Visualizing spatial and temporal heterogeneity of single molecule rotational diffusion in a glassy polymer by defocused wide-field imaging

Defocused wide-field fluorescence microscopy was used to follow the 3D molecular rotational diffusion of a fluorescent probe molecule in a polymer thin film. The technique allows for visualizing the molecular reorientation both in-plane and out-of-plane. The local environmental change driven by heterogeneous dynamics of the polymer can be probed on a scale of 1 µm as demonstrated by parallel imaging of several molecules. A multi-component rotational diffusion decay is observed which might reflect both different relaxation regimes of the polymer as well as rapid changes of the local environment.

Unraveling Excited-State Dynamics in a Polyfluorene-Perylenediimide Copolymer

Insight into the exciton dynamics occurring in a polyfluorene-perylenediimide (<b>PF-PDI</b>) copolymer with a reaction mixture ratio of 100 fluorene units to 1 <i>N,N′</i>-bis(phenyl)-1,6,7,12-tetra(<i>p-tert</i>-octylphenoxy)-perylene-3,4,9,10-tetracarboxylic acid diimide (<b>PDI</b>) is presented here. Time-correlated single photon counting and femtosecond transient absorption spectroscopy measurements on the <b>PF-PDI</b> copolymer have been employed to investigate the excited-state properties of the polyfluorene subunit where the exciton is localized (<b>PF</b>) and the incorporated <b>PDI</b> chromophore. The experimental results were compared with those obtained from a polyfluorene polymer (<b>model PF</b>) and a <i>N,N′</i>-bis(2,6-diisopropylphenyl)-1,6,7,12-tetra(<i>p-tert</i>-octylphenoxy)-perylene-3,4,9,10-tetracarboxylic acid diimide (<b>model PDI</b>) which were used as reference compounds. Because of the high polydispersity of the <b>PF-PDI</b> copolymer, there is a polymer fraction present that contains no <b>PDI</b> chromophores (polyfluorene polymer fraction (<b>PF polymer fraction</b>), and wide-field imaging of single polymers chains of the synthesized <b>PF-PDI</b> copolymer was used to estimate this <b>PF polymer fraction</b>. Following the primary excitation of the <b>PF</b> in the <b>PF-PDI</b> copolymer, energy hopping between <b>PF</b>'s can occur. A fraction of the energy of the absorbed photons will be transferred to a <b>PDI</b> chromophore via energy transfer from a <b>PF</b>. In a polar solvent, a charge transfer state having the S₁ of the <b>PDI</b> moiety as a precursor state is found to form with high efficiency on a nanosecond time scale. The data suggest that a fraction of the absorbed energy is directed, transferred, and used in charge separation, providing a clear view of a multistep mechanism of exciton dissociation into charges

A surface-bound molecule that undergoes optically biased Brownian rotation

Developing molecular systems with functions analogous to those of macroscopic machine components, such as rotors, gyroscopes and valves, is a long-standing goal of nanotechnology. However, macroscopic analogies go only so far in predicting function in nanoscale environments, where friction dominates over inertia. In some instances, ratchet mechanisms have been used to bias the ever-present random, thermally driven (Brownian) motion and drive molecular diffusion in desired directions. 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 mechanism in which photoexcitation of the rotor strongly inhibits its diffusion rate.