In Search for the Best Environment for Single Molecule Studies

Letter pubs.acs.org/JPCL

In Search for the Best Environment for Single Molecule Studies: Photostability of Single Terrylenediimide Molecules in Various Polymer Matrices Hubert Piwoński,*,† Adam Sokołowski,† and Jacek Waluk*,†,‡ †

Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44, 01-224 Warsaw, Poland Faculty of Mathematics and Science, Cardinal Stefan Wyszyński University, Dewajtis 5, 01-815 Warsaw, Poland

‡

S Supporting Information *

ABSTRACT: Photobleaching is the main limiting factor in single molecule studies by optical techniques. We investigated the dependence of photostability of terrylene diimide (TDI) derivative on its environment using confocal fluorescence microscopy. Seven different polymers were tested. Depending on the matrix, photobleaching quantum yields vary by 2 orders of magnitude. Their values correlate with parameters characterizing oxygen mobility in polymers: diffusion coefficient and permeability. Poly(vinyl chloride) (PVC) and poly(methyl methacrylate) (PMMA) exhibit the lowest photodestruction quantum yields. Additional enhancement of photostability can be achieved by aging of PVC or by flushing the sample with nitrogen, which confirms the involvement of oxygen in photodestruction. Different character of the time traces of the intensity of emission from single TDI molecules is observed for different polymer matrices, ranging from intense blinking in the least stable polycarbonate, to practically no blinking in the most stable PVC. These results suggest a photodegradation mechanism involving self-sensitized photooxidation in oxygen complexes of TDI. he possibility of observing fluorescence from a single chromophore at room temperature, first demonstrated in the last decade of the 20th century,1 has led to a rapid development of single molecule spectroscopy (SMS) techniques.2 One of the main advantages of SMS over conventional fluorescence spectroscopy is that it avoids averaging over a large ensemble. Thus, the behavior of individual molecules can be monitored over time, which allows, for instance, observation of a chemical reaction without applying an external stimulus, as is the case for measurements in the bulk. When applied for analytical purposes, SMS provides ultimate analytical sensitivity and a possibility of spatial localization of the emitting species. Moreover, determination of the orientation of the chromophore in 3D can be achieved by using polarized light in excitation and/or emission. The main drawback of fluorescence-based SMS is the limited lifetime of the probed chromophore. In order to collect a number of photons sufficient for, at least qualitative, conclusions, a molecule has to undergo tens or hundreds of thousands of excitation/emission cycles. Therefore, photostability becomes a crucial issue. Even very stable and strongly emitting molecules can usually be observed for a period of seconds, or even less, before they decompose. One way to alleviate the problem with decomposition is to repeat many times the observation of a single molecule, as, e.g., in fluorescence correlation spectroscopy; in this technique, molecules enter the focal volume of a microscope one by

T

© XXXX American Chemical Society

one, from a dilute solution. However, in many cases, one would like to record a time trace of the same chromophore over the longest time possible. In such experiments, the molecule under study is immobilized in a rigid matrix, usually a polymer, and the duration of the experiment is limited by the photostability of the chromophore in a given environment. Many different mechanisms of photobleaching have been discussed.3−15 An extremely important role in photostability is played by oxygen. In cases when oxidation is not the only photodestruction pathway, it can, depending on conditions, either enhance or decrease the photostability.10,11,14 One can significantly improve the conditions for single molecule detection by optimizing the concentration of oxygen in the sample environment.16 Because both the solubility and mobility of oxygen vary for different polymers, it is obvious that the feasibility of a particular SMS experiment depends on a proper selection of a host polymer matrix. With this in mind, we carried out a study in which the goals were to (i) compare the photostability of single molecules embedded in polymers most often used in SMS experiments, (ii) find a polymer matrix which would provide the most suitable host for long-lived single Received: May 21, 2015 Accepted: June 11, 2015

2477

DOI: 10.1021/acs.jpclett.5b01060 J. Phys. Chem. Lett. 2015, 6, 2477−2482

Letter

The Journal of Physical Chemistry Letters

molecules that survived the first raster scan of the sample was determined for different laser powers, separately for each polymer matrix (Figure 2S). This procedure allowed us to select laser powers that did not significantly change the population of fluorescing TDI molecules in a single scan. These powers were subsequently used to obtain time traces from single chromophores. The analysis of the photostability parameters was based on calculating the probability distributions of the total number of photons emitted before photobleaching, obtained from the integral over a time trace. The detection efficiency of our experimental setup, estimated as 1%, was used to calculate the total number of the emitted photons. Figure 2 shows representative time traces of TDI fluorescence recorded in four different polymer matrices: PC,

chromophores, and (iii) understand the mechanism of photodestruction. As a probe chromophore, we used terrylenediimide (TDI), one of the SMS “standards”.17,18 TDI has been widely used in single molecule investigations of diverse phenomena, such as blinking and photobleaching,5,6,19 spectral dynamics,20−22 diffusion,23−25 energy transfer,26−33 electron transfer,34 relaxation dynamics in polymers,35molecular imaging,36,37 cellular labeling,38,39 detection of single oxygen,40,41 and photodegradation.42 In this work, single molecule fluorescence of TDI was studied in seven different polymers: poly(methyl methacrylate) (PMMA), poly(vinyl butyral) (PVB), polycarbonate (PC), poly(vinyl alcohol) (PVA), polyethylene (PE), polystyrene, (PS), and poly(vinyl chloride) (PVC). The choice of the latter matrix was stimulated by a suggestion that it may substantially extend the lifetime of single chromophores due to its low permeability to oxygen.43 Indeed, our results indicate the highest photostability of TDI in PVC, making this polymer a matrix of choice for experiments in which the photobleaching yield crucially depends on oxygen concentration and mobility. Single molecule fluorescence spectra were obtained with a home-built scanning confocal optical microscope. The 594 nm line of a He−Ne gas laser was used for excitation. The detailed description of the optical setup is provided in the Supporting Information. TDI (terrylene diimide, N-(2,6-diisopropylphenyl)-N′-(noctyl)-terrylene-3,4:11,12-tetracarboxidiimide, see Figure 1 for

Figure 1. Absorption and fluorescence spectra of TDI solution in tetrahydrofuran at 293 K. The vertical orange line shows the excitation wavelength used in the experiments (λex = 594 nm).

the formula) was kindly provided by Prof. Jerzy Sepioł. The samples were prepared from cosolutions of a selected polymer (10 mg/mL) and TDI (5 × 10−10 M) in an appropriate solvent, by spin coating onto a cleaned microscope coverslip. Information about polymers and solvents and the detailed description of sample preparation is given in the Supporting Information. In the initial stage of the investigation of photobleaching, wide field images of single TDI molecules were obtained as a function of irradiation time (Supporting Information Figure 1S). Next, optimal conditions for irradiation of single chromophores were tested. Using too large laser powers may lead to biased statistics, as a large number of molecules may be destroyed in the initial scan. To avoid that, the fraction of

Figure 2. Representative time traces of fluorescence intensity of TDI in four different polymer matrices.

PVB, PMMA, and PVC. One immediately notices very different character of intensity variations. The blinking is very strong in PC, much less pronounced in PVB and PMMA, and practically absent for TDI embedded in PVC. The other difference regards the average lifetime of a single molecule before bleaching. It is the shortest for PC and the longest for PVC matrix. We made sure that the sudden drop in intensity is really due to photodestruction of the molecule by extending the recording of time traces to periods several times longer than those for which the chromophore exhibited fluorescence. 2478

DOI: 10.1021/acs.jpclett.5b01060 J. Phys. Chem. Lett. 2015, 6, 2477−2482

Letter

The Journal of Physical Chemistry Letters In order to obtain a reliable determination of photodestruction quantum yield, histograms were constructed, representing the values of photobleaching quantum yields (Ybl) measured for up to several hundred of molecules for each polymer matrix. Ybl is defined as Φfl/Nph, where Φfl is the fluorescence quantum yield (0.73 for TDI19) and Nph is the number of photons emitted by a single chromophore before bleaching occurs. The latter parameter is related to the number of photons detected, Ndet, by the relation Nph = Ndet/ϕdet; ϕdet is the collection efficiency of our system. Since it is estimated with limited accuracy, the relative values of Ybl seem more reliable than the absolute ones. We note, however, a very good agreement between our value of Ybl in PMMA and the value obtained by other authors,19 who found that the photobleaching quantum yield of TDI in PMMA is wavelengthdependent; the reported values were 2 × 10−10 and 2 × 10−8 for excitation at 647 and 520 nm, respectively. In our case, λex = 594 nm, so an intermediate value of Ybl can be expected. Indeed, we find Ybl = 4.4 × 10−10. Examples of the histograms are shown in Figure 3. They reveal differences in the photostability of TDI in various

Table 1. Photobleaching Quantum Yields polymer matrix

photobleaching yield/10−11

PVC PVC (aged) PMMA PMMA (flushed with N2) PVA PE PS PVB PC

5.7 ± 0.3 2.3 ± 0.6 44 ± 4 2.4 ± 0.1 75 ± 12 140 ± 10 230 ± 30 240 ± 40 580 ± 10

The literature values of 2.2 × 10−7, 1.4 × 10−8, and 1.3 × 10−8 cm2 s−1 for PS, PMMA, and PVC, respectively,48 decrease in the same order as the photostabilities increase. We have additionally proved the role of oxygen by comparing the photobleaching quantum yields in PMMA in air and flushed with nitrogen. Flushing resulted in enhancing the photostability by a factor of 18 (Table 1). It would be unrealistic to expect a strict correlation between the above-mentioned parameters and photodegradation quantum yield. First, the values reported in the literature for the same polymer substantially differ. Even more important, our experiments are performed for extremely thin (20−30 nm) films; under such conditions, the interface effects may be very significant. Still, plotting the values of oxygen solubility, diffusion coefficient, and permeability of the polymers used in this work against the values of photobleaching quantum yield (Figure 4) clearly shows a correlation between photostability

Figure 3. Histograms showing the values of photobleaching quantum yields for different polymers and for various experimental conditions.

polymers exceeding 2 orders of magnitude. The values of photodegradation quantum yields measured for all tested polymers, including the data obtained under different experimental conditions (vide inf ra) are given in Table 1. The lowest photobleaching quantum yields were found for PVC and PMMA matrices. These results strongly suggest that the main factor responsible for photodegradation is oxygen. Comparison of permeabilities of O2 at 293 K reveals low values for PVC and PMMA and high ones for PS and PC. For example, permeability of oxygen in PC and PS is reported to be about 30−40 times higher than for PVC and PMMA.44−46 Other authors estimate the PC/PMMA oxygen permeability ratio as about 8.47 These data correlate well with the relative photostabilities of TDI in respective polymers. Similar correlation also exists with the oxygen diffusion coefficients.

Figure 4. Relative oxygen solubilities, diffusion coefficients, and permeabilities plotted against the values of photobleaching quantum yields. See Table 1S for the data used.

and dynamic parameters: diffusion coefficient and permeability. On the other hand, solubility does not seem to correlate with photostability. Possible mechanisms of photodestruction of TDI and similar chromophores have been discussed in several papers.5−7,12,19,26,28,40−42 However, the exact picture of the process is far from being complete. There is no doubt that the oxygen can participate in the photodegradation. Actually, TDI has been used as a probe of airborne singlet oxygen on a single molecule level.40,41 Photooxidation of single terrylene 2479

DOI: 10.1021/acs.jpclett.5b01060 J. Phys. Chem. Lett. 2015, 6, 2477−2482

Letter

The Journal of Physical Chemistry Letters molecules has been monitored by fluorescence spectroscopy.7 The process is accompanied by formation of various nonfluorescent and fluorescent species; exo- and endoperoxides, as well as a diepoxide were considered as possible photoproducts. Other authors studied photoblinking and photobleaching of TDI and perylene diimide in PMMA.19 They proposed radical cations arising from higher triplet states as the species responsible for blinking. The argument was based on the observation that blinking does occur at shorter excitation wavelength (520 nm) but not at 647 nm. The increase in blinking was correlated with the decrease of photostability. Therefore, the radical cations were assigned to the precursors of photobleaching. However, direct photobleaching from a higher triplet state could not be excluded. Our results seem to be compatible with the model of selfsensitized photooxidation, initiated by the generation of singlet oxygen from the T1 (or, less probably S1) state of TDI. An observation that is relevant in this context is a very different pattern of blinking observed for different polymers (Figure 2). “Off” states with lifetimes much longer than that of the triplet state are observed for PC and PVB, matrices that reveal lower photostability. PMMA and, in particular, PVC, do not reveal such behavior: No long-lived dark states are observed, and the transition from a fluorescent to a nonfluorescent form, i.e., photobleaching is very sharp. This can be explained as evidence of two consecutive steps leading to photodestruction. The first involves photosensitized generation of singlet oxygen, the second, its reaction with TDI. The reaction may be reversible; moreover, as postulated for terrylene,7 various products are possible. In the environment with high oxygen mobility, multiple oxidation/dissociation steps can occur, with some of the products being fluorescent, some not. This leads to blinking. On the other hand, in a low mobility matrix, singlet oxygen and TDI will remain at a particular mutual arrangement for a longer time. If this arrangement is favorable for creation of a nonfluorescent product, one step transition with no blinking is expected. We have checked the dependence of photobleaching yield on the laser power. For three different values of excitation intensity, 5, 17, and 53 kW/cm2, the obtained values of photobleaching quantum yields were 5.1, 5.7, and 6.9 × 10−11, respectively. These values do not indicate the quadratic dependence expected for a two-photon process involving formation of radical cations from a higher triplet state. While most of the polymers we investigated conform to the above model, a notable exception exists. Of all the polymers studied in this work, PVA has the lowest oxygen permeability and diffusion coefficient. While the photostability of TDI in PVA is quite good, it is surpassed by PVC and PMMA matrices. We interpret this as an indication of other channels present in PVA competing in photodegradation. Indeed, it has been reported that PVA can react with excited fluorescein.49 In another work, the formation of radicals via intermolecular electron transfer has been postulated for single molecules of 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate (DiI) embedded in PVA.50 It is known that aging of polymers can lead to a decrease in free volume, which, in turn, affects the oxygen diffusion coefficient.51,52 Slower diffusion of oxygen in an aged polymer should enhance photostability. We checked for PVC that this is really the case. Aging for several weeks resulted in nearly 3-fold reduction in photobleaching yield (Table 1), once again confirming oxygen involvement in the process.

In summary, we demonstrated a close correlation between the photostability of a chromophore and the parameters related to oxygen mobility in the polymer matrix in which the molecule is embedded. We recommend PVC and PMMA as very good matrices for single molecule fluorescence studies. We also note that there exist polymers that exhibit permeability to oxygen much lower than PMMA and PVC: polyacrylonitrile, polyvinylidene chloride, or ethyl vinyl alcohol copolymer.46,53 Extrapolating the results of this work, one could speculate that using these polymers as matrices for SMS may further enhance the photostability of single chromophores. We note, however, that such strategy may not work in cases where oxygen is not directly involved in photodestruction, but serves as a quencher of long-lived dark states (triplets or radicals). In this situation, oxygen can either enhance or decrease the photostability. Such behavior has been observed for ionic dyes,10,11,14 whereas aromatic hydrocarbons usually exhibit greater photostability in the absence of oxygen.

■

ASSOCIATED CONTENT

S Supporting Information *

Details of experimental procedures and data analysis. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b01060.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Polish National Science Centre Grants DEC-2011/02/A/ST5/00043 and DEC-2013/ 10/M/ST4/00069.

■

REFERENCES

(1) Trautman, J. K.; Macklin, J. J.; Brus, L. E.; Betzig, E. Near-Field Spectroscopy of Single Molecules at Room-Temperature. Nature 1994, 369, 40−42. (2) Themed collection: Single-Molecule Optical Spectroscopy. Chem. Soc. Rev., 2014, 43, 963−1340. (3) Eggeling, C.; Widengren, J.; Rigler, R.; Seidel, C. A. M. Photobleaching of Fluorescent Dyes under Conditions Used for Single-Molecule Detection: Evidence of Two-Step Photolysis. Anal. Chem. 1998, 70, 2651−2659. (4) Fleury, L.; Sick, B.; Zumofen, G.; Hecht, B.; Wild, U. P. High Photo-Stability of Single Molecules in an Organic Crystal at Room Temperature Observed by Scanning Confocal Optical Microscopy. Mol. Phys. 1998, 95, 1333−1338. (5) Göhde, W.; Fischer, U. C.; Fuchs, H.; Tittel, J.; Basché, T.; Bräuchle, C.; Herrmann, A.; Müllen, K. Fluorescence Blinking and Photobleaching of Single Terrylenediimide Molecules Studied with a Confocal Microscope. J. Phys. Chem. A 1998, 102, 9109−9116. (6) Panzer, O.; Göhde, W.; Fischer, U. C.; Fuchs, H.; Müllen, K. Influence of Oxygen on Single Molecule Blinking. Adv. Mater. 1998, 10, 1469−1472. (7) Christ, T.; Kulzer, F.; Bordat, P.; Basché, T. Watching the PhotoOxidation of a Single Aromatic Hydrocarbon Molecule. Angew. Chem., Int. Ed. 2001, 40, 4192−4195. (8) Hübner, C. G.; Renn, A.; Renge, I.; Wild, U. P. Direct Observation of the Triplet Lifetime Quenching of Single Dye Molecules by Molecular Oxygen. J. Chem. Phys. 2001, 115, 9619− 9622. 2480

DOI: 10.1021/acs.jpclett.5b01060 J. Phys. Chem. Lett. 2015, 6, 2477−2482

Letter

The Journal of Physical Chemistry Letters (9) Hou, Y. W.; Higgins, D. A. Single Molecule Studies of Dynamics in Polymer Thin Films and at Surfaces: Effect of Ambient Relative Humidity. J. Phys. Chem. B 2002, 106, 10306−10315. (10) Zondervan, R.; Kulzer, F.; Orlinskii, S. B.; Orrit, M. Photoblinking of Rhodamine 6G in Poly(vinyl alcohol): Radical Dark State Formed through the Triplet. J. Phys. Chem. A 2003, 107, 6770−6776. (11) Zondervan, R.; Kulzer, F.; Kol’chenko, M. A.; Orrit, M. Photobleaching of Rhodamine 6G in Poly(vinyl alcohol) at the Ensemble and Single-Molecule Levels. J. Phys. Chem. A 2004, 108, 1657−1665. (12) Hoogenboom, J. P.; van Dijk, E. M. H. P.; Hernando, J.; van Hulst, N. F.; Garcia-Parajó, M. F. Power-Law-Distributed Dark States Are the Main Pathway for Photobleaching of Single Organic Molecules. Phys. Rev. Lett. 2005, 95, 097401−1−4. (13) Gensch, T.; Böhmer, M.; Aramendia, P. F. Single Molecule Blinking and Photobleaching Separated by Wide-Field Fluorescence Microscopy. J. Phys. Chem. A 2005, 109, 6652−6658. (14) Renn, A.; Seelig, J.; Sandoghdar, V. Oxygen-Dependent Photochemistry of Fluorescent Dyes Studied at the Single Molecule Level. Mol. Phys. 2006, 104, 409−414. (15) Stein, I. H.; Capone, S.; Smit, J. H.; Baumann, F.; Cordes, T.; Tinnefeld, P. Linking Single-Molecule Blinking to Chromophore Structure and Redox Potentials. ChemPhysChem 2012, 13, 931−937. (16) Piwoński, H.; Kołos, R.; Meixner, A.; Sepioł, J. Optimal Oxygen Concentration for the Detection of Single Indocarbocyanine Molecules in a Polymeric Matrix. Chem. Phys. Lett. 2005, 405, 352− 356. (17) Mais, S.; Tittel, J.; Basché, T.; Bräuchle, C.; Göhde, W.; Fuchs, H.; Müller, G.; Müllen, K. Terrylenediimide: A Novel Fluorophore for Single-Molecule Spectroscopy and Microscopy from 1.4 K to Room Temperature. J. Phys. Chem. A 1997, 101, 8435−8440. (18) Weil, T.; Vosch, T.; Hofkens, J.; Peneva, K.; Müllen, K. The Rylene Colorant Family-Tailored Nanoemitters for Photonics Research and Applications. Angew. Chem., Int. Ed. 2010, 49, 9068− 9093. (19) Haase, M.; Hübner, C. G.; Nolde, F.; Müllen, K.; Basché, T. Photoblinking and Photobleaching of Rylene Diimide Dyes. Phys. Chem. Chem. Phys. 2011, 13, 1776−1785. (20) Mais, S.; Basché, T.; Müller, G.; Müllen, K.; Bräuchle, C. Probing the Spectral Dynamics of Single Terrylenediimide Molecules in Low-Temperature Solids. Chem. Phys. 1999, 247, 41−52. (21) Kiraz, A.; Ehrl, M.; Bräuchle, C.; Zumbusch, A. Low Temperature Single Molecule Spectroscopy Using Vibronic Excitation and Dispersed Fluorescence Detection. J. Chem. Phys. 2003, 118, 10821−10824. (22) Jung, C.; Hellriegel, C.; Platschek, B.; Wöhrle, D.; Bein, T.; Michaelis, J.; Bräuchle, C. Simultaneous Measurement of Orientational and Spectral Dynamics of Single Molecules in Nanostructured Host− Guest Materials. J. Am. Chem. Soc. 2007, 129, 5570−5579. (23) Seebacher, C.; Hellriegel, C.; Deeg, F. W.; Bräuchle, C.; Altmaier, S.; Behrens, P.; Müllen, K. Observation of Translational Diffusion of Single Terrylenediimide Molecules in a Mesostructured Molecular Sieve. J. Phys. Chem. B 2002, 106, 5591−5595. (24) Zürner, A.; Kirstein, J.; Döblinger, M.; Bräuchle, C.; Bein, T. Visualizing Single-Molecule Diffusion in Mesoporous Materials. Nature 2007, 450, 705−709. (25) Feil, F.; Cauda, V.; Bein, T.; Bräuchle, C. Direct Visualization of Dye and Oligonucleotide Diffusion in Silica Filaments with Collinear Mesopores. Nano Lett. 2012, 12, 1354−1361. (26) Cotlet, M.; Gronheid, R.; Habuchi, S.; Stefan, A.; Barbafina, A.; Müllen, K.; Hofkens, J.; De Schryver, F. C. Intramolecular Directional Förster Resonance Energy Transfer at the Single-Molecule Level in a Dendritic System. J. Am. Chem. Soc. 2003, 125, 13609−13617. (27) Métivier, R.; Christ, T.; Kulzer, F.; Weil, T.; Müllen, K.; Basché, T. Single-Molecule Spectroscopy of Molecular Aggregates at Low Temperature. J. Lumin. 2004, 110, 217−224. (28) Cotlet, M.; Vosch, T.; Habuchi, S.; Weil, T.; Müllen, K.; Hofkens, J.; De Schryver, F. Probing Intramolecular Fö rster

Resonance Energy Transfer in a Naphthaleneimide−Peryleneimide− Terrylenediimide-Based Dendrimer by Ensemble and Single-Molecule Fluorescence Spectroscopy. J. Am. Chem. Soc. 2005, 127, 9760−9768. (29) Hinze, G.; Haase, M.; Nolde, F.; Müllen, K.; Basché, T. TimeResolved Measurements of Intramolecular Energy Transfer in Single Donor/Acceptor Dyads. J. Phys. Chem. A 2005, 109, 6725−6729. (30) Melnikov, S. M.; Yeow, E. K. L.; Uji-i, H.; Cotlet, M.; Müllen, K.; De Schryver, F. C.; Enderlein, J.; Hofkens, J. Origin of Simultaneous Donor−Acceptor Emission in Single Molecules of Peryleneimide−Terrylenediimide Labeled Polyphenylene Dendrimers. J. Phys. Chem. B 2007, 111, 708−719. (31) Hinze, G.; Métivier, R.; Nolde, F.; Müllen, K.; Basché, T. Intramolecular Electronic Excitation Energy Transfer in Donor/ Acceptor Dyads Studied by Time and Frequency Resolved Single Molecule Spectroscopy. J. Chem. Phys. 2008, 128, 124526. (32) Curutchet, C.; Mennucci, B.; Scholes, G. D.; Beljonne, D. Does Förster Theory Predict the Rate of Electronic Energy Transfer for a Model Dyad at Low Temperature? J. Phys. Chem. B 2008, 112, 3759− 3766. (33) Mackowski, S.; Wörmke, S.; Ehrl, M.; Bräuchle, C. LowTemperature Spectral Dynamics of Single TDI Molecules in n-Alkane Matrixes. J. Fluoresc. 2008, 18, 625−631. (34) Izquierdo, M. A.; Bell, T. D. M.; Habuchi, S.; Fron, E.; Pilot, R.; Vosch, T.; De Feyter, S.; Verhoeven, J.; Jacob, J.; Müllen, K.; Hofkens, J.; De Schryver, F. C. Switching of the Fluorescence Emission of Single Molecules between the Locally Excited and Charge Transfer States. Chem. Phys. Lett. 2005, 401, 503−508. (35) Braeken, E.; De Cremer, G.; Marsal, P.; Pèpe, G.; Müllen, K.; Vallée, R. A. L. Single Molecule Probing of the Local Segmental Relaxation Dynamics in Polymer above the Glass Transition Temperature. J. Am. Chem. Soc. 2009, 131, 12201−12210. (36) Kukura, P.; Celebrano, M.; Renn, A.; Sandoghdar, V. SingleMolecule Sensitivity in Optical Absorption at Room Temperature. J. Phys. Chem. Lett. 2010, 1, 3323−3327. (37) Celebrano, M.; Kukura, P.; Renn, A.; Sandoghdar, V. SingleMolecule Imaging by Optical Absorption. Nat. Photonics 2011, 5, 95− 98. (38) Jung, C.; Müller, B. K.; Lamb, D. C.; Nolde, F.; Müllen, K.; Bräuchle, C. A New Photostable Terrylene Diimide Dye for Applications in Single Molecule Studies and Membrane Labeling. J. Am. Chem. Soc. 2006, 128, 5283−5291. (39) Jung, C.; Ruthardt, N.; Lewis, R.; Michaelis, J.; Sodeik, B.; Nolde, F.; Peneva, K.; Mullen, K.; Brauchle, C. Photophysics of New Water-Soluble Terrylenediimide Derivatives and Applications in Biology. ChemPhysChem 2009, 10, 180−190. (40) Naito, K.; Tachikawa, T.; Cui, S. C.; Sugimoto, A.; Fujitsuka, M.; Majima, T. Single-Molecule Detection of Airborne Singlet Oxygen. J. Am. Chem. Soc. 2006, 128, 16430−16431. (41) Naito, K.; Tachikawa, T.; Fujitsuka, M.; Majima, T. Real-Time Single-Molecule Imaging of the Spatial and Temporal Distribution of Reactive Oxygen Species with Fluorescent Probes: Applications to TiO2 Photocatalysts. J. Phys. Chem. C 2008, 112, 1048−1059. (42) Liao, Z. Y.; Hooley, E. N.; Chen, L.; Stappert, S.; Müllen, K.; Vosch, T. Green Emitting Photoproducts from Terrylene Diimide after Red Illumination. J. Am. Chem. Soc. 2013, 135, 19180−19185. (43) Vargas, F.; Hollricher, O.; Marti, O.; de Schaetzen, G.; Tarrach, G. Influence of Protective Layers on the Blinking of Fluorescent Single Molecules Observed by Confocal Microscopy and Scanning Near Field Optical Microscopy. J. Chem. Phys. 2002, 117, 866−871. (44) Liu, R. Y. F.; Bernal-Lara, T. E.; Hiltner, A.; Baer, E. Polymer Interphase Materials by Forced Assembly. Macromolecules 2005, 38, 4819−4827. (45) Bhadha, P. M. How Weld Hose Materials Affect Shielding Gas Quality. Weld. J. 1999, 78, 35−40. (46) Oxygen and Water Permeability, INNOVATIVE PLASTICS, 2013 . Accessed 2015 Apr.15. (47) Boersma, A.; Cangialosi, D.; Picken, S. J. Mobility and Solubility of Antioxidants and Oxygen in Glassy Polymers. III. Influence of 2481

DOI: 10.1021/acs.jpclett.5b01060 J. Phys. Chem. Lett. 2015, 6, 2477−2482

Letter

The Journal of Physical Chemistry Letters Deformation and Orientation on Oxygen Permeability. Polymer 2003, 44, 2463−2471. (48) Charlesworth, J. M.; Gan, T. H. Kinetics of Quenching of Ketone Phosphorescence by Oxygen in a Glassy Matrix. J. Phys. Chem. 1996, 100, 14922−14927. (49) Talhavini, M.; Atvars, T. D. Z. Photostability of Xanthene Molecules Trapped in Poly(vinyl alcohol) (PVA) Matrices. J. Photochem. Photobiol. A: Chemistry 1999, 120, 141−149. (50) Ishitobi, H.; Kai, T.; Fujita, K.; Sekkat, Z.; Kawata, S. On Fluorescence Blinking of Single Molecules in Polymers. Chem. Phys. Lett. 2009, 468, 234−238. (51) Klinger, M.; Tolbod, L. P.; Gothelf, K. V.; Ogilby, P. R. Effect of Polymer Cross-Links on Oxygen Diffusion in Glassy PMMA Films. ACS Appl. Mater. Interfaces 2009, 1, 661−667. (52) Tiemblo, P.; Guzmán, J.; Riande, E.; Mijangos, C.; Reinecke, H. Effect of Physical Aging on the Gas Transport Properties of PVC and PVC Modified with Pyridine Groups. Polymer 2001, 42, 4817−4823; Erratum. Polymer 2001, 42, 8321. (53) Mark, J. E. Polymer Data Handbook; Oxford University Press, Inc.: Oxford, U.K., 1999.

2482

DOI: 10.1021/acs.jpclett.5b01060 J. Phys. Chem. Lett. 2015, 6, 2477−2482