Research Progress

1.     p-conjugated system stemmed out from diazocines and the development of related synthetic methodology.

Scheme 1-1. Synthesis of diazocines

Dibenzodiazocines, a compound with hormone-like activity, could also be a building block for for artificial muscles due to their reversible conformational changes occurring during electrochemical redox processes. However, synthetic strategies toward diazocines arequite limited. We developed a novel synthetic methodology toward diazocines via a facile dimerization and cyclization of 2-benzoylbenzoyl azides (Scheme 1-1). The reaction condition was later optimized to catalytic acid conditions and the reaction mechanism was clarified. (Org. Lett., 2011, 13, 709；Tetrahedron, 2012, 68, 9665.)  The success synthesis of diazocines led to the preparation of its laddered oligomer. The laddered oligodiazocines contains 4-6 repeating units as indicated by Maldi-TOF analysis and its electrochemical reduction behavior was confirmed (J. Polym. Sci., Part A. Polym. Chem., 2013, 51, 4694).

Scheme 1-2. Formation of diazocine cyclic host.

The dimerization of diazocines via either an acetylene bridge or a thiophene bridge was also realized (Scheme 1-2). The formed cyclic host could interact with C60 to form co-crystals. The interaction was confirmed by fluorescence quenching effectof C60 on cyclic dimer 2 (Tetrahedron Lett. 2014, 55, 3545).

Scheme 1-3. Novel synthetic methodology toward heterocycles bearing pyrrolo[3,2-b]pyrrole core.

In the exploration of diazocine related reaction, we later found out chlorinated diazocines could undergo reductive ring closure cyclization to give dihydroindolo[3,2-b]indole (Scheme 1-3). This paves an efficient and novel way toward fully conjugated heteroaromatics with pyrrolo[3,2-b]pyrrole core. Pyrrolo[3,2-b]pyrrole is the most electron-rich heterocycles among 10p electron system, which enable the heteroaromatics based on it the potential materials for OFET and OPV devices. The scope and limitation of this methodology was fully investigated, and small molecular OFET devices were fabricated and tested(Chem. Commun., 2012, 48, 12225; J. Org. Chem., 2014, 79, 11339).

Scheme 1-4. Structure-Stability relationship of antiaromatic pyrrolo[3,2-b]pyrroles.

Very interestingly, in our attempts to alkylate dihydroindolo[3,2-b]indole to make , we found that it could be easily oxidized to its antiaromatic form, indolo[3,2-b]indole. Mild condition such as air oxidation works for this transformation (Scheme 1-5). However, the success of this oxidation is strongly dependent on the structure of the starting materials. The structure-stability relationship was then investigated, and it was found that the location of the fusion has dramatic effects on the stability of the resultant antiaromatics. Theoretical calculations explained these experimental results clearly from several aspects, including the change of the frontier orbital symmetry, bond-length evolution, resonance structure and the change in antiaromaticity, and predicted the most stable antiaromatic derivatives (Chem. Commun., 2014, 50, 3324; J. Phys. Chem. A, 2015, 119, 3762).

2.     Modification and manipulation on isoindigo core structure and related conjugated polymers.

Scheme 2-1. p-Extended isoindigo derivatives.

Isoindigo-based conjugated small molecules/polymers are potential materials for OFET and OPVs. We focus on the manipulation of isoindigo core structure for better performance. We managed to synthesize thiophene-fused isoindigo via a “three-step in one”(alkylation, oxidation and condensation) protocol (Scheme 2-1). Benzothiophene and benzofuran-fused isoindigo were also synthesized. These molecules exhibit either ambipolar or n-type charge transfer properties with mobility in 10^-2cm²V^-1s^-1 range (Tetrahedron Lett., 2014, 55, 1040; RSC Advances, 2015, 5, 8340).

Scheme 2-2. Ambipolar conjugated polymer based on thiophene-fused isoindigo

Thiophene-fused isoindigo for polymerization has to be synthesized via an alternative way due to the difficulties encountered in bromination in later stage. It was then copolymerized with thiophene via Stille coupling polymerization. The obtained polymer exhibits ambipolar charge transfer properties with mobility in 10^-1 cm²V^-1s^-1 range (Polym. Chem., 2016, 7, 235).

Scheme 2-3. Homopolymer of thiophene-fused isoindigo.

Interestingly, the brominated thiophene-fused isoindigo undergoes “one-pot” Suzuki and Stille homopolymerization(Scheme 2-3). Compared with copolymers, this homopolymer exhibits much more ordered structure in thin-film state and better ambipolar charge mobility compared with homopolymer of isoindigo (RSC Advances, 2017, 7, 25009).

Scheme 2-4. Unexpected reduced form of pyrazinoisoindigo

In our attempt to synthesize more electron poor isoindigo derivatives by incorporation of benzopyrazine moiety, we found that only the reduced form of pyrazinoisoindigo could be obtained (Scheme 2-4). Contrary to electron-deficient isoindigo, this reduced pyrazinoisoindigo is a stable electron-rich donor, and exhibits hole mobility in 10^-2 cm²V^-1s^-1. The synthetic mechanism was also discussed (Eur. J. Org. Chem., 2016, 2603).

Scheme 2-5. Isomerization of isoindigo to a novel building block for wide bandgap OPVs.

We also developed a novel isomerization strategy to convert isoindigo into dibenzonathphalidione (DBND) in three steps (Scheme 2-5). Conjugated polymers based on DBND are wide bandgap polymer with Eg >2.2eV, but still exhibit high power conversion efficiency (PCE) up 6.32%, the best value among the polymers with bandgap larger than 2.2eV (Chem. Mater., 2016, 28, 6196).

Scheme 2-6. Opposite influences of ortho- vs para- fluorination on DBND based polymers.

We further modified the DBND core structure by introducing F atom, to adjust the LUMO/HOMO energy levels of the corresponding polymers and improve their coplanarity for better OPV performance. Unexpectedly but interestingly, we found that ortho-fluorination and para-fluorination have completely opposite effect on the PCE of the corresponding polymer: while o-fluorination largely decreases PCE to a value below 2%, p-fluorination boosts the PCE from 5.75% to 6.55%(Scheme 2-6). We found that different fluorination positions lead to different dipole moments and different interchain interactions, which eventually changes the solubility of the two polymers and the phase separation in the active layer (Chem. Mater., 2017, 29, 9162).

Scheme 2-7. N- vs. O- alkylation influence on OFET performance of DBND based polymers.

With an improved synthetic strategy, we prepared the N-alkylated isomer of DBND (N-DBND), which gave us a chance to compare it with O-alkylated DBND (O-DBND) on their influence on the OFET performance of the corresponding polymers. It was found that the polymer based on N-DBND exhibits a much higher hole mobility (0.55 cm² V⁻¹ s⁻¹), almost 100 times greater than the one based on O-DBND (0.006 cm² V⁻¹ s⁻¹), although both polymers exhibits very similar coplanarity and crystalline patterns, as shown in Scheme 2-7. The reasons for such a huge difference were thoroughly investigated. It was found that repeating unit in the polymer based on N-DBND exhibits a much higher dipole moment (1.56 D) than that based on O-DBND (0.49 D), which results in a much stronger intermolecular binding energy (-57.2 Kcal mol^-1 vs -30.0 Kcal mol^-1), and leads to shorter p-p stacking distance (3.63 Å vs. 3.68 Å). We come to the conclusion that the more polar amide bond in N-DBND is the major factor which governs the charge transport properties, which overwhelms the side-chain engineering effect that O-alkylation might bring in (Macromolecules, 2017, 50, 8497).

Scheme 2-8. N- vs. O- alkylation influence on OPV performance of DBND based polymers.

The influence of N- and O-alkylation of DBND on the OPV performance of the corresponding polymers was further studied. With a longer alkyl sidechain which guarantees the solubility of the three copolymers, we found that polymer based on half-N-alkylated-half-O-alkylated DBND (P(N,O-DBND-2T)) exhibited the best performance with a PCE around 5%. The higher dipole moment and the better solubility due to the asymmetrically alkylated DBND core may account for the better nano-phase separation of the polymer when blended with PC₇₁BM (RSC Adv., 2019, 9, 12310). 

Scheme 2-9. Synthesis towards bromo-thiazoloisoindigo and its stability study.

In our continuous efforts to manipulate isoindigo core, we finally achieved the synthesis of thiazoloisoindigo (TzII). A lithium tetramethylpiperidine (TMP)-promoted nucleophilic cyclization strategy was established to synthesize the challenging thiazoloisatin, which in turn underwent facile dimerization to afford the brominated thiazoloisoindigo. Thiazoloisoindigo was shown to ehxibit a deeper LUMO energy level than isoindigo and thienoisoindigo, which makes it a potential building block for n-type semiconductors. Moreover, brominated thiazoloisoindigo is sensitive toward nucleophilic attack, and its reaction with aniline resulted in an unexpected much more stable acceptor witha LUMO energy level as low as -3.82 eV(Org. Chem. Front., 2018, 5, 422.).

Scheme 2-10.  Polymer based on thiazoloisoindigo and its OFET performance.

The synthetic route was further optimized:  the electron-poor nature of the thiazole ring mediate was mediated by attaching a thiophene ring to it, which underwent Frediel-Crafts intramolecular acylation easily and eventually led to the synthesis of polymerizable thiophene-flanked TzII, and the polymer based on TzII is therefore obtained for the first time. As what we have designed, TzII possesses the merits of both ThII and TzII: it is not only easy to form quinoidal structures as ThII, which causes the large redshift of the UV-vis absorption of the corresponding polymers, but also highly electron-deficient as DAII, which is suitable to construct high performance ambipolar OFETs. Its copolymer with thiophene, P(TzII-TTT), exhibits well balance ambipolar transport characteristics with the highest hole and electron mobilities of 3.93 and 1.07 cm² V^-1 s^-1, respectively. This work shows the great potential of TzII as a novel electron-deficient and quinoidal-structure-preferred building block for conjugated polymers, which may have a wide range of promising applications in organic electronics not limited to OFETs, but also other areas such as photo-detection and photoacoustic imaging (Chem. Eur. J., 2018, 24, 9807.).

3.     Learn from spider: oligopeptide directed self-assembly of functional materials.

Silk of spiker is one of the strongest natural fiber with high ductility, owning to its alternating block structure consisting of highly crystalline domains and flexible linkers. The crystalline domains are mainly composed by the oligopeptides which form b-sheets. Inspired by these oligopeptides, we synthesized a clickable tetrapeptide (N₃GVGVOMe) which could easily form organogels by self-assembly into b-sheets (Supramol. Chem. 2013, 25, 269). This oligopeptide was then used to modulate the self-assembly properties of functional small molecules and macromolecules.

Scheme 3-1. Multi-stimuli responsive organogels.

N₃GVGVOMe was attached to dithienylcyclopentene via click chemistry to form a organo-gelator, which forms stable gels in THF, acetone and acetonitrile. The organogel is multi-responsive to various external stimuli including temperature, light, chemicals, andmechanical force (Scheme 3-1). Moreover, in the presence of catechol, this gelator forms a more robust organogel, accompanied by a dramatic change of the assembly manner and rheological properties (Soft Matter, 2013, 9, 7538).

GVGV tetrapeptide was attached oligothiophenes via click chemistry to form peptide–thiophene–peptide (PTP) and thiophene–peptide–thiophene (TPT) to investigate the influence of peptide contentration and location on the nanostructures and properties of the assemblies. Both conjugates formed organogels consisting of left-handed twisted nanostructures; however, anti-parallel b-sheets were observed in PTP while parallel b-sheets were obtained for TPT, although in both cases oligothiophenes adopted an H-like stacking mode. Obvious solvent-induced supramolecular chirality inversion from the oligothiophene segment was observed for PTP while such phenomenon was not clear for TPT (Scheme 3-2). PTP and TPT gels also showed different stabilities towards temperature increase, as evidenced by variable-temperature circular dichroism study (Supramol. Chem., 2014, 26, 383).

Scheme 3-2. Peptides’ content and location influence chirality transfer.

N₃GVGVOMe tetrapeptide was also attached to terthiophene via click chemistry, which was then polymerized either in solution state or in organogel state to give the corresponding polythiophene with oligopeptide side chain. Nanorods was formed with controllable size, as shown in Scheme 3-3 (Supramol. Chem., 2014, 26, 383). 

Scheme 3-3. Oliogpeptide controlled self-assembly of polythiophenes.

Scheme 3-4. Molecular weight dependent assembly behavior.

More interestingly, the alternating copolymer of N₃-GVGV-N₃with oligothiophene exhibits strong molecular weight-dependent self-assembly behaviors (Scheme 3-4). The copolymerwithlowest molecular weight (M_W = 7400) assembles into well-ordered fibrous nanostructures, while the one with highest molecular weight (M_W = 16980) assembles into nanoballs. The one with medium molecular weight (M_W= 14800), exhibits more complicated self-assembly behaviors, more like a transition state between the other two (Macromol. Chem. Phys., 2014, 215, 906.).

Scheme 3-5. Inclusion of Cu nanoparticle inside a C3-symmetric artificial oligopeptide fiber.

N₃-GVGV-OMe oligopeptide was attached to a benzene 1,3,5-tricarboxamide (BTA) derivative viaclick chemistry to afford a C₃-symmetric artificial oligopeptide. The key feature of this oligopeptide is that the binding sites (triazole groups formed by click reaction) are located at the center, while the three oligopeptide arms with a strong tendency to assemble are located around it, which provides inner space toaccommodate nanoparticles via self-assembly. The inclusion of Cu nanoclusters and the formation of one-dimensional (1D) arrays inside the nanofibers of the C₃-symmetric artificial oligopeptideassembly were observed (Scheme 3-5), which is quite different from the commonlyobserved nanoparticle growth on the surface of the preassembledoligopeptide nanofibers via the coordination siteslocated outside (Nanoscale, 2015, 7, 20369.).

4.     Learn from mussel: realization of strong underwater bonding via the design of mussel-inspired adhesives.

Mussels could attach themselves onto rocks under severe underwater conditions, which inspires scientists to design novel mussel-inspired adhesives. Although many progresses have been made, there are still two major obstacles remaining: 1. to realize strong underwater adhesion; and 2. to lower the cost of the preparation of such bio-inspired materials.

Scheme 4-1. Mussel-inspired adhesives with polyoxetane sidechain.

In our initial efforts, we focused on the preparation of polymeric mussel-inspired adhesives with polyether backbone. Polyoxetane was chosen since it exhibits biocompatibility similar with poly(ethylene glycol) (PEG), while it has sidechains that could be grafted with catechol moieties, which mimics the active component in mussel foot proteins (Mfps). Catechol moieties was then grafted onto it to form our first-generation mussel-inspired adhesives(Polymer2014, 55, 1160), which exhibits strong adhesion strength at dry conditions (Scheme 4-1). Bonding at humid conditions was realized by attaching phosphorate groups onto the catechol moiety to make the polymer water-soluble, and bonding strength up to 0.3 MPa was realized (Macro. Chem. Phys., 2015, 216, 450)

Scheme 4-2. Mussel-inspired adhesives with stronger underwater bonding strength than at dry conditions.

To further improve the underwater bonding strength of the bio-inspired adhesives, we next chose poly(N-vinylpyrolidone) as the backbone, which mimics the polypeptide backbone of Mfps. Catechol moiety was then attached to this backbone to finish our 3rd-generation mussel-inspired adhesives. Very interestingly, this novel adhesives exhibits stronger underwater bonding strength than at dry conditions (Scheme 4-2). The possible mechanism for such phenomena was discussed(Chem. Commun., 2015, 51, 9117).

Scheme 4-3. 4th-generation of mussel-inspired adhesives.

Aiming at solving the sophisticated preparation-steps problem for mussel-inspired adhesives, we further developed a one-step synthesis of a hot-curing adhesive with strong bonding strength up to 17 MPa from commercially available polyvinyl alcohol and 3,4-dihydrobenzaldehyde. Such bonding strength surpasses most commercially available adhesives (Macromol. Rapid Commun., 2016, 37, 545). Such a low-cost, solvent-free and easy-to-use hot-curing adhesives may find real-world applications (Scheme 4-3).

Scheme 4-4. Robust Zn²⁺-releasing coating based on PVA-g-DBA.

This easy-to-prepare adhesive can also be used to make robust coatings.The catechol moiety in this polymer was used to chelate with Zn²⁺, which could continuously release zinc ions at high release rate when immersed into artificial seawater (ASW). We can also utilize the unreacted alcohol group on PVA-g-DBA to introduce anti-fouling ingredients such as PEG via isocyanate chemistry, which shows adjustable resistance toward protein adsorption and bacterial adhesion. Both coating exhibits high mechanical strength, strong adhesion to stainless steel, and excellent anti-abrasion properties, as shown in Scheme 4-4 (J. Mater. Chem. B, 2017, 5, 1742). 

Scheme 4-5. The contribution of the polarity of the polymers in underwater bonding.

We further disclosed the contribution of the polarity of the mussel-inspired adhesives in the realization of strong underwater bonding. Four mussel-inspired adhesives with similar catechol contents and molecular weights but different polymer backbones were synthesized and their underwater adhesion properties were compared. It was found that the underwater bonding strength increases with the increase of the amide/lactam bond content on the backbone, as shown in Scheme 4-5. Dielectric constant was introduced to evaluate the polarity of the mussel-inspired adhesives, which provides a semi-quantitative parameter to correlates the contribution of polymer polarity to the underwater bonding properties. Polymer with PVP backbone shows the highest ε′ value, exhibits the best underwater adhesion behavior, the underwater bonding strength of which exceeds 1.0 MPa, five times stronger than that of polymer with polystyrene backbone. Our results indicate that the polarity of the polymer backbone is a factor that might be added to the consideration sheets for the design of more effective mussel-inspired underwater adhesives (ACS Biomater. Sci. & Eng. 2017. 3, 3133).