I have one basic question regarding polyaniline. Polyaniline salt is green in color and polyaniline base is blue in color. Is it due to its conjugated back bone? Please share your answers with relevant reference. Thanks in advance
yes, because of the conjugated backbone, but also because of 3-dimensional interaction of the electron-rich system (you don't just have isolated chains, you have a complicated 3-dimensional structure which in case of the salt is (if well done) metallic; when neutralizing, you have a different backbone structure (different conjugation) and also 3-D interactions, but no conductivity, hence the bandgap has widened.
You can find something about the PAni structure here:
https://www.researchgate.net/publication/49593603_New_Insight_into_Organic_Metal_Polyaniline_Morphology_and_Structure?ev=srch_pub&_sg=sMQtv5a3YgJQIlCF%2BK039M%2FsQYEXL6clityGjDWRmCy%2F1FZ0UGc8CPxC3FhsfYyo_loiRoDD86g%2FzPumkCDoXZM2g6Y1eiKeYZo%2Fofm5i2vmBCC3nryLX4%2Feq369iaQBB_6oblhdUgVxaJn7KGT482yUua29u%2FvkYnnarLRkXMq4B5Sj1Fo%2BftIAausS4GXnoJ
Article New Insight into Organic Metal Polyaniline Morphology and Structure
My understanding is that conjugate backbone does not play any role in the color, its just the change in protonation that gives color.
CONDUCTING POLYMERS PREPARED BY
OXIDATIVE POLYMERIZATION: POLYANILINE
N. GOSPODINOVA, L. TERLEMEZYAN
Prog. Polym. Sci., Vol. 23, 1443–1484, 1998
sorry, that is not the correct description. The neutral (not at all protonated) Emeraldine base is blue which is determined by conjugated backbone and by the particle size. With protonation, the conjugated backbone (and the 3D intrachain!) conjugation is changed which leads to another colour, it happens to be green. Depending on how well the 3D conjugation is established, the green colour has a blueish or a yellowish shade (tone), the more metallic - the more yellowish (and the absorption extends into the near IR showing metallic properties).
https://www.researchgate.net/publication/49593603_New_Insight_into_Organic_Metal_Polyaniline_Morphology_and_Structure (and references cited therein, most of them accessible on my RG page as much as I am the author)
For the other oxidation states which are not protonated, like
- Leucoemeraldine (the fully reduced form): it is colourless transparent - no conjugation
- and the oxidized forms (purple / violett, partially conjugated)
you find different colours.
Article New Insight into Organic Metal Polyaniline Morphology and Structure
Thanks Dr Wessling for explanation. 3 dimensional structure means, intra/inter molecular H bonding interactions between polyaniline chains, and pi-pi interactions forms such 3D assembly ? Or crosslinking between the chains afford 3D assembly of polyaniline? Or all these three occurs for 3D assembly?
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