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
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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
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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.
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