Iterative Binomial Synthesis of Monodisperse Polyfluorenes up to 64-mers and Their Chain-Length-Dependent Properties

Abstract

C onjugated polymers have emerged as one class of advanced materials due to their applications in optoelectronic devices and sensors. 1-4 However, various fundamental issues, such as crystalline structures and chain-length-related properties, still need further study for understanding insight from the polymer viewpoint. [5][6][7][8][9][10] In principle, polymer samples with well-defined chain length and chemical structures and narrow molecular weight distribution are preferable in studying these fundamental issues. However, synthesis of well-defined conjugated polymers by polymerizations has been only realized for limited conjugated systems to date. 11,12 As one class of the most important conjugated polymers, polyfluorenes with defined molecular weights and narrow molecular weight distributions are still difficult to be prepared.Monodisperse conjugated oligomers (MCOs) are characterized with well-defined and uniform chemical structures. 13,14 These characteristics enable them as the ideal model compounds to attain specific information, such as effective conjugation length (ECL), of their corresponding polydisperse high molecular weight analogues by extrapolation. 15,16 However, the molecular weights of the most of MCOs reported today locate in the oligomeric region since the synthesis of MCOs is a multistep process and requires carefully optimizing the reaction condition and synthetic strategy. 17-23 Their molecular weights are not high enough to serve as models of the corresponding conjugated polymers in all cases. In this regard, synthesis of monodisperse conjugated polymers (MCPs) with molecular weights beyond 10 000 g/mol is very important. Two series of MCPs based on thiophene and fluorene units with the molecular weights beyond 20 000 g/mol have been reported. 24,25 However, these MCPs were separated from the polymerization mixtures of macromonomers by preparative gel permeation chromatography (PGPC), and the methods do not allow preparing MCPs in a relatively large quantity.The iterative binomial approach is most efficient in the synthesis of MCOs. 13,17,18 However, it has not been used in the preparation of MCPs yet. After carefully optimizing the reaction conditions, in the current paper, we first successfully synthesized monodisperse poly(9,9-dioctylfluorene)s (POFs) with molecular weights up to 25 018 g/mol in half-gram scale via iterative binomial approach. In THF, the R value of POFs in the Mark-Houwink equation was measured to be 1.15. Absorption spectroscopic measurements indicate that absorption maxima of POFs do not reach the convergent limit yet up to 64-mer, indicating that POFs are highly conjugated.The structures of the monodisperse POFs, i.e. SiFnSi and Fn, are depicted in Chart 1, where n and Si represent the number of fluorene units and the terminal SiMe 3 group, respectively. Synthesis of SiFnSi and Fn is outlined in Scheme 1. In general, two orthogonal protecting groups are required in iterative binomial synthesis of MCOs, as exemplified by the synthesis of oligo(p-phenyleneethynylene)s...

Strong main-chain length-dependence for the β-phase formation of oligofluorenes

“…1b). 5,[23][24][25][26] The final products obtained after careful purification using the recycling gel permeation chromatography (GPC) were characterized by the 1 H and 13 C NMR and high-resolution ESI-MS measurements (for details, see the Experimental section and Fig. S1-S7 †).…”

“…1-3 So far, various polyfluorenes, oligofluorenes and their derivatives have been synthesized, and in particular, oligofluorenes with well-defined chain lengths are considered as notable materials for understanding the structure-property relationship of the fluorene materials because typical polyfluorenes have difficulty in precise characterization due to polydispersity and conformational complexity owing to the long-chain polymer structures. 4,5 A noteworthy structure of the main-chain conformation is the β-phase 1,3,6-9 that is formed by an expanded conjugation structure that shows red-shifted absorption and emission which enable many applications such as in organic lasers. [10][11][12] Regarding this feature, therefore, the chain-length dependence of the oligofluorenes is a crucial issue to clarify the inherent properties of fluorene polymers and oligomers and to design highly-functionalized fluorenebased materials.…”

“…[10][11][12] Regarding this feature, therefore, the chain-length dependence of the oligofluorenes is a crucial issue to clarify the inherent properties of fluorene polymers and oligomers and to design highly-functionalized fluorenebased materials. Several researchers have reported the synthesis of chain-length-controlled oligofluorenes 5,[13][14][15][16][17] and discussed the β-phase formation. [18][19][20][21][22] As an example of β-phase formation using relatively long oligomers, Lidzey et al used oligomer mixtures containing a maximum length of 19-mers and found that the electronic conjugation length of the β-phase was consistent with the persistence length of the main chain.…”

π-Conjugated polymer-layered structures: synthesis and self-assembly

“…Because the degree of polymerization of the original polymers was sufficiently large and similar to the reported effective conjugation length, 45,46 the intramolecular interactions within the introduced polymers had little influence on their electronic state. On the other hand, significant differences were observed in the cast films (Figure 3).…”

Size Control of Spherical and Anisotropic Fluorescent Polymer Nanoparticles via Precise Rigid Molecules

“…An illustrative exception is Geng et al's synthesis of a polyfluorene 64-mer by Suzuki coupling in an iterative binomial approach. 28 The obtained oligomers, which feature a reasonable molecular weight distribution of M w /M n of 1.09, were not studied further in terms of structure formation.Within the aforementioned range of rod-like conjugated polymers, poly(phenylene ethynylene)s 29 stand out in that the repeat units are linear and thus linkages are particularly stiff. The synthesis of monodisperse oligo(arylene ethynylene)s in a step-by-step buildup by Pd-catalyzed Sonogashira crosscoupling has been the subject of numerous reports.…”