Protonation-Induced Charge Transfer and Polaron Formation in Organic Semiconductors Doped by Lewis Acids

Lewis acid doping of organic semiconductors (OSCs) opens up new ways of p-type doping and has recently become of significant interest. As for the mechanistic understanding, it was recently proposed that upon protonation of the OSC backbone, electron transfer occurs between the protonated polymer chain and a neutral chain nearby, inducing a positive charge carrier in the latter [Nat. Mater. 18, 1327 (2019)]. To further clarify the underlying microscopic processes on a molecular level, in the present work, we analyze the influence of protons on the electronic properties of the widely used PCPDT–BT copolymer. We find that single protonation of the polymer chain leads to the formation of a polaron coupled to the position of the proton. Upon protonation of the same chain with a second proton, an intrachain electron transfer occurs, leaving behind a polaron largely decoupled from the proton positions. We also observe the possibility of an interchain electron transfer from a neutral chain to a double protonated chain in agreement with the mechanism recently proposed in the literature. The simulated vertical excitation spectra for an ensemble of protonated species with different amounts of protons enable a detailed interpretation of experimental observation on PCPDT–BT doped with the Lewis acid BCF. Our results further suggest that multi-protonation plays an important role for completing the mechanistic picture of Lewis acid doping of OSCs.

Solvent-Free Formation of Cyclodextrin-Based Pseudopolyrotaxanes of Polyethylene Glycol: Kinetic and Structural Aspects

Pseudopolyrotaxanes (PPRs) are supramolecular structures consisting of macrocycles able to thread on a linear polymer chain in a reversible, non-covalent way, often referred to in the literature as “molecular necklaces”. While the synthesis and reaction mechanisms of these structures in solution have been widely described, their solvent-free production has received little attention, despite the advantages that this route may offer. We propose in this work a kinetic mechanism that describes the PPR formation in the solid phase as a process occurring in two consecutive stages. This mechanism has been used to investigate the spontaneous formation of a PPR that occurs when grinding α-Cyclodextrin (α-CD) with polyethylene glycol (PEG). In the threading stage, the inclusion of the polymer and subsequent release of the water molecules lodged in the cavity of the macrocycle cause vibrational changes that are reflected in the time-dependence of the FTIR-ATR spectra, while the further assembly of PPRs to form crystals produces characteristic reflections in the XRD patterns, due to the channel-like arrangement of CDs, that can be used to track the formation of the adduct in crystalline form. The effects that working variables have on the kinetics of the reaction, such as temperature, feed ratio, molar mass of the polymer and the introduction of an amorphous block in the polymer structure, have been investigated. The rate constants of the threading step increase with the temperature and the activation energy of the process increases at lower proportions of CD to PEG. This is attributed to the lower degree of covering of the polymer chain with CDs that reduces the hydrogen-bonding driven stabilization between adjacent macrocycles. The formation of crystalline PPR, which takes place slowly at room temperature, is markedly promoted at higher temperatures, with lower proportions of CD favoring both the formation and the growth of the crystals. The molar mass of the polymer does not modify the typical channel-like arrangement of packed PPRs but the conversion into crystalline PPR diminishes when using PEG1000 instead of PEG400. At a microscopic level, the crystals arrange into lamellar structures, in the order of hundreds of nm, embedded in an amorphous-like matrix. The introduction of a polypropylene oxide block in the structure of the polymer (Pluronic L62) renders poorer yields and a considerable loss of crystallinity of the product of the reaction. The methodology here proposed can be applied to the general case of inclusion complexes of CDs with drugs in the solid phase, or to multicomponent systems that contain polymers as excipients in pharmaceutical formulations along with CDs.

Poly(ʟ-lactide)s with Tetraphenylethylene: Role of Polymer Chain Packing on Aggregation-Induced Emission Behavior of Tetraphenylethylene

Development of biocompatible and biodegradable fluorescent polymers and understanding their fluorescent characteristics are highly desirable. In this work, we synthesized one-armed, two-armed and four-armed poly(ʟ-lactide)s by ring-opening polymerization of ʟ-lactide...

Absolute negative mobility of active polymer chains in steady laminar flows

We investigate the transport of active polymer chains in steady laminar flows in the presence of thermal noise and an external constant force. In the model, the polymer chain is...

Study of the microstructure of low-molecular rubbers

The oligomer microstructure, reflecting the configuration of the elementary units (cis-1,4-, trans-1,4-, and 1,2-) and their distribution order in the polymer chain, decisively affects the physicochemical and rheological properties of the oligomer. Parameters of microstructure-dependent transitions (glass transition) characterize the oligomer behavior under abnormal low-temperature conditions. We present the results of studying the microstructure of low-molecular rubbers. We determined the content of cis-1,4- and trans-1,4-structural links of poly-divinyl-isoprene-urethane-epoxy oligomer. The structure of polybutadiene HTPB-IV with terminal hydroxyl groups was analyzed using NMR and IR spectroscopy. A comparative analysis of the microstructure of the poly-divinyl-isoprene-urethane-epoxy oligomer and low molecular rubbers with a known content of structural units has been carried out. The obtained results can be used to obtain oligomers with the desired physicochemical and mechanical properties.