Conformation and physics of polymer chains: a single-molecule perspective

Abstract

N anotechnology has been one of the driving forces of science and technology over the past 20 years. Scaling down the size of an object to nanometer dimensions can result in dramatic changes to its physical properties. With recent progress in the nanoscience and nanotechnology of polymeric materials, new methods are now required for the characterization of polymer structures and properties. Despite the great success of electron microscopy and scanning probe microscope techniques, there remains a need for a non-invasive method that could provide information not only on the structure but also on the physical phenomena occurring on the nanoscale level. One method that has recently emerged as an excellent tool for nanoscale characterization is single-molecule optical detection and spectroscopy. Single-molecule spectroscopy relies on the detection and measurement of the fl uorescence signal and spectra from single isolated molecules or polymer chains (Figure 1). Th ese can either be immobilized in a solid matrix or be diff using in a solution or melt. In all cases, the concentration of the emitting molecules must be suffi ciently low to ensure that the spatial separation between individual molecules is larger than the optical resolution of the setup, typically on the order of micrometers. As the fl uorescence signal from a molecule is strongly infl uenced by the nanoscale environment, measuring many molecules one by one provides the distribution of physical properties of the respective nano-environments. Moreover, monitoring a molecule over a period of time reveals dynamic changes in its environment. Th ese static and dynamic heterogeneities of nanoscale properties are averaged and hidden in the usual ensemble experiments. Th e strength of fl uorescence detection is in its sensitivity and low background level -emissions of just a few hundred photons per second can be readily detected. Th e technique also off ers remarkable dynamic range, from sub-nanosecond timescales to seconds and longer, and is non-invasive, making it possible to detect emissions from molecules buried deep in a sample.Single-molecule spectroscopy has evolved since the beginning of the 1990s from low-temperature, high-resolution optical spectroscopic

Structure and Morphology Control in Thin Films of Conjugated Polymers for an Improved Charge Transport

“…Besides, the coil-to-rod (or rod-to-coil) transition of chains conformed to an intra-chain mechanism because of the independence of this red-shift on concentration of solution [138]. Compared with "expanded" coil-like conformations, the anisotropy of chains in "collapsed" rod-like conformations is enhanced [29,139] due to the further elongated conjugation length and segment size driven by the intrinsic tendency for minimizing energy (Figure 17). Then, the enhanced intra-chain conjugation length strengthens the interchain π-π stacking interactions of chains, which conforms to an inter-chain mechanism.…”

“…However, clarifying the detailed, bottom-up relationship between materials and performance has been always challenging since the discovery of semi-conducting conjugated polymers. After the related research for several decades, a great progress has been made in this field [2,3,[19][20][21][22][23][24][25][26][27][28][29], i.e., proposing more clear and efficient synthesis guidelines, concluding principles to improve morphological order and revealing the importance of morphological and structural features in films to the final performance. Now, synthesis of more efficient π-conjugated systems and improvement of order in films have become the essential problems for the development of opto-electronic functional conjugated polymers in terms of chemistry and physics, respectively.…”

Recent developments in polymeric superoleophobic surfaces

“…More importantly, responsive polymers can change their chemical or physical properties reversibly with the external stimuli exerted on them. 26,38 Due to this feature, polymers have been used to prepare smart superoleophobic surfaces (that can switch between superoleophobicity and superoleophilicity reversibly using external stimuli like temperature, pH, light, electricity, magnetism, and so on), with attractive applications such as controllable oil/water separation and bioseparation. 39 In this review, we will cover the design, fabrication, and recent developments of polymer superoleophobic surfaces.…”

Single-molecule electroluminescence and photoluminescence of polyfluorene unveils the photophysics behind the green emission band

“…Part of the reason that there has not been a clear consensus on the topic is that the amorphous polyfluorene films are heterogeneous systems, and conventional spectroscopic and other characterization methods provide only bulk-averaged values. Single-molecule spectroscopy is known as an effective method that removes the ensemble averaging and has been successfully used in studies of polymers and other complex soft matter [26][27][28] , including the observation of electrically induced chemiluminescence from single conjugated polymer nanoparticles 29 . In particular, it is the ability of singlemolecule spectroscopy to reveal static distributions in spectra of individual molecules (inhomogeneous broadening) and temporal spectral changes (spectral diffusion) that make it well suited for the study of the inhomogeneous green band.…”