ISSN 10628738, Bulletin of the Russian Academy of Sciences. Physics, 2014, Vol. 78, No. 3, pp. 184–188. © Allerton Press, Inc., 2014. Original Russian Text © S.V. Orlov, A.V. Naumov, 2014, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2014, Vol. 78, No. 3, pp. 280–284.
Manifestation of Tunneling TLS Dynamics of a Polymer Matrix in SingleMolecule Fluorescence Blinking S. V. Orlova and A. V. Naumova, b a
Institute for Spectroscopy, Russian Academy of Sciences, Troitsk, Moscow, 142190 Russia b Moscow State Pedagogical University, Moscow, 119991 Russia email:
[email protected]
Abstract—Blinking (stochastic intermittence) of fluorescence is a quite common phenomenon that accom panies the emission of single quantum objects—organic chromophore molecules, quantum dots, and nano crystals. It is demonstrated that fluorescence blinking of single organic molecules embedded into a polymer matrix including the occurrence of “grey” states is due to tunneling transitions in the twolevel systems (TLSs) of the matrix. The repeated registration of fluorescence excitation spectra of single molecules (SMs) is used for our analysis. The statistics of fluorescence blinking of an SM is directly related to conformational changes (tunneling transitions in TLSs) in its immediate vicinity. Individual parameters of the corresponding elementary excitation are also determined. DOI: 10.3103/S1062873814030150
INTRODUCTION Intermittent light emission (blinking) is a natural property of a wide range of quantum objects [1] that manifests itself as a random switch between the light emitting (on) and nonemitting (off) states. The fundamental processes that occur both inside the quantum object and in its immediate vicinity deter mine the characteristic properties of blinking. Hence analysis of the fluctuations of fluorescence intensity (blinking statistics) allows us to investigate the nature of intra and intermolecular interactions. Moreover the understanding of the microscopic nature of the phenomenon offers possibilities to control the blink ing. In particular blinking can be used for farfield luminescence microscopy with subdiffraction resolu tion [2]. On the other hand eliminating (or control ling) blinking will allow the development of new types of singlephoton light sources [3, 4]. The following questions must be answered to explain blinking: What is the nature of the intra and intermolecular processes that result in the fluorescence blinking of a single emitter? How do the blinking parameters depend on the experimental conditions? How can the blinking of single objects be controlled? Why are objects pho tobleached over time? These questions (among the blinking’s fundamen tal and practical aspects) have attracted the interest of multiple research groups in the blinking of various nanoobjects: fluorescent molecules [5, 6], quantum dots and nanocrystals [7–12], emitting nitrogen vacancies (NV centers) in diamonds [13–15], emit ting centers in conjugated polymers and biological sys tems [16–18], molecular complexes [19, 20], donor– acceptor pairs [21–23], and Jaggregates [24].
A variety of spectroscopic techniques is used to study the above systems (see introduction in [25]): confocal and widefield luminescence microscopy, nearfield microscopy and correlation spectroscopy. The objects are investigated under different condi tions: in the solid state, in solutions, on substrates; at room and cryogenic temperatures; under vacuum and in noble gas atmospheres; by changing humidity and acidity of the medium; depending on the intensity of the excitation light and its wavelength; and with differ ent times of observation and time resolutions. The distributions of the duration of on and off periods are the most intensively studied characteristics of blinking. Analysis of these distributions shows that in many cases they are governed by a power law or by an exponential/multiexponential law. These results allow identification of the different processes that cause fluorescence blinking. Several mechanisms in particular have been observed that result in an emitter transitioning to the dark state: (a) photochemical processes such as photo isomerization or photoionization followed by the elec tron capture or reversible oxidation by oxygen; (b) interaction with another embedded molecules; (c) light or heatinduced conformational changes of the molecule; (d) rotation of the molecule; (e) spectral changes caused by the environment of the molecule (spectral diffusion); (g) transitions to the triplet state. Spectroscopy of single molecules (SMS) embed ded into an disordered matrix (glasses, polymers) is one area of research in which fluorescence blinking is still intensely debated. One of the most effective ways of analyzing fluorescence blinking is to record SM spectra at cryogenic temperatures another embedded since the dynamics of the emitter and its local environ
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EXPERIMENTAL The correlation between SM fluorescence blinking and the spectral dynamics of ZPL was analyzed in this work. The fluorescence of single TBT molecules in PIB with molecular weight Mw = 420 000 g/mol at cryogenic temperatures was investigated for this pur pose. Total sample thickness was up to several hun dreds of nanometers. A detailed description of the experimental setup and measurement procedures are given in [31]. Only the basic experimental parameters are presented here. The fluorescence excitation spectra of the SM were repeatedly recorded using a confocal microscope of our own design equipped with a singlemode tunable dye laser with an effective width of the spectral line of approximately 2–3 MHz as an excitation source. Each sample was held in a cryostat in an atmosphere of cold gaseous helium. A temperature of 7 K was controlled with accuracy of ±0.5 K. The laser wavelength was 577 nm. The laser frequency in each scan was tuned in the range of 36 GHz. Each scan contained 500 points; the length of the exposure of each point was 20 ms.
Signal intensity, rel. units
ment is simplified considerably at low temperatures. Moreover the narrow zerophonon lines (ZPLs) observed at cryogenic temperatures are highly sensitive to both intra and intermolecular processes. Multiple studies have been conducted in this area [6]. Among other things it was found that the SM spectral dynam ics in longchain polymer matrices at T < 4 K was described reasonably well within the context of the standard tunneling model and the stochastic sudden jump model [26]. Dynamics in the range of a few to several tens of degrees Kelvin is explained by interac tion between chromophores and quasilocalized low frequency vibrational modes (LFMs) [27, 28]. At the same time the behavior of the SM spectra in low molecular weight glasses and oligomers does not fit these models as they exhibit additional dynamics and more pronounced fluorescence blinking [29, 30]. Hence one of the most common reasons for the fluo rescence blinking of single molecules in lowtempera ture matrices are conformational changes in the vicin ity of an emitter and in particular tunneling transitions in the TLSs. It is shown in this work that analysis of single mol ecules spectra allows clear identification of the reasons for SM fluorescence blinking, and determination of the characteristic parameters corresponding to the respective elementary excitations. Experimental results from measuring spectral traces (repeatedly recorded fluorescence excitation spectra of SM) of a single dye molecule (tetratertbutylterrylene, TBT) embedded into an amorphous polymer matrix (poly isobutylene, PIB) are used in such analysis. Possible mechanisms of the fluorescence blinking of a single emitter in the investigated dyematrix system are dis cussed based on the obtained experimental data.
Laser frequency tuning, GHz
MANIFESTATION OF TUNNELING TLS DYNAMICS OF A POLYMER MATRIX Scan number 300 600
0 30
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900
(a) ν1
20
ν2
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0 50 (b) 40 1 30 20 2 10 3 0 50 (c) 40 1 30 20 2 10 3 0 30 60 90 0 120 150 Duration of SM spectrum observation, min
Fig. 1. (a) Spectral trajectory of a single TBT molecule in PIB at Т = 7 K. Measurement parameters: 500 points per scan of excitation laser frequency in the range of 36 GHz with 20 ms exposure for one point. Temporal evolution of PLL (blinking) with the molecule excitation at fixed laser frequencies (b) ν1 and (c) ν2 (depicted in (a) by horizontal lines). The levels of signal intensity indicated by 1, 2, and 3 correspond to on, grey and offstates.
The fluorescence emission was recorded with an ava lanche photodiode in the photoncounting mode. The repeated recording of the SM fluorescence excitation spectra in a chosen spectral region was used in [32–34]. This technique allows us to obtain the so called spectral traces (trajectories) of SM spectra reflecting the ZPL dynamics in time. These data are usually presented as twodimensional images (see Fig. 1a). The vertical axis (laser frequency tuning) cor responds to the excitation wavelength; the horizontal axis represents the number of scans or total time of spectra acquisition. The intensity of the registered flu orescence signal is represented by a grey color gradi ent. Data represented in the form of a twodimen sional image allow unambiguous determination of the individual spectra of different SMs and the avoidance of spectral distortions that can emerge due to spectral diffusion in a spectrum recorded only once. RESULTS AND DISCUSSION One often discussed reason of SM fluorescence blinking in condensed media is reversible conforma tional (structural) changes in the vicinity of a chro
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ORLOV, NAUMOV Intensity, rel. units 100 (a) 80 60 40
20
20
0 100 80
40 60 Duration of onintervals, s
(b)
60 40
20
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20 40 60 Duration of offintervals, s
Fig. 2. Distribution of (a) on and (b) offinterval dura tions corresponding to Fig. 1b approximated with expo nential functions (straight lines). For details see the text.
mophore. A classic example of such processes is inter action between the chromophore and the tunneling TLSs in an amorphous matrix. It has suggested in the context of the standard tunneling model and the model of stochastic sudden jumps that SM–TLS interaction results in ZPL jumps along the frequency between the two fixed spectral states separated by the interval [35]
(
)
υ = 2πα ( A E ) cos θ r 3 ,
(1)
where α is the constant of SMTLS interaction; А, E, θ, and r represent TLS asymmetry, TLS splitting energy, the SM–TLS orientation parameter, and the distance between them, respectively. E is defined as
E = A 2 + J 2, where J is a tunneling matrix element. The rate of transitions between the upper and lower levels in a TLS is determined by Fermi’s golden rule: ku = 1 τ u = cJ 2 E (e β E − 1), 2 −β E kd = 1 τ d = cJ E (1 − e ),
(2)
where ku and kd are rate constants of TLS transitions in the upper and lower states, respectively; τu and τd are the characteristic lifetimes of a TLS in these states;
β = 1/kBT, where kB is the Boltzmann constant and T is temperature; and с is the constant of TLS– phonon interaction (set at 4.41 × 1077 J–3 s–1 for PIB) [35]. ZPL spectral jumps caused by the transitions in a nearby TLS are presented in Figs. 1b and 1c in the form of SM fluorescence blinking as if the measure ments were conducted at a fixed laser frequency. The spectra are presented as twodimensional images (see Fig. 1a) for the detailed investigation of this phe nomenon. It can be seen from the figure that the ZPL jumped between several spectral states within the scanning range. Such behavior is in complete agree ment with the standard tunneling model and the sto chastic suddenjump model. In the case of Fig. 1a the spectral jumps could easily be due to interaction with three TLSes. Multiple distant and/or weakly interact ing TLS also contribute to the width of this ZPL. The transition rate in all three of the strongly inter acting TLSs is considerably lower than the scanning rate. Only one of them (the one shifting the SM ZPL shift least) is fast enough to create a distribution of transition times (on and offintervals) with sufficient statistical accuracy. Therefore only spectral jumps caused by this TLS are considered below. The example in Fig. 1 demonstrates the main advantage of the spec tral traces technique in studies of SM blinking. During the dark periods at fixed excitation frequency ν1 the molecule is in neither the dark nor the weakly fluores cent state, but its ZPL shifts to the second absorption frequency, ν2. It is impossible to detect this transition upon excitation at a fixed laser frequency (ν1 or ν2) (Fig. 1b, and 1c), which can lead to wrong conclusions regarding the reason of blinking. For example so called grey states are observed on the time trajectories of intensity, the nature of which is now widely debated in the literature [36]. An undeniable advantage of the spectral trajectory technique is that the obtained data can be analyzed using both temporal and spectral scales. For example, it is easy to recreate the time trajectory for any chosen frequency as was done in Figs. 1b and 1c for excitation frequencies ν1 and ν2. The on and offintervals for these frequencies correlate negatively as the jumps are caused by switching within the same TLS. The nature of the grey states in the time trajectories of the SM luminescence intensity upon excitation at fixed fre quencies becomes obvious from an analysis of the spectral trace (Figs. 1b and 1c). Spectral diffusion results in a ZPL jump relative to the laser excitation frequency by a value comparable to the ZPL width when an SM transitions to the grey state. Hence the SM is excited through the wing of the ZPL band less effectively, reducing the intensity of molecule fluores cence. It is possible to perform a statistical analysis of the duration of the on and offintervals. The durations of the on and offintervals at frequency ν1 obey the exponential distribution exp(–t/τ) (Fig. 2), which is
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also in agreement with the standard TLS model. Approximation of the data by the exponential depen dence allows us to obtain the characteristic lifetimes of the on and offstates (24.41 and 20.93 s, respec tively) which corresponds to the rates of transitioning to the upper and lower states (ku = 0.041 s–1 and kd = 0.048 s–1). Parameters А and J for the considered TLS can be calculated using Eq. (2). Assuming that the constant of TLS–phonon interaction c = 4.41 × 1077 J–3 s–1 [35], we obtain A/kBT = 0.044 and J/kBT = 3.34 × 10–7 (A = 0.214 cm–1; J = 1.625 × 10–6 cm–1). Moreover Eq. (1) allows calculation of the distance between the TLS and SM (normalized to the unknown parameter θ). Considering that the spectral splitting between ZPL states ν1 – ν2 = υ = 1.2 GHz in Fig. 1a and the TLS– SM interaction constant α = 25 GHz nm3, r = 3 cos θ = 5.1 nm for the TBT/PIB [35]. Hence detailed analysis of the SM spectral trails allows not only unambiguous determination of the cause of the blinking at the fixed excitation frequency (in the considered examples this comprises interaction of the electron transition of the dye molecule with the tunneling TLS), but also calculation of the individual parameters of this TLS. CONCLUSIONS It was demonstrated that the repeated detection of fluorescence excitation spectra (spectral trails) can be used to investigate the fluorescence blinking of single molecules of organic chromophores in solid lowtem perature matrices. Combined analysis of the temporal evolution of spectral characteristics and blinking sta tistics allows us to understand the microscopic nature of transitions between the on and offstates. It was shown by the example of a single molecule of tetra tertbutylterrylene in amorphous polyisobutylene that the reason for fluorescence blinking and the presence of grey states is the wellknown stochastic process of ZPL spectral jumps caused by conformational changes in the vicinity of an SM (tunneling transitions in closely located twolevel systems). For the selected SM fluorescence blinking effect was clearly related to tunneling transitions in a nearby TLS. Individual parameters of the TLS were found, and the distance between the TLS and the selected SM was estimated. The approach described in this work can easily be used to investigate the fluorescence blinking of many types of emitters (quantum dots, nanocrystals, molec ular complexes, emitting regions in macromolecules) at both cryogenic temperatures and under normal conditions. ACKNOWLEDGMENTS This work was supported by the Research Founda tion of Germany, project no. KO 1359/241; by the
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Russian Foundation for Basic Research, projects nos. 120233027mol_a_ved and 110200816a; RF Presidential Grants MD465.2012.2 and NSh 1049.2012.2; and by the program Fundamental Opti cal Spectroscopy and Its Applications, Branch of Physical Sciences, Russian Academy of Sciences. The authors are grateful to L. Kador at the University of Bayreuth, Germany; and to Yu.G. Vainer at the Insti tute for Spectroscopy, Russian Academy of Sciences, in collaboration with whom the experimental results used in our analysis were obtained [25]. REFERENCES 1. Riley, E.A., Hess, C.M., and Reid, P.J., Int. J. Molec. Sci., 2012, vol. 13, p. 12487. 2. Rust, M.J., Bates, M., and Zhuang, X.W., Nature Methods, 2006, vol. 3, p. 793. 3. Lounis, B. and Moerner, W.E., Nature, 2000, vol. 407, no. 6803, p. 491. 4. Kako, S., Santori, C., Hoshino, K., et al., Nature Mater., 2006, vol. 5, p. 887. 5. Orrit, M. and Moerner, W.E., High Resolution Single Molecule Spectroscopy in Condensed Matter, Physics and Chemistry at Low Temperatures, Singapore: Pan Stan ford Publ., 2011, p. 381. 6. Naumov, A.V., Usp. Fiz. Nauk, 2013, vol. 183, p. 633. 7. Barnes, M.D., Mehta, A., Thundat, T., Bhargava, R.N., Chhabra, V., and Kulkarni, B., J. Phys. Chem. B, 2000, vol. 104, p. 6099. 8. Kuno, M., Fromm, D.P., Hamann, H.F., Gallagher, A., and Nesbitt, D.J., J. Chem. Phys., 2000, vol. 112, p. 3117. 9. Shimizu, K.T., Neuhauser, R.G., Leatherdale, C.A., et al., Phys. Rev. B, 2001 vol. 6320, p. 205316. 10. Ambrozevich, S., Van der Auweraer, M., Dirin, D., et al., J. Russ. Laser Res., 2008, vol. 29, p. 526. 11. Ren, T., Erker, W., Basche, T., and Schartl, W., Lang muir, 2010, vol. 26, p. 17981. 12. Bruhn, B., Valenta, J., Sangghaleh, F., and Linnros, J., Nano Lett., 2011, vol. 11, p. 5574. 13. Bradac, C., Gaebel, T., Naidoo, N., et al., Nature Nano technol., 2010, vol. 5, p. 345. 14. Kuhn, S., Hettich, C., Schmitt, C., Poizat, J.P.H., and Sandoghdar, V., J. MicroscopyOxford, 2001, vol. 202, p. 2. 15. Wrachtrup, J., Nature Nanotechnol., 2010, vol. 5, p. 314. 16. Dickson, R.M., Cubitt, A.B., Tsien, R.Y., and Moerner, W.E., Nature, 1997, vol. 388, no. 6640, p. 355. 17. Weber, W., Helms, V., McCammon, J.A., and Lang hoff, P.W., Proc. Nat. Acad. Sci. USA, 1999, vol. 96, p. 6177. 18. Feist, F.A. and Basche, T., Angewandte Chem.Int. Ed., 2011, vol. 50, p. 5256. 19. Yip, W.T., Hu, D.H., Yu, J., et al., J. Phys. Chem. A, 1998, vol. 102, p. 7564. 20. Ernst, D., Hildner, R., Hippius, C., et al., Chem. Phys. Lett., 2009, vol. 482, p. 93.
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