Detecting Magnetic Resonance through Quantum Jumps of Single Molecules

Abstract

The fluorescence of a single molecule exhibits light and dark periods which reveal the jumping back and forth between the singlet and triplet manifold. In this paper we demonstrate how microwaveinduced changes to the distribution of the triplet-residence times can be used to detect magneticresonance transitions between triplet sublevels and to determine their kinetics. By synchronizing resonant microwave pulses with the quantum jumps of a terrylene molecule it proved possible to record transient-nutation signals with a high contrast. [S0031-9007(98)05942-0] PACS numbers: 39.30. + w, 33.20.Bx, 76.70.Hb In recent years, optical spectroscopy of single molecules has allowed a wide range of experiments on an increasing number of molecular systems [1]. Still, magnetic-resonance experiments involving a paramagnetic state were as yet demonstrated only for single pentacene molecules [2,3] and individual N-V centers in diamond [4]. Since the signal detected in single-molecule experiments usually consists of the fluorescence emitted by the molecule upon laser excitation, systems apt for single-molecule spectroscopy should have a high fluorescence rate. The fluorescence concerns a transition between the singlet ground state ͑S 0 ͒ and the lowest excited singlet state ͑S 1 ͒. The lowest triplet state T 1 (where two unpaired electrons combine to form a spin S 1) lies below S 1 and becomes occupied, with a low degree of probability, from the S 1 state through a spin-forbidden process called intersystem crossing (ISC). The T 1 state is relatively long lived. While present in T 1 , the molecule does not participate in excitation/emission cycles and thus ceases to fluoresce. For this reason molecules with a low ISC, which visit the triplet state infrequently, are favored in single-molecule spectroscopy. However, this very property makes it hard to perform optically detected magnetic-resonance (ODMR) spectroscopy on the triplet state of a single molecule. In such an ODMR experiment the resonant absorption of microwaves connecting two triplet sublevels is detected through a change of the fluorescence rate. The fluorescence rate may change when two triplet sublevels with different lifetimes are coupled, causing a change in the average triplet lifetime.Instead of detecting the average fluorescence, our method involves the detection and analysis of the length of the dark periods that reflect the presence of the molecule in the triplet state. Such an experiment was performed on terrylene embedded at low concentration in a p-terphenyl host crystal (Fig. 1). Terrylene is known to have a quantum yield for ISC from S 1 to T 1 as low as 10 25 [5] and as yet it proved impossible to observe the magnetic-resonance transitions in its T 1 state. We detected single terrylene molecules by cooling a crystal to 1.8 K and spectrally selecting molecules using a single-mode cw dye laser. Molecule 1 was selected at 17286.08 cm 21 , in the red wing of the origin of the X 2 site [5]. Owing to the short fluorescence lifetime of terrylene (3.8 ns [6]) and...

A Stochastic Theory of Single Molecule Spectroscopy

“…absorbed photons (20) provided that (20) is a necessary condition for the present theory to hold, and it means that the large number of absorbed photons is approximately equal to the large number of emitted photons (i. e. we have neglected any non-radiative decay channels). When there are non-radiative decay channels involved, one may modify Eq.…”

“…When there are non-radiative decay channels involved, one may modify Eq. (20) approximately by taking into account the fluorescence quantum yield, φ, the ratio of the number of emitted photons to the number of absorbed photons,…”

“…Time dependent fluctuation phenomena in SMS occur in many ways, such as spectral diffusion [6,7,8,9,12,17] and fluorescence intermittency [18,19,20,21,22,23]. The physical mechanisms causing these fluctuation phenomena vary depending on the dynamical processes a SM is undergoing, including: triplet state dynamics [18,19,20], energy transfer processes [24,25,26], exciton transfer processes [17], chemical reactions [27,28,29,30,31,32], conformational changes [33,34,35,36], rotational dynamics [37,38], and diffusion processes [39]. Thus SMS provides a unique microscopic tool to investigate the dynamical processes that a SM and its environment undergo during the measurement time.…”

Purely Electronic and Vibronic Fluorescence Excitation of Single Terrylene Molecules in a Naphthalene Crystal

“…Such studies were already performed for single Tr molecules in a p-terphenyl crystal, and the relation k 23 k 31 was found [4,14]. The saturation intensity I s is inversely proportional to the absorption cross--section [1] of considered transition, but it is also dependent on the relative widths of the absorption and excitation lines.…”

Single-molecule fluorescence autocorrelation experiments on pentacene: The dependence of intersystem crossing on isotopic composition

“…This information can be exploited to determine the intersystem crossing rates between the singlet and triplet manifold either by recording the intensity autocorrelation of a molecule's fluorescence 5 or by directly timing the singlet and triplet residence of a molecule through the presence or absence of fluorescence. 6,7 In previous work we have explored the effects of isotopic composition of pentacene through optical excitation spectroscopy and magnetic resonance. 8,9 Here we extend this work and refine the involved techniques so as to examine the dependence of the intersystem crossing ͑ISC͒ of pentacene on its isotopic composition.…”