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Molecular defects in organic materials

Unravelling the relationships between defects and material properties is a daunting task, but increased analysis and knowledge could be beneficial in many applications.

Solid inorganic materials contain defects that can significantly influence mechanical strength, ionic or electronic transport, and other physical properties. These defects also moderate chemical potential and affect reactivity. It can even be argued that defect engineering is pivotal in the fabrication of metals, semiconductors and ceramics.

Organic synthesis, however, is fundamentally different — organic chemists tend to target discrete molecules and aim to precisely correlate their structures and properties. For macromolecules, this correlation is weaker because deviations from the ‘perfect’ structure are commonplace and often remain unknown. Moreover, the degree to which these imperfections cause erratic variations in properties is uncertain. It is thus understandable that many organic chemists restrict their attention to small molecules in solution, for which it is possible to know almost everything, but cast doubt on organic macromolecules and materials. However, with the knowledge that complex function often stems from complex structures, we must not retreat to simple molecules, but must take on the challenges of structural perfection, purity and systems integration of macromolecular entities.

Achieving structural precision

Many polymer scientists still accept a level of defects that scales with molecular size and complexity. With this acceptance, several pertinent questions remain unanswered. For example, do polymers, and materials derived thereof, possess truly characteristic, intrinsic properties? And does this lack of knowledge cause a knock-on effect on how we strive to create useful materials and the scientific rigor involved? In the polymer community, terms like ‘well-defined polymers’ or ‘precision synthesis’ (Ref. 1) are used in cases that fall short of absolute perfection. This lack of precision hampers investigations into the properties of materials; for example, studying the elasticity of polymer networks is difficult because of the lack of ‘ideal’ models with monodisperse chains between crosslinks and without the presence of dangling ends.

Polymerization techniques have been developed that aspire to give the chemist control over molecular weight and structure. In polyolefin chains, the position and configuration of substituents sensitively influence thermal and mechanical properties. Anionic polymerization of activated olefins offers ‘living’ ends that are exploitable for end-capping or block copolymer formation; however, ‘living’ ends are more often hope than reality. In radical polymerization, various additives are used to achieve ‘control’ over chain growth and suppress side reactions. In the synthesis of conjugated polymers by various carbon–carbon coupling protocols, undesired chemical side reactions limit the achievable molecular weights, leading to the inclusion of sp3 centres that interrupt π conjugation or produce functional groups that trap excitation energy. These features can severely compromise light-emitting behaviour or charge-carrier transport.

The properties of organic materials also depend on molecular packing and hence structural imperfections in the solid state. A high degree of supramolecular order is beneficial for conduction in organic semiconductors that are active components of field-effect transistors. A packing defect can act as a scattering site and obstruct charge transport, despite being undetectable by modern methods of structure determination. Single crystals — for example, of organic semiconductors — are considered as the ideal case of highly ordered, pure materials. In reality, crystals contain various defects that influence catalytic properties or compromise charge transport.

Supramolecular architectures, as present in thin-film devices, rely on an interplay of many weak, non-covalent intermolecular forces. Although concepts such as ‘side chain engineering’ for semiconductor polymers have been invoked, targeting the right polymorph upon processing remains a matter of trial and error. Further, it is not clear whether kinetically trapped ‘wrong’ states can be healed using post-treatments such as thermal annealing. An example of useful supramolecular order is discotic mesophases that have columnar channels for charge or exciton transport in which a dynamic structure is combined with the desired order. In this case, however, defects such as bifurcation of columns can neither be avoided nor healed.

Although herein the focus is on defects present at the level of molecules or their supramolecular packing, it should be noted that polymers can form microscopic defects under the influence of environmental stresses. These defects can develop into macroscopic voids — namely, cracks — that severely hamper mechanical properties. Healing of these defects becomes possible with a mobile phase that fills the cracks and then solidifies, a repair that occurs either autonomously or subject to external stimuli.

Nature's way

Although nature is the master of precision macromolecular synthesis, ‘mistakes’ have contributed to species diversity and to health problems of individuals. DNA damage can result from a violation of base pairing rules or from chemical processes such as pyrimidine dimer formation. A gene carries the recipe for the synthesis of proteins and, hence, a mutation may prevent protein expression, such as that of tumour suppressor protein p53, or alter the structure of the expressed protein. One single nucleotide error is the cause of sickle cell anaemia, an illness that compromises the entire bloodstream. At the upper echelon of the hierarchy, protein misfolding and defects are known as causative agents for neurodegenerative diseases. Nonetheless, defects at both the transcriptional and translational levels can be rectified by systems of proteins. The synthetic world has attempted to design corrective materials capable of resolving defects, but breakthroughs have been marginal. These attempts have involved the exploitation of molecular or macromolecular symmetry together with dynamic bonds to induce self-correction. The molecules used in an assembly are semi-rigid, thus reducing conformational flexibility and allowing any structural misfits or errors to be corrected through reversible intermolecular forces. Therefore, planar (macro)molecules and assemblies might serve as an ideal platform. Indeed, defect-free planar fractals have been made on surfaces by the deposition of small oligophenyl molecules with a 120° kink and by using a symmetry match between substrate and adsorbate2.

The role model of graphene

A good example for balancing defect tolerance with synthetic perfection is graphene. This 2D polymer can be fabricated in a top-down manner by exfoliating graphite via intercalation processes or oxidation to dispersible graphene oxide. Alternatively, graphene can be synthesized in a more controlled bottom-up approach by fusing together benzene rings to form increasingly large graphene-like ‘honeycomb’ lattices3. The fabrication of graphene from graphene oxide is popular amongst materials scientists, although this precursor is structurally ill-defined.

Turning again to semiconducting properties, the high charge-carrier mobility of graphene holds enormous promise for its use in field-effect transistors. However, the vanishing electronic bandgap of graphene does not allow a turn-off behaviour of the devices. The necessary bandgap opening becomes possible by a geometric confinement, as occurs in graphene nanoribbons. Top-down protocols towards synthesizing nanoribbons, such as unzipping carbon nanotubes, cannot give perfect edges required for a defined electronic band structure. This emphasizes the important role of precision synthesis. By contrast, it is possible to intentionally introduce defects, such as vacancies, into graphene and to control electronic, mechanical and catalytic properties.

Future perspectives

A chemist is generally trained to rely on strict structure–property correlations, however, if there is no such thing as a perfect structure, then the rational design of materials properties becomes more difficult. Also, defects could be of varying importance and, as a result, it may be possible to define a hierarchy of structural errors. The following guidelines are useful to take on the challenge of defects:

  • Explore improved synthetic protocols and building blocks to make defined materials with high precision and reproducibility. Also, find methods of sensitively detecting, characterizing and theoretically understanding structural deviations. It may be that the properties of a real organic material do not represent its intrinsic characteristics because we are just a defect or two away from the ideal molecular, supramolecular, morphological and environmental conditions. In electronic devices, the functions and performances of defect-free materials could stand head and shoulders above what we have now.

  • Design materials that can self-heal their defects. Redistributing weak covalent or non-covalent junctions of polymer segments in networks and gels using environmental stimuli holds promise to improve properties. Also, reshuffling polymer structures has become possible in the course of living supramolecular polymerization in which assembly occurs far from thermodynamic equilibrium4.

  • Increase the structural complexity of materials so that they can tolerate unavoidable defects. For example, in a 1D electrical conductor, a defect obstructs the percolation of charges, but an increase in dimensionality could make the transport isotropic and open pathways to bypass the defect.

  • When synthetic materials perfection is impossible, turn vice into virtue and employ ‘defect engineering’ to achieve inaccessible properties. Rewarding examples are crystalline metal–organic frameworks5 or graphene. Their electronic structures, although different in origin, are believed to sensitively depend on the nature and number of defects or on heteroatom doping.

Although macromolecular synthetic perfection may continue to evade us for decades, it should be looked on as an exciting task rather than a reason for pessimism or a plea for negligence. Although the transition from covalent to non-covalent bonding has been highlighted, other fields of chemistry also demand attention, such as reactions under extreme conditions or in confining geometries. In any case, striving to master chemical complexity seems to need more advanced defect management.

References

  1. 1.

    et al. (eds) Sequence-Controlled Polymers: Synthesis, Self-Assembly and Properties (ACS Symposium Series 1170, 2015).

  2. 2.

    et al. Assembling molecular Sierpinski triangle fractals. Nat. Chem. 7, 389–393 (2015).

  3. 3.

    et al. New advances in nanographene chemistry. Chem. Soc. Rev. 44, 6616–6643 (2015).

  4. 4.

    et al. Living supramolecular polymerization realized through a biomimetic approach. Nat. Chem. 6, 188–195 (2014).

  5. 5.

    et al. Defect-engineered metal–organic frameworks. Angew. Chem. Int. Ed. Engl. 54, 7234–7254 (2015).

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Affiliations

  1. Max Planck Institute for Polymer Research, 55128 Mainz, Germany.

    • Klaus Müllen

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The author declares no competing interests.

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Correspondence to Klaus Müllen.