Single Pentacene Molecules Research Articles

Theory of non-Markovian relaxation of single triplet electron spins using time- and frequency-domain magnetic resonance spectroscopy measured via optical fluorescence: Application to single pentacene molecules in crystallinep-terphenyl

Fluorescence-detected magnetic resonance (FDMR) allows one to monitor magnetic resonance phenomena via fluorescence. Experimental FDMR data obtained using single triplet-state chromophore guest molecules in a low-temperature organic host matrix are analyzed using a stochastic approach to describe triplet electron spin dephasing resulting from frequency fluctuations ${U}^{t}$ induced by host-matrix proton spin dynamics. Modeling the fluctuations ${U}^{t}$ by a sum of N independent random telegraph processes with the same jump rate $\ensuremath{\nu}$ but different variances ${\ensuremath{\sigma}}_{k}$ we construct an exact set of equations for the density matrix of a five-level molecule averaged over fluctuation histories ${U}^{t}.$ These equations provide a basis to study non-Markovian effects of microwave- (MW-) field-dependent dephasing in the FDMR response of a molecule undergoing slow fluctuations ${U}^{t} ({\ensuremath{\sigma}}^{2}/{\ensuremath{\nu}}^{2}>~1, {\ensuremath{\sigma}}^{2}=\ensuremath{\sum}{\ensuremath{\sigma}}_{k}^{2})$ to a MW field that is resonant with a transition between triplet spin substates. Both frequency- and time-domain FDMR phenomena such as (i) power-broadened FDMR line shapes, (ii) FDMR Hahn echo signals, and (iii) FDMR free induction decay are studied. Analytical expressions for the FDMR response are obtained in the case $\ensuremath{\nu}\ensuremath{\gg}{k}_{j}^{i}$ where ${k}_{j}^{i}$ is an intersystem crossing rate. Experimental data on power-broadened line shapes for a pentacene+p-terphenyl pair which demonstrate a pronounced effect of MW-field-suppressed dephasing are explained in the context of the theory.

Single molecule spectroscopy in He gas using a highly efficient mirror arrangement

We present a new experimental setup for single molecule spectroscopy using a high efficient mirror arrangement with a geometrical efficiency of 0.77. Signal-to-noise (S/N) ratios up to 90 for single pentacene molecules in a p-terphenyl crystal demonstrate that increasing the detection efficiency improves the S/N ratio if the detection geometry ensures a strong stray light suppression. This is realized by the presented mirror arrangement. The experiments were done in a He contact gas cryostat, which is able to work below the lambda point. Temperatures down to 1.7 K at the sample were obtained.

Stochastic theory of optically detected single-spin coherent phenomena: Evidence for non-Markovian dephasing of pentacene inp−terphenyl

The optically detected coherent response of a single chromophore molecule in an organic host matrix to a microwave radiation resonant to a transition between triplet spin substates is analyzed using a model in which N independent random telegraph processes produce fluctuations of the spin transition frequency. Measurements of ~i! Hahn echoes, ~ii! power-broadened line shapes, ~iii! transient nutation, and ~iv! second-order correlation functions for single pentacene molecules in a p-terphenyl crystal are explained within the context of the model, assuming the fluctuations to be slow. The failure of the standard Bloch equations for this system is demonstrated and the effects of microwave-suppressed dephasing are discussed. @S0163-1829~97!03625-4# Optically detected magnetic resonance ~ODMR! allows one to monitor the changes an individual triplet electron spin of a single chromophore molecule embedded in an organic matrix undergoes as a result of its interaction with a microwave ~MW! field. 1,2 This technique has been applied successfully to study the line shapes of transitions between different triplet substates 1‐4 and triplet population and depopulation kinetics 4 for single pentacene ~Pc! molecules in a p-terphenyl ~PT! crystal at 1.4 K. Additionally, quantum jump phenomena, resulting from shelving of the fluorescing single molecule in a metastable triplet state, have been observed and the second-order fluorescence intensity correlation function g (2) (t) has been measured for this guest-host

Stochastic theory of the optically detected single-spin coherent phenomena Application to the pentacene molecule in p-terphenyl crystal

The optically detected coherent transient response of a single chromophore molecule in an organic matrix is analyzed using a model in which N-independent random telegraph processes produce fluctuations of the resonant spin transition frequency. A wide range of experimental data obtained for single pentacene molecules in p-terphenyl crystal can be explained within the context of the model, assuming the fluctuations to be slow.

Magnetic resonance on single nuclei

The magnetic resonance signal of individual hydrogen nuclei has been detected. The experiments have been performed on a single pentacene molecule with the aid of optically detected electron nuclear double resonance. Two nuclear magnetic resonance lines of 30 kHz width have been observed as a 3% change in the fluorescence intensity of a single molecule. The results can be reproduced by a spin Hamilton operator describing the interaction of a single electron spin with two hydrogen nuclei.

Single molecule polarization spectroscopy: pentacene in p-terphenyl

Polarized light spectroscopy has been performed on single pentacene molecules in p-terphenyl single crystals. Thin crystal flakes of p-terphenyl doped with a very low concentration of pentacene have been grown by sublimation. The orientation of the transition dipole moment of the pentacene molecules in the a′b′-plane of the crystal was determined by taking the birefringence of the crystalline matrix into account. An assignment of the pentacene sites O1, O2, O3 and O4 to the crystallographic position M1, M2, M3 and M4 of the p-terphenyl triclinic unit cell is proposed.

Optical Spectroscopy of Single Pentacene Molecules in a Naphthalene Crystal

Abstract Dilute mixed molecular crystals of pentacene in naphthalene were prepared by cosublimation of the two compounds. Single pentacene molecules could be detected in the thin platelets by fluorescence excitation spectroscopy. By analyzing the exponential decay of the fluorescence intensity autocorrelation function for a single molecule in the microsecond time regime we could determine the population (k23) and depopulation (k31) rate of the metastable triplet state. The increase of the population rate k23 by a factor of ∼ 4 when compared to pentacene in p-terphenyl is responsible for the weaker fluorescence signals of single pentacene molecules in naphthalene crystals.

Variation of triplet state parameters of single pentacence molecules in a p-terphenyl single crystal

Zero field triplet state fine structure parameters of single pentacene molecules in p-terphenyl differ in magnitude by some tenth of a percent among each other. For most of the cases a relationship to the O 1 and O 2 sites of pentacene holds. Intersystem crossing and triplet decay rates vary by up to one order of magnitude for individual molecules. No relationship among these parameters or optical transition frequencies and fine structure have been found.

Fluorescence spectra of single pentacene molecules in p-terphenyl at 1.7 K

A new optical scheme for single molecule spectroscopy, based on a confocal design and a high-quality parabolic mirror enables us to efficiently image fluorescence light from single molecules onto the slit of a spectrograph. Owing to the enhanced sensitivity, the emission spectra of single pentacene molecules in a p-terphenyl crystal can be recorded with unprecedented detail. The main spectral lines are remarkably constant from molecule to molecule. Comparison of single-molecule fluorescence spectra to those of O 1 and O 2 sites, which differ slightly, allows unambiguous identification of the molecular site in most cases. Some molecules present original features which do not appear in O 1 and O 2 spectra. We think they arise from defect-induced redistribution of intensity, although we cannot rule out isotopic substitutions completely.

Fluorescence detection of single molecule magnetic resonance for pentacene in p-terphenyl. The hyperfine interaction of a single triplet spin with a single 13C nuclear spin

We report the observation of the broadening of a magnetic-resonance transition between triplet sublevels of a single pentacene molecule owing to the interaction of the triplet electron spin with a single 13C nuclear spin. Analysis of this broadening allows the assignment of particular features in the fluorescence—excitation spectrum to pentacene molecules containing 13C isotopes in specific positions.

Optical spectroscopy of single pentacene molecules in an ordered and a disordered p-terphenyl crystal

We investigated pentacene-doped p-terphenyl crystals with different degrees of disorder as evidenced by the inhomogeneous broadening of the O 1-site distribution. Single pentacene molecules were detected in the wings of O 1 by fluorescence excitation spectroscopy. The ISC rates for a number of single molecules were determined by measuring the intensity correlation function of the fluorescence photons. We found a dramatic dependence of the distribution of ISC rates on the crystal disorder. The fully saturated fluorescence emission rates as calculated using the SC rates were found to be consistent with the experimentally measured photocount rates.

Kinetics of optically detected magnetic resonance of single molecules

Various time-resolved experiments on optically detected magnetic resonance (ODMR) of single pentacene molecules in p-terphenyl at 1.8 K are described and discussed with the help of a model based on the optical-microwave Bloch equations. Intersystem crossing rates for this system are deduced from the ODMR spectra, from the fluorescence correlation function, which varies nonmonotonically with the microwave power, and from the analysis of the fluorescence recovery transients. We also discuss the enhancement of the ODMR effect on using fluorescence photons from single molecules as a time base to trigger the microwaves in fluorescence recovery experiments.

Single molecule fluorescence: From excitation spectra to time correlation

Fluorescence excitation spectra of small and dilute samples of solid solutions at helium temperatures show the spectrally resolved lines of single solute molecules. The signal-to-noise ratio is sufficient for several spectroscopic and time-correlated experiments, which are briefly reviewed here. In particular, paramagnetic resonance within the triplet manifold was demonstrated recently for single pentacene molecules in a p-terphenyl crystal.

A model for the spectral diffusion of single molecule optical transitions in solids

We propose a microscopic theoretical model to explain the recently observed spectral diffusion of the transition frequency of single pentacene molecules inp-terphenyl crystal. Our analysis yields detailed microscopic information about the nature and spatial distribution of the localized excitations responsible for the observed spectral diffusion.

Single molecule spectroscopy: Stark-, pressure-, polarization-effects and fluorescence lifetime measurements

Properties of single molecules at low temperatures can be studied by fluorescence excitation spectroscopy. Stark effect, pressure-induced spectral shifts, polarization effects, and time-resolved measurements have been performed on single pentacene molecules embedded in a p-terphenyl crystal at low temperature.

Line width study in fluorescence excitation spectra of single pentacene molecules introduced as impurities in p-terphenyl crystal

We report results on line widths in the fluorescenceexcitation spectra of single pentacene molecules inp-terphenyl crystal flakes for the 01 and 02 sites at 1.85 K. A setup for the measurement of fluorescence excitation spectra of single impurity molecules with an overall quantum efficiency of about 0.1% has been designed. Spectra obtained with a signal-to-noise ratio up to 1O:l were recorded at different laser excitation intensities in the range from 1 to 5 mW/cm2. The line widths measured for different pentacene impurity molecules and extrapolated to zero probing intensity show remarkable variation in the range from 5 to 15 MHz. A reason why some lines are narrower than the lifetime-limited one (8 MHz) is discussed.

Spectral diffusion of single molecule fluorescence: A probe of low-frequency localized excitations in disordered crystals.

Spectral diffusion of the transition frequency of single pentacene molecules in p-terphenyl crystal was recently observed experimentally. In this Letter we propose n microscopic theoretical model to explain this phenomenon. Analysis of the experimentnl data with our model yields detailed microscopic information about the nature and spatial distribution of the localized excitations responsible for the observed spectral diffusion

Optically detected spin coherence of single molecules.

Optically detected electron paramagnetic resonance of single molecules of pentacene in a p-terphenyl crystal at 1.8 K is presented. Transient nutation of a single electronic spin is demonstrated, showing a coherence damping within several microseconds. The fluorescence photons of a single molecule can be used as an internal timebase to trigger the application of microwave pulses. Because of this it is possible to enhance or switch off the optically detected magnetic resonance effect, depending on the delay between the triggering photon and the microwave pulse.

Single molecule spectroscopy: fluorescence excitation spectra with polarized light

The orientations of the transition dipole moments of single pentacene molecules located in the O 1 and O 2 site of p-terphenyl were investigated. Two distinct orientations were found for most of the molecules. The results are discussed with respect to the crystallographic structure of the p-terphenyl host crystal.

Single molecule spectroscopy: pressure effect on pentacene in p-terphenyl

The hydrostatic pressure-induced shift of the zero-phonon line of single pentacene molecules in a p-terphenyl host crystal was studied at cryogenic temperatures. Five molecules in the inhomogeneously broadened S 1 ← S 0 absorption band of the O 1 site (592.32 nm) were selected. In the low-pressure regime (Δ p ⩽ 600 hPa), all molecules showed a linear and reversible frequency shift to the red, varied from −0.74 to −1.0 MHz/hPa.