In organic chemistry, contorted aromatics, or more precisely contorted polycyclic aromatic hydrocarbons, are polycyclic aromatic hydrocarbons (PAHs) in which the fused aromatic molecules deviate from the usual planarity.
Introduction
The comparison of structures of graphene and fullerene can help comprehend the origin of curved pi surfaces and contortion in PAHs. Both graphene and fullerene bear resemblance in having sp2 hybridized carbons; however, exhibit different geometries (allotropes of carbon). The fact that fullerenes have five-membered rings incorporated among six-membered rings, makes them spherical whereas graphene remains planar due to the presence of exclusively all six-membered rings in it. In general, the ideal bond length and angle for or is about 1.42 Å and 2π/3 respectively, whereas in corannulene structure, a five membered ring is surrounded by six-membered rings contributing to non-planarity in the molecule. This leads to change in bond angles and bond lengths rendering non-planarity in the structure. Such type of contortions in the structure of PAHs are known as 'arching distortions'. The hydrogen and carbon atoms of the PAH molecule which are either closest to one another due to non-planar structure or suffer from angle strain are known as the "saturated" ones and may serve as a source of 'splitting distortions' (see fig. 2).
The region of a contorted molecule having saturated hydrogens and carbons is known as bay region (having white spheres or saturated hydrogens) (see fig. 4).
Another reason for making the PAH molecules contorted may be the size of these molecules. Theoretical vibrational frequency studies of (n = 2-12) on coronenes using quantum chemical calculation (Hartree-Fock and DFT) show accelerated loss of planarity on increasing size of PAH molecules in-spite of having a stable structure due to conjugation, delocalization and aromatization. A gas phase coronene can be expected to switch to non-planar geometry around n = 9-12. The combined strain of arching and splitting distortion is thought to force the PAH molecules out of the plane and generate contortions. A switch to umbrella geometry may occur at n = 12 (see fig. 3).
Figure 4 illustrates the strain energies carried by three different PAH molecules. The white spheres represent saturated hydrogens and the grey spheres as the leading carbons contributing to strain energy due to non-planar distortions.
Table 1 shows the energy values for non-planarity (strain) of PAHs in kcal/mol for five-membered and six-membered ring molecules with common helicene and coronene units with reference to fullerenes C60 = 483.91 kcal/mol and C70 = 492.58 kcal/mol. The shaded parts indicate the bay regions in PAH molecules bearing the most strain. The strain energy is expressed in units of 10-2 kcal/mol. The non-planar strain (contortion) energies indicate how much a PAH molecule deviate from the standard planar structure in terms of bond angles and bond lengths. The standard for PAHs is usually graphite. An is introduced in the molecular structure whenever it deviates from the standard structural geometries to get the strain relief. The values represented for a variety of PAHs in Table 1 are shown. Two types of contortions or Enp can be observed based on the data in Table 1. The PAH molecules shown in table 1 are based on two kinds of motifs. The ones based on helicenes and the others on corannulenes. The small values (ranging from 0.25-8 kcal/mol) represented by the helicene based molecules and are because of weak contortions in their structures. The contributing factor seems to be the presence of only splitting distortions in the bay region of these PAHs. However, the other group based on corannulenes show higher values (ranging from 1.86-116 kcal/mol). The higher values suggest stronger contortions which may have originated due to the higher number of fused rings and a five-membered ring core in PAH molecules along with the bay region contortions.
Based on the above discussion it can be cautiously inferred that the combined effects of non-planar strain caused by arching and splitting distortions as well as the size of PAH molecules give birth to contortions in PAHs structures.
These unparalleled contorted molecules exhibit wide absorption spectrum and enhanced charge transport properties which make them potential candidates for electronic and optoelectronic applications. The curvature in these molecules cause slight shifting of the frontier orbitals from ideal parallel symmetry. The departure from parallel overlap induces modification in HOMO and LUMO eventually giving birth to changed optical properties. Inversion of these molecules in solution may allow changes in orbital geometry, thereby, broadening the absorption and emission spectra which is useful for light-emitting diode (LED) applications. Some derivatives of corannulenes act as blue emitters. The efficient charge transport in contorted aromatic molecules is related with self-assembly and crystal packing. The discotic contorted molecules in crystals tilt relative to the columnar axis because of the interaction of protons with the electronic cloud of neighboring PAHs molecules. This tilt forbids the latitudinal stacking of molecules and allows only the longitudinal stacking (see fig 6). This property enhances the linear overlap of orbitals and promote the charge carrier mobility. All these properties make the contorted PAHs excellent candidates for semiconductor, organic field effect transistors (OFETs), and organic photovoltaic (OPV) devices applications.
Discs and ribbons (see Fig. 1) constitute the major classes of contorted aromatic structures. The discs possess a concave π molecular surface and upon substitution with a hydrocarbon side chain can be self- assembled to turn into columns figure5. These columns can express the desired properties of nanoscale phase separation, charge separation and charge transport in films. The concave discs can behave as molecular sensors for electron deficient aromatic molecules. Once in contact with electron acceptors like fullerenes they show typical characteristics of a p-n junction in shape of a ball and socket joint model Figure 7.
The ribbons, on the other hand, can be envisioned as pieces of graphene being spun into ribbons owing to contortions. They are good materials for OFETs as electron transporting materials. With the incorporation of donor polymers they behave as potential alternative for non-fullerene based organic solar cells. The focus on developing the non-fullerene organic semiconductor is because of some potential
downsides of fullerene-based materials for n-type organic molecules. Fullerenes fail to fulfil the basic criterion of good organic semiconductors by not having good absorption in UV-Vis region and poor tunability upon substitution with various groups. These demands have fuelled research towards finding new materials including contorted aromatic molecule which possess fundamental qualities of wide absorption range and better charge transportation. These molecules also provide controlled π-π stacking in small domains and excellent charge percolation pathways.
History The Nobel Prize in Chemistry of 1953 won by Hermann Staudinger for characterizing macromolecules as polymers opened the gateway for a new field of materials. Ever since that time the polymers have been replacing conventional materials like wood, metals and now are widely used as conductors and semiconductors in commercialized products. The age of plastic based society really began in the wake of 1953's Nobel prize. It was later revealed in 1970 that some polymers can exhibit appropriate electrical conductivity. The thought would have been consolidated by the fact that graphite a purely carbon-based material was capable of electric conduction due to its expansive π conjugation system. The organic conductors are however, not as good as metal-based conductors. By late 1970s efforts were made for fabricating polymer-based power transmission lines, light weight motors and novel approaches for achieving superconductivity. Currently massive worldwide efforts for achieving higher power conversion efficiencies in OPVs, better hole carrier mobilities in OFETs in π-conjugated polymer domain are underway. The discovery of iodine doped polyacetylene and its electrical properties in 1977 fuelled the research work towards finding better and more efficient conjugated organic polymers. The '2000' Nobel Prize by Alan J. Heeger, Alan G. Macdiarmid, and Hideki Shirakawa in recognition of their contribution towards unravelling the polyacetylene figure electronics set another milestone in the journey of finding conducting polymers. Over the past two decades developments have been made in synthesizing new polymers like polythiophenes, polyphenylenes and polyphenylene sulphides and small organic molecules. The contorted aromatic molecules due to their properties of making ladder polymers, self-assembly and innate charge percolation pathways because of small domains and π stacking other than providing conjugations throughout their structure became another focal point for their use in a variety electronic application. These contorted molecules offer a delicate balance and phase separation which can promote the power conversion efficiency (PCE) close to theoretical limit of 20%. Widespread futuristic studies on synthesizing new small contorted aromatic molecules capable of being stable over longer period and higher PCE are being carried out.
From inorganic semiconductors to Contorted Aromatic Small molecules One may keep getting pestered by the simple question of why we need the organic solar cells? No wonder if the commercial inorganic silicon based solar cells had been doing fine no one would have ever thought of an alternative way of getting at the same point. The answer is very simple. Inorganic solar cells have their own pros and cons. But the point of concern would be to address the issues which have limit their wide spread use so far. One of the major drags in commercializing this technology on massive scale is the cost. Highly purified silicon production needs a lot of energy which would add to the cost of this technology on one hand and hurt the green chemistry on the other. Efforts are underway to bring about new ways to aggregate as much stacks of silicon material as possible to increase the PCE which does not seem to help as far as cost reduction is concerned. The inorganic solar panels are heavy and non-flexible. These factors count towards limiting this technology for mass adaptation and may not be able to compete with other green and cheap energy sources like wind and hydro power. Multi junction silicon based solar cells however, are the best so far for providing highest PCE 44.4%. All the above-mentioned challenges have been pushing the scientific community to seek for new opportunities. The organic solar cells seem to supersede their predecessors in many aspects but the challenge of competing with PCE remains. Unlike the inorganic solar cells, the material requirement for fabrication of OPV is much smaller. They can be printed out using printing tools which would help in low cost commercialization of these materials. Solution processable organic semiconductors can be easily fabricated into thin films. In short OPVs, OFETs and organic light emitting diodes OLEDs are light weight, flexible, easy to fabricate, inexpensive and tunable to demand as compared to the conventional inorganic materials. The downsides of organic small contorted molecules as semiconductors is their less reliable long-term usability and low PCE so far. The highest PCE for organic solar cell achieved so far is 11%.
When it comes to the reasons that limit OPVs power conversion efficiencies, we need to understand the structural and chemical limitations that existing organic small molecules offer.