A Versatile and Efficient Strategy to Discrete Conjugated Oligomers

Article pubs.acs.org/JACS

A Versatile and Efficient Strategy to Discrete Conjugated Oligomers Jimmy Lawrence,†,‡ Eisuke Goto,†,§,‡ Jing M. Ren,† Brenden McDearmon,† Dong Sub Kim,† Yuto Ochiai,§ Paul G. Clark,∥ David Laitar,∥ Tomoya Higashihara,§ and Craig J. Hawker*,† †

Materials Research Laboratory and Departments of Materials, Chemistry, and Biochemistry, University of California, Santa Barbara, California 93106, United States § Department of Organic Materials Engineering, Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa 992-8510, Japan ∥ The Dow Chemical Company, Midland, Michigan 48674, United States S Supporting Information *

ABSTRACT: An efficient and scalable strategy to prepare libraries of discrete conjugated oligomers (Đ = 1.0) using the combination of controlled polymerization and automated flash chromatography is reported. From this two-step process, a series of discrete conjugated materials from dimers to tetradecamers could be isolated in high yield with excellent structural control. Facile and scalable access to monodisperse libraries of different conjugated oligomers opens pathways to designer mixtures with precise composition and monomer sequence, allowing exquisite control over their physical, optical, and electronic properties.

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grow dimer, tetramer, octamer, etc. repeat series.25−28 While such stepwise synthetic methods have been successful in producing oligomeric materials with precise length, they remain inefficient for preparing large-scale libraries of discrete oligomers and are challenging targets for nonexperts (Scheme 1). In contrast, a separation-based strategy employing reversed-

INTRODUCTION Conjugated polymers are an attractive class of materials owing to their unique optoelectronic properties and promising application in organic electronics and nanoscale assemblies.1−6 While conjugated polymers are generally synthesized as broadly disperse mixtures (Đ > 2.0),7 synthetic methods to access conjugated polymers with narrow dispersity (Đ < 1.2) have been developed.8−11 However, narrow dispersity polymers remain a statistical mixture of molecular weights, with small differences in the degree of polymerization and/or dispersity having a significant impact on properties and optical behavior.12−14 This impact is more pronounced for conjugated materials with a degree of polymerization (DP) 12. 13736

DOI: 10.1021/jacs.7b05299 J. Am. Chem. Soc. 2017, 139, 13735−13739

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Journal of the American Chemical Society

oligomer. MS analysis confirmed the molecular weight change corresponding to the mass difference between the Br- and vinyl chain ends (53 amu) with NMR spectroscopy also showing the expected appearance of resonances for the vinyl group (Figure 4, see the Supporting Information).

Figure 4. (Top) Synthesis of vinyl terminated o3HT2 hexamer. (Bottom) MS spectra of o3HT2 hexamer (black) and vinyl-o3HT2 hexamer (blue).

In addition to oligomer lengths having a significant impact on optical performance, molecular weight dispersity can also play a role in defining physical properties. This can be easily observed in the PL color difference between hexamer samples that have discrete (green) and disperse (yellow) molecular weights. In this case, the red-shifted appearance of the disperse sample is attributed to ensemble distribution of species and cascade energy transfer between the different oligomer lengths present in the mixture. Access to a library of discrete oligomers then opens the intriguing possibility of preparing artificial oligomer blends with precise control of both DP and dispersity leading to unique and tunable optical properties. Using a library of discrete oligothiophenes with PL color progressing from blue to orange-red, a wide range of artificial mixtures can be simply prepared by direct mixing to obtain materials with unique optical properties. For example, mixing a 3:1 molar ratio of the discrete oligothiophene tetramer (DP = 4) and dodecamer (DP = 12) affords a blend that is nominally a hexamer, DP = 6. More importantly, this artificial blend leads to near-white emission which is in striking contrast to the bright green emission of the discrete hexamer and yellow emission of the asprepared, disperse hexamer. Of equal significance is that a series of “artificial” oligothiophene hexamer samples can be prepared by mixing the tetramer with varying amounts of octamer, decamer, and dodecamer (Figure 5). From these isomeric materials, different emissions and variable tuning of the optical properties can be obtained. This ability to use mixtures of discrete oligomers to control properties underlines the simplicity of this strategy and the influence of dispersity on the properties of conjugated oligomers. Besides providing an efficient access to precision blends, a library of discrete oligomers can be used to construct a set of novel materials with precise structures, for example, isomers with specific monomer sequence. To demonstrate this, we examined the effect of positioning acceptor-type benzothiadiazole (BT) at different locations within the backbone of a donortype 3-hexylthiophene (3HT) octamer. Three isomers were prepared: (3HT)8-BT (D8A), (3HT)6-BT-(3HT)2 (D6AD2), and (3HT)4-BT-(3HT)4 (D4AD4). Purification of these asymmetric and symmetric donor−acceptor materials using a

Figure 3. (a) Synthesis of oligofluorene (oF) and o3HT2. (b) 1H NMR spectra for the discrete tetramer, hexamer, and decamer o3HT2. All spectra are normalized.

the 3,4′-dihexyl-2,2′-bithiophene repeat unit. Isotope analysis of these discrete molecular ions also confirmed the structural purity of the isolated discrete o3HT2 samples with each oligomer showing a single set of molecular ions that correlates with the expected molecular structure (Br- and H-chain ends). The structural purity of the discrete o3HT and o3HT2 samples was further reinforced by 1H NMR analysis, which reveals spectra changes that correspond to the structural evolution from tetramer to decamer. In all cases, as the oligomer size increases, the relative intensity of aromatic and aliphatic backbone protons increases in a linear fashion when compared with the chain-end protons (Figure 3b, Figure S4). For example, the methylene signal adjacent to the thiophene ring (2.63−2.82 ppm) gradually shifts downfield, and the overall peak shape for the backbone protons also broadens as the oligomer series progresses to the decamer, with the higher molecular weight oligomers (dodecamer and tetradecamer) resembling the characteristic spectrum for high molecular weight poly(3-hexylthiophene). We noted that while the adopted synthesis protocol gives functional chain-ends, it introduces a tail−tail linkage at the end or inside the oligomer chain. Nevertheless, this overall strategy should be applicable to other synthesis of conjugated materials.34 In addition to the controlled number of repeat units, a key feature of these discrete conjugated oligomers is the presence of well-defined chain-ends. This leads to increased versatility and makes these materials synthetically powerful building blocks. As an illustrative example, Stille coupling of the hexameric o3HT2 with vinyl tri-n-butyltin affords the vinyl terminated oligothiophene hexamer, opening further chemistry (e.g., thiol−ene functionalization) while retaining the discrete nature of the 13737

DOI: 10.1021/jacs.7b05299 J. Am. Chem. Soc. 2017, 139, 13735−13739

Article

Journal of the American Chemical Society

potential utility of asymmetric materials for targeted applications.

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CONCLUSION In summary, the combination of controlled polymerization and automated chromatographic separation represents a highly efficient and powerful approach to the preparation of discrete conjugated oligomer libraries. The increased availability of different conjugated oligomers with high structural purity and in synthetically useful quantities offers unprecedented opportunities for tuning electronic and photonic properties. By simple mixing of discrete oligomers (e.g., all-thiophene mixture) or preparing novel materials with unique monomer sequence, dramatic changes in PL are achieved, illustrating the significant potential of these discrete conjugated building blocks for designer materials and nanoscale assemblies.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b05299. Experimental procedures and characterization data for all compounds (PDF)

Figure 5. Comparison of PL for discrete, disperse, and artificial mixtures of oligothiophene with the same average molecular weight (DP = 6) but varying dispersity (*: ĐSEC).

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combination of normal-phase and reversed-phase chromatography afforded pure materials with distinct optical properties (Figure 6). Specifically, as the series progresses from D8A to

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Jimmy Lawrence: 0000-0003-4455-6177 Tomoya Higashihara: 0000-0003-2115-1281 Craig J. Hawker: 0000-0001-9951-851X Author Contributions ‡

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the MRSEC program of the National Science Foundation (DMR 1121053, C.J.H.), the Dow Chemical Company through the Dow Materials Institute at UCSB (J.L. and C.J.H.), and the Institute for Collaborative Biotechnologies through grant W911NF-09-0001 from the U.S. Army Research Office (C.J.H.). The content of the information does not necessarily reflect the position or the policy of the U.S. government, and no official endorsement should be inferred. E.G. thanks the support by Grant-in-Aid from JSPS, Research Fellowship for Young Scientists (15J00430) and Innovative Flex Course for Frontier Organic Material Systems (iFront) at Yamagata University. J.M.R. thanks the Victorian Endowment for Science, Knowledge, and Innovation (VESKI) for a postdoctoral fellowship.

Figure 6. Top: asymmetric and symmetric donor−acceptor materials, D8A, D6AD2, and D4AD4 (D = 3-hexylthiophene, A = benzothiadiazole). Bottom: UV−vis and PL spectra of D8A (black), D6AD2 (blue) and D4AD4 (red).

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D4AD4, absorbance peak splitting occurs (unimodal to bimodal), and the onset of absorption redshifts while the absorbance maxima blue-shifts, in agreement with timedependent DFT calculations (Figure 6, see the Supporting Information). Interestingly, the small change in the location of the BT unit from D8A to D6AD2 results in a large emission difference (yellow, 583 nm, to red, 658 nm), which not only reinforces the significance of structural purity and monomer sequence35 for tuning material properties but also highlights the

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DOI: 10.1021/jacs.7b05299 J. Am. Chem. Soc. 2017, 139, 13735−13739