The orientation of the S 1 r S 0 π,π* transition dipole moments of oxonine (Ox + ), pyronine (Py + ), and POPOP (5,5′-diphenyl-2,2′-p-phenylenebis(oxazole)) in the channels of zeolite L crystals was investigated by means of fluorescence microscopy and single-crystal imaging. Qualitative observations led to the result that the transition moment of POPOP is aligned along the c-axis of the hexagonal crystals whereas the fluorescence of Ox + and Py + is not. More detailed investigations on Ox + showed a cone-shaped distribution of the transition moments with a half-cone angle of 72°. The orientation of the transition dipole moment for all of these molecules is parallel to the molecules' long axis. We found by means of space-filling arguments that POPOP, the van der Waals length of which is about 21 Å, can only be aligned along the channel axis. This is in full agreement with the observed fluorescence anisotropy. For Ox + and Py + , geometrical arguments based on the zeolite L structure give room for only two possible arrangements of the molecules' long axis: a half cone angle of up to 40°for Ox + and up to 30°for Py + , and an angle of about 90°for both of them with respect to the c-axis of zeolite L. The surprising discrepancy between geometrical considerations and the results of the fluorescence measurements can be explained by assuming that Ox + and Py + are exposed to a considerable anisotropic electrical field in the zeolite channels.
Resorcin[4]arene cavitands with four quinoxaline bridges are a family of macrocycles that adopt, at elevated temperature, a contracted, vase‐type conformation, capable of guest inclusion, whereas at low temperature they switch to an expanded, kite‐type conformation with a large flat surface. The present investigations lay the foundation for the use of such dynamic cavitands as miniaturized mechanical grippers for supramolecular construction at the single‐molecule level. New vase–kite switching modes, stimulated by pH changes or stoichiometric metal‐ion complexation, have been discovered and monitored by 1H NMR and optical absorption spectroscopy. The solid‐state geometries of the two states have been revealed by X‐ray crystallography, and the kinetics and thermodynamics of the switching processes in solution as well as their solvent dependency has been investigated in great detail. Monolayers of the cavitand in the vase form have been studied by scanning tunneling microscopy at molecular resolution; conformational switching is also observed in Langmuir monolayers at the air/water interface. Synthetic protocols have been developed for preparation of partially and asymmetrically bridged resorcin[4]arene cavitands, which are also shown to undergo conformational switching. These synthetic advances pave the way to new, dynamic molecular receptors for steroids, tetrathiofulvalene‐bridged grippers with the potential to undergo electrochemically induced conformational switching, and systems with greatly extended, rigid cavity walls functionalized at the termini by dipyrrometheneboron difluoride dyes. The latter cavitands are shown by fluorescence resonance energy transfer to undergo geometrically precisely defined motions between a contracted (≈ 7 Å linear extension) and a strongly expanded (≈ 7 nm linear extension) state.
We study the interaction of a biased, metallized tip in close proximity to single fluorescent molecules at cryogenic temperature. By scanning over the sample, the tip’s inhomogeneous electric field induces Stark shifts of the zero-phonon lines of nearby molecules. When illuminated with an off-resonant laser, molecules are tuned into resonance for specific tip positions located on circular patterns around the molecules’ spatial positions. The origins of circles belonging to different molecules can be determined with high precision. We demonstrate a spatial resolution of neighboring individual molecules of ∼50nm using a tip scanned in a distance of ∼3μm above the sample.
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