is the dynamic quenching constant. The relationship illustrated in eq 1 indicates that the Stern−Volmer plot for both steady-state and time-resolved PL data would be a straight line with a y intercept of 1 and a slope defined by kq. Often, the simple linear relationship described above is complicated by an additional quenching process, termed static quenching, whereby the fluorophore and quencher form a ground state complex to create a subpopulation of fluorophores that are nonemissive. In such a scenario, the time-resolved PL quenching data can still be described using eq 1, but the quenching of the steadystate PL is more pronounced, leading to a nonlinear dependence on the quencher concentration that is given by F0 = 1 + (kqτ0 + KS)[Q ] + kqτ0KS[Q ]2 F = 1 + (KD + KS)[Q ] + KDKS[Q ]2

(2)

where KS is the association constant for fluorophore−quencher complex formation. An alternative model to describe the nonlinear quencher concentration dependence of the Stern−Volmer plot for the steady-state PL data is an apparent static quenching process where fluorophores that are adjacent to the quencher at the moment of excitation are instantaneously quenched, such that those fluorophores appear to be dark. In this model, the quencher sits at the center of a “sphere of action” within which an excited fluorophore is quenched quantitatively, and the Stern−Volmer behavior of the steady-state PL data is given by ⎛ VN [Q ] ⎞ F0 = (1 + kqτ0[Q ])exp⎜ A ⎟ ⎝ 1000 ⎠ F ⎛ 4πR3N [Q ] ⎞ A ⎟ = (1 + kqτ0[Q ])exp⎜ 3000 ⎠ ⎝

rgy = (3)

∑i ( ri − R cms + radd)2 mi ∑i mi

(4)

where ri is the position of the considered atoms with mass mi, RCMS is the center of mass of the considered atoms in a given molecule, and radd is an additional length to account for the quenching radius around the considered atomic species.

where NA is the Avogadro constant, and V and R describe the volume and radius of the quenching sphere of action, respectively. 12543

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Figure 3. (a) Steady-state PL spectra and (b) PL decays for the perylene fluorophore (4.7 mM in acetonitrile) and (c) Stern−Volmer plots of F0/F (blue) and τ0/τ (red) as a function of TEMPO radical moiety concentration for PTMA-100.

Table 1. Parameters Extracted from the Quenching Sphere of Action Model for Quenching of the Perylene Fluorophore by 4oxo-TEMPO and n-PTMA Oligomers radical concentrationa 4-oxo-TEMPO 14-PTMA 25-PTMA 49-PTMA PTMA-100

oligomer concentrationb

kq (× 108 M−1 s−1)

KD (M−1)

R (nm)

kq (× 108 M−1 s−1)

KD (M−1)

R (nm)

183 18.9 12.5 8.45 8.25

101 10.4 6.86 4.65 4.54

2.3 2.2 2.1 2.2 2.2

183 255 304 444 −

100 140 167 244 −

2.3 5.3 6.1 7.9 −

a

The quencher concentration for the PTMA oligomers was calculated as the concentration of TEMPO radical moieties, making the assumption that these units are homogeneously distributed in solution. bIn this case, the quencher concentration is calculated assuming that each of the PTMA oligomers is a single quencher unit. The true quencher concentration is a reflection of the fact that multiple TEMPO radical moieties are bound together and partake in quenching. Therefore, these two models represent the upper and lower limits to the PTMA quenching radii.

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RESULTS AND DISCUSSION Quenching of Perylene by 4-Oxo-TEMPO. Figure 2 shows the steady-state (a) PL spectra and PL (b) decays for the perylene fluorophore in an acetonitrile solution (4.7 mM) as a function of the addition of 4-oxo-TEMPO. Here the reduction in both the PL intensity and the PL lifetime are depicted, while Figure 2c shows the Stern−Volmer plots derived from the steady-state and time-resolved PL measurements. The Stern− Volmer plot for the time-resolved data (τ0/τ) exhibits classic behavior for a dynamic (collisional) quenching process (linear dependence on quencher concentration), whereas that derived from the steady-state data (F0/F) clearly indicates the presence of an additional apparent static quenching process (nonlinear dependence on quencher concentration). The PL decays (Figure 2b) also show evidence for the apparent static quenching process, which manifests itself as a reduction in the peak PL intensity with increasing 4-oxo-TEMPO concentration. Stern−Volmer analysis of the PL lifetime data using eq 1 (see Experimental Methods) allows us to determine the bimolecular quenching constant for the diffusion-limited collisional quenching process, kq = 1.83 × 1010 M−1 s−1. Although this value is of the same order of magnitude as for other collisional quenching processes, it is a little larger than expected as compared to that observed for common fluorophore quenchers such as oxygen.28 However, in addition to the diffusion constants of the colliding species, kq is also dependent on the sum of their interaction radii, which appears to be significantly larger than normal in the case of the 4-oxo-TEMPO radical quencher (vide infra). The analysis of the τ0/τ data allows us to assess the applicability of the two models for the nonlinear F0/F data: the combined dynamic and static quenching model (eq 2)

and the quenching sphere of action model (eq 3). In the case of static quenching resulting from ground-state complex formation, we calculate the association constant for complex formation, KS = 43 M−1, which implies a rather strong interaction between perylene and the 4-oxo-TEMPO radical. In the alternative model, we obtain a radius for the quenching sphere of R = 2.3 nm, which is significantly larger than the dimensions of the 4-oxo-TEMPO radical, implying that the radical species is able to quench the perylene fluorophore without the need for close contact. Quenching of Perylene by PTMA Oligomers. Moving from the free 4-oxo-TEMPO model compound to PTMA-100, where 100% of the methacrylate units contain pendent TEMPO radicals, a distribution of macromolecules with varying molecular weight results in clear differences in the observed quenching behavior, as shown in Figure 3. The effect is significantly more noticeable in the PL decays (Figure 3b), where the PL lifetime is relatively unaffected by the addition of the PTMA-100 quencher, whereas the steady-state PL spectra are fairly strongly quenched. This observation suggests that the apparent static quenching process begins to dominate the quenching behavior, as highlighted in Figure 3c. When one assumes that the quencher concentration is given by the concentration of TEMPO moieties in solution, and not the concentration of PTMA-100 chains, a comparison of the bimolecular quenching rate coefficients, kq, for 4-oxo-TEMPO and PTMA-100 indicates that the macromolecule, with its bound TEMPO moieties, is a far less efficient quencher (Table 1). This can be interpreted in terms of the restricted motion of the PTMA-100 in solution in conjunction with the proclivity for polymer folding, which both contribute to limited access of 12544

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the fluorophore to TEMPO radical quenching units (i.e., the effective concentration of the TEMPO moieties is reduced). Given the rather broad distribution of molecular weights in this sample of PTMA-100 (Mn = 18000 g/mol; PDI = 2.6), it was necessary to quantitatively probe the molecular weight (chain length) dependence of the quenching behavior for PTMA. Size-selective fractionation of a sample of PTMA-100 (see Experimental Methods) yielded three discrete oligomer samples with average chain lengths of 14, 25, and 49 monomers, spanning molecular weights from ∼3300 g/mol to ∼11700 g/mol. The molecular weight distributions of these fractions, termed 14-PTMA, 25-PTMA, and 49-PTMA, respectively, were all quite narrow (PDI = 1.2) and are illustrated in Figure S1 of the Supporting Information). The steady-state PL spectra and PL decays as a function of the PTMA oligomer concentrations, for the different n-PTMA samples are shown in Figures S2−S4 of the Supporting Information. The Stern−Volmer plots extracted from these data, again assuming that the quencher concentration is given by the concentration of TEMPO moieties in solution and not the concentration of n-PTMA chains, are shown in Figure 4 (panels a and b), and the parameters obtained using the quenching sphere of action model are given in Table 1.

(collisional) process. This is expected, as a change in polymer size and, in turn, a change to solution-phase morphology would be expected to have a significant effect on quenching efficiency. The largest polymers will diffuse more slowly and have more radicals self-protected inside the molecular packing in solution, thus lowering the probability of interactions with the fluorophore, while the shorter PTMA chains will be able to diffuse more quickly through the solvent, hence enhancing the rate of collisions with excited perylene fluorophores. Also, the smaller oligomer length will afford less shielding to the TEMPO moieties. Self-diffusion coefficients for poly(methyl methacrylate) (PMMA) polymer dispersed in acetonitrile are well-known.30 Using the known diffusion behavior of PMMA, we expect the diffusion coefficients for PTMA at these molecular weights to span a similar range from ∼1.8 × 10−09 to 6.2 × 10−10 m2/s, assuming the absence of significantly different solvent interactions as a result of the pendant radical groups. Extrapolating the data, one can estimate that a ∼5-fold decrease in the mobility of a PTMA particle in acetonitrile effectively cuts the quenching capability in half. Additionally, higher molecular weight polymers fold more readily in solution, and this folding presents new steric hindrances for the fluorophore. Theoretically, we expect barriers to accessing all of the TEMPO moieties within the polymer coil as predicted for the various oligomer lengths using the molecular simulations shown in Figure 5. Interestingly, despite the fact that the quenching entity is now significantly larger than the 4oxo-TEMPO model compound, the radius of the quenching sphere of action remains relatively constant over the series of PTMA molecular weights. This suggests that only one TEMPO moiety within the oligomer chain can participate in the quenching process, so the radius is effectively the same as that of the 4-oxo-TEMPO model compound. An alternative model to describe the observed quenching behavior is to treat each oligomer as a single quenching unit. Stern−Volmer analysis where the concentration of oligomers (“oligomer view”) in eq 3 was used instead of the radical moiety concentration is shown in Figure 4 (panels c and d). The results from this approach are given in Table 1 and indicate that in this scenario, where the effective concentration of quenchers is much lower than in the “monomer view”, the quenching radius of action increases with increasing oligomer length. In a purely qualitative look at the Stern−Volmer sphere of action model (See experimental eq 3), the curvature in the fit is achieved purely by the exponential term in eq 3. Therefore, as the effective quencher concentration decreases, the radii of interaction necessarily increase to offset this change and give the unique curvature observed in the plot of F0/F. The physical implications of the increasing radius with oligomer chain length can be attributed to access of the perylene fluorophore to multiple quenching TEMPO radical units at the surface of the singular macromolecule. This approach results in an upper limit for the radii of the quenching sphere for each oligomer. Overall, the true quenching radius for n-PTMA oligomers lies between the “monomer” and “oligomer” views presented here, and additional analysis is underway to determine the true quenching behavior exhibited by n-PTMA macromolecules. In an effort to elucidate the solvent effects on macromolecule morphology, and the subsequent changes to quenching behavior, experiments in CH2Cl2 were also performed. For CH2Cl2, with a dielectric constant of 8.93 (compared to that of 37.5 for CH3CN), we hypothesized that changing the polarity of the solvent would have an impact on the efficiency of the

Figure 4. (a−b) Stern−Volmer plots of F0/F and τ0/τ as a function of the TEMPO radical moiety concentration for the n-PTMA oligomer series. (c−d) Stern−Volmer plots of F0/F and τ0/τ as a function of the oligomer concentration. Also shown for comparison is the equivalent data for 4-oxo-TEMPO.

A comparison with the PL quenching data for 4-oxoTEMPO and PTMA-100 indicates that quenching of the perylene fluorophore by the n-PTMA oligomers is dominated by the apparent static process, suggesting that, even for the shortest oligomer chains, the diffusion of these macromolecules is somewhat inhibited. A clear trend in the value for the bimolecular quenching rate coefficients, kq, with the oligomer chain length is observed, with the shorter chains able to more effectively quench the perylene fluorophore through a dynamic 12545

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Figure 5. Molecular dynamics simulations of PTMA oligomers. The predicted polymer folding and the expected sphere of interaction as a function of oligomer length are shown.

Figure 6. Stern−Volmer plots of (a) F0/F and (b) τ0/τ as a function of the TEMPO radical moiety concentration for the PTMA-X oligomer series. Also shown for comparison is the equivalent data for 4-oxo-TEMPO.

by the “monomer” and “oligomer” views, the precise values for these radii are further complicated by the unknown fraction of TEMPO units that can participate in the quenching process. For instance, for fluorophore quenching that occurs by a resonant energy transfer (RET) mechanism, the distancedependence of the rate coefficient for the quenching process is dependent on the “dimensionality” of the quenching entity.31 When the quencher is represented by either a 2-dimensional “plane” or 3-dimensional “slab”, the RET process can occur over significantly extended distances, compared to the scenario where both the fluorophore and quencher are represented by point dipoles.31 If this mechanism is in play between perylene and n-PTMA this could manifest itself as an increase in the quenching radius of the macromolecule compared to 4-oxoTEMPO. Theoretical calculations are underway to shed more light onto the possibility of enhanced quenching and to extract a more rigorous value for the quenching radius of the oligomers. Varying TEMPO Density in the PTMA Oligomers. In addition to probing the effect of chain length on the quenching behavior, another variable that can be tuned is the density of

quenching process if it was preceded by a mechanism involving charge transfer between interacting species. However, no such changes to the quenching efficiency were observed in CH2Cl2, indicating both that the apparent macromolecule morphology remains unaffected and the quenching process proceeds by a mechanism that does not necessarily require stabilization of a polar species. In order to verify those observations made in our physical measurements, molecular modeling was also performed. Molecular simulations of various oligomer chains confirm the increase in quenching radius as a function of oligomer length as shown in Figure 5. Using classical molecular dynamics, the geometries of PTMA chains of 12, 24, and 48 monomers in length were predicted in acetonitrile. The radius of gyration of the sphere of action was calculated by considering the distance of each TEMPO nitrogen from the center of mass of the nPTMA macromolecule and then adding 2.3 nm to account for the quenching sphere of the individual TEMPO moieties. The radii of gyration of the spheres of action for the 12, 24, and 48 length oligomers were found to be 3.4, 3.5, and 4.3 nm, respectively. Though these values fall between those predicted 12546

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quenching process since it is also possible that some dye molecules can become trapped within the coil and permanently associated with the quenching radical moieties. Quenching efficiency as a function of polymer morphology and the large quenching sphere of action provide valuable experimental insight as we proceed to look at the specifics of the inter- and intramolecular electron-hopping mechanisms within the TEMPO polymer during oxidation−reduction reactions. Consequently, this work now makes it possible to utilize the incorporation of an organic fluorophore into the radical polymer system to advance understanding of the redox charge transfer mechanism through advanced spectro-electrochemical investigations.

TEMPO units incorporated into the oligomer chain. Two samples were prepared by the random copolymerization of the 2,2,6,6-tetramethylpiperidine methacrylate monomer with methyl methacrylate to form a PTMA sample where 20% (PTMA-20) and 60% (PTMA-60) of the methyl methacrylate “repeat” units contain the TEMPO group. The Stern−Volmer plots extracted from the steady-state and time-resolved PL data, once again assuming that the quencher concentration is given by the concentration of TEMPO moieties in solution and not the concentration of PTMA-X chains, are shown in Figure 6. The parameters obtained using the quenching sphere of action model are given in Table 2.

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Table 2. Parameters Extracted from the Quenching Sphere of Action Model for Quenching of the Perylene Fluorophore by 4-Oxo-TEMPO and PTMA-X Oligomers radical concentration 8

kq (× 10 M 4-oxo-TEMPO PTMA-20 PTMA-60 PTMA-100

183 27 9.4 8.3

−1

−1

s )

S Supporting Information *

Gas phase chromatography traces of the n-PMTA fractions separated from as-prepared PTMA-100. Photoluminescence decays and spectra of perylene as a function of TEMPO concentration for the various n-PTMA oligomers. Comparison between “static” and “quenching sphere of action” (apparent static) fits to the steady-state PL quenching data for n-PTMA. This material is available free of charge via the Internet at http://pubs.acs.org.

a

−1

KD (M )

R (nm)

101 14 5.2 4.5

2.3 2.3 2.25 2.2

ASSOCIATED CONTENT

a

The quencher concentration for the PTMA oligomers was calculated as the concentration of TEMPO radical moieties, making the assumption that these units are homogeneously distributed in solution.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

In this case, the PTMA-X oligomers are all less effective collisional quenchers, as compared to the model compound, which is as expected for macromolecules with inhibited diffusion. As the density of the TEMPO moiety is increased, and therefore the average polymer molecular weight becomes larger, the efficiency of the dynamic quenching process is decreased. This observation can be attributed to hindered diffusion of the higher molecular weight material. Though the quenching efficiency decreases, the calculated radii of the quenching sphere is consistent with those observed for 4-oxoTEMPO and implies that the quenching process does not require intimate contact between the reacting species. With the lowest molecular weight, PTMA-20 exhibits a bimolecular quenching rate constant of 27 × 108 M−1 s−1, which is more than 3 times as efficient as that seen for PTMA-100, and PTMA-60 falls nonlinearly between the two.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Dr. Justin Johnson and Dr. Garry Rumbles for helpful discussions. This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division under contract DE-AC36-08GO28308. The research was performed using resources sponsored by the Department of Energy’s Office of Energy Efficiency and Renewable Energy, located at the National Renewable Energy Laboratory.

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CONCLUSIONS In conclusion, this study is the first to demonstrate that the efficient quenching of perylene dye by TEMPO-containing polymers is dependent on a number of polymer properties including length and morphology. Furthermore, fluorophore quenching occurs by a process that requires the fluorophore and the quencher to be within ∼2.2 nm of each other in the ideal scenario, when each component is allowed to freely diffuse through solution, as is the case for the model compound, 4-oxoTEMPO. The dynamics of this interaction are changed greatly once the quencher is polymer-bound, and now for the same radical concentrations in solution, quenching of perylene emission occurs less frequently, as seen by the variation in quenching efficiency observed for TEMPO-100. Also, the mechanism of quenching transitions away from the diffusionlimited process as the molecules become larger and diffuse more slowly. The presence of larger coiling macromolecules may also further contribute to the appearance of a static

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