Peculiar Aggregation Features in Poly(3-hexylthiophene)/Chlorobenzene Solutions

A skeletal randomization strategy for high-performance quinoidal-aromatic polymers

Enhancing the solution-processability of conjugated polymers (CPs) without diminishing their thin-film crystallinity is crucial for optimizing charge transport in organic field-effect transistors (OFETs). However, this presents a classic "Goldilocks zone" dilemma, as conventional solubility-tuning methods for CPs typically yield an inverse correlation between solubility and crystallinity. To address this fundamental issue, a straightforward skeletal randomization strategy is implemented to construct a quinoid-donor conjugated polymer, PA4T-Ra, that contains para-azaquinodimethane (p-AQM) and oligothiophenes as repeat units. A systematic study is conducted to contrast its properties against polymer homologues constructed following conventional solubility-tuning strategies. An unusually concurrent improvement of solubility and crystallinity is realized in the random polymer PA4T-Ra, which shows moderate polymer chain aggregation, the highest crystallinity and the least lattice disorder. Consequently, PA4T-Ra-based OFETs, fabricated under ambient air conditions, deliver an excellent hole mobility of 3.11 cm2 V-1 s-1, which is about 30 times higher than that of the other homologues and ranks among the highest for quinoidal CPs. These findings debunk the prevalent assumption that a random polymer backbone sequence results in decreased crystallinity. The considerable advantages of the skeletal randomization strategy illuminate new possibilities for the control of polymer aggregation and future design of high-performance CPs, potentially accelerating the development and commercialization of organic electronics.

Gels of Semiconducting Polymers in Benign Solvents

Gels of semiconducting polymers have many potential applications, including biomedical devices and sensors. Here, we report a self-assembled gel system consisting of isoindigo-based semiconducting polymers with galactose side chains in benign, alcohol-based solvents. Because of the carbohydrate side chains, the modified isoindigo polymers are soluble in alcohols. We obtained thermoreversible gels in 1-propanol using these polymers and di-Fmoc-l-lysine, a molecular gelator. The polymers and molecular gelators have been selected in such a way that they do not have significant physical interactions. The molecular gelator self-assembled to form a fibrous structure that confines the polymer chains in the interstitial spaces of the fibers. The polymer chains formed local aggregations and increased the shear moduli of the gels significantly. Bulky galactose side chains and the less planar nature of the polymer backbone hindered the formation of long-range assembled structures of the polymers. However, the dispersion of polymers throughout the gel samples resulted in a percolated structure in the dried gel films. The bulk electrical conductivity of dried gels confirmed the presence of such percolated structures. Our results demonstrated that carbohydrate-containing conjugated polymers can be combined with molecular gelators to obtain gels in eco-friendly solvents.

Conformational Relaxation Dynamics of Poly(3-hexylthiophene) Photoexcited in Solution as Studied by Femtosecond Time-Resolved Stimulated Raman Spectroscopy in 1190-1550 nm Region

When a conjugated polymer is photoexcited in solution, its effective conjugation length in the singlet exciton state often increases through the conformational relaxation of the polymer main chain and/or hopping of the excitation. We measured femtosecond time-resolved near-IR stimulated Raman spectra of poly(3-hexylthiophene) (P3HT) photoexcited in four organic solvents for understanding the dynamics of the exciton elongation through the conformational relaxation separately from that through the exciton hopping. In the ring CC stretch frequency region, a band appears at around 1415 cm-1 and decays, while a new band rises at around 1370 cm-1. The average time constant of the change is estimated to be 8.7-19 ps and correlated almost linearly with the viscosity of the solvents. These results suggest that the main chain of P3HT in the singlet exciton state relaxes from a twisted form to a planar form in the 0-100 ps range when it surmounts an activation barrier of 5.8-7.8 kJ mol-1, generated possibly by the steric effect of the hexyl side group. When the rise of the 1370 cm-1 band is analyzed in detail, it is reproduced with two exponential rise functions with time constants of 0-3.3 and 16-22 ps. The two rise components suggest that a portion of P3HT forms a cluster in solution, while the other portion of P3HT is isolated.

Ordered Solid-State Microstructures of Conjugated Polymers Arising from Solution-State Aggregation

Controlling the solution-state aggregation of conjugated polymers for producing specific microstructures remains challenging. Herein, a practical approach is developed to finely tune the solid-state microstructures through temperature-controlled solution-state aggregation and polymer crystallization. High temperature generates significant conformation fluctuation of conjugated backbones in solution, which facilitates the polymer crystallization from solvated aggregates to orderly packed structures. The polymer films deposited at high temperatures exhibit less structural disorders and higher electron mobilities (up to two orders of magnitude) in field-effect transistors, compared to those deposited at low temperatures. This work provides an effective strategy to tune the solution-state aggregation to reveal the relationship between solution-state aggregation and solid-state microstructures of conjugated polymers.

Controlling the Microstructure of Conjugated Polymers in High-Mobility Monolayer Transistors via the Dissolution Temperature

It remains a challenge to precisely tailor the morphology of polymer monolayers to control charge transport. Herein, the effect of the dissolution temperature (Tdis ) is investigated as a powerful strategy for morphology control. Low Tdis values cause extended polymer aggregation in solution and induce larger nanofibrils in a monolayer network with more pronounced π-π stacking. The field-effect mobility of the corresponding monolayer transistors is significantly enhanced by a factor of four compared to devices obtained from high Tdis with a value approaching 1 cm2  V-1  s-1 . Besides that, the solution kinetics reveal a higher growth rate of aggregates at low Tdis , and filtration experiments further confirm that the dependence of the fibril width in monolayers on Tdis is consistent with the aggregate size in solution. The generalizability of the Tdis effect on polymer aggregation is demonstrated using three other conjugated polymer systems. These results open new avenues for the precise control of polymer aggregation for high-mobility monolayer transistors.

PBTTT-C16 sol-gel transition by rod associations and networking

A low-molecular-weight poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) (designated as Lw-pBTTT-C₁₆) in a fair solvent (chlorobenzene, CB) displays peculiar structural, mechanical, and electronic features during sol-gel transition. Using comprehensive (multiscale) dynamic/static analysis schemes, the Lw-pBTTT-C₁₆/CB solution (10 mg mL^-1) is shown to capitalize on rod associations and networking to form a gel, in stark contrast with its high-molecular-weight companion previously reported to form gels through hierarchical colloidal bridging. The present study reveals, however, that the molecular weight of pBTTT-C₁₆ has a subtle impact on the gelation behaviors through the rarely recognized, contrasting supramolecular conformations (rod-like vs. wormlike) of the aggregate clusters fostered in the pristine solution. The ac conductivity nearly doubles as a result of improved (mesoscale) packing of cylindrical aggregates near the gel state as well as enhanced backbone rigidity of the constituting chains. Other distinguishing features include: (1) there is no real crossover of the dynamic moduli (G' and G'') upon increasing the temperature from gel (T = 15 °C) to solution (T = 80 °C) states. (2) The gel is about a hundredfold softer in dynamic modulus, yet more resilient with a fivefold increase in the yield strain. Both viscoelastic features are expected to greatly benefit the gel processability. (3) The coexistent microgels and cylinder (aggregate) bundles form a peculiar gel network that has not been reported previously with polymer or colloidal gels. The overall findings provide new mechanistic insight into the phenomenological effects of molecular weight for the pBTTT-C_n series in solution, sol, gel, and thin film.

Solvent Effects on Morphology and Electrical Properties of Poly(3-hexylthiophene) Electrospun Nanofibers

Herein, poly(3-hexylthiophene-2,5-diyl) (P3HT) nanofiber-based organic field-effect transistors were successfully prepared by coaxial electrospinning technique with P3HT as the core polymer and poly(methyl methacrylate) (PMMA) as the shell polymer, followed by extraction of PMMA. Three different solvents for the core polymer, including chloroform, chlorobenzene and 1,2,4-trichlorobenzene, were employed to manipulate the morphologies and electrical properties of P3HT electrospun nanofibers. Through the analyses from dynamic light scattering of P3HT solutions, polarized photoluminescence and X-ray diffraction pattern of P3HT electrospun nanofibers, it is revealed that the P3HT electrospun nanofiber prepared from the chloroform system displays a low crystallinity but highly oriented crystalline grains due to the dominant population of isolated-chain species in solution that greatly facilitates P3HT chain stretching during electrospinning. The resulting high charge-carrier mobility of 3.57 × 10⁻¹ cm²·V⁻¹·s⁻¹ and decent mechanical deformation up to a strain of 80% make the P3HT electrospun nanofiber a promising means for fabricating stretchable optoelectronic devices.