Lowest unoccupied molecular orbital polymers

Conjugated polymers are generally poor conductors unless they have been doped (oxidized or reduced) to generate mobile charge carriers. This can be explained by the schematic band diagrams shown in Fig. I.23 Polymerization causes the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the monomer to split into n and n bands. In solid-state terminology these are the valence and conduction bands, respectively. In the neutral forms shown in Structures 1-4, the valence band is filled, the conduction band is empty, and the band gap (Eg) is typically 2-3 eV.24 There is therefore little intrinsic conductivity. 

The optical and electronic functions of polysilanes owe to their delocalized highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) that are occupied by holes and conduction electrons, respectively. The polymer does not show high conductivity or optical nonlinearity if the electrons or holes are localized on a small part of the polymer chain. To elucidate the structure of HOMO and LUMO is therefore important for the molecular design of polysilanes as functional materials. 

The molecular orbital (MO) calculations within the PM3 method, using a MOP AC package, provided an explanation of the advantages of a new redox system, poly(l,4-phenylene-l,2,4-dithiazolium-3, 5 -yl) (PPDTA), as a cathode material for high-capacity lithium secondary batteries in comparison with three typical polymer conductors (poly-/>-phenylene, polypyrrole, and polythiophene). The MO calculation revealed that the S-S bond in the 1,2,4-dithiazo-lium moiety of PPDTA caused gap narrowing and a downshift of HOMO and LUMO levels, which is consistent with the electrochemical experiment (HOMO = highest occupied molecular orbital LUMO = lowest unoccupied molecular orbital) <2001MI2305>. 

There are several properties of luminescent materials that need to be controlled in order to make efficient LEDs and lasers. The first is the colour of the emission, which is primarily determined by the energy difference (band-gap) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), but in the solid state is also affected by interactions between the molecules or polymer chains which can lead to red-shifts in the emission due to formation of aggregates. This can be controlled by manipulating both the polymer backbone and the substituents. Polyphenylenes are intrinsically blue-emitting materials with large HOMO-LUMO gaps, but as we will show, by copolymerisation with other materials it is possible to tune the emission colour across the entire visible spectrum. Even without the incorporation of comonomers it is possible to tune the... 

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