Colors Of Indophenol

[tylerngo]

Hi
I can't understand why indophdnol is blue in a basic medium and red in an acid one. Shouldn't it be the opposite? The structures in the to mediums are in the attachment. In a basic medium the conjugated system is bigger hence wave length would be bigger than indophenol in an acid medium.

 

Brumer

[exchemist]

Hi

I can't understand why indophdnol is blue in a basic medium and red in an acid one. Shouldn't it be the opposite? The structures in the to mediums are in the attachment. In a basic medium the conjugated system is bigger hence wave length would be bigger than indophenol in an acid medium.

 

Brumer

Attachment doesn't seem to have come through. I'm rusty on this but doesn't the colour come from a transition between bonding or non-bonding and antibonding  π MOs? So the bigger the gap between the 2 involved in the absorption, the bluer the colour. 

 

So I suppose we'd need to work out what MOs exist in the 2 forms and their occupancy in protonated and deprotonated versions.

 

I imagine it could be that if the π - system is extended when the molecule loses a proton, then the bonding energy levels fall a bit, leaving a bigger gap between them and the π* levels, in which case the colour would be bluer than when the proton is attached.

 

Does that make sense? I suppose it would help to see your pictures.