[jbrookterrell]

How/why does extended conjugation decrease the HOMO LUMO energy gap?

[chiddler]

Because the more conjugated the substance is, the lower in energy it is (ie more stable) and as a result, excitation will result in a lower wavelength emission.

[jbrookterrell]

Wait, that doesn't make sense to me.... I thought adding conjugation would increase the energy of a molecule. For example, say you are talking about a molecule with 3 conjugated double bonds, this is more stable than a molecule with 3 isolated double bonds but it is less stable than a molecule with 2 conjugated double bonds. (I am basing this on  :delta: H values for heats of hydrogenation.)

I could be wrong, but either way I still don't get how the HOMO/LUMO gap is decreasing.