Modern chemistry enables the synthesis of complex molecules that are comprised mainly of carbon, hydrogen, oxygen and nitrogen atoms. These atoms are linked together by energetically strong covalent bonds in which two neighboring atoms each each one electron. In addition, non-covalent interactions inspired from biology such as hydrogen bonding can be used to build complex supramolecular structures. Such bottom-up approaches are at the basis of nanotechnologies. The choice of molecular building blocks with varying electron affinities and the control of their interactions allow for the desing of materials with tunable optical, electronic, and mechanical properties.
Carbon can form four bonds with neighboring carbon atoms or other atoms. In some simple molecules such as methane (CH4), four valence electrons occupy four sp3 hybridized orbitals and can form four covalent bonds. However, in more complex molecules three of the four valence electrons occupy sp2 hybridized orbitals that are in a plane and the fourth electron occupies a p orbital that is perpendicular to this plane. When two such electrons are on adjacent carbon atoms they form an energetically weak bond that is called a p bond. This bond together with a bond formed between two electrons in sp2 orbitals form a double bond. In molecules that are called conjugated, atoms are linked together by alternating single and double bonds. An example of such a conjugated molecule is the benzene molecule shown below in which six carbon atoms form a closed loop.
The wavefunctions associated with these electrons are highly delocalized and provide the molecule with unique properties: first this delocalization leads to high polarizability under the influence of an electric field that is reponsible for strong nonlinear optical properties; second the weaker binding energy allows for electron transfer reactions between molecules that allow charge to be transported. The hopping of these p electrons between molecules confer semiconducting properties to these materials.
Molecules with small molecular weights can be processed from the vapor phase and are usually not very soluble. In contrast, polymers that consist of long chains of identical molecules can easily be processed from solution.