Stefan Hecht

Laws of motion are of fundamental importance for the understanding of dynamic systems. At the atomic scale, the kinetic energy of molecules controls chemical reactions. The velocity of individual molecules, however, has never been measured, because it is difficult to determine time–distance relationships. Here, we show how the real speed of a single molecule moving across a Ag(111) surface is measured at the microsecond time scale. We found that atomic defects extend the travelling time by orders of magnitude, even at distances of few nanometers from the molecule and with high sensitivity to their precise location. Statistical measurements, obtained by repetitive translation of a single molecule in an oscillating electric field, give insight into the time scales of molecular motion in a confined environment. The strong influence of the defects underscores the need for their investigation during chemical reactions or collision experiments at variable velocities and geometries on a surface.

Laws of motion are of fundamental importance for the understanding of dynamic systems. At the atomic scale, the kinetic energy of molecules controls chemical reactions. The velocity of individual molecules, however, has never been measured, because it is difficult to determine time–distance relationships. Here, we show how the real speed of a single molecule moving across a Ag(111) surface is measured at the microsecond time scale. We found that atomic defects extend the travelling time by orders of magnitude, even at distances of few nanometers from the molecule and with high sensitivity to their precise location. Statistical measurements, obtained by repetitive translation of a single molecule in an oscillating electric field, give insight into the time scales of molecular motion in a confined environment. The strong influence of the defects underscores the need for their investigation during chemical reactions or collision experiments at variable velocities and geometries on a surface.

Collisions of atoms and molecules are required for bond formation and are thus key to any chemical reaction. Their outcome is determined by the collision energy, relative orientation, and impact parameter of the reactants. Collision studies in the gas phase have shown that energies are easily controlled, whereas it is difficult to steer the molecular orientations and completely impossible to control the impact parameter. Here, we study the collision of individual molecules at a surface where each involved compound is imaged before and after collision by scanning tunneling microscopy, allowing precise control of orientations and impact parameter. We observe that the molecules must collide in a narrow cone of reaction and that surface atom displacements can enable a reaction even for disfavored pathways.

Achieving multi‐site photoisomerization within a single π‐conjugated scaffold while retaining luminescent functionality remains a formidable challenge, as electronic coupling between isomerizable units typically promotes intramolecular energy transfer that suppresses sequential switching, and the additional nonradiative pathways inherent to multiple isomerizable bonds further impede emissive behavior. Herein, we report a methine‐bridged trithiophene bearing 2,6‐dichlorophenyl substituents, in which two C═C double bonds embedded in a thienoquinoid framework undergo photoisomerization to afford three interconvertible geometrical isomers ( ZZ , EZ , and EE ). The steric constraint imposed by the ortho ‐chloro substituents suppresses nonradiative torsional relaxation, yielding a fluorescence quantum yield of 0.40 for the ZZ ‐isomer in cyclohexane, which decreases upon photoisomerization to the EZ ‐ and EE ‐isomers—a significant enhancement over the corresponding derivative lacking ortho ‐substituents on the aryl groups, accompanied by a markedly reduced Stokes shift. The photostationary state composition varies systematically with the irradiation wavelength, primarily reflecting differences in the molar absorption coefficients of the three isomers, thereby enabling wavelength‐selective multi‐state control over isomer distributions inaccessible under thermodynamic equilibrium. Fluorescence lifetime measurements revealed isomer‐dependent excited‐state lifetimes, confirming that molecular geometry governs the photophysical properties within this conjugated framework. This study establishes a molecular design strategy for luminescent multi‐state photoswitches applicable to multi‐level optical information processing.

Molecular design is critical in order to both study and exploit chirality in two‐dimensional (2D) self‐assembled molecular networks at surfaces and interfaces. Here, we investigate the supramolecular structures of two prochiral π‐conjugated molecular systems on a graphite surface by scanning tunneling microscopy at the solid–liquid interface. By systematically extending the periphery of such molecular systems, we demonstrate how the expression of chirality of the resulting supramolecular networks can be rationally controlled, driving the assembly from an achiral to a chiral organization. Our approach provides fundamental insights into the role of molecular geometry in governing the physical properties of 2D self‐assembled molecular networks, and it offers a rational bottom‐up strategy for the fabrication of chiral surfaces from prochiral building blocks.

Achieving multi-site photoisomerization within a single π-conjugated scaffold while retaining luminescent functionality remains a formidable challenge, as electronic coupling between isomerizable units typically promotes intramolecular energy transfer that suppresses sequential switching, and the additional nonradiative pathways inherent to multiple isomerizable bonds further impede emissive behavior. Herein, we report a methine-bridged trithiophene bearing 2,6-dichlorophenyl substituents, in which two C=C double bonds embedded in a thienoquinoid framework undergo photoisomerization to afford three interconvertible geometrical isomers ( ZZ , EZ , and EE ). The steric constraint imposed by the ortho -chloro substituents suppresses nonradiative torsional relaxation, yielding a fluorescence quantum yield of 0.40 for the ZZ -isomer in cyclohexane, which decreases upon photoisomerization to the EZ - and EE -isomers—a significant enhancement over the corresponding derivative lacking ortho -substituents on the aryl groups, accompanied by a markedly reduced Stokes shift. The photostationary state composition varies systematically with the irradiation wavelength, reflecting differences in the molar absorption coefficients of the three isomers, thereby enabling wavelength-selective multi-state control over isomer distributions inaccessible under thermodynamic equilibrium. Fluorescence lifetime measurements revealed isomer-dependent excited-state lifetimes, confirming that molecular geometry governs the photophysical properties within this conjugated framework. This study establishes a molecular design strategy for luminescent multi-state photoswitches applicable to multi-level optical information processing.

Achieving multi-site photoisomerization within a single π-conjugated scaffold while retaining luminescent functionality remains a formidable challenge, as electronic coupling between isomerizable units typically promotes intramolecular energy transfer that suppresses sequential switching, and the additional nonradiative pathways inherent to multiple isomerizable bonds further impede emissive behavior. Herein, we report a methine-bridged trithiophene bearing 2,6-dichlorophenyl substituents, in which two C=C double bonds embedded in a thienoquinoid framework undergo photoisomerization, affording three interconvertible geometrical isomers ( ZZ , EZ , and EE ). The steric constraint imposed by the ortho-chloro groups suppresses nonradiative torsional relaxation, yielding a fluorescence quantum yield of 0.402 for the ZZ -isomer in cyclohexane, which decreases upon photoisomerization to the EZ - and EE -isomers—a significant enhancement over the corresponding derivative lacking ortho-substituents on the aryl groups—accompanied by a markedly reduced Stokes shift. The photostationary state composition varies systematically with the irradiation wavelength, reflecting the wavelength-dependent differences in molar absorption coefficients among the three isomers, thereby enabling wavelength-selective multi-state control over isomer 2 distributions that are inaccessible under thermodynamic equilibrium. Fluorescence lifetime measurements reveal isomer-dependent excited-state lifetimes, confirming that molecular geometry governs the photophysical properties within this conjugated framework. This work establishes a molecular design strategy for luminescent multi-state photoswitches applicable to multi-level optical information processing.

Xolography is a volumetric 3D printing technology utilizing two different wavelengths of light to produce objects with micrometer resolution and excellent surface quality within minutes. Dual‐color photoinitiators (DCPIs) are crucial for spatial control of polymerization. However, their performance has mainly been assessed indirectly by evaluating printed objects. In this study, we present a methodology using in situ Raman spectroscopy to characterize the dual‐color curing response of a representative spiropyran‐based DCPI in a methacrylate‐based resin composition. A dual‐wavelength irradiation setup has been developed to monitor the degree of curing (DoC) as a function of the applied light doses, providing quantitative, real‐time insights into bond conversion and polymerization kinetics, as well as enabling the Xolography process window to be derived. The DCPI exhibited a contrast of 89%, showing improved performance compared to first generation systems, while its efficiency was benchmarked against a benzophenone reference. Spatially resolved Raman mapping of volumetric test prints revealed DoC gradients ranging from 40% to 80% directly after printing. We demonstrate that subsequent posttreatments can eliminate these nonuniformities, increasing the DoC to > 90%. This approach provides a framework for evaluating novel dual‐color photoinitiator systems, facilitating the rational design of resins, illumination protocols and optimized configurations for volumetric 3D printing.

Weak dipole–dipole interactions drive the formation of 2D honeycomb porous networks at interfaces. STM and simulations uncover how subtle molecular design and interfacial forces control stability, switching behavior, and guest inclusion.