Absorption and Emission
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This article will describe the vibronic progression that can be seen in the absorption and emission spectra. Vibronic progression means that there is a coupling to vibrational modes in your polymer or oligomer as you have an excitation from the ground state to the excited state, or emission from the excited state down to the ground state.
Consider the first optically allowed excited state of the transpolyacetylene which has a 1 Bu symmetry. As the molecule moves from the ground state to that first optically excited state, there is a shift of the double bonds from one bond to the next resulting in a shift of the pi bond densities from one bond to the next. This is described as the reversal of the bond length alternation pattern. If there is a single excitation on a long polyacetylene chain, that modification will be localized somewhere. For PPV, you start with a chain where the rings are aromatic-like, with single, double, single bonds between the rings. The wave functions of the HOMO and the LUMO, and the pi bonding and anti- bonding pattern dictates that as you go to the excited state and an electron leaves the HOMO and goes into the LUMO, you will shift the pi-bond densities in such a way that you end up with a quinoline-like structure. In between the rings, there is a double bond, single bond, double bond, or a larger factor of that kind .
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