Conjugated polymers

The light that a polymer can absorb depends among other things on how it works as a conductor. The different conductors are classified according to their band gap (Eg), which defines the energy level between the highest occupied molecular orbital (HOMO) or conducting band and the lowest unoccupied molecular orbital (LUMO) or valence band,DOI:10.1016/S1631-0705(02)01335-X presented in Figure 1.

In metals both the HOMO and the LUMO bands are partially occupied by electrons and there is no energy gap between the highest level, the Fermi level, and the lowest empty one, and the two levels are therefore shown as one box in Figure 1. Semiconductors and insulators have on the contrary defined band gaps. Insulators are normally seen as a material that cannot conduct electricity, which means that the bandgap is relative high. Semiconductors are situated in between metal-like conductors and insulators. A semiconducting polymer absorbs light with an energy that is equal to or higher than the $E_g$ between HOMO and LUMO. If the polymer is illuminated with an energy that is lower than the band gap it will not lead to an excitation but if the energy is equal to or higher than the energy of the band gap it can lead to an excitation, with excess energy lost in the form of heat.

Figure 1. A representation of the energy level between HOMO and LUMO in a metal, a semiconductor and an insulator.

In polyacetylene, that is the simplest conjugated polymer and therefore can be considered as a prototype, it is the electrons of the $\pi$ – bonds, which are located in the HOMO, that can be transferred to the LUMO. DOI:10.1016/S0079-6700(01)00043-0 When a photon with certain energy interacts with a polyacetylene molecule the electrons can be transferred from $\pi$ to a $\pi*$ excited state, this is known as an excitation, DOI:10.1126/science.258.5087.1474 shown in Figure 2.

Figure 2. A molecular orbital diagram of an alkene. A) showing the ground state. B) is showing the excited state, where the molecule has absorbed energy and an electron is therefore excitated from the $\pi$ orbital to the $\pi*$ orbital.

For a conjugated polymer the delocalization of double bonds varies, as shown in Figure 3, which means that there will be periodic bond alternation and variation in charge density within the polymer chain, known as Peierls effect. Peierls effect changes the polymer from a metal-like conductor with half-filled bond, to a semiconductor with a band gap.DOI:10.1016/S0079-6700(01)00043-0DOI:10.1016/S0927-796X(00)00029-2 In such a polymer the pz orbitals are oriented perpendicular to the polymer backbone allowing for an electronic interaction between the double bonds. This interaction results in a delocalization which contributes to the conducting mechanism of the conjugated system. DOI:10.1117/12.662829

As a common simplification; a conjugated polymer is a polymer with alternating single and double bonds, and if the polymer backbone does not have this alternation it is not a conjugated polymer.

Figure 3. Schematic representation of the delocalization of the double bonds throughout the polymer chain. A) total delocalization of the double bonds, this means that the polymer can be seen as a metal-like conductor. B) periodic bond length alternation of shorter double bonds and longer single bonds.