High-resolution spectroscopy of organic molecules in solids: from

J. Phys. Chem. 1993,97, 10256-10268

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FEATURE ARTICLE High-Resolution Spectroscopy of Organic Molecules in Solids: From Fluorescence Line Narrowing and Hole Burning to Single Molecule Spectroscopy M. Orrit’ and J. Bernard Centre de Physique Molbculaire Optique et Hertzienne, C.N.R.S. et Universitb Bordeaux I, 33405 Talence Cedex, France

R. I. Personov Institute of Spectroscopy of the Academy of Sciences of Russia, Troitzk, 142092 Moscow Region, Russia Received: May 20, 1993: I n Final Form: July 22, 1993’ We give a short overview of the selective spectroscopy of organic molecules in solid solutions, starting from Shpol’skii matrices up to single molecule spectroscopy. We discuss the general principles of selective spectroscopy and different applications of this technique to molecular and solid-state studies. W e examine in more detail two new fields to which we have contributed: persistent spectral hole burning in Langmuir-Blodgett (LB) films and the study of individual molecules. We show how persistent spectral hole burning provides information about structure and dynamics of LB films and how energy transfer can be studied in concentrated films. We probed the dynamics of the LB matrix as a function of the depth of the dye in a multilayer. We show that the surface monolayer presents specific dynamics, which we attribute to the long hydrophobicchains. The shift and broadening of a spectral hole under an applied electric field allows us to determine the orientation and direction of the chromophore axes. We then present the new field of single molecule spectroscopy, including the latest results. So far, the observations were made in a molecular crystal and in a polymer. We first consider the general appearance of fluorescence excitation lines and the sudden jumps of their resonance frequencies. The external electric field effects are then discussed. The correlation properties of the light emitted by single molecules give new insight about intramolecular dynamics and spectral diffusion, which would be impossible to obtain in experiments with ensembles of molecules. We demonstrate how single molecule spectroscopy gives truly local information, eliminates averages and populations, and gives access to distributions of molecular parameters in solids.

1. Introduction 1.1. Brief HistoricalOverView. As is well-known, theelectronic spectra of complex organic compounds in solutions and solids are characterized by an ill-resolved electron-vibrational (vibronic) structure. Absorption and emission spectra usually contain several broad bands whose widths are hundreds or thousands of wavenumbers. Their smoothnessmakes these spectra of low value for analysis, characterization, science, and applications. Therefore, spectroscopists started long ago to search for means and conditions to obtain more informative fine-structured spectra of complex molecules. The first examples of electronic spectra of organic compounds with well-resolved vibronic structure have been obtained about 40 years ago for some aromatic hydrocarbons (benzene, naphthalene, and some derivatives) in crystals at low temperature. They were described in the studies of Obreimov and Prikhot’ko (see review I), Pesteil? and McClure and colleagues.’ In 1952, Shpol’skii et al.4dissolved some aromatic hydrocarbons in n-paraffins (n-hexane, n-heptane, etc.). At low temperatures, they obtained fluorescence and absorption spectra consisting of several comparatively narrow bands or quasi-lines with a width of 2-20 cm-l instead of broad bands. Investigations about the nature of quasi-line spectra led to the conclusion that the lines correspond to optical zero phonon transitions (an optical analog of the resonance y-line in the MGssbauer effect) and that they possess all the theoretically expected features of such transitions (see e.g. refs 5-9). By now, several hundreds of compounds are ‘Abstract published in Advance ACS Abstracts. September 1, 1993.

0022-365419312097-10256$04.00/0

known to give quasi-linespectra in n-paraffin matrices. However, powerful as it is, the Shpol’skii method does not succeed in all cases and is not the only means to obtain fine-structured spectra. Many molecules give broad-band spectra even in n-paraffin matrices at low temperature. Taking into account thegreat variety of organic compounds and the wide choice of organic solvents, one can say that the majority of solutions of complex molecules possess broad-band spectra even at helium temperatures. In 1972,Personov et al. found that the low-temperaturespectra of many organicsolutionsare broadened mainly inhomogeneously and possess concealed fine structure. The spectra consist of the sum of many zero phonon lines.lOJ1In emission spectra, this line structure can be revealed by selective laser excitation, a method called fluorescence line narrowing. In that work, it was also shown that the zero phonon line’s intensity drops during laser irradiation; Le., some photochemical or photophysical “burning out” of the selectivelyexcited centers takes place. Later, in 1974, two groups showed that this process leads to the appearance of narrow persistent holes in absorption bandslzl) (persistent spectral hole burning, hereafter called simply hole burning). After a few years, fluorescence line narrowing and hole burning techniques started to be used for very fine and precise spectroscopic measurements of impurity centers in solids. Then appeared the first studies of the homogeneous line width1k16and of external field effects (Stark17-19 and Zeeman20J1 effects), the first application of these techniques to spectrochemical a n a l y s i ~ , 2 ~ . ~ ~ etc. Even at helium temperatures, molecular lines are not fixed in the inhomogeneous spectrum but drift over many time scales, a process called spectral diffusion. In refs 24-27, hole burning started to be applied to the study of spectral diffusion in organic 0 1993 American Chemical Society

Feature Article glasses. Many different applications of fluorescence line narrowing and hole burning have been discussed in reviews.2*-33We will consider some of them below. It is necessary to stress that fluorescence line narrowing and hole burning techniques (called together site selective or energy selectivespectroscopy)cannot remove all inhomogeneity. In fact, impurity molecules having the same optical transition energy may differ from each other in their host-guest interactions, their excited-state lifetime, their homogeneous line width, etc. Therefore, even energy selective spectroscopy at very high resolution provides average information about ensembles of molecules. An important advance in this direction was the recent discovery by two research groups of a new branch of high-resolution spectroscopy of the solid-state, single molecule spectroscopy. In 1989, Moerner and Kador published results of the first optical detection of single pentacene molecules in a p-terphenyl crystal via a sophisticated technique of doubly-modulated ab~orption.3~J~ In 1990, Orrit and Bernard36J7 succeeded in detecting single molecules in the same system by using fluorescence excitation. They showed that this much simpler and very sensitive method provided dramatic improvementof the signal-to-noiseratio. This technique has opened a novel spectroscopy of the individual molecule in solids, in which all kinds of inhomogeneity and averaging are suppressed. Before going to the latest results and their discussion, we shall briefly recall somegeneral principlesof the selective spectroscopy of solids. 1.2. Principles of Selective Spectroscopy. 1.2.1. Theoretical Bases. The main differences between the optical bands of a molecule in a solid matrix and those of the same molecule in the gas phase stem from the interaction with lattice vibrations, which we call phonons hereafter. The profile of vibronic bands is determinedby the electron-phonon coupling. At low temperature, every vibronic band consists of a narrow zero phonon line and a relatively broad phonon wing, lying at the long-wave side of the zero phonon line in emission and at its short-wave side in absorption. The zero phonon line corresponds to a transition of an impurity molecule without any change in the number of phonons in the matrix. Phonon wings are related to the phototransitions of the molecule in which matrix phonons are created or annihilated. A detailed theoretical analysis of the optical properties of an impurity in a solid may be found in refs 38 and 39 and references therein. Here, we recall only the few points which are necessary to understand the spectra. In the frame of the adiabatic and Franck-Condon approximations, the intensity Zof the optical band profile may bedescribed in the form

The zero phonon line has a Lorentzian shape with width I'(T). The ratio of zero phonon line and phonon wing intensities can be characterized by the Debye-Waller factor a(T), which depends on temperature according to

When temperature is increased, s ( T ) increases, so that Zzp~and CY(?") decrease rapidly. Therefore, zero phonon lines can be observed at low temperatures only. But spectra usually show no fine structure even at low temperature. The reason for this is the inhomogeneous broadening due to somewhat different local conditions around impurity centers, which cause a statistical distribution of their energy 1evels.N The microscopic sources of inhomogeneous broadening are electrostatic, polarization, and dispersion interactions, shortrange repulsion, and specific interactions like hydrogen bonds. Their effect on the optical band depends strongly on matrix structure. In ordered systems, random strains and stresses due

The Journal of Physical Chemistry, Vol. 97, No. 40, 1993 10257

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to defects and to impurities slightly shift the zero phonon lines of individual molecules. Therefore, inhomogeneous broadening is small, but significant, in crystals (Av 1-20 cm-l), in the quasi-line spectra of molecules in n-paraffin matrices, or in specially selected host-guest couples. Glassy or amorphous matrices are characterized by their lack of long-range order and the broad variety of local environments. The spread of zerophonon line frequenciesis therefore especially large in amorphous matrices (Av 102-103 cm-l). Since the inhomogeneous band is built from the superposition of many narrow homogeneous lines, specific experimental techniques are needed to eliminate the inhomogeneous broadening. 1.2.2. Experimental Methods. To eliminate the inhomogeneous broadening, we must selectively affect the sample so that it becomes possible to separate a homogeneous component of the spectrum. In emission this can be done by the selectiveexcitation of one type of center with an absorption zero phonon line at the laser frequency. In this case, narrow lines appear at all vibronic frequencies of these centers in the fluorescence spectrum (fluorescence line narrowing,). One example of such spectra is given in Figure la. Under selective excitation, resonant molecules may undergo photoinduced processes. These can be either photochemical processes, leading to large frequency shifts of the photoproducts, or photophysical processes, whose products absorb within the inhomogeneousband. In both cases, narrow persistent dips appear in the absorption spectrum at all vibronic frequencies of the transformed molecules. These dips are usually called spectral holes and the technique, persistent spectral hole burning. The lifetime of the holes is determined by the rate of the back-reaction (if it is reversible). At low temperatures and in the dark, persistent holes may last many hours or days. An example of a hole burning spectrum is given in Figure 1b. Photochemical hole burning mechanisms include, in particular, proton and electron phototransfer, photoionization, and photodissociation. For photostable molecules, only photophysical transformations involving the matrix take place at low temperature. This nonphotochemical hole burning can be observed in many polar glassy and polymer matrices. The usual model for such a process involvesthe coupling of the impurity with a so-called two-level system (TLS) in the glass, describing a structural rearrangement. The microscopic nature of the TLS's and the coupling mechanism are ill-known in most cases. When discussing the shape of the narrow structures observed in fluorescenceline narrowing or hole burning, the broad phonon wings should also be taken into account. They give rise to structures from "nonresonantly" excited centers (see Figure IC), which in fluorescence spectra lead to an increase of the longwave phonon wing (Figure ld, hatched area) and to a reduction of the observed Debye-Waller factor. In hole burning spectra, they cause an increase of the short-wave phonon wing of the hole and the appearance of a specific and intense long-wave wing near the narrow zero phonon line hole (Figure le, hatched area). In this article, we will consider only the narrow features of the optical bands caused by zero phonon lines. For many applications, it is important to analyze the shape of narrow holes. For low laser intensity and short burning time, the hole profile is described by the expression

-

-0=

~~(",,)2(m~b)2f(u+O)

e(vb-v0) dvo dQ

(3)

Here, AD is the change in the optical density of a sample in the region of the narrow hole. Ub is the burning frequency, m, np,and nb are unit vectors in the directionsof the transition dipole moment of the molecule and the polarization of the probing and burning light, respectively. t(t+vg) is the homogeneous profile of the zero phonon line, and Q is the set of Eulerian angles for molecules in the laboratory coordinate frame. If ~(v-vo) is Lorentzian, its width r Z p L is related to the hole width rh by I'h = 2I'zp~. This

Orrit et al.

10258 The Journal of Physical Chemistry, Vol. 97, No. 40, 1993 I

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ABSORPTION

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Figure 1. Examplcs of fluorescence line narrowing and persistent hole burning spectra (a, b) and diagrams explaining how zero phonon line and phonon wings structures arise (c, d, e). (a) Fluorescence spectra of protochlorophyll in ether under selective laser excitation (T= 4.2 K).23 The broken line spectra are recorded with ordinary broad-band excitation. (b) Hole burning spectrum (differencein the optical density before and after hole burning) of perylene in ethanol (T = 4.2 K). The broken line is the absorption spectrum before hole b~rning.”~ relationship is used for measurements of rmL and of its temperature dependence. At helium temperature, for purely electronictransitions, typical values of I’mL are around le2 cm-’. Many important details concerning fluorescenceline narrowing and hole burning spectroscopies and their applications can be found in review^.^^-^^ A few years ago, new experiments have demonstrated the possibility of the optical detection of single molecules in solids at helium temperature, either by a b ~ o r p t i o n 3or ~ *by~fluorescence ~ excitation.3627 In contrast to experiments at room temperature in liquids,41142 the solid state at low temperature allows fine spectroscopicstudies on single molecules, called single molecule spectroscopy. Single molecule spectroscopyopens up promising perspectives in trace detection, in the spectroscopy of localized neighborhoods, and in the optical addressing of local areas in solids. The details of single molecule spectroscopy together with some new results and applications will be discussed later in this paper (see section 3). 1.3. Some Applications of Selective Spectroscopy. Selective spectroscopy allows precise measurements to be performed within inhomogeneously broadened bands or lines. The spectral resolution can be improved in this way by a factor 103-105, This is now a rapidly growing field of molecular and solid-state physics. Many interesting and important results were obtained by selective spectroscopy of inorganic systems (see ref 31 and references therein), but here we restrict ourselves to organic materials. Let us point out here some of the many different applications of these methods. I .3.1. Spectroscopy of Vibronic Levels and Relaxations in Complex Organic Compounds. Fluorescenceline narrowing and hole burning techniquesallow precise determination of molecular vibronic levels in ground and excited electronic states and investigation of the different intramolecular relaxation processes. These methods have been applied to the study of a number of

aromatic hydrocarbons, heterocylic compounds, porphyrins, phthalocyanines, and some dyes. In recent years, selective methods have also been used for some new systems: ionic forms of molecules,43 conjugated polymers,M important biological systems such as the reaction centers of photosynthesis,’w8 and biopolymers.49.50 1.3.2. Spectroscopy of Disordered Solids. Many properties of amorphous solids (for example, specific heat, thermal conductivity, etc.) at low temperatures differ drastically from those of crystal^.^' These differencesare explained by the existenceof many local minima in the complex potential energy landscape of a glass. The dynamical properties of glasses can be described by modeling the transitions from one local minimum to another by a simpletwo-minima model (the so-called two-level system, TLS). These TLS’s result in the presence in amorphous media, beside phonons, of another special kind of low-energy excitations with a significant density of states at low enough temperature. The optical properties of impurities dissolved in glassy and crystalline matrices also differ strongly at low temperature. It was demonstrated lately that hole burning and fluorescenceline narrowing techniques (together with photon echo measurements) are very promising tools to study the dynamics of amorphous solids. Let us first consider the molecular frequencies as fixed in the inhomogeneous spectrum and discuss the loss of coherence (dephasing) due to fast fluctuations in the molecule’s neighborhood. The homogeneous line width r of the zero phonon line is

r = (2,q-I +(q‘)-l

(4) where TI is the population decay time of the excited state and Tz’is the pure dephasingtime determined by fast residualthermal motion. The temperature dependence of r arises from that of Ti. In crystalline matrices, dephasing is caused by electron-

Feature Article


phonon coupling. For Debye acoustic phonons, the dependence r on temperature T is r a Ik where n = 7 for T > 8,8being the Debye temperature (see for example refs 38 and 39 and references therein). If a local or pseudolocal mode in the crystal is responsible for the dephasingprocess (which happens in many cases), r(T)decreases exponentially when T 0. In amorphous matrices at liquid helium temperatures, the zero phonon line width is 1 or 2 orders of magnitude larger than in crystals and in most cases has a dependence r a P,with 1 In I2.2”32 The theoretical approaches to dephasing in glasses are based on the TLS model, which may also explain spectral diffusion as discussed below. Disordered solids, or glasses, are nonequilibrium systems and evolve with time. The relaxation processes of glasses produce so-called spectral diffusion. Because of the evolution of the surrounding matrix, the resonance frequency of the molecules drifts (diffuses) within the inhomogeneous spectrum. Therefore, results of a line width measurement will depend on the time scale of the experiment. Indeed, careful comparison of the experimental data obtained by hole burning to those obtained by photon echoes showed that “homogeneous” line widths are much smaller when measured by photon ech0es.2~ These data prove that spectral diffusion processes slower than dephasing take place in glasses in the time span between hole burning and hole recording. By now, optical experiments have confirmed the broad distribution of characteristic times for spectral diffusion in organic glasses, from pico~econds52.5~ to hours.25 Recently, the study of the longlived stimulated photon echo at 1.5 K has proved a very convenient method for the study of spectral diffusion processes between picoseconds and milli~econds.5~ But the detailed mechanism of spectral diffusion and the molecular nature of theTLS‘s in organic glasses are still ill-known, and much work should be done to understand them better. Single molecule spectroscopy seems very promising in this respect (see section 3.4.3). 1.3.3. External Field Effects. The investigation on the influence of external electric and magnetic fields (Stark and Zeeman effects) on complex molecules in solids (especially in glasses) is usually limited by the large width of theinhomogeneous spectrum. With the hole burning technique, owing to the very small hole width, the sensitivity of the measurement increases by several orders of magnitude. On applying an external field F (electric or magnetic), all zero phonon lines are shifted by

of

-

+

AV = -(Ap.F FAF) (5) where A p is the change in electric or magnetic dipole moment on electronic transition and A is the change in the polarizability or magnetic susceptibility tensor. The first term corresponds to a linear effect and the second one to a quadratic effect. From eqs 3 and 5, one can obtain the hole profile under an external field. It depends on the angle y between A p and m, on the orientation of the axes of tensor A, on the light polarization and on the geometry of the experiment. The theoretical analysis of the hole profile under a field has been performed for most important case~.18,5>5~For isotropic systems, it was shown that the hole profile broadens symmetricallyunder a linear Starkeffect without any shift, whereas the hole shifts and becomes asymmetric under a quadratic effect. By now many precise measurements of the Stark effect via hole burning have been performed for different organic molecules. Among the most important results is the fact that centrosymmetric molecules imbedded in polymers or glasses show a linear Stark effect (instead of a quadratic one!). This effect arises because the molecular symmetry is broken by the interaction with the disordered matri~.~~*60-a3 One can deduce the average value of the induced dipole moment difference A p from these results and estimate the average value of the matrix internal field. It was also shown that A p increases from the blue to the red edge of the inhomogeneous absorption band. Theoretical analyses lead to

the conclusion that this variation of A p and the solvent shift of the absorption lines are dominated by the dispersion interaction, which is larger than the electrostatic part by 2 orders of magnitude.64 Recently,the weak Zeeman effect on singlet-singlet transitions was investigatedusing hole burning. These studies required high pulsed magnetic fields (up to 50 T ) . 6 5 ~An ~ ~interesting result of this kind of experiment is the determination of an average value of the relatively small Jahn-Teller splitting which cannot be seen directly in a spectrum because of the large inhomogeneous broadening.66 Several papers were devoted to the investigationof the strainfield effect via hole burning. They show that pressures of the order of 1G-l MPa give significant changes of the hole position and cont0ur.5~-~7-@ In such experiments, a frequency-selected ensemble of molecules in a disordered solid (with well-defined solvent shift) can be studied under pressure. The pressure tuning of spectral holes gives the possibility to determine the compressibility, the vacuum absorption frequency, and solvent shift, etc., of complex molecules. I .3.4. Other Applications of Selective Spectroscopy. In this article, it is impossible to discuss all the applications of selective spectroscopy,which include such interesting items as, for example, phosphorescence hole burning,’O selective spectroscopy of adsorbates71 and of biological s y s t e m ~ , 4 ~ ~molecular . ~ * ~ ~ 3computing,74 and many others. Here, we are going to consider the two following topics: the hole burning spectroscopy of ultrathin molecular films and the selectivespectroscopy of single molecules. Both of them are based on a very sensitive spectroscopic technique and have led to interesting new results.

2. Hole Burning Spectroscopy of Dye-Doped Langmuir-Blodgett Films

2.1. Introduction. Since the pioneering works of Langmuir and B l ~ d g e t t ,where ~ ~ , ~ultrathin ~ films of fatty acid soaps have been prepared and transferred to solid substrates, these systems (LB films) drew the attention of many researchers. LB films are very interesting for both science and applications (in particular for molecular electronics, see for example refs 77-81 and references therein). Assembly of LB films is one of the few thin film techniques that really permit manipulation of materials at the molecular level. The advantage of these films is that they are a good basis to assemble molecules in a planned way, including the creation of superlattices and of quasi-bidimensionalsystems. LB monolayers afford a unique possibility to probe surfaces and interfaces, e.g., as chemical sensors. Optical methods are important tools to characterize LB films. They were used from the very beginning to measure the thickness and refractive indices of the film^.^^,^^ Later, spectroscopic methods were applied to investigate the orientation of dye molecules in the films and the influence of aggregation on absorption and emission spectra, to study energy and electron transfer, e t ~ . More ~ ~ ~recently, 1 the development of laser sources has opened the way to such new techniques as second harmonic, sum- and difference-frequency generation, four-wave mixing, short light pulses, etc., as applied to LB films.82 This section is about another and rather new type of optical experimentson LB films: hole burning spectroscopy of dye-doped LB films. We will show here that this method could become a powerful tool to investigate two-dimensional systems and interfaces. Bogner et al.83 reported the first observation of fluorescence line narrowing in spectra of LB films (Cd arachidate doped with perylene). The authors noted the decrease of the fluorescence intensity during laser irradiation. Later, we reported the first direct measurements of spectral holes in LB films doped with impurity molecules (resorufin, tetraazaporphin, its complex with

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10260 The Journal of Physical Chemistry, Vol. 97, No. 40, 1993

Mg, and a cyanine dye).84-86 The latest papers were devoted to the Stark effect in hole burning spectra of dye-doped LB film~8~~88 and energy transfer.89 These works (see also refs 90 and 91) have shown that hole burning spectroscopy applied to LB films provides important information about their structure and dynamics. Below, we will present and discuss some of these results. 2.2. Experiment. Most of the LB films studied here were Cd arachidate assemblies doped with low concentrations of ionic dyes (usually 1 dye for 500 matrix molecules). The reason for this choice was that pure Cd arachidate films are the best characterized LB films. Further, dye-doped Cd arachidate LB films have been studied extensively in the group of Kuhn and M b b i ~ s ~by ~ 9absorption, ~8 linear dichroism, energy transfer, etc., and most of the samples were prepared in their laboratory. The dye molecules were selected so that their absorption matched the spectral range of our laser (from 570 to 640 nm). The amphiphilic cyanine dye S20 used in refs 86-88 and 91 (see sections 2.3.1 and 2.4.1) was synthesized many years ago in Kuhn’s group. The studies on the amphiphilic porphyrin85+a9were made on samples prepared by Dr. Samoilenko in Moscow. The concentration of these samples was higher (1 dye for 100 matrix molecules), allowing intermolecular interaction to take place. All multilayer assemblies were deposited head-to-head and tail-to-tail (the so-called Y-deposition, where contacts between hydrophilic and hydrophobic parts are excluded). This procedure gives the most stable films for strongly amphiphilic molecules such as fatty acid soaps, so that rearrangements of the films are unlikely. The films were deposited in standard conditions on thin glass slides (surface pressure 25 mN/m, transfer speed a few cm/min). To improve the signal/noise ratio in the case of single monolayers, a stack of about 30 coated slides was tilted at Brewster incidence for the exciting light. LB films were characterized by absorption and fluorescence excitation before hole-burning measurements. In all cases, broad structures were observed, as expected for inhomogeneously broadened bands. For Stark effect measurements (see section 2.4.1) a multilayer was sandwiched between a transparent IT0 (indium-tin oxide) electrode and an opaque aluminum electrode deposited by vacuum evaporation. Electrical contacts were ensured by thin gold threads. The persistent spectral holes were burnt by a single-frequency dye laser (CR 699-21) and usually recorded in the fluorescence excitation spectrum for an optimal sensitivity. This was particularly important in the case of a single monolayer. Fluorescence photons were filtered by a red-pass colored glass and detected at a rate which did not exceed 3 X lo5 counts/s. In the hole burning method, the excitation spectrum recorded before irradiation is compared to that recorded after irradiation. Drifts and fluctuations of the exciting beam direction and intensity were limited by a passive spatial filter and by active power stabilization. Spectral holes can be power-broadened or fluencebroadened. The burning power used was low, of the order of a few pW/cm2 to a few mW/cm2. It depended on the burning mechanism active in the sample. For ionic dyes, for example, burning was very efficient, and we attributed it to a hydrogenbond rearrangement. The laser power used for recording was normally one hundredth of the burning power. To eliminate saturation broadening of the holes, a series of holes were recorded a t increasing irradiation durations (usually seconds to minutes). A plot of hole width as a function of hole depth was then extrapolated to zero hole depth.84,85s90 In the systems investigated, the hole depth never exceeded 20%, especially for ionic dyes. We attribute this to a low value of the DebyeWaller factor, Le., to a fairly strong coupling of the optical electron to matrix phonons. 2.3. The Hole Width and Its TemperatureDependence. 2.3.1. Low- Temperature Dynamics of LB Monolayers and Multilayers. The structure and low-temperature dynamics of amorphous systems are very different from those of crystals, because of the presence of TLS’s (see section 1.3). We started to investigate

-1 5

0

15

FREQUENCY SCAN (GHz) Figure 2. Comparison of the profiles of holes burnt at 615.5 nm in the excitationspectrum of porphin imbedded either in a monolayer (bottom

spectrum) or in a multilayer (20 layers, top spectrum) of Cd arachidate at 2 K. The concentration was 1 dye molecule per 100matrix molecules. The hole in the monolayer is about 20 times broader than in the multilayer. Similar observationswere made for different dyes, though the ratio between monolayer and multilayer was somewhat less. This indicates that the broadening mechanism is linked to the LB matrix. LB films by hole burning in order to compare their behavior to those of glasses and crystals. The results show that LB films present specific dynamical properties which have no equivalent in bulk systems. Let us consider this in detail. All dye-doped LB films which we investigated have broad (300500 cm-I) structureless absorption and fluorescence bands, even at helium temperature. Our experiments haveshown that narrow persistent holes can be burnt in the broad absorption band by laser irradiation. This demonstrates the inhomogeneous nature of the spectral broadening and the existence of narrow zero phonon lines in these systems. One of the striking results obtained is the large difference in hole width between a monolayer and a multilayer at helium temperature. This observation is common to all systems investigated so far (resorufin, cyanine dye, and porphin in Cd arachidate; porphyrins in poly(hepty1 cyanoacrylate)) and must be regarded as a property of the LB matrix. As an example, Figure 2 presents two holes burnt in the spectra of porphin in a multilayer and in a monolayer of Cd arachidate. As one can see, the ratio of the widths is about 20. We must stress that this difference cannot stem from a saturation effect. (Power or fluence saturation is a common source of broadening in hole burning experiments.) This has been checked in measurements of the hole width as a function of its depth. To determine the origin of the large difference between monolayer and multilayer, we investigated the hole width and its temperature dependence as a function of the position of the dye in a LB multilayer. This was done on specially prepared multilayer assemblies of 11 monolayers of Cd arachidate with different positions of the dyedoped layer. (The dye used was the amphiphilic cyanine S20; see molecular structure in Figure 5.) We number the samples according to the position of the dye-doped monolayer in the undoped multilayer assembly, following the deposition order; e.g. in (1) the S2O-doped layer lies against the hydrophilic glass slide, and in (11) the S2O-doped layer is the last layer lying at the surface of the sample. The resultsof the hole width measurements and its temperature dependence obtained for positions 1, 5, and 11 are shown in Figure 3. The temperature behavior of the hole width obtained for position 5 reproduces those of the multilayer,

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ine Journal of rhysrcal chemisrry, Vol. Y7, No.40, I Y Y j 1U261

10

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TEMPERATURE (K) Figure 3. Temperature dependence of the width of holes burnt at 591 nm in the fluorescenceexcitationband of thecyaninedye S20(see structure in Figure 5 ) included in a Cd arachidate LB film. The concentration is 1 dye molecule for 500 arachidate ions. The temperature dependences for samples 1, 5 , and 11 are markedly different, with the broadest holes at low temperature for sample 11.

for which interface effects are negligible. In this case, the hole is narrowest. When S20 is in layer 1 (in contact with the glass), the temperature dependence of the hole width is roughly linear and gives a small width at T = 2 K (close to the value obtained for ( 5 ) ) . This result is similar to those in disordered bulk matrices (glass or polymer) and indicates that the glass-monolayer interface is more disordered than the hydrophilic interface between identical layers in a multilayer. A similar conclusion was reached independently from infrared spectroscopy of LB films.92-93 Finally, the surface position 1 1 shows broad holes and slow dependence with a large residual width of about 2 GHz a t T = 2 K. Additional experiments showed that the hole width did not change between the time of the hole burning (a few minutes) and several hours, although the hole depth decreased. This means that the hole width is due to either dephasing (motion faster than 1 ns) or spectral diffusion on a time scale shorter than a few minutes. The main conclusion from the above results is the existence of an additional broadening of the homogeneous zero phonon line of molecules in the surface monolayer. This broadening can only arise from specific motion of the molecules which has no equivalent in the bulk of the assembly: we can speak of specific surface dynamics. To get insight into the nature of this motion, we investigated other positions. The results for positions 7 and 9 were identical within experimental error to those for ( 5 ) . The holes burned for position 10,however, showed a somewhat lower width than for (1 l), although the chromophore of S20 lies in both cases a t the same distance (27A) from the surface. This indicates that the hydrophobic chains of S20 are involved in the surface motion, because the direction of these chains is the only difference between (10)and (1 1). However, the motion cannot be restricted to the chains of the dye molecule; otherwise, the hole width should be the same for positions 9 and 10. We think that the surface motion involves the long saturated chains of the LB film. The motion cannot be harmonic (phonons), because the temperature behavior of the width should be much steeper (see discussion in section 1.3.2). Therefore, we assume that a faulty packing of the chains can lead to the appearance of two-dimensional TLS's with low-energy difference between the wells. Because of the partial interdigitation of chain ends in the bulk of the multilayer, finding isoenergetic positions for the chains is much more difficult than at the surface, and the corresponding motion is effectively blocked in the bulk.

Figure 4. Wavelength dependence of the hole width in the red wing of the (M absorption band of substituted tetraazaporphyrinin a LB film. For these measurements, samples with four different impurity concentrations were prepared: sample 1, c = lo-' M/M (stars); sample 2, c = 3X M/M (squares);sample 3, c = 1p2M/M (circles); sample 4, c = 3 X 1 k 2M/M (triangles). The inset shows examples of holes burnt with fixed laser power (20 mW/cm2) at X = 630 nm for samples 1-4. The width and burn times are as follows: (1) 0.7 GHz, 10 s; (2) 0.7 GHz, 10 s; (3) 1 GHz, 10 s; (4) 4 GHz, 100 s. The dashed line is the inhomogeneous absorption profile. (The figure was adapted from ref 90).

2.3.2. Energy Transfer. To perform high-resolution spectroscopy in ultrathin films doped with dye molecules, one needs relatively concentrated samples, especially in the case of weakly emitting molecules. Then, the average distance between chromophores can become comparable to the Forster radius for energy transfer. This transfer can interfere with other processes under investigation such as optical dephasing and spectral diffusion. On the other hand, the study of energy transfer in LB films using high-resolution techniques is interesting in itself. Energy transfer in complex molecular systems often considerably reduces the lifetime of the donor. As was demonstrated on some biological antenna systems and reaction centers,4548the transfer rate can be deduced from hole burning measurements. This shows the interest of investigating concentration effects in hole burning spectra of dye-doped LB films. A few years ago, we studied LB films with a fairly high concentration of an amphiphilic porphyrin (heptyl cyanoacrylate doped with a derivative of the free-base tetraazaporphyrin at mol/mol). The hole width depended on the concentration burning wavelength within the inhomogeneous excitation spectrum. Weattributed theincreaseofthe holewidth withdecreasing wavelength to energy transfer toward lower-energy centers.85 A similar dependence of a homogeneous zero phonon line width within an inhomogeneous profile was observed previously in some inorganic systems via fluorescence line narrowing and photon echoes94995 and in some conventional bulk systems via hole b~rning.~~.~~ Recently, more detailed investigations were performed89 with the same dye-doped LB material as in ref 85. The influence of impurity concentration on excitation and emission spectra, fluorescence decay rates, and hole burning parameters was studied. In these experiments, a shift of the fluorescence band and nonexponential decay in highly concentrated samples provide evidence for energy transfer. The hole width of heavily doped samples depends strongly on wavelength, increasing for higher excitation energy. Figure 4 presents data illustrating this point. For example, at a concentration of 3 X mol/mol, the hole width increases by a factor 6 when the wavelength changes from 635 to 628 nm. This hole broadening is accompanied by a strong reduction in hole burning efficiency. Due to this fact, the hole becomes very shallow for highly concentrated samples (see inset in Figure 4), and it is very difficult to detect the holes in the short-wave part of the absorption band. The analysis of the experimental data shows that there is no

Orrit et al.


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Figure 5. Diagram of the cell used for Stark effect measurements and structure of the cyanine dye S20. The alternate LB assembly is of the type (AB)m B being a pure Cd arachidate layer and A a mixture of Cd arachidate and methyl arachidate doped with the cyanine S20.

simple correlation between the observed hole broadening and the decrease in fluorescence lifetime due to energy transfer. The fluorescence decay measurements indicate that energy transfer reduces the T1value from 5 to 1 ns, correspondingto a broadening less than 0.15 GHz (see eq 4). However, as the minimum hole width measured in these experiments is 0.5 GHz, T I processes cannot be responsible for the observed broadening. A hole broadening could appear in highly doped LB films if hole burning takes place in an acceptor chromophore, after quasiresonant energy transfer took place. More probably, a decrease of the dephasing time Tz can explain the experimental results. Coulomb interactions with resonant close neighbors could build a delocalized exciton state which would be much more sensitive to coupling to phonons than a localized impurity state. Similar effects were reported for systems containing rare earth ions as impurities and were explained by electronic level shifts due to excitation of neighbor centers.98 2.4. External Field Effects. 2.4.2. The Stark Effect. We mentioned above (see section 1.3.3) that hole burning is used with success for Stark effect measurements in conventional bulk systems. It is promising to use the Stark effect on spectral holes to study LB films. In principle, this can give important information concerning the value and direction of the dipole moment differenceof impurity molecules, the internal field within molecular layers, the orientation of chromophores in the film, etc. We started such experiments recently on a multilayer LB film.87J@We consider here some results of Stark effect measurements obtained with the noncentrosymmetric cyanine dye S20,imbedded at low concentration in a Cd arachidate multilayer assembly. The structure of S20 and the diagram of the assembly are shown in Figure 5. As will be shown below, the Stark effect on the holes can be used as a very sensitive tool to get information about the orientation of the chromophores in the LB film. To do this, we prepared an alternate multilayer in which all dye moleculeshave thesameorientation with respect to the electrodes. We then expect both a broadening and a shift of the hole under an external electric field. Figure 6 shows the change of the profile of a hole burned in the excitation spectrum of S20 when the field is applied. In addition to the considerable broadening of the hole, a small shift is clearly visible. The data show that the Stark effect is linear, as expected for a noncentrosymmetric dye. From the large broadening and relatively small shift, we conclude that the angular distribution of the dipole moment difference Ap is rather broad and nearly symmetricwith respect to the layer plane. The long chromophore axis carrying the transition dipole moment is known to lie parallel to the LB film surface.99 Since the hole shape directly reflects the distribution of the projection of the short chromophore axis (carrying AN) onto the surface normal, the orientation of the chromophore is determined entirely. To analyze the results quantitatively, we need the magnitude Ap of the dipole moment change. We determined this quantity

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Figure6. Stark effect on a hole burnt at 592nm in the excitation spectrum of S20 in the alternate Cd arachidate LB film of Figure 5 (T = 2 K). The solid lines are Lorentzian fits to the hole profiles, drawn to evaluate the width and shift of the holes. The slanting straight line joins the hole centers and should be compared to the vertical axis to estimate the hole shift under electric field (adapted from ref 89).

by measuring the Stark effect on a sample of bulk Cd arachidate (therefore isotropically disordered). We deduced from these measurements A p = 0.35 D and the average absolute angle between Ap and the surface, about 18O, whereas the average bias angle (responsible for the shift) is much smaller, about 2'. The cyaninechromophoreisthus lying nearly flat on the layer surface, with larger disorder for its short axis than for the long one. This result agrees with a simple picture where ionic chromophores tend to lie flat on the surface but can be tilted by Cd ions or residual water molecules. Thus, the method discussed is sensitive enough to determine the orientation of molecules in LB films, even when the dipole moment is nearly perpendicular to the applied field. 2.4.2. Effect of Surface Perturbations. The molecular engineering capability of the LB technique makes LB films good elements for the construction of sensors. For instance, LB films were used in many gas sensing systems (seelooand references therein). In the case of a monolayer doped with a hydrophilic dye, the chromophores lie some 27 A away from the surface and are therefore sensitive even to small changes occurring across the interface. Hole burning spectroscopy affords a very clear demonstration of this sensitivity,as the following experiment (see Figure 7) will show. First, a hole was burnt and recorded for a surface monolayer of Cd arachidate doped with S20 in a lowpressure helium atmosphere(spectrum 1 in Figure 7). Then, the sample was immersed in superfluid helium and the hole was recorded again (spectrum 2). Finally, the hole was recorded once more after the liquid helium was evacuated (spectrum 3). As appears in Figure 7, the hole of spectrum 2 is red-shifted and broadened with respect to the other holes. At least two effects can account for the observed spectral changes. The first is caused by van der Waals interaction between the dye molecules and helium atoms. This effect can be modeled simply by replacing liquid helium by a dielectric half-space, 27 A away from the molecules. The molecular transition dipole moment interacts with its image in the half-space. This is therefore a dynamic effect. Its sign and order of magnitude are in agreement with the observed red shift. The second effect arises from the field of the image of the static charge distribution around the dye molecule and is much more difficult to model. It could explain the broadening of the hole if the charge distribution is sufficiently disordered. We conclude that a spectral hole in a LB


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Figure 7. Shape of a spectral hole burnt at 591 nm in the excitation spectrumof S20included in the last (surface) monolayer of a Cd arachidate LB film (position 1 1 of Figure 3). The holeshapechangesupon immersion of the sample in superfluid helium. The concentration was 1dye molecule per 500 chains: (1) sample in helium gas atmosphere, (2) sample in superfluid helium, (3) sample in helium atmosphere after evaporation of the liquid. The red shift and the broadening of the middle spectrum are reversible (top). These spectralchanges occur because the chromophore of the dye feels the change in environment through the saturated chains, 27 A away.

monolayer is very sensitive even to a slight change in permittivity (from 1.00 to 1.057), 27 A away. 2.5. Concluding Remarks. The preceding results, obtained within a few years, demonstrate the value of high-resolution spectroscopy for the study of LB films. We have shown the existence of a specific surface dynamics of these films and attributed it to motion of the long chains. As is the case in biologicalsystems, the hole width in LB films is sensitive to energy transfer and to intermolecular interaction. The Stark effect is easy to study in LB films because they are thin and regular layers. By studying this effect with hole burning spectroscopy, we have determined the orientation of the dye chromophore completely, even though it is nearly perpendicular to the applied field. We may propose several directions for future work in this field. First, the low-temperature dynamics of the surface monolayer could be examined in more detail. The characteristic activation energy for this motion is lower than a few kelvin and could probably be determined by measurements with 3He, above 0.3 K. If bidimensional TLS’s are confirmed to be the cause of the surface dynamics, they could serve as models for the more complex TLS’s of three-dimensional glasses. Second, hole burning studies of nonresonant energy transfer could extend the classical work of Kuhn et al.77 with improved sensitivity. The intermolecular interactions in concentrated systems leading to non-Fiirster behavior must also be investigatedmore closely. Third, the Stark effect could help determine the local symmetry breaking of molecules included in different parts of a LB film, in particular as a function of the LB matrix molecules (acids, esters, alcohols, etc.). A particularly appealing field is the realization of organic superlatticesin which interesting electroopticaleffects might take place. Fourth, due to the possibility of building planned assemblies, LB films are interesting materials for the applications of hole burning to the optical processing and storage of information. The preceding experimentsshow the sensitivityof hole burning spectroscopy: The hole signal stemmingfrom impurities included at low concentration in a single monolayer could be measured. Yet, the number of molecules contributing to the hole signal in these experiments is on the order of lo7. The information they

FREQUENCY

Figure 8. Numerical simulation showing the optical absorption spectra of a solid sample containing small numbers of colored molecules (the spectra being normalized to the number of absorbing molecules, they are extinction coefficient spectra). The center frequenciesof the Lorentzian homogeneous lines of single molecules were chosen at random in the smooth quasi-Gaussian probability profile (smooth fine line). (A) 10, (B) 100, (C) IOOO, (D) 10 OOO molecules. For large molecule numbers, we recoverthesmooth inhomogeneous profile, with statisticalfluctuations. For small numbers, and in the wings of the distribution,single molecule lines are completely resolved.

provide still contains contributions from many local situations, as illustrated by the orientational distribution of dipole moments (section 2.4.1). As we will see in the next section, fluorescence excitation allows us to reach the ultimate sensitivity limit, the detection of a single molecule.

3. Single Molecule Spectroscopy 3.1. Introduction. As we noted it in the general introduction (sections 1.1 and 1.2.2), it is possible to resolve the sharp resonances of single impurity molecules in the inhomogeneous optical bands of very dilute and small samples. After pioneering w0rk,3~3~ single molecule spectroscopy (SMS)is now a fast growing field of solid-state spectroscopy. Although its discovery was partly inspired by the workon hole burning, singlemoleculespectroscopy seems to be an original branch of spectroscopy, more related to fluorescence line narrowing than to hole burning. Its main advantage are, on the one hand, the possibility of addressing in a truly local way a probe in a doped solid and, on the other hand, the elimination of all averaging over ensembles of centers. In the 3 years since these experiments started, many fascinating results were obtained in this field. Among them are the determination of the homogeneous line shape of one molecule in a crystalline matrix (pentacene i11p-terphenyl3~J~~JO2) and in a polymer matrix (peryleneI03Jwand terrylenelos in polyethylene),the temperature dependence of the single molecule line width,lsl09 the direct observation of spectral theStarkeffect,105JlI the observationof photon b u n ~ h i n g ~ ~ Jand ~ J antibunching,ll3 ~J~2 and that of individual two-level systems.*09J14 A review of the main results in single molecule spectroscopy has appeared recently.115 Below, after a short description of the principle and of the experimental technique of single molecule spectroscopy, we will discuss and summarize the main results obtained so far. 3.2. Principle and Experiment. In an inhomogeneous absorption band, the homogeneous resonance line of any molecule is shifted at random according to its particular microenvironment. Figure 8 shows the evolution of an inhomogeneous absorption spectrum when the number of absorbing molecules is reduced. For large numbers, the spectrum is smooth, with negligible relative

Orrit et al.


absorption fluctuations. When the number of molecules decreases, statistical absorption fluctuations appear, which scale like the inverse square root of the number of molecules. Such a statistical finestructurewasfirst observedin 1987 byMoernerandCarter.lI6 Finally, by selecting a sufficiently small volume of sample, we must be able to resolve spectrally the lines of all molecules in the sample (Figure 8). Since the excitedvolume is practically limited by the diffraction spot of the exciting light (for example, a few hundred pm3, as in the case of pentacene in p-terphenyl), the concentration of impurities in the sample must be low enough (on the order of 106-10-9 mol/mol). The most difficult condition to fulfill in order to detect single molecules is the sensitivity requirement: The fluorescence excitation m e t h ~ d , ~working ~J~ on a dark background, dramatically improves the signal/noise ratio with respect to direct absorption, even with the help of sophisticated modulation techniques.34~35 So far, only the fluorescenceexcitationtechniqueis used for the practical exploitation of single molecule lines. Hereafter, we discuss a few conditions which can help in selecting the most favorablehost-guest systems for single molecule spectroscopy among the wide variety of possible couples offered by organicmaterials. First, each molecule should interact strongly with photons. This points to strong allowed transitions and to a small homogeneousline width to concentratetheoscillator strength on a narrow and intense line. The latter condition will be best satisfied ina rigid matrix at low temperature. Second,thestability of the sample must be as high as possible to avoid photophysical hole burning and spectral diffusion. The photoinduced or spontaneous processes responsible for these effects will shift the resonance frequency of the molecule and thereby stop the acquisition of the weak signal. For example, H-bonded systems are known to present photoinduced bond rearrangements which probably will make single molecule detection more difficult than in nonpolar matrices. Third, bottleneck states like triplet excited states or metastable ground-state configurations will limit the average rate of photon absorptionsand therefore the useful signal. In addition to these conditions, a good fluorescence quantum yield of the impurity molecule is an obvious requirement of the fluorescence excitation method. The first system in which single molecule spectroscopy was demonstrated is the molecular crystal p-terphenyl (pTP) doped with pentacene (Pc). The spectroscopicproperties of this system have been studied in detail in the l i t e r a t ~ r e . ~ ~Of~ the - l ~four ~ possible insertion sites of Pc in pTP, only the two lower-energy ones (0'and 02) are of interest because their triplet yield is lowest. Single molecule spectroscopy was later extended to perylene (Pr) lo3 and to terrylene (Tr)"-'5 included in polyethylene (PE), a semicrystallinepolymer, Pr and Tr were chosen because of their stability and of their high fluorescence yields. PE is a polymer matrix giving narrow spectral holes with a variety of guest molecules at low temperatures. The Pc/pTP crystals were prepared by sublimation of a dilute load. Pr/PE and Tr/PE samples can be prepared by diffusion of the aromatic compound into the molten polymer. The optical setup for single molecule spectroscopyis a classical one, based on a single-mode dye laser for excitation. Fluorescence photons are counted after wide-angle collection by a parabolic mirror and elimination of the exciting light by a filter. Different sample mounting schemes have been proposed to reduce the volume of the sample excited. The focused exciting light can be sent to the sample through an optical fiber37 or through an aspherical lens,lWJM or else the incoming wave can be simply limited by a pinhole.ll1 In all these schemes, the light beam has a diameter of a few micrometers, and the sample is a crystal flake or film, some tens of micrometers thick. The excited volume is of a few hundreds of pm3 (say 300 pm3), containing from 200 to 200 000 impurity molecules if the concentration varies from le9 to lv mol/L. In the experiments described hereafter, the

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Figure 9. Example of a section of the fluorescence excitation profile of a small crystal of p-terphenyl doped with pentacene at 1.8 K. These 1line. The frequency scale spectra were recorded in the red wing of the 0

is enlarged toshow theshapeof the homogeneouslineof a singlepentacene molecule. The width of the line is approximately the natural width of 8 MHz. Note the presenceof broader structures,probably corresponding to molecules with defects in their environment. exciting intensity varied between 0.1 and 100 mW/cm2. The maximum detection rate of fluorescence photons is fixed at saturation by the intersystem crossing (ISC) parameters of the impurity molecule. This rate is around 5000/s for Pc/pTP and more than 100 OOO/s for Tr/PE. 3.3. Single Molecule Lines. 3.3.1, Line Width and Spectral Jumps. So far, single molecule spectroscopy has been demonstrated with aromatic molecules dissolved either in a molecular crystal (Pc/pTP) or in a polymer (Pr/PEor Tr/PE). The results for these two kinds of systems will be discussed in parallel. Figure 9 shows the excitation line of a single Pc molecule in pTP crystal at 1.8 K. The line width (about 8 MHz) corresponds to the homogeneous width deduced from photon echoes117J18 and also almost to the natural line width (or inverse fluorescence lifetime). This low value of the width of the line is a strong argument for its attribution to a single Pc molecule. Many other features like the intensity and distribution of the lines confirm this attribution, but perhaps the clearest argument is provided by the observationof strong photon correlation and of spectraljumps (see section 3.4). Most molecules, particularly in the center of the 0 1 line, exhibit the lifetime-limited width, but some others have broader lines,'07 up to 20 MHz. We attribute their additional broadeningto spectral diffusion induced by crystal defects. These may be more numerous in samples stuck on solid substrates than in free-held samples. In the polymer samples (Pr/PE, Tr/PE), similar isolated excitation lines can be f o ~ n d . ~ O ~The - ~ Odistribution ~ of observed line widths is much broader and the proportion of molecules having the lifetimelimited width at 1.8 K is very small, but nonvanishing. One of our first observations on some single molecule lines is the sudden intensity changes attributed to spectral jumps.37This phenomenon was studied in detail in Pc/pTP by Moerner's group,lM who showed that the jumps are spontaneous. Spectral jumps are extremely variable according to molecules. There are large jumps, in which the line disappears outside the 30-GHz laser scan, and small jumps with shifts comparable to the line width. There are slow jumps, appearing as irreversible on the time scale of measurements (minutes), and fast reversible ones. (The fastest jumps clearly documented so far-by correlation in Tr/PE-lie in the microsecond range; see section 3.4.4.) There are spontaneousjumps'" and, in the polymer matrix, photoinduced

Feature Article ones, which can be reversible or n0t.103~1~ Due to this diversity, much better statistics are needed before spectral jumps can be classified and a census of them can be given. The temperature dependence of single molecule lines was investigated by different groups. For Pc/pTP, the general trend is activated broadening above 3 or 4 K.lM The activation energy, 27 cm-1, corresponds to a local phonon.118Jl9 However, the detailed temperature behavior varies from molecule to molecule. Some lines were observed to narrow as the temperature increased, and two neighboring lines were seen to merge at high temperature, a possible indication for motional narrowing in the optical domain.lo7 In the polymer, even morecomplexbehaviorsappear, with different laws for line width as a function of temperature." These results show the tremendous sensitivity of single molecule lines to low-temperature motion in the matrix. This type of phenomenon is known in bulk systems as spectral diffusion.Single molecule spectroscopy shows directly that the elementary step for spectral diffusion is a spectral jump. (It also revealed the presence of spectral diffusion in a crystal host.) Conventional methods (photon echoes, hole burning) only give an average width, while single molecule spectroscopy gives the distribution of widths109 Theorigin of the broadening with respect to the natural width could be either dephasing or spectral diffusion. Because spectral jumps are observed on many different time scales (see section 3.4.4.),we feel that spectral diffusion must account for the larger part of this broadening. 3.3.2. Stark Effect. When a static electric field is applied to a sample, singlemolecule lines shift without broadening, in contrast to spectral holes. Let us first discuss the case of Pc/pTP. The Pc molecule is centrosymmetric and is included in a crystal site which is itself centrosymmetric. We therefore expect a quadratic Stark effect, which was indeed observed by Meyling et a1.120The first Stark experiments on single Pc molecules in pTP were carried out by Wild et al.111 They found a dominant quadratic Stark effect, approximately the same for all molecules, and a very weak but variable linear effect, correspondingto a dipole moment change A p = 10-4 D. This effect may be explained by a slight symmetry breaking by the crystal defects responsible for inhomogeneous broadening of the crystal lines. A linear Stark effect had previously been exploited in ref 35 and attributed to charge injection into the sample by the electrodes. In polymer solutions and glasses, the molecular symmetry is broken by microscopic disorder, and the Stark effect is found to be linear in all cases. Because the impurity molecules and the polymer chains are oriented at random, the linear shift has random magnitude and sign for different molecules. We measured the linear shifts for a number of Tr molecules in PE. The results are presented in Figures 10 and 11. The most surprising result is the comparatively large value of the induced Ap, of the order of 0.5 D. A few molecules show an even stronger Ap of about 2.5 D. Such large values could be explained by the presence of free charges in the vicinity of these molecules. Sometimes,the molecule linejumped during the measurement of the Stark effect. The value of A p did not change after the jump. This could beexpected, since the optical frequency change is small as compared to the inhomogeneous bandwidth. A change in A p of the same relative magnitude is much smaller than the experimental accuracy and is thus impossible to measure. The dipole moment change can therefore be used as a fingerprint of the molecule when its frequency jumps throughout the inhomogeneous spectrum. 3.3.3. Optically Detected Magnetic Resonance. At high exciting power, the fluorescence intensity from a single molecule is limited by the triplet bottleneck. The mechanism of this limitation will be discussed in section 3.4.3. Because the rates of intersystem crossing from the different triplet sublevels to the ground singlet differ, the application of a resonant microwave will modify the average dwell time in the triplet manifold and


ELECTRIC FIELD (kV/cm)

Figure 10. Spectra of single terrylene molecules in a polyethylene film were recorded under different appliedelectric fields (T=2 K, wavelength around 580 nm). The lines shift without broadening, and the shifts are plotted as functions of the applied field. The Stark effect is seen to be linear (showing that the molecular symmetry is broken by the matrix). The slopes depend on the particular molecule because their orientation with respect to the applied field is random (adapted from ref 105). 10

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Figure 11. Histogram of the dipole moment changes of single terrylene molecules,deduced from plots such as thost of Figure 10. Most molecules have dipole moment changes of the order of 0.5 D, but some high values (up to 2.5 D) are present. These could arise from highly distorted molecules or from molecules with charges in their environment.

thereby the average fluorescenceintensity. It is therefore possible to detect paramagnetic resonance lines optically for a single molecule, as was recently demonstrated independently by two groups.121J22 This experiment will be discussed in section 3.4.2. 3.4. Correlation Effects. The correlation method provides a way to analyze the intensity fluctuations of a light source.123 The correlation is the distributiong(2)(7)of pairs of photons separated by a time delay 7 , g(2)(~)= ( I ( t ) I ( t + 7 ) ) / ( I ( t ) ) 2 .This timeresolved method is best adapted to slow and intermediate time scales (1 ns to seconds or longer). In some cases, like RayleighBrillouin scattering, the scattered electric field fluctuates around zero. Then, the intensity fluctuates strongly, because it vanishes very often. The corresponding correlation function exhibits a continuous decrease with time, characteristic of chaotic light. However, the contrast g(2)(0)/g(2)(=) of the correlation never exceeds the limit of 2. In the case of fluorescence, only the intensities add. This means that, for a large number of independent emitters, the fluctuations must vanish. On the contrary, they can be very strong for a single system. Hereafter,


we give examples of the specific information that can be drawn from fluorescence fluctuations of single molecules. 3.4.1. Phoron Antibunching. If the fluorescence intensity I ( t ) was a continuous classical function of time, it would follow from the definition of g'2) that g(2)(0) 1 g'2)(m). For a single molecule in a coherent laser field, however, the true quantum evolution of the molecule's state must be considered. Quantum evolution introduces discontinuous jumps after measurement (collapse of the wave packet). This discontinuous behavior can lead to correlation functions such that g'2)(0) < g'2)(m) (photon antibunching), a signatureof quantum mechanics.lZ4 Themechanism of antibunching is very simple for a two-level molecule. (Vibrational relaxation is much faster than the processes considered here, so that only the lowest vibrational levels of each electronic state may be considered.) The measurement of a fluorescence photon projects the wave function into the molecule's ground state. Therefore, immediately after detection of a photon, the probability of detecting a second photon is nil (d2)(0) = 0). The molecule must be excited again before a second photon can be measured, which takes (on average) half the Rabi period of oscillation in the laser field. Photon antibunching was demonstrated experimentally 20 years ago on single atoms in beams and later on single ions in traps. Morerecently, experiments by Basch6 and Moerner demonstrated it on a single Pc/pTP molecule.113 The measured correlation function shows the expected dip at short times. At high exciting intensity, oscillations appear on the correlation when Rabi oscillations become faster than spontaneous emission. The measured correlation profile is in good agreement with the theory, and the Rabi frequency determined from the plots coincides with the one deduced from the parameters of the experiment. 3.4.2. Bunching Due to Intersystem Crossing. The triplet manifold plays an important role in the photodynamics of the Pc molecule by limiting the number of photons the molecule can absorb before relaxing to the triplet subspace. The relaxation into and out of the triplet manifold (intersystem crossing, ISC) is slow and can be described by rates k23 and k31, respectively. To simplify the discussion, we consider only one triplet sublevel at first. After the laser has been turned on, the molecule absorbs and emits photons, giving intense fluorescence. Then, a transition to the triplet takes place, and the fluorescence stops altogether during the lifetime of the triplet sublevel. Finally, the triplet level decays to the ground singlet, and the whole cycle can start over again. The fluorescence photons are therefore bunched in packets, separated by dark intervals which last for the triplet lifetime on average. The theory of the three-level system predicts an exponential decay of the correlation function for times longer than 1 ps, Le., outside the domain where antibunching is observed. While the average duration of the dark intervals is always the triplet lifetime, that of the bright intervals depends on the exciting intensity, which determines the average occupation of the excited singlet. For strong excitation, the bright intervals are short, the correlation contrast is high, and the decay time is short. (The decay rate is k3I + k23/2 because the population of S1 is l / 2 at saturation.) For weakexcitation, the correlation contrast is weak, and the correlation decay rate k31 is fixed by the average triplet lifetime. The correlation of the fluorescence of single molecules of Pc/ pTP was demonstrated in ref 37 and used as a proof that the signal arose from a single molecule. Later, correlation functions were investigated for several single molecules of Pc/pTP.112 Some examples of correlation functions at different exciting intensities are presented in Figure 12. The correlation decay rate changes with excitation intensity as predicted by the theory, and the ISC rates can be determined for each molecule. These rates were found to differ significantly (by up to 20%) from molecule to molecule, which weattribute to theinfluenceof neighboringcrystal

Orrit et al.

TIME (ps)

Figure 12. Correlation functions of the fluorescence of single pentacene molecules in a p-terphenyl crystal. The exponential decay of the

correlation (see fitted solid lines) arises from intersystem crossing transitions into and out of the triplet manifold. The correlationfunctions were recorded together with the line shapes (insets) at different exciting intensities(see the number of counts on theverticalscales). On saturation of the three-levelsystem,thecorrelationdecay becomes faster, thecontrast increases, and the line profile broadens. defects on Pc molecules. That the sensitivity of intramolecular ISC to surroundings is abnormally high in Pc/pTP is proved by the difference of 2 orders of magnitude in the rate k23 when passing from sites 01,Oz to sites 03,0 4 of the system Pc/pTP. It is therefore likely that ISC amplifies the slight site distortions which are responsible for inhomogeneous broadening and linear Stark effect (see section 3.3.2). In fact, the triplet manifold comprises three sublevels, whose degeneracy is lifted by spin-spin interaction (zero-field splitting), In general, the different sublevels have different rates of population from SIand of relaxation to So. By applying a resonant microwave to the optically excited molecule, we can modify the occupation of the sublevels and thereby modify the average dwell time in the triplet manifold, i.e., the average duration of the dark intervals. The average fluorescence intensity will therefore change, and the magnetic resonance can be detected optically. (This method is called fluorescence detected magnetic resonance (FDMR)). Recently, this experiment was carried out independently by two groups.121J22Two FDMR transitions were found, which connect the highly populated sublevels X and Y to the weakly populated, slowly decaying sublevel Z. The analysis of the correlation function under microwave irradiation confirmed the lengthening of the dark intervals when either the X-Z or the Y-Z transition was excited.I22 The investigation of the correlation as a function of optical and microwave powers will give access to the ISC rate constants for all the sublevels. 3.4.3. Bunching Due to Spectral Jumps. As was noted in section 3.3.2, single molecules are coupled to low-energy excitations (TLS's) in their environment. In the simplest case of spectral diffusion, a single molecule is coupled to a single TLS. As the TLSjumps from one well to the other, the single molecule's resonance also jumps between two particular frequencies. There must therefore appear two separate lines in the molecule's excitation spectrum. If the exciting laser is now tuned to one of these resonances, the intensity will jump between high and low values. These intensity jumps will give rise to photon bunching (very similar to that due to ISC) and will appear as a decay of the correlation function. Correlation functionsof many single molecules of Tr/PE were measured in our [email protected] Some 20-30% of the molecules studied presented significant correlation in the experimental time window (1 ps to minutes). Depending on the molecule under study, both the shape of the correlation function and the time range of the correlation varied widely, displaying the variety in

Feature Article 3~

The Journal of Physical Chemistry, Vol. 97,No. 40, 1993 10267

I

T=l.7 K

1

.1

10

1000

I00000

TIME T (ms)

Figure 13. Examples of correlation functions of the fluorescence of a single terrylene molecule in polyethylene. The correlation functions are exponential within experimentalerror (see the fitted solid lines and note the logarithmictime scale). Here, photon bunching is due to fluctuations in the molecule’ssurroundings. The molecule’sexcitation spectrum(inset) presentstwopeab, attributed totwometastableconfigurationsofanearby defect (TLS) of the polymer. When the defect jumps from one well to the other, so does the molecule’s optical line. The similarity of the correlationfunction in the two peaks is strong support for this model. The attribution of the two peaks to the same molecule was further confirmed when both lines jumped simultaneously to a new frequency, several gigahertz away, where they had the same correlation function.

local environments on a nanoscopic scale. Figure 13 shows the example of a two-peaked single Tr molecule. We attribute the two peaks to the same molecule on the basis of their identical correlation functions. This assignment was confirmed by several jumps of the peaks, without change of their frequency difference, which also confirmed the model of a molecule coupled to a hierarchy of TLS’s. In most cases, however, the second line could not be identified within the frequency range accessible to our laser, indicating that the TLS molecule couplingusually leads to frequency shifts larger than 10 GHz. When a monoexponential decay has been singled out in a correlation function, we attribute it to a single TLS. It then becomes possible to investigate the jumping dynamics of this TLS as a function of external parameters, like temperature, laser power, etc. The power dependence shows that spontaneous as well as photoinduced jumps occur,lw but this study of difficult because Tr molecules usually undergo large irreversible jumps under strong excitation. The temperature dependence of the jump rate was easier to study. The results for a few molecules confirm the expected mechanism for the TLS jumps, tunneling assisted by acoustic phonons. During a TLS jump, the energy difference between the wells is made up by emission or absorption of one acoustic phonon or by a Raman process, in which one acoustic phonon is absorbed and another emitted.1WJl4 Finally, a particular TLS showed a more complex activated behavior, pointing to a strong coupling between the TLS and acoustic phonons.lwJ14 3.5. Conclusion. A wide variety of experiments were carried out recently on single molecules. They show that the signal-tonoise ratio obtained by fluorescence excitation is sufficient, not only to detect the molecules in solids but also to study their spectroscopy. Singlemolecule spectroscopyoffers unique possibilities to study the distribution of molecular parameters, where most conventional methods only give average values. The data obtained so far (histograms of ISC rates, dipole moments, line widths, etc.) show that deeper insight on nanoscopicstructure and dynamics can be obtained in this way. Another advantage of single molecule spectroscopy is that it opens the way to new experiments which have no equivalent for ensembles. This is the case of the observation of spectral jumps,

and of the correlation effects, which vanish for large numbers of independent molecules. These methods can directly give timeresolved information in the time range from 1 ns to minutes or longer and are therefore extremely important for the study of nanoscopic dynamics. Finally, single molecule spectroscopyaddresses single sites in a transparent solid by means of a macroscopicparameter, which is the frequency of the exciting laser. This property could be exploited in devices with parallel addressing of several local sites. Consideringthe number of results obtained in just a few years, it is safe to predict that single molecule spectroscopy will still develop quickly in the near future. The main effort should bear now on the generalization of this method to many more host-guest couples in addition to the three systems already known (Pc/pTP, Pr/PE, Tr/PE). More particular, a probe molecule like Tr could be included in different matrices, and the structure and dynamics of local sites in these matrices could be compared. Space and time resolutions could still be improved to study short processes, intermolecular interactions, etc. Much better statistics are needed to obtain useful information from histograms of molecular parameters. Automatic data collection would be a very helpful next step. Optical spectroscopyincludes many experiments like energy-, electron-, or proton-transfer, intermolecular interactions, nonlinear optics, intramolecular relaxation, and photochemistry,etc., which could all be tried on single molecules. The field of nanophysics was opened some years ago by the scanning tunneling microscope. Single molecules are an alternative type of probe from the sharp tips used in such techniques, presenting advantages of their own: They involve relatively weak perturbations of the system, they can be addressed macroscopically, and they can be placed in the bulk of the system to be probed. They might therefore be used to probe the microscopic processes at work in future nanoelectronic devices.

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High-resolution spectroscopy of organic molecules in solids: from

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