Browsing by Author "Li, Chen"

Developing molecular systems with functions analogous to those of macroscopic machine components, such as rotors1,2, gyroscopes3 and valves4, is a long-standing goal of nanotechnology. However, macroscopic analogies go only so far in predicting function in nanoscale environments, where friction dominates over inertia5,6. In some instances, ratchet mechanisms have been used to bias the ever-present random, thermally driven (Brownian) motion and drive molecular diffusion in desired directions7. Here, we visualize the motions of surface-bound molecular rotors using defocused fluorescence imaging, and observe the transition from hindered to free Brownian rotation by tuning medium viscosity. We show that the otherwise random rotations can be biased by the polarization of the excitation light field, even though the associated optical torque is insufficient to overcome thermal fluctuations. The biased rotation is attributed instead to a fluctuating-friction mechanism8,9 in which photoexcitation of the rotor strongly inhibits its diffusion rate.

We report a rationale for identifying superior dyes for stimulated-emission depletion (STED) microscopy. We compared the dyes pPDI and pTDI, which displayed excellent photostability in single-molecule spectroscopy. Surprisingly, their photostability and performance in STED microscopy differed significantly. While single pTDI molecules could be visualized with excellent resolution (35 nm), pPDI molecules bleached rapidly under similar conditions. Femtosecond transient absorption measurements proved that the overlap between the stimulated-emission band and the excited-state absorption band is the main reason for the observed difference. Thus, assessment of the excited-state absorption band provides a rational means of dye selection and determination of the optimal wavelength for STED.