Synthesis of Novel Heterocyclic Building Blocks

 

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The Jeffries-EL group develops novel building blocks for the synthesis of conjugated polymers. These molecules are beneficial to the field because of their tunable properties and facile synthesis. We have prepared both electron-deficient (benzobisazole) and electron-rich (benzodifuran) monomers. Collectively these can be used with appropriate comonomers for the synthesis of donor/acceptor copolymers. The properties of these polymers are readily tuned by substitution affording narrow band-gap materials for use in organic solar cells and wide-band gap materials for use in light emitting diodes.

Synthesis of electron-deficient monomers based on benzobisazoles.

Although the literature is filled with examples of π-conjugated systems, functional electron-deficient systems for use in the synthesis of new polymers are limited. Accordingly, we have decided to pursue the synthesis of new materials based on the benzobisazoles moiety because this electron-deficient heterocycles is present in a number of materials with properties such as efficient electron transport, third-order optical non-linearity and photoluminescence. In spite of these exceptional properties materials containing benzobisazoles have not been widely investigated because of the harsh conditions required for their synthesis, which limits the types of substituents that can be introduced onto the ring system. To circumvent the limitations of traditional benzobisazole synthesis, she developed a mild approach based on ortho esters (J. Org. Chem. 2010, 75, 495; Org. Lett. 2008, 10, 4915). This versatile approach enabled the synthesis of a variety of 2,6-disubstituted benzobisoxazoles, Scheme 1. The benefits of the method are 1) the low reaction temperatures; 2) the favorable yields; 3) the ease of separation and purification of the desired product; and 4) the versatility of the method. 

Scheme 1. General approach for the synthesis of functional benzobisazoles and their copolymers.


The resulting benzobisazoles can be used as building blocks for new conjugated materials via a variety of well-known coupling reactions, simply by using monomers with orthogonal functional groups, Scheme 1. For example, the chloro-methyl derivatives can be used to synthesize phosphonate esters, which can be polymerized via Horner-Wadsworth-Emmons (HWE) coupling reactions with various aldehydes (J. Poly. Sci. A, 2010, 48 (6), 1456; Phys. Chem. Chem. Phys. 2011, 13, 1338; Pure Appl. Chem. 2012, 84, 991; J. Polym. Sci., Part A 2013, 51, 916; Macromolecules 2011, 44, 248) or via metal catalyzed cross-coupling of the thiophene derivative (Polym. Chem. 2011, 2, 2299; Macromolecules 2011, 44, 9611; Aust. J. Chem. 2014, 67, 711). The tunable nature of this system is exemplified by a series of poly (arylene vinylene) benzobisazoles that were synthesized by polymerizing benzobisazole phosphonate esters with various aromatic dialdehydes. The use of weakly electron-donating co-monomers, such as fluorene, produced poly(fluorenevinylene-co-benzobisazoles), wide band gap materials with low-lying LUMO levels, suitable for use in OLEDs as detailed below. Whereas strong electron-donating co-monomers such as dithienopyrrole afforded materials with smaller band gaps appropriate for use in OPVs, (Figure 2). 

Figure. 2. Structures of the poly(arylene vinyelene) trans-benzobisoxazoles (left), solution UV-Vis spectra of the same polymers (middle), THF solutions of the polymers under 254nm UV light (left). Solution of the ditheinopyrrole polymer is not shown.