Nanoscale mobility mapping in semiconducting polymer films

This work was supported by grant No 19-12-00066 of the Russian Science Foundation.Local electrical properties of thin films of the polymer PTB7 are studied by conductive atomic force microscopy (C-AFM). Non-uniform nanoscale current distribution in the neat PTB7 film is revealed and connected with the existence of ordered PTB7 crystallites. The shape of local I-V curves is explained by the presence of space charge limited current. We modify an existing semi-empirical model for estimation of the nanoscale hole mobility from our experimental C-AFM measurements. The procedure of nanoscale charge mobility estimation was described and applied to the PTB7 films. The calculated average C-AFM hole mobility is in good agreement with macroscopic values reported for this material. Mapping of nanoscale hole mobility was achieved using the described procedure. Local mobility values, influenced by nanoscale structure, vary more than two times in value and have a root-mean-square value 0.22 × 10−8 m2/(Vs), which is almost 20% from average hole mobility.PostprintPeer reviewe

Ultrafast Through-Space Electronic Energy Transfer in Molecular Dyads Built around Dynamic Spacer Units

A pair of complementary molecular dyads have been synthesized around a 1,2-diaminocyclohexyl spacer that itself undergoes ring inversion. Despite these conformational exchange processes, the donor and acceptor occupy quite restricted spatial regions, and they are not interchangeable. The donor and acceptor pair comprise disparate boron dipyrromethene dyes selected to display favorable electronic energy transfer (EET). Steady-state emission spectroscopy confirms that through-space EET from donor to acceptor is almost quantitative, aided by the relatively short separations. Ultrafast time-resolved fluorescence spectroscopy has allowed determination of the rates of EET for both dyads. Surprisingly, in view of the close proximity of donor and acceptor (center-to-center separations less than 20 Å), the EET dynamics are well-accounted for in terms of the computed molecular conformations and conventional Förster theory. One dyad appears as a single family of conformations, but EET for the second dyad corresponds to dual-exponential kinetics. In this latter case, an intramolecular hydrogen bond helps stabilize an open geometry, wherein EET is relatively slow

Solubilisation of lipid membranes by detergents: probing the three-state model at the single vesicle level

The solubilisation of lipid membranes by detergents is a common technique in the purification and isolation of proteins and in the study of membrane proteins. Biophysical studies of these solubilisation processes using ensemble-averaging techniques have provided an integrated picture of the solubilisation mechanism that involves three states: the detergent is taken up in stage I without solubilisation (vesicle regime); stage II refers to the coexistence of detergent-saturated membranes and mixed detergent-lipid micelles and finally stage III corresponds to complete membrane solubilisation leading to the formation of mixed micelles (micellar regime). Although the three-state hypothesis is a didactic and simple thermodynamic model, it is known that detergent lipid-interactions induce a much more complex and diverse dynamic transition between both regimes. These dynamic changes include transmembrane lipid motion (flip-flop), swelling and breakdown of the membrane permeability barrier and fusion between vesicles. To date, whether these processes are independent of each other and take place sequentially, or if they are somehow interconnected is an open question. Here, we have used single-molecule FRET to monitor the solubilisation dynamics of single POPC/POPS vesicles induced by the non-ionic detergent Triton-X 100. Using this approach we have been able to unambiguously separate within a single FRET trajectory the swelling, permeabilization and lysis steps of the solubilisation process and their kinetic details above and below the critical micellar concentration. The present strategy should help in the design of more efficient applications of vesicle solubilisation