Single Terrylene Molecules Research Articles

Optical characterization of a single molecule complete spatial orientation using intra-molecular triplet-triplet absorption.

Progress in single molecule fluorescence experiments have enabled an in-depth characterization of fluorophores, ranging from their photophysical rates to the orientation of their emission dipole moments in three dimensions. However, one crucial spatial information remains elusive: the molecule orientation relative to its emission dipole moment. One can retrieve the latter only by the use of another non-colinear transition dipole moment. We experimentally demonstrate the optical retrieval of this information for single terrylene (Tr) molecules in a 30 nm thin para-terphenyl matrix. We show, through second-order correlation measurements at varying excitation power and polarization, that Tr molecules experience an optically induced deshelving of their triplet states, mediated by two orthogonal intra-molecular triplet-triplet absorption dipole moments. We take advantage of these two transition dipole moments to retrieve the full orientation of the Tr molecule, employing a 3-level scheme for the molecule photophysics and analytical calculations for the exciting electric field distribution. This modelling approach enables us to accurately describe both varying power and polarization measurements, giving access to the molecule's photophysical rates and to its complete orientation in three dimensions. This includes the orientation of the singlet emission dipole moment in the laboratory frame, and the orientation of the molecule plane with respect to the singlet emission dipole moment.

Sharp zero-phonon lines of single organic molecules on a hexagonal boron-nitride surface

Single fluorescent molecules embedded in the bulk of host crystals have proven to be sensitive probes of the dynamics in their nano environment, thanks to their narrow (about 30–50 MHz or 0.1–0.2 μeV) optical linewidth of the 0-0 zero-phonon line (0-0 ZPL) at cryogenic temperatures. However, the optical linewidths of the 0-0 ZPL have been found to increase dramatically as the single molecules are located closer to a surface or interface, while no 0-0 ZPL has been detected for single molecules on any surface. Here we study single terrylene molecules adsorbed on the surface of hexagonal boron-nitride (hBN) substrates. Our low-temperature results show that it is possible to observe the 0-0 ZPL of fluorescent molecules on a surface. We compare our results for molecules deposited on the surfaces of annealed and non-annealed hBN flakes and we see a marked improvement in the spectral stability of the emitters after annealing.

Dispersion of Lifetimes of Excited States of Single Molecules in Organic Matrices at Ultralow Temperatures

Abstract: Fluorescence excitation spectra of single terrylene molecules in transparent naphthalene and polyethylene matrices at ultralow (30–100 mK) temperatures have been studied under the conditions where the widths of zero-phonon spectral lines are determined only by the lifetime T of the excited electronic state. The experimentally observed dispersion of T values for identical molecules is attributed to local field effects, which are responsible for the dependence of T on the effective value of refractive index n of the matrix, which is characteristic of the localization region of each molecule. It has been shown that the dependence T(n) for ensembles of point emitters in organic matrices is satisfactorily explained within the model of a “virtual cavity” around the emitter inside a continuous medium, as well as within the developed quantum kinetic approach including various contributions of the local environment to the lifetime T. The recalculation of average T values to the corresponding n values with the use of the expressions obtained for T(n) has shown that the difference of the calculated refractive indices of naphthalene and polyethylene from the well-known tabulated n values is less than 1%.

Coherent interaction of single molecules and plasmonic nanowires

Quantum plasmonics opens the option to integrate complex quantum optical circuitry onto chip scale devices. In the past, often external light sources were used and nonclassical light was coupled in and out of plasmonic structures, such as hole arrays or waveguide structures. Another option to launch single plasmonic excitations is the coupling of single emitters in the direct proximity of, e.g., a silver or gold nanostructure. Here, we present our attempts to integrate the research of single emitters with wet-chemically grown silver nanowires. The emitters of choice are single organic dye molecules under cryogenic conditions, which are known to act as high-brightness and extremely narrow-band single photon sources. Another advantage is their high optical nonlinearity, such that they might mediate photon–photon interactions on the nanoscale. We report on the coupling of a single molecule fluorescence emission through the wire over the length of several wavelengths. The transmission of coherently emitted photons is proven by an extinction type experiment. As for influencing the spectral properties of a single emitter, we are able to show a remote change of the line-width of a single terrylene molecule, which is in close proximity to the nanowire.

Anomalous doping of a molecular crystal monitored with confocal fluorescence microscopy: Terrylene in a p-terphenyl crystal.

Highly terrylene doped single crystals of p-terphenyl, obtained by co-sublimation of both components, showed bright spots in the confocal fluorescence images. Polarization of the fluorescence excitation spectra, blinking and bleaching, and saturation behavior allowed us to attribute them to single molecules of terrylene anomalously embedded between two neighbor layers of the host crystal, in the (a,b) plane. Such an orientation of terrylene molecules results in much more efficient absorption and collection of the fluorescence photons than in the case of previously investigated molecules embedded in the substitution sites. The above conclusion was supported by quantum chemistry calculations. We postulate that the kind of doping considered in this work should be possible in other molecular crystals where the host molecules are organized in a herringbone pattern.

Single molecules of terrylene in di-substituted naphthalenes crystallizing in the herringbone pattern

2,3-Dichloronaphthalene and 2,3-dibromonaphthalene were synthesized, their crystal structures determined, and vibronic spectra of single terrylene molecules in the crystals indicated lowering of the guest symmetry.

Coherence Studies of Photons Emitted from a Single Terrylene Molecule Using Michelson and Young’s Interferometers

Coherence length (time) is a key parameter in many classical and quantum optical applications. Two interferometers – Michelson and Young’s double-slit – are used to characterize the temporal coherence of single photons emitted from single terrylene molecules. For quantitative analysis, a dispersion-related distortion in the interference pattern of a Michelson interferometer is carefully corrected by a simple dispersion compensation. Additionally, it has been demonstrated that Young’s interferometer can be used in temporal coherence studies at the single photon level with high accuracy. The pros and cons of the two systems are discussed. The measured coherence lengths in the two systems are consistent with one another under the self-interference interpretations.

Enhancing the radiative emission rate of single molecules by a plasmonic nanoantenna weakly coupled with a dielectric substrate.

Enhancing the spontaneous emission of single emitters has been an important subject in nano optics in the past decades. For this purpose, plasmonic nanoantennas have been proposed with enhancement factors typically larger than those achievable with optical cavities. However, the intrinsic ohmic losses of plasmonic structures also introduce an additional nonradiative decay channel, reducing the quantum yield. Here we report on experimental studies of a weakly coupled dielectric substrate and a plasmonic nanoantenna for enhancing the radiative decay rate of single terrylene molecules embedded in an ultrathin organic film. We systematically investigate how the refractive index of the dielectric substrate affects the lifetime and the quantum efficiency and show that the coupled structure could moderately enhance the radiative decay rate while maintaining a high quantum efficiency.

Photon Statistical Properties of Single Terrylene Molecules in P-Terphenyl Crystals**Supported by the Project of Xuzhou Institute of Technology under Grant No XKY2014309, and the National Natural Science Foundation of China under Grant Nos 11304266 and 11447149.

We consider the photon emission statistical properties of a single molecule under pump-probe field driving, using the generating function method. The first- and second-order moments of statistical quantities are presented. Derived from the first-order moment, the line shapes are in good agreement with the experimental results. Derived from the second-order moment, Mandel's Q parameters show an obvious quantum effect of photon statistical distribution, i.e., the anti-bunching effect.

Electron Energy Loss of Terrylene Deposited on Au(111): Vibrational and Electronic Spectroscopy

We have investigated the vibrational and electronic properties of terrylene by high-resolution electron energy-loss spectroscopy (HREELS), Fourier-transform infrared spectroscopy, and low-temperature single-molecule fluorescence spectroscopy. Terrylene thin films were sublimated in an ultrahigh vacuum on the Au(111) surface in order to record the HREEL spectra. Polycrystalline p-dichlorobenzene was used as a matrix to isolate a single terrylene molecule at 1.5 K and record its fluorescence spectrum. The infrared spectrum, the vibrational components from the fluorescence spectrum, and density functional theory calculations were used for the assignment and identification of the active modes found in HREELS. Finally, we report a loss signal around 17 000 cm–1 (2.1 eV) for the first singlet electronic excited state in agreement with optical spectroscopy. The HREEL spectra show both IR- and Raman-active vibration modes because of specific surface selection rules. Energy-loss spectroscopy could be used as a complementary technique to explore some other degrees of freedom that are not accessible by optical means.

Stable single-molecule lines of terrylene in polycrystalline para-dichlorobenzene at 1.5 K.

The spectroscopic properties of single terrylene (Tr) molecules are studied in a polycrystalline matrix of para-dichlorobenzene (p-DCB) at 1.5 K. Samples grown in a glass capillary show a very strong site at 597 nm, which is redshifted by more than 700 cm(-1) from the observed transition energy for Tr in p-DCB prepared as a film on a coverslip (572 nm). Each of these two sites is characterized by measuring their single-molecule spectroscopic parameters at 1.5 K. Lifetime-limited linewidths of 45±5 MHz are found for both sites. Fluorescence detection rates reach 8×10(4) count s(-1) at saturation. The spectral trails of the majority of single molecules show no spectral jumps, indicating an absence of interacting two-level systems; however, the small distribution of linewidths may indicate weak interactions with low-frequency modes. Frequency jumps are observed for 10 % of the molecules. The complete emission spectra from two different single molecules at the center of each of the two sites is presented. Debye-Waller factors of αDW=0.33±0.05 for the normal site (572 nm) and αDW=0.30±0.05 for the red site (597 nm) are reported. This new host-guest system provides a quick and easy way to obtain lifetime-limited single-molecule lines.

Emission rate and collection efficiency of photons from single terrylene molecules: Dependency on the substrate and the collection lens

Photons emitted from a single emitter are promising candidates for an efficient single-photon source, which is essential for many novel nano- and quantum-optical applications. As a photon source having a larger flux and high reliability is highly desirable. In this paper, we quantitatively studied the dependencies of the numbers of photons emitted and collected from single terrylene molecules on different experimental conditions, the refractive index of the substrate and the numerical aperture of the collection lens. Also, back-focal plane images are used to visualize the emission pattern of a single emitter.

Statistical analysis of photons from a single terrylene molecule for the study of the energy level scheme

Single photons from single emitters are good candidates for a high-quality single-photon source which is essential in current quantum optical experiments and applications. Because the statistics of emitted photons is mainly determined by the energy level structure of the system, knowing the correct level scheme and the transition rates between levels is of primary importance. In this work, the energy level scheme and the photon-emission statistics of a single terrylene molecule are studied by measuring the second-order correlation function and the time trace of the emitted photons. The details in the measurement and analysis are discussed, and a more advanced energy-level structure of the terrylene molecule is suggested.

Spin–lattice relaxation and intersystem crossing in single molecules of terrylene embedded in a p-terphenyl crystal

Thermal evolution of fluorescence intensity correlation functions of single molecules of terrylene in a p-terphenyl crystal was studied and double-exponential decay at 5K was found to change to a mono-exponential decay above 17.5K. Such behavior was attributed to spin–lattice relaxation (SLR), which couples the long-lived zero-field triplet spin sublevel TZ with the short-lived and experimentally unresolved TX and TY sublevels. For two of the studied molecules, which were probably deformed from planar symmetry, we observed overpopulation of the spin sublevel TZ and activation of the SLR at temperatures above 20K. The SLR rate constant exhibited Raman relaxation.

Single-molecule fluorescence of terrylene embedded in anthracene matrix: A room temperature study

We characterized room-temperature fluorescence properties of single terrylene molecules embedded in anthracene thin films that were prepared by spin-coating on glass substrates. Our results show that terrylene molecules embedded in anthracene matrix are oriented nearly parallel to the substrate. Typical measured fluorescence lifetime of terrylene was 3.6ns and photon count rate at saturation was 750kHz. Our analyses of 104 molecules indicate that we can detect on average 1.6×106 photons from a single terrylene molecule before it photobleaches. These results make terrylene–anthracene a promising guest-host system for room temperature single molecule experiments.

Statistical properties of blinking kinetics in single molecule

We study the statistical properties of blinking kinetics of single terrylene molecule in p-terphenyl crystal with intersystem crossing process (ISC) via generating function. The analytical expression of the waiting time distribution of the ‘bright’ and ‘dark’ period is obtained, which demonstrates that the behaviors of the ‘dark’ period show bi-exponential decaying because of two kinds of sublevels. The probability of the single terrylene molecule in the ‘bright’ and ‘dark’ states are also investigated. Our theoretical results are in good agreement with experimental results.

Single-molecule probing of incommensurate biphenyl

Our data on the distribution of purely electronic linewidths of single molecules of terrylene in incommensurate biphenyl crystals are compared with the data obtained by other groups for different low-temperature organic solid hosts and with results of numerical simulations. The first two moments of the distributions measured within a narrow temperature interval have been used to calculate a single dimensionless parameter characterizing each of the respective hosts—the variation coefficient. It appears that different amorphous hosts have similar values of this coefficient, but the value obtained for the incommensurate crystal of biphenyl is significantly different. One can conclude that the remarkable single-molecule line broadening in biphenyl at 1.8K cannot be solely explained by the interaction with two-level systems, which is considered to cause the broadening in amorphous hosts.

Control and imaging of single-molecule spectral dynamics using a nano-electrode

We study the influence of a scanning nano-electrode on the fluorescence excitation spectra of single terrylene molecules embedded in thin p-terphenyl films at cryogenic temperatures. We show that applied voltages of less than 10 V can result in reversible Stark shifts of up to 100 times and a linewidth increase greater than 10 times the natural linewidth. We discuss the potential of our experimental scheme for the direct imaging of individual two-level systems in the nanometer vicinity of single molecules.

Photo-stability of single terrylene molecules in 2,3-dimethylnaphthalene crystals

Fluorescence excitation spectra and fluorescence intensity traces of single terrylene molecules in 2,3-dimethylnaphthalene crystals were studied at 5 K and also at temperatures between 200 and 290 K. Terrylene molecules were found to be relatively photo-stable in crystals obtained by cosublimation of both components under an argon atmosphere whereas these molecules were more easily photo-bleached in crystals obtained under a helium atmosphere. It is propose that the diverse behaviour in both groups of crystals is due to different penetration by oxygen and bleaching of terrylene molecules by photo-oxidation. The fully saturated fluorescence detection rates for the (0, 0) and vibronic transitions were studied.

Motion of Single Terrylene Molecules in Confined Channels of Poly(butadiene)−Poly(ethylene oxide) Diblock Copolymer

The motion of terrylene probe molecules in confined PB channels of an asymmetric PB-PEO diblock copolymer has been investigated by single molecule tracking. The one-dimensional diffusion coefficients were found to be significantly smaller and had a narrower distribution compared to two-dimensional diffusion coefficients in PB. The trajectories of some single molecules showed unusual behavior of directed motion where mean square displacement had a parabolic dependence on lag time. The likely origin of this behavior is discussed in terms of local variations in the PB channel width and the resulting change in the local density. The results show the effect of nonuniformities and heterogeneities in the channels on the motion of single molecules and demonstrate the sensitivity of single molecule tracking in characterizing self-assembled block copolymer morphologies.