[CrimpJiggler]

In college, I halogenated anthracene and I spotted the product, along with both reagents onto a TLC plate. I did this 3 times and messed it up every time for some reason but I noticed that each time, when I visualised the plate in a UV chamber, there was a very small turquoise spot in the anthracene lane. Does anthracene have turquoise fluorescence?

[Arkcon]

Yup.

http://en.wikipedia.org/wiki/Anthracene

You're lucky you got to make anthracene, it has a very distinctive visible fluorescence.  And distinctive visible fluorescence is always notable, like I've said before.

[CrimpJiggler]

Thats pretty cool. This is the 2nd time I've seen fluorescence on a TLC plate. First time it was dark blue fluorescence, exhibited by an alkene with an anthracene ring attached to it. I see now that the anthracene molecule is responsible for the fluorescence. I found anthracenes flourescent emission spectrum just there:

I wonder why the halogenated anthracene compound wasn't fluorescent.

[Arkcon]

I wonder why the halogenated anthracene compound wasn't fluorescent.

Why indeed.

 http://pubs.acs.org/doi/abs/10.1021/ja8040493

And that's just a starter from a quick Google search

[CrimpJiggler]

Sorry I meant a substitution product, not addition product. The degree of conjugation should be the same, the only difference is one of the H atoms is replaced by a halogen atom.

[Arkcon]

Yes, but halogens are electron withdrawing groups, when they're in the vicinity of conjugated double bond, they disrupt it.  (Right?  Halogens are electron withdrawing, or do I have that backward?)

[CrimpJiggler]

Actually, it seems halogens enhance the conjugation of the system. I'm comparing the UV spectrum I obtained for anthracene with the one I obtained for its 9-bromo analogue and you can clearly see a bathochromic shift has occured (meaning the 9-bromo analogue absorbs at higher wavelengths). Halogens have inductive withdrawing properties but they have resonance donating properties. I'm guessing its mainly their resonance donating properties that matter when it comes to the influence they have on the degree of conjugation of a conjugated system. Makes sense to me since sigma and pi bonds are kinda separate things. When there is no hyperconjugation going on that is. The brominated analogue is bright yellow so it clearly absorbs in the visible spectrum.

EDIT: The only explanation I can find for 9-bromoanthracenes lack of fluorescence is that heavy atoms quench the fluorescence of conjugated systems.