Replacing Alkyl with Oligo(ethylene glycol) as Side Chains of Conjugated Polymers for Close π–π Stacking

Abstract

We synthesize and systematically study a series of conjugated polymers with oligo(ethylene glycol) (OEG) or alkyl chain as the side chain and poly [2,7-fluorene-alt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)] as the polymer backbone. Replacing alkyl chain with OEG chain can decrease the π−π stacking distance of polymer backbone in thin film from 0.44 to 0.41 nm because OEG chain is more flexible than alkyl chain. As the result, the conjugated polymer with OEG side chain exhibits higher hole mobility, red-shifted absorption spectrum in thin film and smaller bandgap than those of the conjugated polymer with alkyl side chain. With the increase of the length of OEG side chain, the resulting conjugated polymers exhibit unchanged π−π stacking distance and decreased hole mobility. Moreover, owing to the large polarity of OEG chain, OEG side chain makes the conjugated polymer suitable for polymer solar cell (PSC) devices processed with polar nonhalogenated solvent, methoxybenzene. A power conversion efficiency of 4.04% is demonstrated with the resulting PSC devices. This work provides the new insight into the effect of OEG side chain on conjugated polymer, which can be used in the molecular design of novel conjugated polymer materials with excellent optoelectronic device performance. ■ INTRODUCTIONConjugated polymers have been widely used in optoelectronic devices, such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and polymer solar cells (PSCs), with the great advantages of low cost, flexibility, and solution processing. 1 For solubility of conjugated polymers, a rigid conjugated polymer backbone is always equipped with an alkyl side chain. 2 Oligo(ethylene glycol) (OEG) chain is well-known for its hydrophilicity and is always used to endow molecules/ polymers solubility in water or polar organic solvents. 3 Thus, OEG chain has been used as side chain of conjugated polymers for efficient OLED, OFET, and PSC devices processed with polar nonhalogenated solvents. 4 As optoelectronic properties of conjugated polymers are affected by the side chain, 5 the effect of OEG side chain on properties of conjugated polymers is less investigated and poorly understood. Researchers may doubt whether hydrophilic OEG side chain would disturb the π−π stacking of hydrophobic polymer backbone in conjugated polymers. This manuscript aims to provide an insight into the effect of OEG side chain on conjugated polymers.We note that OEG chain is more flexible than alkyl chain. As shown in Scheme 1, the two hydrogen atom in CH 2 unit act as steric hindrance for the rotation of CH 2 −CH 2 unit in alkyl chain, while the two lone electron pair in oxygen atom result in no steric hindrance for the rotation of O−CH 2 in OEG chain. 6 Therefore, the energy barrier of the rotation of O−CH 2 in OEG chain (E = 0.08 eV) is smaller than that of CH 2 −CH 2 in alkyl chain (E = 0.11 eV), 6 making OEG chain more flexible than alkyl chain. For conjugated polymers, alkyl side chain impedes the close π−π Scheme 1. Schematic Il...

Effect of hexyl substituent groups on photophysical and electrochemical properties of the poly[(9,9‐Dioctyluorene)−2,7‐diyl‐alt‐(4,7‐bis (3‐Hexylthien‐5‐Yl)−2,1,3‐Benzothiadiazole)−2′,2″‐diyl]

“…It is evident that the redshift effect is more intense with a decreasing of hexyl substituents on F8TBT polymer chain. This behavior is consistent with results obtained by Meng et al These authors analyzed the presence of different substituent groups linked to fluorene rings in poly[2,7‐fluorene‐alt‐5,5‐(4,7‐di‐2‐thienyl‐2,1,3‐benzothiadiazole)] and concluded that the alkyl substituents affect the polymer backbone organization, increasing π‐π stacking distance between polymer chains in solid state which consequently reduces redshift.…”

Glycolated Thiophene‐Tetrafluorophenylene Copolymers for Bioelectronic Applications: Synthesis by Direct Heteroarylation Polymerisation

“…Due to the strong positive mesomeric effect of the oxygen atom attached directly to the thiophene, an increase in the push‐pull effect can be expected, resulting in a higher rigidity of the conjugated backbone. Furthermore, in comparison with commonly used alkyl chains, glycol chains are more flexible and possess constant dipole; both factors can result in a shortening of the π‐π stacking distance between the polymer backbones . Another essential property is that the glycol chains have a high affinity for polar solvents and ions, allowing their penetration from the aqueous electrolyte into the polymer thin film, resulting in the possibility of mixed conduction of both electrical charges and ions in the solid state .…”

“…Furthermore, in comparison with commonly used alkyl chains, glycol chains are more flexible and possess constant dipole; [37] both factors can result in a shortening of the π-π stacking distance between the polymer backbones. [38,39] Another essential property is that the glycol chains have a high affinity for polar solvents and ions, allowing their penetration from the aqueous electrolyte into the polymer thin film, resulting in the possibility of mixed conduction of both electrical charges and ions in the solid state. [40] Ion conduction is a crucial prerequisite for creating a functioning organic electrochemical transistor (OECT).…”

Synthesis of amphiphilic isoindigo co‐polymers for organic field effect transistors: A comparative study

“…Amphiphilic polymers, poly(am‐ i Ind‐DT) and poly(am‐ i Ind‐TT) have lower oxidation potential than their regular counterpart. This phenomenon could be attributed to the electron donating ethylene glycol side chains . The E ox and E Homo energy levels of poly(am‐ i Ind‐DT) and poly(am‐ i Ind‐TT) were calculated to be 0.37 V/–5.17 eV and 0.36 V/–5.16 eV, respectively, whereas, poly(reg‐ i Ind‐DT) and poly(reg‐ i Ind‐TT) have the respective values of 0.53 V/–5.33 eV and 0.52 V/–5.32 eV.…”

“…On further note, the oligo ethylene glycol chains can also be easily functionalized and can be substituted on the CPs. Unlike alkyl chains, oligoethylene glycol chains are hydrophilic in nature which facilitates the intermolecular interaction via reducing the π‐π stacking distance . Recently, oligoethylene glycol substituted diketopyrrolopyrrole (DPP) homopolymers have shown high charge carrier mobility .…”