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Optical Detection of Single Molecules in a Solid A New Frontier in Optical Spectroscopy.

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Optical Detection of Single Molecules in a Solid:
A New Frontier in Optical Spectroscopy?
By Dietrich Haarer* and Lothar Kador*
On examining the emission spectrum of atomic hydrogen
B u h e r discovered a sequence of narrow spectral lines which
didn't fit into the picture of classical physics. This observation triggered the birth of quantum mechanics. A comparable success story in molecular spectroscopy has not occurred
since or, if there is such a story, the development stages have
been smaller and less spectacular. This can be explained on
the following grounds.
Firstly, larger molecules have a much larger variety of
excited electronic states and, hence, the Hamiltonian cannot
be solved without major approximations. Secondly, the
molecular energy states in the condensed phase are, other
than in diluted gases, not sharply defined but smeared out
over a large energy interval due to intermolecular interactions. This phenomenon which smears out the well defined
molecular states to broad, structureless bands is called inhomogeneous broadening.['. 'I It is much more pronounced in
amorphous materials than in ordered crystals. In spectroscopic experiments, it obscures the 'intrinsic' molecular information as is shown schematically in Figure 1. Recording
w-
a
b
Fig. 1. Schematic representation of an ensemble of dye molecules in a disordered solid (a) and the corresponding distribution of their absorption frequencies (b)[ll]. Although all the absorption lines belong to the same electronic
transition of a set of identical molecules, their exact positions are affected by a
variation of the dye-matrix interactions in the different local environments. In
a straightforward optical experiment, the envelope of the full ensemble of lines
yields the observed inhomogeneously broadened spectrum. A = absorption.
homogeneous, molecular absorption lines, i.e. absorption
lines of single dopant molecules, has therefore until recently
been an unfulfilled dream in the world of molecular spectrosCOPY.
In the past, scientists had to live with the above-mentioned
'nuisance' of inhomogeneous broadening and, hence, the
success of straightforward optical spectroscopy in the solid
state was rather limited. Following the advent of the laser
with its unique capabilities of generating light of extremely
narrow bandwidth or ultrashort pulses, a number of methods were devised as approaches to overcome this problem.
They operate either in the time domain like tw~-pulse[~]
and
accumulated photon echo[41or in the frequency domain like
[*] Prof. Dr. D. Haarer, Dr. L. Kador
Pbysikalisches Institut and Bayreuther Institut fur
Makromolekiilforschung (BIMF) der Universitat
Postfach 101251, W-8580Bayreuth (FRG)
540
0 VCH Verlogsgesellschofi mbH. W-6940 Weinheim. 1991
fluorescence line narrowingts1and persistent spectral hole
burning.L6*'I
The experiments of the first group probe more or less all
the absorbing centers in the inhomogeneous distribution and
eliminate the differences of their absorption frequencies by
sophisticated sequences of short light pulses, forcing the centers to emit coherent pulses of fluorescence radiation whose
characteristics are solely determined by the homogeneous
line shape. These methods are based on the similar technique
of generating spin echoes in magnetic resonance.IB1They
measure the homogeneous absorption linewidth on the time
scales of picoseconds (two-pulse photon echo) and microseconds (accumulated photon echo) and are thus able to reveal
fast time dependences which may occur in disordered matrices due to structural relaxation p r o c e ~ s e slo]
. ~ On
~ ~ the other hand, it is difficult to decide with these methods if the
limiting 'natural' linewidth, the homogeneous width, is really
the same for all the molecules in the sample or if there is some
distribution. In the latter case, the measurements yield an
average value.
The second group of the above-mentioned experimental
techniques selects and probes only a small subset of the absorbing centers whose transition frequencies are identical to
within roughly one homogeneous linewidth. The common
feature of these methods is that the sample is illuminated
with laser light of very small bandwidth so that only those
dye molecules are promoted to the excited state whose absorption lines are in resonance with the laser. Since the excitation frequency can be varied throughout the inhomogeneous distribution, it is possible to investigate correlations
between optical properties of the dopant molecules and their
exact transition frequencies. With the technique of persistent
spectral hole burning" 'I it was shown, for instance, that
in some dye-matrix systems the matrix-induced electric
dipole moments of the absorber^,"^. l4]the pressure shifts of
their transition frequen~ies,[''l and partly even the homogeneous linewidth~['~,
161 vary with the position in the inhomogeneous band, i.e. with the solvent shift and, thus, the transition frequency. These results indicate that imperfections and
disorder in a solid do not only influence the absorption frequencies but also a number of other properties of the dopant
molecules. Therefore, the question arises as to whether at
least a subset of absorbers with identical transition frequency
is characterized by a number of coinciding physical parameters.
In order to address this point, several spectroscopic experiments of ultra-high sensitivity have been carried out during
the past three years, which were aimed at detecting and probing very few or, ideally, only one single absorber in a solid at
low temperatures. In vacuum, the spectroscopy of single
metal ions trapped in Paul cages has meanwhile become a
well-established technique" 'I since the fluorescence signals
of these particles can be detected very sensitively without any
background light present. In condensed matter, the situation
0570-0833/9ljOSOS-0540$3.50+ ,2510
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Angew. Chem. I n f . Ed. Engl. 30 (1991) No. S
is distinctly more difficult. Due to diffraction effects, the
probing light cannot be focussed to a volume much smaller
than 1 pm3 which still contains roughly 10" matrix
molecules. The latter give rise to Rayleigh and/or Raman
scattering and, in fluorescence experiments, tend to obscure
the desired signal of the dopant, the 'needle in the haystack'.
Thus, the first ultra-sensitive measurement of this type118]
had a detection limit of five Sm2@ions in a CaF, crystal. The
excitation of the ions was hereby performed using a fixedfrequency laser. The first spectra of single molecules were
recorded in an absorption experiment with a double-modulation techniq~e.l'~I
The sample was a molecular host crystal
of p-terphenyl doped with pentacene. Single absorption lines
of dopant molecules were selected by tuning the laser frequency to positions way out in the wings of the inhomogeneous absorption band. The same system was subsequently
investigated by a different group of researchers using an
improved fluorescence technique.[''] They observed that
some of their single-molecule signals disappeared and reappeared in the course of a few seconds and they explained this
effect as being due to single-molecule hole burning. However, the latest investigation of pentacene in p-terphenyl[zll
revealed a far more fascinating origin of the unstable signals.
Ambrose and Moerner found that there are two classes of
absorbing pentacene molecules in the crystal : Class I has
stable absorption frequencies, whereas the transition lines of
the Class I1 molecules jump spontaneously between two or
more frequency positions.
Figure 2 shows a sequence of 22 fluorescence excitation
spectra which were recorded successively in a spectral region
around 592-593 nm. These data clearly indicate the presence
of three Class I molecules whose fluorescence signals appear
as stable ridges, and one absorber of ClassII. The absorption
frequency of the latter varies from one scan to the next. In
fact, the corresponding absorption line can change its position several times in a 2-minute laser scan and, as a consequence, it gives rise to one or more broad fluorescence signals in some spectra and seems to be totally absent in others.
No influence of the intensity of the probing light on the
frequency jumps was detected.
This is the first reportrz1]of spectral diffusion of single
molecules and, most interestingly, the effect was observed on
a crystal rather than an amorphous sample. It shows that
new and unexpected physical phenomena can be found in
2500f
I
Fig. 2. Time dependence of the fluorescence excitation signals o f single pentacene molecules in ap-terphenyl crystal [21]. Left: three-dimensional representation of 22 individual spectra which were recorded successively; x = number
of photons per s; right: contour plot of the signal intensities as a function of
excitation frequency and time. The recording time was 2 minutes for each
spectrum.
Angew. Chem. Int. Ed. Engl. 30 (1991) No. 5
0 VCH
ultra-sensitive optical experiments which probe single, separate absorbing centers. In conventional site-selective techniques such as persistent spectral hole burning, these effects
are not immediately visible since the optical experiment averages over many molecules.
So far, most of the extremely sensitive spectroscopic experiments which were capable of detecting signals from individual centers were performed on the molecular crystal p-terphenyl doped with pentacene. In several respects this system
is ideally suited, because it is characterized by a large optical
absorption cross section, narrow homogeneous lines, and an
extremely low quantum yield for photochemical reactions.
Similar conditions may be found in a number of other organic mixed crystals containing strongly absorbing dye molecules. It will be an experimental challenge, however, to apply
such a technique to inorganic or amorphous systems, since
the oscillator strengths of inorganic ions are usually smaller
by several orders of magnitude and ions or molecules in
glasses tend to exhibit far higher hole-burning quantum
yields. On the other hand, the study of spectral-diffusion and
hole-burning processes on a single-molecular level in a glass
would certainly contribute to our present understanding of
the amorphous state in general. Another interesting direction in which the experiments could be extended would be
the investigation of single homogeneous lines close to the
center of their absorption band rather than far out in the
wings. For this purpose, samples of much lower dopant concentrations would have to be used. These measurements
should reveal details about the dye-matrix interaction for
molecules sitting in almost unperturbed equilibrium positions. However, the chemical purity of the samples will be of
utmost importance in these investigations. The results obtained thus far on the pentacenelp-terphenyl system are very
encouraging and can be considered as a step towards molecular spectroscopy as described by 'Molecular Hamiltonians'.
German version: Angew. Chem. 103 (1991) 553
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1151 S. Reul, W. Richter, D. Haarer, unpublished.
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(211 W.P. Ambrose, W E . Moerner, Nature 349 (1991) 225.
Verlagsgesellschaji mbH. W-6940 Weinheim. 1991
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