How rigid are conjugated non‐ladder and ladder polymers?

Abstract

Persistence length is commonly used to quantitatively describe the chain rigidity of macromolecules, which represents an important structural parameter governing many physical properties of polymers. Although the mathematical models and experimental measurements on the chain rigidity of conventional single stranded polymers have been well explored and documented, those of the more rigid yet highly intriguing multiple stranded polymers, especially conjugated ladder polymers, are yet not well established. This article introduces the fundamental concepts on macromolecular chain rigidity, as well as the corresponding experimental methods, models, and simulations. Subsequently, representative examples of works done on the chain rigidity of nonladder conjugated polymers and conjugated ladder polymers are reviewed. Last but not least, it provides outlooks on the challenges with respect to the less-investigated chain rigidity of conjugated ladder polymers, including new models to describe and predict chain conformation, synthetic control on structural defects, and insights into the correlation of rigidity and applications.

Chain Stiffness of Donor–Acceptor Conjugated Polymers in Solution

“…While more rigid chains also tend to enhance short contacts between chains enhancing π–π stacking and intermolecular charge transport, ,, the exact microstructure of the polymer in the active layer is critically important. ,, For example, a higher content of planar fused rings correlates with enhanced electrical conductivity, , although ring expansion alone is not sufficient to encourage coplanarity; persistence lengths and charge-carrier mobilities are decreased if the bulky aromatic rings cause steric hindrance and additional torsion elsewhere in the chain. ,, Additionally, a larger degree of conformational freedom (often lowering the persistence length) can actually assist the chain segments in achieving higher crystallinity, and thus higher charge-carrier mobility, as is suspected for polythiophenes. , It is suggestive that the optical absorption for such flexible-in-solution polymers (thiophene and CPDT-containing) changes dramatically upon solidification as the chains order, while the intrinsically higher persistence length polymers (DPP and IDT-containing) exhibit little change in their optical spectra. ,,− Collectively, these results reinforce that chain rigidity of constituent conjugated polymers, as quantified by the persistence length, is one of several factors that contribute to high performance organic electronic devices. , …”

“…6,14,18 For example, a higher content of planar fused rings correlates with enhanced electrical conductivity, 57,69 although ring expansion alone is not sufficient to encourage coplanarity; persistence lengths and charge-carrier mobilities are decreased if the bulky aromatic rings cause steric hindrance and additional torsion elsewhere in the chain. 58,70,71 Additionally, a larger degree of conformational freedom (often lowering the persistence length) can actually assist the chain segments in achieving higher crystallinity, and thus higher charge-carrier mobility, as is suspected for polythiophenes. 72,73 It is suggestive that the optical absorption for such flexible-in-solution polymers (thiophene and CPDT-containing) changes dramatically upon solidification as the chains order, while the intrinsically higher persistence length polymers (DPP and IDT-containing) exhibit little change in their optical spectra.…”

Increasing the Charge Transport of P(NDI2OD-T2) by Improving the Polarization of the NDI2OD Unit along the Backbone Direction and Preaggregation via H-Bonding

“…The backbone conformation of D–A conjugated polymer strongly depends on the rotational freedom of the single bonds connecting the π-conjugated units and arene moieties. , To limit the torsion of the single bonds and promote interchain interactions, there are many effective strategies, such as lowering side chain steric hindrance, , conformational lock to restrict the main chain torsion, − improvement of conformation adjustment ability, , external forces such as printing flow and many others. Among them, H-bonding interactions have demonstrated promising effects on the improvement of coplanar conformation. − The formation of H-bonding needs the following three key conditions: (i) It must contain lone electron pairs from strong electronegativity atoms such as oxygen (O), nitrogen (N), and fluorine (F).…”

Nanoconfined synthesis of conjugated ladder polymers

“…1). [1][2][3][4][5] Owing to their highly extended π-conjugation systems, CLPs exhibit several fascinating opto-electronic properties, such as narrow band gaps, high carrier mobilities, and long exciton diffusion lengths. 3,[6][7][8][9][10][11][12][13] Hence, CLPs are prime targets for numerous applications in optoelectronic devices including organic photovoltaics, light-emitting diodes, and thin-film transistors, [14][15][16] continuously stimulating the creativity of chemists to develop synthetic methods for CLPs.…”