13892
J. Phys. Chem. 1996, 100, 13892-13894
Dibenzoterrylene in Naphthalene: A New Crystalline System for Single Molecule Spectroscopy in the Near Infrared F. Jelezko, Ph. Tamarat, B. Lounis, and M. Orrit* Centre de Physique Mole´ culaire Optique et Hertzienne, CNRS et UniVersite´ Bordeaux I, 351 Cours de la Libe´ ration, 33405 Talence, France ReceiVed: March 19, 1996; In Final Form: June 18, 1996X
We report on a new system for single molecule spectroscopy in the near infrared: dibenzoterrylene (DBT) in a naphthalene crystal. Its absorption band (758 nm) is interesting because single-frequency laser diodes may serve as cheap sources for near-infrared light. Single molecules in this system have a high fluorescence yield and narrow homogeneous lines and show neither spectral diffusion nor significant triplet population. All these characteristics make this new system very promising for future quantum optical measurements.
1. Introduction
2. Experimental Setup
spectrum of DBT in benzene solution shows a strong and broad band near 738 nm (extinction coefficient close to 40 000 L/(mol cm)),10 a spectral range that is accessible to our Ti-sapphire laser. We purchased DBT from Dr. W. Schmidt (PAH-Research Institute, Greifenberg, FRG). Naphthalene was extensively zone-refined (100 passages). A small quantity of DBT was dissolved in liquid naphthalene, and a single crystal was grown by the Bridgman method (we did not attempt a sublimation growth, since the molecular weights of the two molecules are very different). Upon crystallization, the major part of DBT was expelled from the crystal, but a small fraction of the molecules remained in the crystal, with a higher concentration at crystal defects, appearing as bright fluorescent zones under excitation at 633 nm. The initial concentration in the liquid was 10-7 M; the final (inhomogeneous) concentration was difficult to estimate, but we think that it was not more than 10-8 M and probably much less. Our setup for the fluorescence excitation of single molecules used a single-frequency Ti-sapphire laser (CR 899-21) pumped by an Argon ion laser. Its output power was regulated with a noise-eater. The sample was excited with the confocal setup described in ref 11: a high-quality aluminum paraboloid focused the exciting laser light on the sample, whose position could be manually adjusted so that the surface coincided with the mirror’s focal plane. One of the advantages of this scheme is that we could use a fairly big piece of crystal from the Bridgman ingot and did not have to prepare a thin unstrained sample, which would have been difficult. A further advantage is the very good imaging of the emitted fluorescence to a spot of a few tens of microns in diameter, allowing us to record the fluorescence spectrum with a spectrograph11 and to use an avalanche photodiode (APD) as a detector, which has low noise and high quantum efficiency. The APD (EG&G, SPCM-AQ131) had a sensitive area of about 0.2 mm in diameter, a low dark rate of 50 counts/s, and a quantum efficiency of about 40% at 800 nm. The nonlinearity of the APD was about 3% for a count rate of 1 MHz and can be neglected in the present measurements. The exciting laser light was cut off by a notch filter (Kaiser Notch 752). Autocorrelation measurements of the fluorescence intensity were performed with a pseudologarithmic correlator (ALV5000, ALV, Langen, FRG).
The structure of 7.8,15.16-dibenzoterrylene (DBT) is schematically shown as an inset in Figure 3. The absorption
3. Results
The spectroscopy of single molecules in solids at liquid helium temperatures has developed quickly in the recent years.1,2 This high-resolution optical spectroscopic method consists of recording fluorescence excitation spectra of small samples of dilute solid solutions. For well-chosen host-guest systems, the signal of single molecules can be isolated as narrow homogeneous lines in the region of the inhomogeneous absorption band. Several new results were obtained by this method, e.g., the direct study of homogeneous line shapes and width of molecules in solids, of the optical saturation,3 the determination of emission,4 dephasing,5 or intersystem crossing rates6 for individual molecules, direct evidence for two-level systems in the optical spectral diffusion,7 the study of magnetic resonance,8 and quantum optical effects.9 All these results, however, were obtained so far with only a few guest molecules (essentially pentacene and terrylene) in various matrices, and extension to more host-guest systems is of utmost importance. One of the big hurdles to the generalization of single molecule spectroscopic studies is the price of the setup involved. On the one hand, liquid helium temperatures cannot be avoided yet if high-resolution spectroscopy of narrow lines is to be performed in condensed matter. On the other hand, highresolution spectroscopy usually requires a pump ion laser and a single-frequency tunable laser, which are the heaviest investment in the experiment. It is therefore very attractive to replace these expensive lasers by cheaper diode laser sources, which work best in the near-infrared spectral range. The purpose of the present paper is to describe a new polycyclic aromatic hydrocarbon, which is very well suited to single molecule spectroscopy and can be included as a guest in a molecular crystal. This molecule, 7.8,15.16-dibenzoterrylene, can be embedded in a naphthalene crystal, where the absorption maximum of its electronic 0-0 line lies at 757.7 nm. This wavelength, which can be tuned to some extent by changing the host, lies in the near-infrared range, where diode lasers are available. For this work, we use an ion laser pumped Tisapphire laser, but single molecule spectroscopy with diode lasers can now be envisaged.
X
Abstract published in AdVance ACS Abstracts, July 15, 1996.
S0022-3654(96)00845-3 CCC: $12.00
3.1. Broad-Band Bulk Spectra. The solid solution of DBT in polycrystalline naphthalene at high concentration (10-6 M) © 1996 American Chemical Society
Letters
J. Phys. Chem., Vol. 100, No. 33, 1996 13893
Figure 2. Plot of the maximum fluorescence signal of a single molecule line vs its line width, for various exciting intensities. The solid line is a fit with the usual saturation expressions that goes through the experimental points. The homogeneous width is 35 MHz for the particular molecule under study.
Figure 1. Fluorescence excitation line of a single DBT molecule in a naphthalene crystal at 2.0 K. The upper spectrum, recorded with low excitation intensity, is nearly unsaturated and has a width of 27 MHz. The lower spectrum, at high intensity, is clearly saturated and has a width of 120 MHz.
shows a sharp single site at 757.7 nm, with a bandwidth of a few cm-1, typical of mixed molecular crystals. This indicates that the bulky DBT molecule is well inserted in the naphthalene lattice, perhaps thanks to its two naphthalene moieties. At 2 K, where all the experiments reported here were made, this band is inhomogeneously broadened with very weak phonon wings. 3.2. Single Molecule Spectra. The excitation lines of single DBT molecules were found around the site wavelength (757.7 nm), spread over a range of about 120 GHz. In this range, we found some 40 molecules in the sample studied. The volume of the focal spot was estimated from the characteristics of the optical image to be a little less than 10 µm3. The excited volume was of course much larger than this, since the exciting light propagates in the crystal. Assuming that all molecules were in the focal spot, we obtain an upper bound for the concentration of 10-8 M, but, as discussed below, the actual concentration was probably much less. We found almost no molecules outside this small interval of 4 cm-1 around the site wavelength. Figure 1 shows an example of a single DBT molecular line, at weak and strong exciting intensity (1 and 300 W cm-2). Both spectra are obtained after a single scan of the laser, to decrease distortion by laser drift. The background is very low, as expected for excitation in the near-infrared range, where very few fluorescent impurities absorb, and from the high purity of the matrix material. The line shapes looked roughly Lorentzian and show clear saturation when the laser power is raised. Contrary to terrylene or other molecules with fairly long-lived triplet states, no quantum jumps12 appeared in fast scans of the spectrum. The lines are very stable: we did not observe any spectral diffusion or any photoinduced jumps, even under heavy illumination, up to 1000 times the saturation intensity (see hereafter). A similar observation was made by Basche´ and coworkers for pentacene in naphthalene.13 The molecules we studied were in the middle of the inhomogeneous distribution. The concentration was too low to study the wings, where spectral diffusion could be more common.
An example of a saturation study of a single molecule line (line intensity vs line width) is presented in Figure 2. For the two molecules studied in detail, we found very good agreement with the expected saturation laws.3 The two fits gave homogeneous widths of 25 and 35 MHz, with errors of a few megahertz (see Figure 2). More statistics are needed before an average value of the homogeneous width can be given, but a value of 30 ( 5 MHz is consistent with crude estimates of the widths of a number of molecules at low power. The saturation intensities deduced from plots of line width vs intensity were of the order of a few tens of W cm-2. This value is too strong to be consistent with a radiative width of 30 MHz, even for a perfect two-level system without any triplet bottleneck. The most likely explanation is that the molecules under study were far from the mirror focus. We made similar observations for terrylene in p-terphenyl, where the guest concentration also was extremely low. The last parameter that can be deduced from saturation plots is the fluorescence count rate at saturation, about 5 × 105 s-1. The order of magnitude of this signal is comparable to that of Tr in p-terphenyl. The detection yield cannot be precisely calculated if the molecules are some distance away from the focus. If we consider the measured width as lifetime limited, the maximum photon emission rate is about 108 s-1, giving an overall detection yield of 5 × 10-3, which is quite reasonable once possible losses due to poor imaging are considered. This estimation allows us to give an upper limit for the burning efficiency of DBT in naphthalene. Since molecules could be excited at resonance for tens of minutes at high power, the burning efficiency must be less than 10-10. We measured the fluorescence autocorrelation functions of the two molecules whose saturation behavior was studied. In neither case could we find any clear correlation in the experimental time window (for times larger than 10 µs, where shot noise was weak enough), even under full saturation. This means that, if the triplet lifetime is longer than 10 µs, the contrast associated with photon bunching must be less than 10-3 at saturation (this limit is given by the level of shot noise and that of fluctuations in laser intensity). This result, and the analysis of ref 6, imply that the population rate of the triplet must be less than 200 s-1, i.e., that the triplet yield must be less than 4 × 10-6. This is the order of the triplet yield of terrylene, for which bunching can still be observed in p-terphenyl.14 In conclusion, either the triplet lifetime is very short (less than 10 µs) or the triplet yield very small. In either case, the triplet is no significant bottleneck. We think that the triplet level of DBT
13894 J. Phys. Chem., Vol. 100, No. 33, 1996
Letters TABLE 1: Vibrational Frequencies (in cm-1) and Intensities (Normalized to that of the Vibrational Line at 292 cm-1, but Not Corrected for Instrument Response) of the Most Intense Fluorescence Lines of Single DBT Molecules in Naphthalene Crystal 2.0 Ka frequency
intensity
frequency
intensity
frequency
intensity
151 163 177 235
0.22 0.27 0.60 0.70
292 403 410 457
1.0 0.31 0.22 0.14
462 525 675 1259
0.16 0.10 0.16 0.19
a
Figure 3. Fluorescence spectrum of a single DBT molecule in a naphthalene crystal at 2.0 K (upper trace). The relative frequencies of the main lines are indicated. The lower trace was recorded by detuning the laser by 1.5 GHz from the excitation maximum of the molecule; the DBT lines almost disappear from this spectrum (apart from weak traces of the strongest lines), and the remaining emissions are Raman lines from the naphthalene crystal. The strength of the Raman lines compared to single DBT lines is a strong indication that the molecule studied was located some distance away from the focus of the paraboloid (the noise in the off-resonance spectrum is stronger because its accumulation time was shorter).
lies very deep and therefore couples very efficiently to the vibrational levels of the ground state, resulting in a very short lifetime. We recorded fluorescence spectra of single DBT molecules in naphthalene crystal; an example is shown in Figure 3. The upper spectrum was recorded with the laser in resonance with the molecule. The lower spectrum with the laser detuned by 1.5 GHz shows Raman scattering lines of the naphthalene matrix with intensities comparable to single molecule fluorescence lines. In general, Raman scattering from the matrix is expected to be orders of magnitude weaker than fluorescence,2 and it was not observed for pentacene in p-terphenyl, for example,11 which is a much weaker emitter than DBT. This large difference cannot be ascribed to the sample thickness only (much larger in the present case), since the spectrograph slit acts as a spatial filter for the sample emission. Rather, the appearance of Raman scattering lines confirms that the molecule studied was located some distance away from the paraboloid focus and thus was excited with a much lower intensity than the focal area. The low-frequency region of the spectrum (below 150 cm-1) is cut off or attenuated by the notch filter. The frequencies and intensities of the main ground state vibrations of DBT are given in Table 1. The spectrograph resolution is about 4 cm-1, and the intensities are not normalized for the response of the CCD detector and of the optical elements. As compared to the fluorescence spectrum of pentacene,11 DBT emits the better part of its fluorescence into low-frequency modes (the spectrograph response can account for part of this difference only). As in broad-band spectra, no significant phonon wings appear in the fluorescence spectrum, in contrast to pentacene.11 The comparison of the spectra of terrylene15 and DBT yields only two obviously similar features. The
The frequency resolution is about 4 cm-1.
intense mode at 235 cm-1 in DBT corresponds to that at 243 cm-1 in terrylene, which was attributed in ref 15 to an Ag skeleton stretch vibration along the long molecular axis. Since the central cycles are nearly immobile in this vibration, the conservation of this mode upon substitution of these cycles is not surprising. A second DBT mode at 1259 cm-1 may correspond to the 1272 cm-1 mode of terrylene, attributed in ref 15 to a stretch of CC bonds parallel to the long axis, mainly in the end naphthalene units. Again, this mode would hardly depend on substitution of the central unit. In conclusion, we have discovered a very promising system for single molecule spectroscopy, presenting all the desired features for a high signal-to-background ratio: narrow, very photostable lines, with high fluorescence efficiency and no triplet bottleneck. This system is particularly interesting for measurements where stability and strong coupling to light are important, such as quantum optical measurements.9 The single molecule lines are found at 758 nm, in the spectral region of laser diode sources. Acknowledgment. This work was supported by Conseil Re´gional Aquitaine. References and Notes (1) Moerner, W. E. Science 1994, 265, 46. (2) Orrit, M.; Bernard, J.; Brown, R.; Lounis, B. In Progress in Optics; Wolf, E., Ed.; North Holland: Amsterdam, 1996; Vol. 35. (3) Ambrose, W. P.; Basche´, T.; Moerner, W. E. J. Chem. Phys. 1991, 95, 7150. (4) Pirotta, M.; Gu¨ttler, F.; Gygax, H.; Renn, A.; Wild, U. P. Chem. Phys. Lett. 1993, 208, 379. (5) Kummer, S.; Mais, S.; Basche´, Th. J. Phys. Chem. 1995, 99, 17078. (6) Bernard, J.; Fleury, L.; Talon, H.; Orrit, M. J. Chem. Phys. 1993, 98, 850. (7) Fleury, L.; Zumbusch, A.; Orrit, M.; Brown, R.; Bernard, J. J. Lumin. 1993, 56, 15. (8) Ko¨hler, J.; Brouwer, A. C. J.; Groenen, E. J. J.; Schmidt, J. Science 1995, 268, 1457. (9) Tamarat, Ph.; Lounis, B.; Bernard, J.; Orrit, M.; Kummer, S.; Kettner, R.; Mais, S.; Basche´, T. Phys. ReV. Lett. 1995, 75, 1514. (10) Clar, E. Polycyclic Hydrocarbons; Academic Press/Springer: New York, 1995; p 213. (11) Fleury, L.; Tamarat, Ph.; Lounis, B.; Bernard, J.; Orrit, M. Chem. Phys. Lett. 1995, 236, 87. (12) Basche´, T.; Kummer, S.; Bra¨uchle, C. Nature 1995, 373, 132. (13) Kummer, S.; Bra¨uchle, Ch.; Basche´, Th. Mol. Cryst. Liq. Cryst. 1996. (14) Kummer, S.; Basche´, T.; Bra¨uchle, C. Chem. Phys. Lett. 1994, 229, 309; 1995, 232, 414. (15) Myers, A. B.; Tche´nio, P.; Zgierski, M. Z.; Moerner, W. E. J. Phys. Chem. 1994, 98, 10377.
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