Terrylene Molecules Research Articles

Intersystem Crossing Mechanisms and Single Molecule Fluorescence: Terrylene in Anthracene Crystals

Single molecule spectroscopy requires molecules with low triplet yields and/or short triplet lifetimes. The intersystem crossing (ISC) rate may be dramatically enhanced by the host matrix. Comparing the fluorescence intensity of single terrylene molecules in para-terphenyl, naphthalene, and anthracene crystals, we found a reduction of the saturation intensity by three orders of magnitude in the latter case. The fluorescence autocorrelation function indicates that the bottleneck state is the terrylene triplet. We propose a ping-pong mechanism between host and guest. This intermolecular ISC mechanism, which can open whenever the host triplet lies lower than the guest singlet, was overlooked in previous single molecule investigations.

Single-photon sources based on single molecules in solids

Single molecules in suitable host crystals have been demonstrated to be useful single-photon emitters both at liquid-helium temperatures and at room temperature. The low-temperature source achieved controllable emission of single photons from a single terrylene molecule in p-terphenyl by an adiabatic rapid passage technique. In contrast with almost all other single-molecule systems, terrylene single molecules show extremely high photostability under continuous, high-intensity irradiation. A room-temperature source utilizing this material has been demonstrated, in which fast pumping into vibrational sidebands of the electronically excited state achieved efficient inversion of the emissive level. This source yielded a single-photon emission probability p(1) of 0.86 at a detected count rate near 300 000 photons s−1, with very small probability of emission of more than one photon. Thus, single molecules in solids can be considered as contenders for applications of single-photon sources such as quantum key distribution.

Dye-doped cholesteric-liquid-crystal room-temperature single-photon source

Fluorescence antibunching from single terrylene molecules embedded in a cholesteric-liquid-crystal host is used to demonstrate operation of a room-temperature single-photon source. One-dimensional (1-D) photonic-band-gap microcavities in planar-aligned cholesteric liquid crystals with band gaps from visible to near-infrared spectral regions are fabricated. Liquid-crystal hosts (including liquid crystal oligomers and polymers) increase the source efficiency, firstly, by aligning the dye molecules along the direction preferable for maximum excitation efficiency (deterministic molecular alignment provides deterministically polarized output photons), secondly, by tuning the 1-D photonic-band-gap microcavity to the dye fluorescence band and thirdly, by protecting the dye molecules from quenchers, such as oxygen. In our present experiments, using oxygen-depleted liquid-crystal hosts, dye bleaching is avoided for periods exceeding one hour of continuous 532 nm excitation.

Aligned terrylene molecules in a spin-coated ultrathin crystalline film of p-terphenyl

We report on the use of a simple spin casting procedure to fabricate very thin crystalline films of p-terphenyl doped with fluorescent terrylene molecules. By performing single molecule studies, we show that the guest molecules are oriented normal to the plane of the film. We find that despite the very low thickness of the p-terphenyl matrix, as thin as only 20 molecular layers, about half of the embedded emitters withstand photobleaching for illumination times of at least a day.

Hole burning and single-molecule spectroscopy of terrylene in incommensurate biphenyl

Techniques of spectral hole burning and single-molecule spectroscopy have been used to study thin films of polycrystalline terrylene-doped biphenyl at temperatures between 1.7 and 2.2 K. A very wide range of probabilities of photoinduced spectral transitions of terrylene molecules are indicated by the strong non-exponentiality of non-photochemical hole burning processes and observation of some extremely stable single-molecule lines. Broad distribution of single-molecule linewidths as well as obvious disagreement between their average value and the corresponding holewidth also demonstrate a large variety of spectral behaviour of terrylene impurity molecules. We attribute these effects to spatial variations of local environment of terrylene molecules in an incommensurate matrix of biphenyl.

Electric-field effects of two-level systems observed with single-molecule spectroscopy

The fluorescence excitation spectra of single chromophores in a solid often consist of several components at low temperature, which is due to the interaction with matrix two-level systems (TLSs). For a number of terrylene molecules in n-hexadecane we observed electric-field-induced changes of the intensity and splitting of the spectral components. With some basic assumptions the data allow us to calculate all the characteristic parameters of a single TLS, viz., the (zero field) asymmetry, the tunnel parameter, and the permanent electric dipole moment. In one case we found an exceptionally large dipole moment of about 8 Debye.

Fluctuating absorption coefficient of single molecule

A quantum mechanical theory for fluctuating absorption coefficient of the impurity center is used to explain spectral trajectory and temporal line broadening of a single terrylene molecule doped polyethylene.

Theory of Near‐field Optical Imaging with a Single Molecule as Light Source

Scanning near-field optical microscopes (SNOM) illuminate a sample in the very near-field using a nanometer sized tip. Ideally, the light source should be point-like and many efforts have been made to optimize tip efficiency (see, for example, the article of Heimel et al in this issue). Very recently, Sandoghdar et al have realized a molecular probe tip in which a terrylene molecule inserted in a paraterphenyl microcrystal is attached at the extremity of the probe tip [1]. The excited molecule behaves as a point-like light source which is raster scanned over an aluminium patterned structure. We propose here an analysis of this experiment based on the field-susceptibility formalism (also called Green's Dyadic Method) [2,3]. In particular, in strong analogy with the Scanning Tunneling Microscope (STM), we will show that the detected signal is proportional to the Local Density of photonic States (LDOS) available at the immediate proximity of the sample. The molecular light source behaves as a dipole p(t), located at rtip and oscillating at the wavelength λ0=2π/ω0=630 nm where u is a unit vector that gives the dipole orientation.The electric field E(r,t) created at the point r below the surface by the dipole p(t) is evaluated thanks to the field-susceptibility S(r,rtip,ω) associated to the experimental geometry Then, the detected signal is the intensity scattered in the solid angle Ω below the surface (see figure 1) Additionally, we demonstrate that for large angle of detection, this signal is directly proportional to the partial LDOS nu(rtip,ω0) [2,3] where the partial LDOS nu(rtip,ω0) depends on the orientation u as following [3] We also propose a simple interpretation of the relationship between the LDOS and the detected signal in strong analogy with the STM [3]. This analogy can be made thanks to the equivalency between the illuminating mode SNOM and a particular collecting mode configuration [3,4]. Finally, the experiment realized by Sandoghdar et al is analysed both numerically and theoretically [1,3]. Moreover, the concept of optical corrals is introduced [5].

Saturation spectroscopy of vibronic transitions in single molecules

A novel technique of saturation spectroscopy is applied to study the individual vibronic spectra of single impurity molecules embedded in a low-temperature solid matrix. Homogeneous linewidths and vibrational frequencies for two lower vibronic levels of the excited electronic state S 1 are determined for several terrylene single molecules in a naphthalene crystal, which indicates a noticeable distribution of these parameters. A certain correlation can be pointed out between vibrational frequency and a purely electronic transition frequency of a terrylene molecule.

Stark spectroscopy of single molecules in the Shpol'skiı̆ matrix n-hexadecane

We present Stark effect measurements of single terrylene and DBATT molecules embedded in the Shpol'skiı̆ matrix n-hexadecane. Upon fast changes of a strong electric field, most single-molecule lines show slow creeping toward a new equilibrium position. The origin of this relaxation effect is not clear at present, yet, it seems that there are at least two contributions with different time constants.

Molecular mechanisms of photo-induced spectral diffusion of single terrylene molecules in p terphenyl

An empirical molecular model of crystals of p-terphenyl doped with terrylene is developed and compared with recent experiments on single guest molecules. The model provides an assignment of the experimental electronic origins to the substitution sites and an interpretation of the photo-induced frequency jumps of single terrylene molecules. The Stark shifts of terrylene in the different sites are estimated by a coupled Hartree–Fock semi-empirical calculation. The topology of the many-dimensional energy surface of the X1 electronic origin is explored with simulated annealing.

Fading Fast and Not So Fast

CHEMISTRY Chemical reactions can be studied spectroscopically at the single-molecule level with the use of fluorescent tracers or dyes. These fluorophores do not last forever; eventually they bleach and become invisible in the fluorescent microscope. Fluorophore destruction is known to be accelerated in the presence of oxygen, but our detailed understanding of the photobleaching process is limited. Christ et al. have studied, one by one, the behavior of several hundred terrylene molecules in air, oxygen, and argon atmospheres. Most of the molecules succumbed to photobleaching, and an argon atmosphere slowed this process by several orders of magnitude. The remaining molecules first switched into a new fluorescent state, with an emission maximum shifted to a shorter wavelength, before subsequently becoming dark. Quantum chemical calculations of possible product molecules indicate that immediate photobleaching and switching into a second fluorescent state are both caused by the covalent addition of oxygen. In the latter case, a second oxidation event then leads to photobleaching, which is consistent with their observation that the lifetime of the secondary photoproduct increases when it is transferred to an argon atmosphere.— JU Angew. Chem. Int. Ed. 40 , 4192 (2001).

Single Nanoobjects as Triggered Single Photons Sources

The generation of non-classical states of light [1] is an important scientific challenge with many potential applications. For example, squeezed states of light [2] have generated much interest in the past decade, because measurements can be performed with noise levels that are lower than those available with classical light. A particularly novel non-classical source of light is a triggered single-photon source: a source that has the property to emit with a high degree of certainty one (and only one) photon at a user-specified time. Such a deterministic source of single photons can be important for quantum information processing [3], quantum cryptography [4] and certain quantum computation problems [5]. A single quantum emitter such as a molecule or an atom is a good potential source because it can emit only one photon at a time. The emitted light is antibunched [6, 7] and consists of single photons separated by random time intervals, which depend on the excited state lifetime and on the pumping rate. Thus triggered emission of single photons can be obtained with controlled excitation of a single emitter. The first demonstration of a single photons source [8] using a single molecule in a solid is based on a rapid adiabatic passage method [9] to excite the molecule. In the effective spin picture, the Bloch vector follows the driving magnetic field adiabatically if the laser-molecule frequency detuning is swept slowly through the resonance. Since the passage inverts the field, the system goes from the ground to the excited state. From this fluorescent state the molecule can emit only a single photon. The molecular resonance is swept by Stark effect at fixed laser frequency. This is done easily by applying a radio frequency electric field to the molecule. The results showed photon states with statistics that differ radically from those of coherent states. But this method requires cryogenic temperatures (2K) since narrow zero phonon lines are needed for the adiabatic following. At room temperature a source of triggered single photons based on the concept of pulsed optical excitation of a single highly fluorescent molecule has been demonstrated [10]. A short pulse of green laser light pumps the molecule from the ground state to a vibronically excited level of the first electronic excited state. After fast (ps) relaxation to the lowest electronic excited state, the molecule subsequently emits a single photon. A frequency-doubled mode-locked Nd-YAG laser is used to pump highly stable single terrylene molecules contained in a p-terphenyl molecular crystal where they are protected from exposure to diffusing quenchers (such as oxygen). The result shows that the photons are emitted after the laser pulses at times separated by the laser repetition time within the lifetime of terrylene in p-terphenyl. Since the pump pulse width is very short compared to the fluorescence lifetime, the probability of two (or more) photon emission per pulse is always zero. At the maximum laser power, 90% of the laser pulses lead to a single photon emission. The parameters of this source (repetition rate and single photon generation probability) were limited only by the laser system used; nevertheless, this performance already surpasses that of previous work. The high performance combined with the vanishing two photon emission probability and the simplicity of this source suggests that it can be considered for a variety of quantum optical experiments and for secure quantum cryptography [11]. Semiconductor quantum dots (QD) are other potential systems for single photon generation [12]. QDs are often referred to as “artificial atoms” because many of their optical properties, such as ultranarrow transitions, arise from discrete, atomic-like energy levels [13, 14]. To reinforce this analogy a study of the photon antibunching in this system was needed. QDs differ from atoms and molecules since the creation of multiple excitons is possible in QDs. Consequently, if two electron-hole pairs can radiatively recombine, the simultaneous emission of two photons from a QD is then possible. Thus different regimes of photon statistics were expected because the probability of multiple exciton creation depends on excitation intensity. The fluorescence intensity correlation function of a single CdSe quantum dot has been investigated [15] using a coincidence setup (start-stop experiment). A strong photon antibunching has been observed over a large range of intensities (0.1-100 kW/cm2). The lack of coincidence at zero time delay indicates a highly efficient Auger ionization process [16], which suppresses multi-photon emission in these colloidal quantum dots. This result shows that room temperature single photon sources can be achieved with colloidal nanoparticles.

Photon statistics in single-molecule fluorescence at room temperature

The fluorescence of single terrylene molecules in a crystalline host is investigated at room temperature by scanning confocal optical microscopy. Photon arrival times are analyzed in terms of inter-photon time distribution, second-order correlation function, and Mandel's Q-function. Nonclassical photon statistics is observed and a reverse intersystem crossing is detected that accelerates linearly with the applied laser power. Rate equations for the time evolution of the molecular level populations are shown to be appropriate for the analysis of the observations.

Photon antibunching in the fluorescence of a single dye molecule embedded in a thin polymer film

We used scanning confocal microscopy to study the fluorescence from a single terrylene molecule embedded in a thin polymer film of polymethyl methacrylate, at room temperature, with a high signal-to-background ratio. The photon-pair correlation function g((2))(tau) exhibits perfect photon antibunching at tau = 0 and a limit of 1.3, compatible with bunching associated with the molecular triplet state. Application of this molecular system to a triggered single-photon source based on single-molecule fluorescence is investigated.

Electronic energy relaxation and transition frequency jumps of single molecules at 30 mK.

Transition frequency jumps for single terrylene molecules in a polyethylene matrix caused by resonant laser irradiation are investigated at 30 mK. These jumps are not accompanied by substantial sample heating. A model for the effect is proposed, based on the interaction of tunneling two-level systems (TLSs) surrounding the single molecule with high-energy nonthermal phonons emitted by the molecule during electronic energy relaxation. The radius of the effective interaction volume is estimated to be r(m) approximately 12.5 nm, and the interaction cross section for nonequilibrium phonon-TLS scattering is estimated as approximately 10(-22) cm2.

Luminescence lifetimes of single molecules in disordered media

Linewidth measurements for single terrylene molecules in polyethylene at a temperature of 30 mK indicate that there is a distribution of lifetimes for the terrylene molecules with a relative standard deviation of ∼20%. An analysis of the linewidth–line area correlation shows that the variations arise from approximately equal radiative and nonradiative contributions. A simple model suggests that the distribution of radiative lifetimes in disordered media is a general effect caused by the same interactions responsible for inhomogeneous broadening. In addition to the transition frequency, the luminescence lifetime of a probe molecule can be used to study the nano-environment of the probe.

Spectral diffusion of individual pentacene, terrylene, and dibenzanthanthrene molecules in n-tetradecane

The spectral stability of three guest molecules embedded in a quickly frozen n-tetradecane matrix has been investigated on a single-molecular level at liquid-helium temperature. In total, about 2500 spectral trajectories of 476 terrylene molecules, 328 dibenzanthanthrene molecules, and 252 pentacene molecules were recorded. Both line broadening and spectral jumps are analyzed and the latter are found to be mainly light induced. The spectral changes in essence reflect the dynamics of the host matrix and the differences between the guest molecules may be reduced to differences in their pumping cycles.

Fluorescence excitation spectroscopy of vibronic transitions in single molecules

Fluorescence excitation spectroscopy is used to study vibronic transitions ( S 1← S 0 n←0) in terrylene molecules doped in a naphthalene crystal. Evidence for the excitation of individual dye molecules is reported. The average linewidth (FWHM) for transitions to the first and second vibronic levels is, in both cases, 30(4) GHz. Linewidths and vibrational frequencies are not the same for all molecules. A method is presented for establishing a one-to-one correspondence between individual lines in the vibronic and the purely electronic band.

Single Photon Light Sources

In the past, generation of single photons for subsequent use in physical or physicochemical experiments was difficult. In the last few years the situation has changed dramatically. Now, such sources based on a number of different techniques are available. Two of them produce antibunching photons by controlled recombination of electrons and holes in semiconductor heterojunctions. A second type of source is based on indium-arsenide quantum dots in high Q factor microcavities. Two further sources use single dibenz-anthanthrene or single terrylene molecules. While all but one sources have to be operated at temperatures close to zero Kelvin, the terrylene source works at room temperature and provides up to 10 billion photons before photodecay, i.e. it can be operated for hours. When the latter is operated at zero Kelvin, it is even more stable and can be used in a microscope to generate complete images. The present review compares and summarizes properties of these single optical photon sources and, in addition, tries to present an argument, why single photons cannot be particle like objects but must have a spatial extension of at least several tens of nanometers.