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Macromolecules Volume 24, Number 13 June 24,1991 @ Copyright 1991 by the American Chemical Society Macromolecules Volume 24, Number 13 June 24,1991 @ Copyright 1991 by the American Chemical Society Reviews A Chemical Approach to the Orbitals of Organic Polymers Roald Hoffmann,' Christoph Janiak, and Christian Kollmar Department of Chemistry and Materials Science Center, Cornell University, Ithoca, New York 14853-1301 Received September 8,1990; Revised Manuscript Received January 8,1991 ABSTRACT The electronic structure of the prototypical unsaturated and saturated organic polymers poly- acetylene and polyethylene is elucidated by a building-up process starting from a linear carbon chain, which is then kinked and finally has one or two hydrogen atoms attached to it. In this process the common features as well as the differences between polyacetylene and polyethylene (semiconductor vs large band gap insulator, unsaturation vs saturation) emerge in a natural way. The emphasis is on bringing together the essential concepts of solid-state physics and simple ideas of chemical bonding. Some topics of special interest, such as bond alternation and solitons in polyacetylene, are also discussed, in as simple and chemical manner as poasible. Synthetic organic polymers mark our time.' The most unsaturated one, one that provides for a conjugated u common such polymer, polyethylene (l),has an extremely system consisting of p orbitals perpendicular to the plane of the backbone. This u system gives rise to interesting electronic properties? thus attracting the attention of both physicists and chemists. For instance, it provides for localized elementary excitations known as soliton^^^^ and polarons6**and nonlinear phenomenag of great interest to theorists. Some of these polymers become highly con- 1 ducting on doping, as conducting as the best metals.6J0 This makes them candidates for practical applications in wide range of applications, responsible for its production the future; recently the mechanical strength of polyacet- volume of -8 billion kilograms in the United States in ylene has reached an impressive magnitude." At the 198ga2 The interest in polyethylene and its derivatives moment, though, the conjugated polymers are interesting derives not from its electronic properties-it is a large mainly for theoretical reasons.12 In the way that reality band gap insulator, generally colorless-but from its and interest have of getting out of phase with each other, strength and mechanical proper tie^.^ These can be tuned one suspects there are more people in academia working over an impressive range, therefore the economic value of on conducting organic polymers (of no industrial value as these synthetics. Even though the electronic properties yet) than there are working on the typical synthetic fibers of polyethylene appear to be less important than its me- and plastics (which represent a good fraction of the GNP chanical ones, the underlying electronic structure deter- of industrialized countries). mines all properties. The rational design of new synthetic The simplest representative of the conjugated polymers polymers based on polyethylene must draw upon our is polyacetylene (2).l3 It is also the one that has been knowledge of how the electrons move in this simplest most thoroughly investigated, both theoretically and polymer. experimentally. The T system of this polymer localizes in There are other polymers that have not yet found their alternating single and double bonds within the carbon way to practical application, but are interesting for backbone, due to a so-called Peierls distortion, a phe- different reasons. These are the conjugated polymers.4 nomenon we will discuss below. Undoped polyacetylene They also have a planar carbon backbone, but now an is a semiconductor with a large band gap of over 1.4 eV." 0024-9291191 /2224-3125$02.5Q JO 0 1991 American Chemical Society 3726 Hoffmann et al. Macromolecules, Vol. 24, No. 13, 1991 How to begin? An aufbau principle has served us well in understanding the structure of atoms, and fragment orbitals are a useful way to look at the bonding of complicated molecule^.^^^^ It seems obvious that one should make use of the common feature of polyethylene and polyacetylene, namely their carbon backbone, if one wants to perceive their similarities. Accordingly, we begin with a simple linear carbon chain, in diagram 3. Finite But it can be doped to be as conducting as copper.1° The whole wealth of interesting electronic phenomena, which is so attractive to theorists, derives from the conjugated ?r system.6 Because polyethylene is both important and so simple in its structure, there have been many theoretical studies of it, at every level of sophisticati~n.~~These calculations are done not only because the unit cell is so small, but also because one needs the electronic structure to understand and interpret the experimental data on the valence and conduction band of polymers, derived from techniques such as UPS18121pn XPS/ESCA17*2S*U(ultraviolet, respec- tively, X-ray photoelectron spectroscopy/electron spec- troscopy for chemical analysis), SEE*% (secondary electron emission), EELS29 (electron energy loss spec- carbon chains, of course, have some interest on their own. troscopy), photocond~ction,~*~~etc. On the side of poly- Carbon clusters were first seen in arc discharges and the acetylene, theoretical calculations of the band structure, tails of comets.& Now they can be generated systematically some of which we have already mentioned, are still more and, for short chains, may have linear structures.“ A abundant.32 synthesis of Cn chains of substantial length has been What we want to do in this contribution is to put the reported.42 electronic structure of these two polymers in their proper To prepare the carbon chain for bonding in the polymers, closely related perspective. Their striking similarities this chain must be kinked. The addition of one or two (C-C and C-H bonding) will emerge, as will their differ- hydrogens per carbon then “generates” polyacetylene or ences (saturation vs conjugation). A discussion of these polyethylene. One of the very, very few advantages of polymers is also part of a pedagogic aim we have, which theory over experiment is that molecules do not have to is to demystify solid-state theory, and to bring to chemists be made in the computer the way they are in the laboratory. psychological comfort and confidence in dealing with So this theoretical construction bears no resemblance to bonding in extended systems.% Such confidence is any realistic synthesis of either polyethylene or polyacet- necessary for chemists to enter as equal partners with ylene. It is, however, a perfectly good way to get at the physicists and engineers in the design of materials with similarities and differences of our two polymers. But unusual desired properties. before we can embark on this theoretical construction we It must be said clearly at the outset that there is little need to introduce the requisite theory. original in this paper. The band structures of polyethylene and polyacetylene we present are available, from better The Tight-Binding Method calculations than ours, in the literature.16*32What is new We need the orbitals of an infinite or long chain of carbon is the pedagogical perspective, and the intent in every way atoms. If N is the number of such atoms (N is large, possible to overcome the barriers of mind and jargon perhaps approachingAvogadro’s number) and each carbon between chemical and physical perspectives of the same has one 2s and three 2p orbitals, then the chains will have molecules. 4N atomic orbitals. From which we need to build 4N mo- Some further words on approximatiom and methodology lecular orbitals. How to do this? may be in place here. Organic, organometallic, and The procedure for accomplishing the task comes from inorganic polymers that crystallize in an extended chain solid-state physics and is called the tight-binding method. form are ideal for both an experimental and a theoretical It is essentially the LCAO-MO method for a translation- analysis if the interchain interactions are weak. Such is ally periodic structure. Let us review the basics of a mo- the case for polyacetylene and polyethylene. This allows lecular orbital (MO)approach. For any system, molecular one in a good approximation to consider only a single orbitals 1c/i are given by a linear combination of atomic isolated chain as determining the electronic structure of orbitals 4r: the bulk material.19*a One dimension is easier than two or three to think about; such single chains (CH), and +i = CCir4r (1) (CH2)” are the subject of our paper. r While we will rely on qualitative arguments throughout 1c/i represents the MO i with energy Ei, 4r is an atomic this paper, we will support these by numerical experiments, orbital. The whole set (&) of the AOs of the atoms that is calculations. On the computational level our composing the molecule constitutes the basis for any type discmsion will be based on the extended Hiickel method,% of calculation within an LCAO scheme. One needs to with which also the oldest band calculation of polyethylene determine the coefficients cir and the energies Ei. was carried out.% The extended HQckel method is a Often certain constraints are placed on the coefficients simple, semiempirical molecular orbital (MO) method, yet cir by the symmetry of the molecule in question. It may quite successful in its agreement with experimental results even happen that the coefficients of the MOs are com- for polyethylene.l7J@0a We would like tostress, however, pletely determined by symmetry and thus independent of that our qualitative considerations are relevant to ab ini- the specific computational method applied. Consider, e.g., tio calculations as we11.37 the hydrogen molecule. If we restrict our basis set to the Macromolecules, Val. 24, No. 13, 1991 Chemical Approach to the Orbitals of Organic Polymers 3727 1s orbitals $1 and 42 of the two hydrogen atoms, we obtain unit cells.
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