- Email: [email protected]
Single molecule spectroscopy of Mg-tetrazaporphyrin in xenon matrix.
Chemical Physics 285 (2002) 121–126 www.elsevier.com/locate/chemphys
Single molecule spectroscopy of Mg-tetrazaporphyrin in xenon matrix. Heavy atom effect A. Starukhin a,c,*, A. Shulga a,c, J. Sepiol b,c, R. Kolos b,c, V. Knyukshto b, A. Renn a, U.P. Wild a a
b
Institute of Physical Chemistry, ETH Zurich, ETH Zentrum, Universit€atstrasse 16, CH-8092 Z€urich, Switzerland Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 220072, F.Scaryna Av. 68, Minsk, Belarus c Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland Received 12 December 2001
Abstract We report initial results on the observation of single Mg-tetrazaporphyrin molecules in solid xenon. Samples were prepared following the conventional matrix-isolation procedure. Essentially inhomogeneous broadening is characteristic for these spectra. Single molecules, detected by fluorescence microscopy, gave remarkably stable signals. Their fluorescence excitation spectra revealed sharp zero-phonon lines, with most represented linewidths around 50 MHz. This is the first study of single molecules of a tetrapyrrolic compound. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Single-molecule spectroscopy (SMS) at liquid helium temperature has revealed a number of unique information about dynamical interaction between molecules of chromophore and the host matrix. For molecules in bulk systems we could observe only the averaging spectral data. Fluorescence microscopy of single molecules is especially suited for the investigation of a whole set of molecules in parallel under identical conditions. Low-temperature microscopy has been employed
*
Corresponding author. E-mail address: [email protected] (A. Starukhin).
as an ideally sensitive method for the determination of changes in the nanoenvironment of molecules. The quest for new molecules of chromophore and matrices is currently central to the development of this new spectroscopic method. Most of the investigated single molecules at liquid helium temperature belong to the class of conjugated aromatic hydrocarbons (see for example [1–3]). These compounds are characteristic ideal parameters for SMS (strongly allowed transition in absorption spectra, high fluorescence quantum yield, a low probability of S1 to T1 transition and/or fast return from T1 to the ground state, etc.) Biologically important complex systems (peripherical light harvesting complex with chlorophyll
0301-0104/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 ( 0 2 ) 0 0 6 9 4 - 8
122
A. Starukhin et al. / Chemical Physics 285 (2002) 121–126
a and b) have been investigated by methods of single molecule spectroscopy at low and room temperatures [4–8]. For the above-mentioned systems caratenoids were the main reasons for an effective quenching of the triplet state of a chlorophyll molecule [7]. In this work, we record single molecules of Mgtetrazaporphyrin (Mg-TAP) embedded in xenon solid matrix at liquid helium temperature. For preparation of the sample we used the matrix isolation technique at low temperature as described in [9,10]. The first goal of this study was to try recording SMS for tetrapyrrolic compounds such as porphyrins. For realization of this idea we need to find a compound with the corresponding spectral parameters and effective quenching of the triplet state. The second purpose was to look for influence of xenon matrix (heavy atom effect) on the photophysical parameters of chromophore. For studying the heavy atom effect on Mg-TAP we carry out experiments for mixture C2 H5 J as model heavy matrix because J and Xe atoms have practically the same atomic weights.
2. Experimental Mg-tetrazaporphyrin was synthesised according to the procedure described in [11]. The compound was purified on an aluminium oxide by chromatographic method. The structure of the Mg-TAP has been established (see Fig. 1) from the 1 H NMR spectrum (Bruker WM-360 spectrometer, 360 MHz). Low-temperature matrices were prepared under direct deposition of the compound of interest on the objective surface. The Mg-TAP was sublimed at 570 K in a stream of noble gas – Xe (from Linde 4.0) and condensed onto the cold objective surface at 55 K. Typical matrix ratios were 1:104 or lower in our experiment. After completing the deposition, the inside of the cold finger was filled with liquid helium. The temperature of the objective reached about 2 K after pumping the vapour of helium. The presence of the compound on the surface of objective was checked by the detection of fluorescence using a tunable dye laser and a
Fig. 1. Fluorescence excitation (solid line, observed at kem ¼ 642 nm) and fluorescence (dashed line, excited at kex ¼ 370 nm) spectra of Mg-TAP in a Xe matrix at 5 K (I). Phosphorescence spectrum (excitation at kex ¼ 585 nm) of Mg-TAP in tetrahydrofuran at 77 K (II). Spectral resolutions were 30 cm1 . The window temperature during the deposition was about 55 K for the Xe matrix.
video camera. A more detailed description of optical cryostat and method preparation of samples for single molecule experiments are given in [9,10]. The optical setup for single molecule detection was similar as described in [3,10]. The beam of a single mode dye laser with Rh6G dye (spectral bandwidth about 2 MHz) was focused on the
A. Starukhin et al. / Chemical Physics 285 (2002) 121–126
matrix near the objective axis (spot diameter ca. 50 lm). The video camera with an image intensifier (Hammamatsu C2400-25) was used for registration of resulting images, produced by individual fluorescing molecules. The Schott RG610 glass filter was placed between a cryostat window and the camera for blocking Rayleigh-scattered laser radiation. To obtain the spectral positions and line shapes of individual molecules the laser wavelength was scanned over 2 GHz in 2 4 MHz steps with the integration time 0.32 s per video frame at each frequency point. Stability of the emission from single molecules was monitored by recording the images generated during the continuous irradiation of the sample at a fixed excitation wavelength. Emission and excitation spectra were measured on a high-resolution spectrometer. The excitation source consisted of an Osram XBO 2.5 kW highpressure xenon lamp combined with a Spex 1402 double monochromator. The sample was placed in the same optical cryostat as for single molecule detection but the objective for these experiments was replaced by a sapphire window on a special holder. The emission light was passed through a second Spex 1402 double monochromator and detected by a cooled Hamamatsu R2949 photomultiplier tube. Data were acquired by a photon counting (SR-400) apparatus coupled to a PC. A more detailed description of this experimental setup is given elsewhere [12]. Phosphorescence and excitation of phosphorescence spectra and kinetic parameters of MgTAP were measured by using a high-sensitive home-built spectrometer that was described in detail previously [13]. The reproducibility of the system is 5%, the accuracy of emission quantum yield ðuÞ measurements is 5–7% for u P 0:1, and the limit of emission quantum yields measured is 105 . The fluorescence quantum yields uF of the systems under investigation were measured by the relative method, tetraphenylporphyrin in toluene (uF ¼ 0:09 at 293 K) was used as the standard.
3. Results and discussion Fig. 1 presents the structure, absorption (fluorescence excitation), fluorescence and phospho-
123
rescence spectra of Mg-TAP in xenon matrix at 5 K for bulk sample. It reveals a strong inhomogeneous broadening of the bands under our conditions of preparation of the sample. The spectrum of Mg-TAP in xenon matrix consists of bands (0–0 transition is located near 17 106 cm1 ) with halfwidths of about 200 cm1 . One video frame out of 600 images was recorded while that scanning the laser over 2 GHz range is shown in Fig. 2. Several molecules were in resonance with the laser there; the positions of these appear as bright spots against the dark background. Analysis of signals coming from single molecules (i.e., from distinct sets of adjacent pixels on the detector surface) as a function of laser frequency gave the individual fluorescence excitation spectra. The positions and widths of resonance lines were notably stable during the time the laser scans. The signal-to-noise ratio was typically higher then 10, with the laser beam intensity of about 0:1 W=cm2 . Shapes of resonance lines were, for the majority of molecules, well reproduced by a Lorentzian fit (see inset of Fig. 2). Spectral profiles for 56 individual Mg-TAP molecules were analysed, as illustrated with the histogram in Fig. 3. The linewidths were distributed within the frequency range 20–130 MHz with a maximum around 50 10 MHz. The fluorescence lifetime of Mg-TAP in solid Xe was not measured yet. At room temperature we obtained sf ¼ 4:5 ns for solution Mg-TAP in tetrahydrofuran. The dependence of the fluorescence lifetime on temperature is usually not significant [14] and the expected lifetime-limited zero-phonon linewidth should be 35 MHz, which is not far from the low frequency histogram cut-off at 20–30 MHz. Influence of heavy atom is more significant. For Mg-TAP in mixture – tetrahydrofuran (1:4) we measured the value lifetime of the singlet state as sf ¼ 1:4 ns. For this case the lifetime limit must be near 110 MHz that cannot be reconciled with the results from Fig. 3. In our opinion situations with heavy atom effect from C2 H5 J and Xe were widely different. It is possible that C2 H5 J as extra-ligand for Mg-atom may have a more appreciable effect on the parameters of Mg-TAP than Xe atom of matrix. In future, we are planning to carry out
124
A. Starukhin et al. / Chemical Physics 285 (2002) 121–126
Fig. 2. Single molecule fluorescence Mg-TAP isolated in a Xe matrix. The traces of individual molecules reproduced as tiny clusters of illuminated CCD detector pixels. The image, accumulated for 0.32 s, corresponds to the excitation with a single-mode laser (2 MHz bandwidth) of around 586.2 nm. Inset shows an exemplary single molecule fluorescence excitation profile (thin curve), approximated with a Lorentzian function (bold).
Fig. 3. The distribution of line-widths for Mg-TAP molecules in a Xe matrix (excitation near 586.15 nm) with the laser beam of 0:1 W=cm2 .
measurements of the kinetic parameters for MgTAP in Xe matrix at low temperatures. The distribution from Fig. 3 has similarities with that for Terylene molecules embedded in nalkane matrix under a directly deposited compound on the surface of a microscope objective [9]. The character of distributions allows us to suggest that the spectral diffusion is not so effective for Mg-TAP in xenon matrix as for Terylene molecules in n-alkanes. It is well known that for Terylene molecules in polymeric materials the respective lines were at least one order of magnitude broader [15]. On the basis of similar characteristics of histograms for Terylene and Mg-TAP it seems reasonable to compare the main spectroscopic parameters of both molecules. For Mg-TAP we found a phosphorescence spectrum (see Fig. 1) at 77 K and determined all
A. Starukhin et al. / Chemical Physics 285 (2002) 121–126
125
Table 1 Spectral and photophysical parameters of Mg-TAP and Terylene molecules
Fluorescence quantum yield Lifetime for first singlet state at 293 K (ns) Phosphorescence quantum yield Lifetime for first triplet state at 77 K (ms)
Mg-TAP (pure TGF)
Mg-TAP (mixture TGF : C2 H5 J)
Terylene (pure TGF)
Terylene (mixture TGF : C2 H5 J)
0.56 4.5 4 105 10
0.04 1.4 1:5 105 0.9
0.95 4.7 – (105 [16]) – (few ms [17])
0.67 4.0 – –
parameters of the triplet state. All our efforts to record the phosphorescence spectrum of Terylene have not been successful. In Table 1 we present the results of new measurements for Mg-TAP and Terylene in pure tertrahydrofuran (TGF) solution and mixture TGF–C2 H5 J (4:1) at 293 and 77 K. We used the mixture with C2 H5 J for modeling the effect of external heavy atom. As it is clear from Table 1 intersystem crossing is important for Mg-TAP and the influence of heavy atom on the singlet and triplet state is very essential. In the xenon matrix for Mg-TAP we also observed a heavy atom effect on the fluorescence quantum yield. On the basis of our evaluation the value quantum yield was about 0.1 or 0.15. Direct measurements of this parameter we could not realise under conditions of the SMS experiment. The phosphorescence quantum yield is not changed, but the lifetime of the triplet state decreased by more than one order. This would lead to a triplet decay rate of 1:1 103 s1 under heavy atom effect, making the SMS of Mg-TAP possible. The molecule of Mg-TAP (similar as for Terylene molecule) has strongly allowed transition (extinction coefficient of about 105 cm1 mol1 [18]) to the lowest excited singlet state. As it is well known [18,19] that for molecules with strongly allowed 0–0 transitions such as perrylene, 3-bromperylene, 3, 9-dibromperylene and 9,10-diphenylanthracene external heavy atom effects (xenon matrix and Br- substitution) are absent. As we can see from Table 1 for Mg-TAP heavy atom effect is more essential than for Terylene molecule, but the effect is weaker than for H2 -porphine [20]. In conclusion, matrix isolation technique was successfully used for single molecule detection MgTAP in xenon matrix. Our experiments to dem-
onstrate new promises for the study of biologically important molecules (porphyrins and phthalocyanins) in rare gas matrices by SMS methods.
Acknowledgements Our thanks also go to Dr. Jan Jasny who constructed the mirror objective, and to Mr. Bruno Lambillotte, for his invaluable technical assistance.
References [1] M. Orrit, J. Bernard, Phys. Rev. Lett. 65 (1990) 2716. [2] Th. Basche, W.E. Moerner, M. Orrit, U.P. Wild (Eds.), Single Molecule Optical Detection, Imaging and Spectroscopy, VCH, Weinheim, 1996. [3] T. Irngartinger, A. Renn, G. Zumofen, U.P. Wild, J. Lumin. 76 (1998) 279. [4] Y. Ishii, T. Yanagida, Single Molecules 1 (2000) 5. [5] M.A. Bopp, Y. Jia, L. Li, R.J. Cogdell, R.M. Hochstrasser, Proc. Natl. Acad. Sci. USA 94 (1997) 10630. [6] A.M. van Oijen, M. Ketelars, J. Kohler, T.J. Aartsma, J. Schmidt, J. Phys. Chem. B 102 (1998) 9363. [7] C. Tietz, O. Cheklov, A. Drabenstedt, J. Schuster, J. Wrachtrup, J. Phys. Chem. B 103 (1999) 6328. [8] C. Tietz, U. Gerken, F. Jelezko, J. Wrachtrup, Single Molecules 1 (2000) 67. [9] J. Sepiol, A. Starukhin, T. Latychevskaia, J. Jasny, A. Renn, U.P. Wild, Chem. Phys. 247 (1999) 35. [10] J. Sepiol, A. Starukhin, R. Kołos, T. Latychevskaia, J. Jasny, A. Renn, U.P. Wild, Chem. Phys. Lett. 311 (1999) 29. [11] R.P. Linstead, M. Whalley, J. Chem. Soc. (1952) 4839. [12] F.A. Burkhalter, E.C. Meister, U.P. Wild, J. Phys. Chem. 91 (1987) 3228. [13] E.I. Zenkevich, E.I. Sagun, V.N. Knyukshto, A.M. Shulga, A.F. Mironov, O.A. Efremova, R. Bonnet, M. Kassem, J. Appl. Spectrosc. 63 (1996) 599. [14] A.T. Gradyushko, M.P. Tsvirko, Opt. Spectrosc. 31 (1971) 291.
126
A. Starukhin et al. / Chemical Physics 285 (2002) 121–126
[15] B. Kozankiewicz, J. Bernard, M. Orrit, J. Chem. Phys. 101 (1994) 9177. [16] S. Kummer, T. Basche, C. Brauchle, Chem. Phys. Lett. 299 (1984) 309. [17] M. Vogel, A. Gruber, J. Wrachtrup, C. von Borczyskovski, J. Phys. Chem. 99 (1995) 14915.
[18] H. Dreeskamp, E. Koch, Chem. Phys. Lett. 31 (1975) 251. [19] E.P Gibson, A. Rest, J. Chem. Phys. Lett. 73 (1980) 294. [20] J.G. Radziszewski, J. Waluk, M. Nepras, J. Michl, J. Phys. Chem. 95 (1991) 1963.
Single molecule spectroscopy of Mg-tetrazaporphyrin in xenon matrix. Heavy atom effect A. Starukhin a,c,*, A. Shulga a,c, J. Sepiol b,c, R. Kolos b,c, V. Knyukshto b, A. Renn a, U.P. Wild a a
b
Institute of Physical Chemistry, ETH Zurich, ETH Zentrum, Universit€atstrasse 16, CH-8092 Z€urich, Switzerland Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 220072, F.Scaryna Av. 68, Minsk, Belarus c Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland Received 12 December 2001
Abstract We report initial results on the observation of single Mg-tetrazaporphyrin molecules in solid xenon. Samples were prepared following the conventional matrix-isolation procedure. Essentially inhomogeneous broadening is characteristic for these spectra. Single molecules, detected by fluorescence microscopy, gave remarkably stable signals. Their fluorescence excitation spectra revealed sharp zero-phonon lines, with most represented linewidths around 50 MHz. This is the first study of single molecules of a tetrapyrrolic compound. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Single-molecule spectroscopy (SMS) at liquid helium temperature has revealed a number of unique information about dynamical interaction between molecules of chromophore and the host matrix. For molecules in bulk systems we could observe only the averaging spectral data. Fluorescence microscopy of single molecules is especially suited for the investigation of a whole set of molecules in parallel under identical conditions. Low-temperature microscopy has been employed
*
Corresponding author. E-mail address: [email protected] (A. Starukhin).
as an ideally sensitive method for the determination of changes in the nanoenvironment of molecules. The quest for new molecules of chromophore and matrices is currently central to the development of this new spectroscopic method. Most of the investigated single molecules at liquid helium temperature belong to the class of conjugated aromatic hydrocarbons (see for example [1–3]). These compounds are characteristic ideal parameters for SMS (strongly allowed transition in absorption spectra, high fluorescence quantum yield, a low probability of S1 to T1 transition and/or fast return from T1 to the ground state, etc.) Biologically important complex systems (peripherical light harvesting complex with chlorophyll
0301-0104/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 ( 0 2 ) 0 0 6 9 4 - 8
122
A. Starukhin et al. / Chemical Physics 285 (2002) 121–126
a and b) have been investigated by methods of single molecule spectroscopy at low and room temperatures [4–8]. For the above-mentioned systems caratenoids were the main reasons for an effective quenching of the triplet state of a chlorophyll molecule [7]. In this work, we record single molecules of Mgtetrazaporphyrin (Mg-TAP) embedded in xenon solid matrix at liquid helium temperature. For preparation of the sample we used the matrix isolation technique at low temperature as described in [9,10]. The first goal of this study was to try recording SMS for tetrapyrrolic compounds such as porphyrins. For realization of this idea we need to find a compound with the corresponding spectral parameters and effective quenching of the triplet state. The second purpose was to look for influence of xenon matrix (heavy atom effect) on the photophysical parameters of chromophore. For studying the heavy atom effect on Mg-TAP we carry out experiments for mixture C2 H5 J as model heavy matrix because J and Xe atoms have practically the same atomic weights.
2. Experimental Mg-tetrazaporphyrin was synthesised according to the procedure described in [11]. The compound was purified on an aluminium oxide by chromatographic method. The structure of the Mg-TAP has been established (see Fig. 1) from the 1 H NMR spectrum (Bruker WM-360 spectrometer, 360 MHz). Low-temperature matrices were prepared under direct deposition of the compound of interest on the objective surface. The Mg-TAP was sublimed at 570 K in a stream of noble gas – Xe (from Linde 4.0) and condensed onto the cold objective surface at 55 K. Typical matrix ratios were 1:104 or lower in our experiment. After completing the deposition, the inside of the cold finger was filled with liquid helium. The temperature of the objective reached about 2 K after pumping the vapour of helium. The presence of the compound on the surface of objective was checked by the detection of fluorescence using a tunable dye laser and a
Fig. 1. Fluorescence excitation (solid line, observed at kem ¼ 642 nm) and fluorescence (dashed line, excited at kex ¼ 370 nm) spectra of Mg-TAP in a Xe matrix at 5 K (I). Phosphorescence spectrum (excitation at kex ¼ 585 nm) of Mg-TAP in tetrahydrofuran at 77 K (II). Spectral resolutions were 30 cm1 . The window temperature during the deposition was about 55 K for the Xe matrix.
video camera. A more detailed description of optical cryostat and method preparation of samples for single molecule experiments are given in [9,10]. The optical setup for single molecule detection was similar as described in [3,10]. The beam of a single mode dye laser with Rh6G dye (spectral bandwidth about 2 MHz) was focused on the
A. Starukhin et al. / Chemical Physics 285 (2002) 121–126
matrix near the objective axis (spot diameter ca. 50 lm). The video camera with an image intensifier (Hammamatsu C2400-25) was used for registration of resulting images, produced by individual fluorescing molecules. The Schott RG610 glass filter was placed between a cryostat window and the camera for blocking Rayleigh-scattered laser radiation. To obtain the spectral positions and line shapes of individual molecules the laser wavelength was scanned over 2 GHz in 2 4 MHz steps with the integration time 0.32 s per video frame at each frequency point. Stability of the emission from single molecules was monitored by recording the images generated during the continuous irradiation of the sample at a fixed excitation wavelength. Emission and excitation spectra were measured on a high-resolution spectrometer. The excitation source consisted of an Osram XBO 2.5 kW highpressure xenon lamp combined with a Spex 1402 double monochromator. The sample was placed in the same optical cryostat as for single molecule detection but the objective for these experiments was replaced by a sapphire window on a special holder. The emission light was passed through a second Spex 1402 double monochromator and detected by a cooled Hamamatsu R2949 photomultiplier tube. Data were acquired by a photon counting (SR-400) apparatus coupled to a PC. A more detailed description of this experimental setup is given elsewhere [12]. Phosphorescence and excitation of phosphorescence spectra and kinetic parameters of MgTAP were measured by using a high-sensitive home-built spectrometer that was described in detail previously [13]. The reproducibility of the system is 5%, the accuracy of emission quantum yield ðuÞ measurements is 5–7% for u P 0:1, and the limit of emission quantum yields measured is 105 . The fluorescence quantum yields uF of the systems under investigation were measured by the relative method, tetraphenylporphyrin in toluene (uF ¼ 0:09 at 293 K) was used as the standard.
3. Results and discussion Fig. 1 presents the structure, absorption (fluorescence excitation), fluorescence and phospho-
123
rescence spectra of Mg-TAP in xenon matrix at 5 K for bulk sample. It reveals a strong inhomogeneous broadening of the bands under our conditions of preparation of the sample. The spectrum of Mg-TAP in xenon matrix consists of bands (0–0 transition is located near 17 106 cm1 ) with halfwidths of about 200 cm1 . One video frame out of 600 images was recorded while that scanning the laser over 2 GHz range is shown in Fig. 2. Several molecules were in resonance with the laser there; the positions of these appear as bright spots against the dark background. Analysis of signals coming from single molecules (i.e., from distinct sets of adjacent pixels on the detector surface) as a function of laser frequency gave the individual fluorescence excitation spectra. The positions and widths of resonance lines were notably stable during the time the laser scans. The signal-to-noise ratio was typically higher then 10, with the laser beam intensity of about 0:1 W=cm2 . Shapes of resonance lines were, for the majority of molecules, well reproduced by a Lorentzian fit (see inset of Fig. 2). Spectral profiles for 56 individual Mg-TAP molecules were analysed, as illustrated with the histogram in Fig. 3. The linewidths were distributed within the frequency range 20–130 MHz with a maximum around 50 10 MHz. The fluorescence lifetime of Mg-TAP in solid Xe was not measured yet. At room temperature we obtained sf ¼ 4:5 ns for solution Mg-TAP in tetrahydrofuran. The dependence of the fluorescence lifetime on temperature is usually not significant [14] and the expected lifetime-limited zero-phonon linewidth should be 35 MHz, which is not far from the low frequency histogram cut-off at 20–30 MHz. Influence of heavy atom is more significant. For Mg-TAP in mixture – tetrahydrofuran (1:4) we measured the value lifetime of the singlet state as sf ¼ 1:4 ns. For this case the lifetime limit must be near 110 MHz that cannot be reconciled with the results from Fig. 3. In our opinion situations with heavy atom effect from C2 H5 J and Xe were widely different. It is possible that C2 H5 J as extra-ligand for Mg-atom may have a more appreciable effect on the parameters of Mg-TAP than Xe atom of matrix. In future, we are planning to carry out
124
A. Starukhin et al. / Chemical Physics 285 (2002) 121–126
Fig. 2. Single molecule fluorescence Mg-TAP isolated in a Xe matrix. The traces of individual molecules reproduced as tiny clusters of illuminated CCD detector pixels. The image, accumulated for 0.32 s, corresponds to the excitation with a single-mode laser (2 MHz bandwidth) of around 586.2 nm. Inset shows an exemplary single molecule fluorescence excitation profile (thin curve), approximated with a Lorentzian function (bold).
Fig. 3. The distribution of line-widths for Mg-TAP molecules in a Xe matrix (excitation near 586.15 nm) with the laser beam of 0:1 W=cm2 .
measurements of the kinetic parameters for MgTAP in Xe matrix at low temperatures. The distribution from Fig. 3 has similarities with that for Terylene molecules embedded in nalkane matrix under a directly deposited compound on the surface of a microscope objective [9]. The character of distributions allows us to suggest that the spectral diffusion is not so effective for Mg-TAP in xenon matrix as for Terylene molecules in n-alkanes. It is well known that for Terylene molecules in polymeric materials the respective lines were at least one order of magnitude broader [15]. On the basis of similar characteristics of histograms for Terylene and Mg-TAP it seems reasonable to compare the main spectroscopic parameters of both molecules. For Mg-TAP we found a phosphorescence spectrum (see Fig. 1) at 77 K and determined all
A. Starukhin et al. / Chemical Physics 285 (2002) 121–126
125
Table 1 Spectral and photophysical parameters of Mg-TAP and Terylene molecules
Fluorescence quantum yield Lifetime for first singlet state at 293 K (ns) Phosphorescence quantum yield Lifetime for first triplet state at 77 K (ms)
Mg-TAP (pure TGF)
Mg-TAP (mixture TGF : C2 H5 J)
Terylene (pure TGF)
Terylene (mixture TGF : C2 H5 J)
0.56 4.5 4 105 10
0.04 1.4 1:5 105 0.9
0.95 4.7 – (105 [16]) – (few ms [17])
0.67 4.0 – –
parameters of the triplet state. All our efforts to record the phosphorescence spectrum of Terylene have not been successful. In Table 1 we present the results of new measurements for Mg-TAP and Terylene in pure tertrahydrofuran (TGF) solution and mixture TGF–C2 H5 J (4:1) at 293 and 77 K. We used the mixture with C2 H5 J for modeling the effect of external heavy atom. As it is clear from Table 1 intersystem crossing is important for Mg-TAP and the influence of heavy atom on the singlet and triplet state is very essential. In the xenon matrix for Mg-TAP we also observed a heavy atom effect on the fluorescence quantum yield. On the basis of our evaluation the value quantum yield was about 0.1 or 0.15. Direct measurements of this parameter we could not realise under conditions of the SMS experiment. The phosphorescence quantum yield is not changed, but the lifetime of the triplet state decreased by more than one order. This would lead to a triplet decay rate of 1:1 103 s1 under heavy atom effect, making the SMS of Mg-TAP possible. The molecule of Mg-TAP (similar as for Terylene molecule) has strongly allowed transition (extinction coefficient of about 105 cm1 mol1 [18]) to the lowest excited singlet state. As it is well known [18,19] that for molecules with strongly allowed 0–0 transitions such as perrylene, 3-bromperylene, 3, 9-dibromperylene and 9,10-diphenylanthracene external heavy atom effects (xenon matrix and Br- substitution) are absent. As we can see from Table 1 for Mg-TAP heavy atom effect is more essential than for Terylene molecule, but the effect is weaker than for H2 -porphine [20]. In conclusion, matrix isolation technique was successfully used for single molecule detection MgTAP in xenon matrix. Our experiments to dem-
onstrate new promises for the study of biologically important molecules (porphyrins and phthalocyanins) in rare gas matrices by SMS methods.
Acknowledgements Our thanks also go to Dr. Jan Jasny who constructed the mirror objective, and to Mr. Bruno Lambillotte, for his invaluable technical assistance.
References [1] M. Orrit, J. Bernard, Phys. Rev. Lett. 65 (1990) 2716. [2] Th. Basche, W.E. Moerner, M. Orrit, U.P. Wild (Eds.), Single Molecule Optical Detection, Imaging and Spectroscopy, VCH, Weinheim, 1996. [3] T. Irngartinger, A. Renn, G. Zumofen, U.P. Wild, J. Lumin. 76 (1998) 279. [4] Y. Ishii, T. Yanagida, Single Molecules 1 (2000) 5. [5] M.A. Bopp, Y. Jia, L. Li, R.J. Cogdell, R.M. Hochstrasser, Proc. Natl. Acad. Sci. USA 94 (1997) 10630. [6] A.M. van Oijen, M. Ketelars, J. Kohler, T.J. Aartsma, J. Schmidt, J. Phys. Chem. B 102 (1998) 9363. [7] C. Tietz, O. Cheklov, A. Drabenstedt, J. Schuster, J. Wrachtrup, J. Phys. Chem. B 103 (1999) 6328. [8] C. Tietz, U. Gerken, F. Jelezko, J. Wrachtrup, Single Molecules 1 (2000) 67. [9] J. Sepiol, A. Starukhin, T. Latychevskaia, J. Jasny, A. Renn, U.P. Wild, Chem. Phys. 247 (1999) 35. [10] J. Sepiol, A. Starukhin, R. Kołos, T. Latychevskaia, J. Jasny, A. Renn, U.P. Wild, Chem. Phys. Lett. 311 (1999) 29. [11] R.P. Linstead, M. Whalley, J. Chem. Soc. (1952) 4839. [12] F.A. Burkhalter, E.C. Meister, U.P. Wild, J. Phys. Chem. 91 (1987) 3228. [13] E.I. Zenkevich, E.I. Sagun, V.N. Knyukshto, A.M. Shulga, A.F. Mironov, O.A. Efremova, R. Bonnet, M. Kassem, J. Appl. Spectrosc. 63 (1996) 599. [14] A.T. Gradyushko, M.P. Tsvirko, Opt. Spectrosc. 31 (1971) 291.
126
A. Starukhin et al. / Chemical Physics 285 (2002) 121–126
[15] B. Kozankiewicz, J. Bernard, M. Orrit, J. Chem. Phys. 101 (1994) 9177. [16] S. Kummer, T. Basche, C. Brauchle, Chem. Phys. Lett. 299 (1984) 309. [17] M. Vogel, A. Gruber, J. Wrachtrup, C. von Borczyskovski, J. Phys. Chem. 99 (1995) 14915.
[18] H. Dreeskamp, E. Koch, Chem. Phys. Lett. 31 (1975) 251. [19] E.P Gibson, A. Rest, J. Chem. Phys. Lett. 73 (1980) 294. [20] J.G. Radziszewski, J. Waluk, M. Nepras, J. Michl, J. Phys. Chem. 95 (1991) 1963.