0030-400X/05/9805- $26.00 © 2005 Pleiades Publishing, Inc. 0681 Optics and Spectroscopy, Vol. 98, No. 5, 2005, pp. 681–686. From Optika i Spektroskopiya, Vol. 98, No. 5, 2005, pp. 745–751. Original English Text Copyright © 2005 by Kol’chenko, Kozankiewicz, Nicolet, Orrit. * 1. INTRODUCTION Since the first detection of single fluorescent mole- cules almost 15 years ago [1, 2], considerable effort has been invested in developing new systems for single molecule spectroscopy at low temperatures. Despite the rather stringent requirements for observing single mol- ecule spectra, a number of suitable host–guest systems based on polycrystalline Shpolskii matrices, crystals, and polymers have been uncovered [3–6]. A good can- didate system for single molecule studies should present a high quantum yield of fluorescence, a strong zero-phonon line, and a small hole burning efficiency. Another important requirement concerns the triplet- state bottleneck. Fluorescence of a single molecule originates from a transition from the lowest excited sin- glet state S 1 to the singlet ground state S 0 (see Fig. 1). The lowest excited triplet state T 1 lies below the S 1 state and is populated with a very low probability through the spin-flipping process called intersystem crossing (ISC). The triplet state is relatively long-lived. While in its triplet state, a molecule does not absorb any more exciting photons and thus cannot emit fluorescence photons. It is therefore virtually “dark.” For this reason, only guest–matrix systems with very low ISC rates and short enough triplet lifetimes can be used in single mol- ecule spectroscopy. In this article, we discuss different manifestations of the triplet-state bottleneck in single molecule spectros- copy. The guest under consideration is terrylene (Tr), embedded in different crystalline matrices. Doped molecular crystals are ideal systems for nonlinear and * This article was submitted by the authors in English. quantum-optical measurements, where long accumula- tion times and high frequency stabilities are needed. When a perfect crystal is cooled down to low tempera- tures, the only degrees of freedom that may affect the optical line shapes of a guest are librations of the host and guest molecules in the lattice. Acoustic phonons are weakly coupled and have a low density of states at low frequencies. Optical absorption lines of dye mole- cules in molecular crystals often show narrow spectro- scopic sites [7], inhomogeneously broadened by crystal defects. These defects usually do not lead to any signif- icant additional dynamics of the host. The emission of single guest molecules should be very stable in fre- quency and intensity. Terrylene exhibits such a simple behavior in naphthalene crystals (for example, [6]), but it has somewhat more complicated properties in para- terphenyl crystals [4]. In the latter matrix, the lines of single molecules in two of the four spectroscopic sites are photostable at low temperatures, whereas they undergo large photoinduced jumps in the two other sites. Single Tr molecules in naphthalene and para-ter- phenyl can emit fluorescence photons at a very high emission rate, up to several hundred thousand counts per second (cps). This behavior indicates an unusually low efficiency of the triplet bottleneck in the optical pumping cycle. However, the ISC yield may strongly depend on the surrounding matrix. A well-known example is pentacene, whose triplet yield strongly depends on the matrix and even on the particular inser- tion site [8, 9]. In para-terphenyl, the triplet yield in sites O 1 , O 2 is smaller by two orders of magnitude than that in sites O 3 , O 4 . This huge difference was attributed to coincidences between singlet and triplet levels Intersystem Crossing Mechanisms and Single Molecule Fluorescence: Terrylene in Anthracene Crystals* M. A. Kol’chenko 1 , B. Kozankiewicz 1, 2 , A. Nicolet 1 , and M. Orrit 1 1 Molecular Nano-Optics and Spins (MoNOS), Leiden Institute of Physics (LION), Huygens Laboratory, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands 2 Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, PL-02668 Warsaw, Poland e-mail: orrit@molphys.leidenuniv.nl Received November 1, 2004 Abstract—Single molecule spectroscopy requires molecules with low triplet yields and/or short triplet life- times. The intersystem crossing (ISC) rate may be dramatically enhanced by the host matrix. Comparing the fluorescence intensity of single terrylene molecules in para-terphenyl, naphthalene, and anthracene crystals, we found a reduction of the saturation intensity by three orders of magnitude in the latter case. The fluorescence autocorrelation function indicates that the bottleneck state is the terrylene triplet. We propose a ping-pong mechanism between host and guest. This intermolecular ISC mechanism, which can open whenever the host triplet lies lower than the guest singlet, was overlooked in previous single molecule investigations. © 2005 Ple- iades Publishing, Inc.