DOI: 10.1002/cphc.200500409 Single-Molecule Spectroscopy of Uniaxially Oriented Terrylene in Polyethylene Jacqueline Y.P. Butter, [a] Brent R. Crenshaw, [b] Christoph Weder, [b] and Bert Hecht* [a] Introduction Single-molecule fluorescence excitation spectroscopy of vari- ous guest molecules in numerous matrices at cryogenic tem- perature has been used extensively to probe low-temperature dynamics of the matrix materials. [1–3] Such studies exploit the sensitivity of the molecular zero-phonon line towards changes in the environment of the fluorescent guest molecules. In highly ordered materials the distribution of spectroscopic pa- rameters due to environmental influences can be minimized and, as a consequence, single molecules at cryogenic tempera- tures behave as nearly ideal, rigidly fixed two-level quantum systems (TLSs). The latter feature makes them attractive model systems for a range of quantum optical experiments in which the position and absorption dipole orientation of the TLS must be well defined; for example, they may be precisely coupled to photonic structures such as cavities [4] or antennas. [5,6] In ad- dition, a favorable orientation results in a maximum absorption cross-section, a high emission rate, and a high fluorescence collection efficiency. [7] Since the first detection of single molecules at cryogenic temperatures, [8,9] a selection of possible matrices has been in- troduced, [2,3] such as molecular crystals, Shpol’skii matrices, and amorphous or semicrystalline polymers. Unfortunately, none of the previously used systems combines all desirable features. In particular, the orientation of the transition dipole moments of guest molecules usually tends to be either random or particu- larly unfavorable, which leads to weak optical absorption in the latter case. Unfortunately, the host systems that show the least environmental influence on the single-molecule behavior, that is, the molecular crystals, either i) tend to incorporate the guest molecules in a way such that the molecular transition dipole moments are nearly parallel to the optical axis (terrylene in single crystals [10,11] and more recently in ultrathin films of p- terphenyl [12] ), or ii) the molecules exhibit long triplet lifetimes (pentacene in p-terphenyl [13] ), or iii) the samples are delicate to prepare and to handle (terrylene in naphthalene [14] ). For the first two cases this leads to unfavorably high saturation intensi- ties due to a reduced absorption cross-section and to a low emission rate, respectively. Mechanical treatment of polymeric materials, such as tensile deformation or rubbing, can be used to induce a high degree of uniaxial orientation of the polymer matrix, and likewise of form-anisotropic fluorescent guest molecules incorporated therein. [15–17] Recent room-temperature experiments on uniax- ially oriented films of ultrahigh molecular weight polyethylene (UHMWPE) doped with a poly(2,5-dialkoxy-p-phenylene ethy- nylene) derivative (EHO-OPPE) [18] have shown that even at the single-molecule level, the (in this case polymeric) fluorescent guest molecules tend to adopt the preferential orientation of the surrounding matrix after uniaxial tensile deformation of the sample. Terrylene in polyethylene is a well-known guest–host system that exhibits narrow zero-phonon excitation lines at cryogenic temperatures. [19–22] Interestingly, all previous studies on this system were performed on disordered materials. We showed earlier that conventional melt-processing and subsequent ten- sile deformation of the resulting molecularly mixed blends of linear low-density polyethylene (LLDPE) and form-anisotropic fluorescent guest molecules allows thin films to be produced Single terrylene molecules doped into linear low-density polyethy- lene can be oriented by tensile deformation of the matrix. In measurements on ensembles at ambient and on single terrylene molecules at cryogenic temperature, strong orientation along the stretching direction was observed by polarization-resolved confo- cal microscopy. At cryogenic temperatures narrow and spectrally stable zero-phonon lines were found. The low saturation intensity of 0.07 Wcm À2 is consistent with an uniaxial orientation of terry- lene in the sample plane. [a] J. Y. P. Butter, Prof. Dr. B. Hecht Nano-Optics Group, National Center of Competence for Research in Nanoscale Science Institute of Physics, University of Basel Klingelbergstr. 82, 4056 Basel (Switzerland) Fax: (+ 41)61-267-3795 E-mail: bert.hecht@nano-optics.ch [b] B. R. Crenshaw, Prof. Dr. C. Weder Department of Macromolecular Science and Engineering Case Western Reserve University 2100 Adelbert Road, Cleveland, Ohio 44106-7202 (USA) ChemPhysChem 2006, 7, 261 – 265 # 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 261