At first an experimental challenge, the ability to conduct experiments with single molecules has been strongly connected with progress in experimental techniques and instrumentation. Scanning probe techniques, such as the scanning tunneling microscope or the atomic force microscope (AFM), use sharp tips in close proximity (10−9 m) to a sample to measure tunneling currents or weak mechanical forces that, in turn, allow generation of a real-space “image” of a single atom or molecule (1). In another approach, macromolecules can be clamped between an AFM tip and a substrate to determine the forces needed to stretch a single polymer chain. Similar experiments are feasible by use of optical tweezers, where a macromolecule is attached to a tiny bead and a substrate. The light force acting on the bead can be used to translate it against a force generated by the macromolecule (2). In the optical domain, the fluorescence emission of single molecules (or more general single fluorophores) in some condensed-phase environments can be imaged by advanced optical microscopies, such as scanning confocal microscopy or near-field scanning optical microscopy (3). At low temperatures, single fluorophores can also be isolated by a frequency selective technique that uses the fact that the sharp optical transition frequencies of dopant molecules are different because of imperfections of the environment (4). These optical techniques allow for detailed spectroscopic investigations at the single-molecule level, taking advantage of spectral, time-resolved, and polarization information.

Single molecule detection has been extended into life sciences by use of strongly fluorescent labels. The green fluorescent protein (GFP) as a self-fluorescent biomolecule has attracted considerable attention. Here, single molecules of the GFP-mutant Glu222Gln are immobilized in a polyvinylalcohol matrix and detected by confocal fluorescence microscopy. Although this mutant stabilizes one of both conformers of the wild-type GFP, the investigation of its fluorescence dynamics reveals strong signal fluctuations. This fluorescence behaviour is-at least partly-caused by reversible photochemical changes of the protein framework, that can relax into the fluorescent state on different timescales. Thus, this protein appears particularly appropriate for studying the microheterogeneity of the macromolecule GFP on a single molecule level.

The controlled manipulation and switching of single atoms and molecules raise the prospect of ultra-high-density data storage. Switching by motion of a single atom has been reported1, and techniques of single-molecule optical detection and spectroscopy2 in the condensed phase have been refined to a degree that allows the modification of the absorption properties of a single chromophore3. Light-induced jumps in single-molecule excitation frequencies have been reported3,4,5, but in none of these cases could the process be controlled: the jumps varied from molecule to molecule, they were interrupted by spontaneous jumps, and the new excitation frequencies could not be identified unambiguously. Here we report light-induced reversible frequency jumps ofsingle molecules of the aromatic hydrocarbon terrylene embedded in a particular site of a p-terphenyl host crystal6 at temperatures of around 2 K. The changes in absorption frequency for different terrylene molecules were identical (within 0.5%) for all samples studied. Thus we were able to switch single-molecule absorption lines in a controlled way between well-defined frequency positions.

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We report on the investigation of temperature dependent optical dephasing of single terrylene molecules in a p-terphenyl host crystal using two different techniques. The temperature dependence of the optical linewidth between 2 and 7.2 K can be described by an exponentially activated process with an activation energy Delta E = 18 +/- 2 cm(-1), which is attributed to optical dephasing of the electronic transition by scattering of a pseudolocal mode. By measuring the fluorescence intensity autocorrelation function we demonstrate that the dephasing time of a single terrylene molecule can also be extracted from coherent transients (Rabi oscillations) appearing in the correlation function in the nanosecond time regime. These oscillations experience an increased damping at temperatures above 2 K due to pseudolocal mode-induced pure dephasing.

BOHR'S notion of quantum jumps between electronic states of an excited atom has now been demonstrated experimentally for single ions confined in radio-frequency traps and interacting with a driving laser field(1-3). In these experiments the fluorescence of a strongly allowed transition was shown to cease abruptly when the ion jumped into a metastable state which was coupled to the common electronic ground state by a weak radiative transition. But attempts to monitor quantum jumps of single molecules have been hampered by the fact that the lifetime of the metastable triplet state was too short in relation to the photon detection rate. By using a system with favourable photophysical parameters-terrylene doped into p-terphenyl crystals(4)-we have now been able to observe directly quantum jumps between electronic states of single terrylene molecules. In contrast to single atoms, here the quantum jumps occur as non-radiative transitions between states of different multiplicity, and are manifested as interruptions of the fluorescence signal. These results demonstrate how single-molecule spectroscopy can reveal truly quantum-mechanical effects in large polyatomic molecules.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTMeasurement of Optical Dephasing of a Single Terrylene Molecule with Nanosecond Time ResolutionS. Kummer and Th. BascheCite this: J. Phys. Chem. 1995, 99, 47, 17078–17081Publication Date (Print):November 1, 1995Publication History Published online1 May 2002Published inissue 1 November 1995https://doi.org/10.1021/j100047a005RIGHTS & PERMISSIONSArticle Views357Altmetric-Citations43LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (426 KB) Get e-Alerts

Single molecule spectroscopy in solids at low temperature is a very rapidly growing field which due to the sensitivity of a single molecule to its truly local environment yields new insights in the structure and dynamics of crystalline and amorphous solids [1,2]. In the first years of SMS a lot of different experimental techniques have been applied to only a few well selected systems. These were the mixed crystalline system pentacene in p-terphenyl, and perylene and terrylene in poly(ethylene). Very recently, it was recognized by several groups that it is important to find new systems to demonstrate the more general applicability of this new and exciting spectroscopy. While there are several - well known - stringent requirements for any specific system to observe single molecule spectra with the fluorescence excitation technique, recent successful experiments proved that there is still a wide variety of systems where SMS can be pursued. Two new systems introduced lately were terrylene in the Shpol’skii matrix hexadecane [3] and Rhodamine 640 in (poly)ethylene [4]. In our group we investigated new crystalline as well as new polymeric systems.

Fluorescence excitation spectroscopy of single dye molecules in crystals offers the possibility to determine molecular paramaters of single absorbers and their distribution instead of ensemble averages [1,2]. Informations about single molecules can be derived from the optical absorption spectra and from the time distribution of the fluorescence photons. The inhomogeneous broadening of an electronic transition reflects the variety of local environments accessible to the guest molecules and is therefore a measure for the degree of disorder or the defect concentration in the host crystal [3,4]. For pentacene molecules in p-terphenyl crystals (c ≈ 10−8mole/mole) we observed different types of inhomogeneous broadening: Some crystals show a narrow (FWHM ≈ 1 GHz) Lorentzian line with a satellite structure at short wavelengths due to 13C substitutional isomers of pentacene. The inhomogeneous line of other crystals is remarkable broadened and neither Lorentzian nor Gaussian. The intersystem crossing (ISC) rates k23 and k31 can be determined by measuring the intensity autocorrelation function of the fluorescence light [5]. In a crystal with low degree of disorder (narrow Lorentzian inhomogeneous broadening) the ISC rates of the molecules show a smaller distribution than in a crystal with higher degree of disorder (broadened line). The fully saturated fluorescence emission rates as calculated using the ISC rates are consistent with the experimentally measured photocount rates.

Temperature-dependent persistent spectral hole-burning measurements are reported on the dye-surface system Quinizarin/gamma-alumina (Q/gamma-Al2O3) and on the mixed matrix-dye-surface systems 3-methylpentane/Q/gamma-Al2O3, methylene chloride/Q/gamma-Al2O3, and n-heptane/Q/gamma-Al2O3. In all cases the temperature dependence of the hole-burning linewidth GAMMA(HB) could be best fitted to a combination of a T(alpha) law and a contribution from an exponentially activated process, indicating that the dynamics of the mixed systems are similar to Q/gamma-Al2O3. For Q/gamma-Al2O3 the power-law term shows clearly the glasslike nature of the gamma-Al2O3 surface. The temperature dependence of GAMMA(HB) yielded a large coupling constant for the exponentially activated term for n-heptane/Q/gamma-Al2O3, indicating a higher degree of crystallinity for the maxtrix n-heptane than for methylene chloride. The mixed matrix-dye-surface systems generally exhibit a smaller contribution of two-level-system modes to exponentially activated processes than does Q/gamma-Al2O3. The distance from the zero-phonon line peak to the phonon side-hole peak nu-0 differs by a factor of 3, demonstrating a distinct dependence of the dominant low-frequency mode on the chromophore environment. A very large nu-0 of almost-equal-to 50 cm-1 was found for the dye-surface system Q/gamma-Al2O3. Nu-0 does not correlate to the frequency of the exponentially activated process. For Q/gamma-Al2O3 stable holes could be burned even at 77 K. The Debye-Waller factor was measured for this system over an extended temperature range.

As was recently demonstrated 1 by using highly efficient fluorescence excitation spcctroscopy with N.A. = 0.98 collection optics, single impurity molecules in both crystals and polymers may now be detected with single-to-noiseratios (SNRs) as high as 30 in a 1 Hz bandwidth at low temperatures. In essence, the solid may be regarded as a trap for the single molecule, which effectively quenches rotation. Our measurements include precise determination of the lifetime limited linewidth and the temperature dependence of the optical dephasing, as well as spectral diffusion (wandering in frequency space due to perturbations by lattice degrees of freedom). For the polymeric host, light-induced changes in resonant frequency of a single molecule can be produced, a process that is normally termed spectral hole-burning when large ensembles of molecules are probed. In recent studies, nonclassical light emission from a single molecule (photon antibunching) has been observed in a fashion analogous to that reported earlier for a single atom in a trap.

Recent advances in absorption1 and fluorescence2 detection now allow high-quality measurement of the optical absorption spectrum of individual single-impurity molecules in both crystals and polymers at liquid-helium temperatures. Here, the solid may be viewed as a trap for the defect center similar to the well-known electromagnetic single-ion traps, except that Doppler and recoil effects are replaced by (static) inhomogeneous broadening and host-guest interactions. Most importantly, molecular rotation is absent because of steric effects at low temperatures.

First preliminary persistent spectral hole-burning (PSHB) investigations of dye-molecules adsorbed onto the disordered surfaces of metal oxide powders or porous glasses 1,2,3 indicated that these systems seem to behave quite similar to doped glasses. The optical absorption is strongly inhomogeneously broadened due to the disorder of the surface and at 1.5 K holewidths in the 1 cm 1 range - even an order of magnitude broader than in glasses - have been reported. In the following we will give a short summary of our latest more detailed investigations of surface adsorbed dye-molecules which on the one hand reaffirm that these systems indeed behave in many respects as 3-D disordered systems but on the other hand point to some remarkable specific features of adsorbed dye-molecules.

Recent advances in the optical detection and spectroscopy of single impurity centers in crystals has provided the possibility of obtaining information about defect-solid interactions on a truly local level. For the system composed of pentacene impurity molecules in the crystal p-terphenyl, single molecules have been studied using both absorption1 and fluorescence excitation2 techniques at liquid helium temperatures. The superior signal-to-noise of the latter technique has led to direct observations of the lifetime-limited Lorentzian homogeneous profile of a single pentacene impurity3 as well as the surprising observation of spontaneous spectral diffusion4. Spectral diffusion, or changes in the resonance frequency of an impurity molecule with time as a result of low-energy excitations (two-level system transitions (TLS)),5 is generally expected in amorphous hosts. Indeed, the presence of TLSs in amorphous solids is intimately connected with nonphotochemical mechanisms for persistent spectral hole-burning (PSHB) in amorphous materials6-8. Using perylene impurity molecules in poly(ethylene), we have observed the optical spectra of single molecules in a polymeric host for the first time. At 1.5K, individual perylene molecules show the expected spectral diffusion; moreover, we observe light-induced changes in resonance frequency, i.e., persistent spectral hole-burning, which allows one to envision optical storage on the single-molecule level.