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Specialty Chemistry Forums => Materials and Nanochemistry forum => Topic started by: beefheart on December 11, 2008, 05:12:36 PM

Title: Which and why will certain chemicals illuminate under UV/Black light?
Post by: beefheart on December 11, 2008, 05:12:36 PM

In part of writing up a large document I need to know which chemicals will illuminate under a UV (black light) light.

I know fairly little about the light and was hoping someone could tell me what exactly triggers the brightness in some objects; blood, semen and lsd-25 for examples and not others.

Would it be a list of certain reacting formulaic components that would just need to be sourced out? For example the formula for lsd-25 is d-lysergic acid diethylamide, would it be a part of that formula?

If it would help, the phosphor typically used in most black lights is either europium-doped strontium fluoroborate (SrB4O7F:Eu2+) or europium-doped strontium borate (SrB4O7:Eu2+)

Thanks for any help much appreciated

Title: Re: Which and why will certain chemicals illuminate under UV/Black light?
Post by: nj_bartel on December 11, 2008, 06:08:32 PM

I believe it has to do with the fact that the electrons of atoms of molecules can become 'excited' (that is, go to a higher energy level) when they are struck with light of a certain wavelength.  When the electrons return to their original energy level, they emit a photon (packet of light - this is what I'm assuming we see when you see the yellowish/whiteish stain on a sheet under blacklight).  I don't know if there's a way to predict which substances will respond to which light, but if there is, it's probably pretty mathematically intense.

Title: Re: Which and why will certain chemicals illuminate under UV/Black light?
Post by: enahs on December 12, 2008, 12:02:47 AM

Conjugated pi electron systems tend to be fluorescent. Weather the emitted light is visible is a different story. I actually remember reading in a book a while back about a way to predict the wavelength of emitted light from a conjugated pi electron system, it essentially treated the system as a particle in a box and the predictions were pretty good too.

One of these days I am going to get not busy and have time to go back and read a bunch of great stuff like that.
Man, if it was not for silly stuff at school taking so much time, I would know 100x more then I know now....!

Title: Re: Which and why will certain chemicals illuminate under UV/Black light?
Post by: nj_bartel on December 12, 2008, 12:04:26 AM

Similar neat technique I like is crystal field theory and predicting colors  :P

Title: Re: Which and why will certain chemicals illuminate under UV/Black light?
Post by: intrepid_nerd on December 27, 2008, 01:23:13 PM

haha...lsd eh?
well C20H25N3O (lsd), blood, semen, etc.  all show conjugated pi systems as nj_bartel said which is what causing you to be able to see these substances under black light.  a conjugated system allows this because of it's delocalization.  black light exists in the near-ultraviolet range (300 - 400 nm) and the chromophores are sections of the molecules that determine what color you're going to see.
http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/UV-Vis/spectrum.htm
that site gives a good synopsis on what you're seeing.
also, you can probably find this book at your local library:
Organic Spectroscopy by L.D.S. Yadav, it's a good intro but is pretty redundant.
regarding predicting what you're going to see:
http://www.kayelaby.npl.co.uk/chemistry/3_8/3_8_7.html
has a decent table but i'm sure you can find a better one.

remember, the greater the conjugated system, the lower the energy difference between the LUMO & the HOMO.

Title: Re: Which and why will certain chemicals illuminate under UV/Black light?
Post by: macman104 on December 27, 2008, 04:20:55 PM

You can use the woodward-fieser rules to predict a rough lambda_max

Go to this page:

http://www.chemistry.ccsu.edu/glagovich/teaching/316/index.html

And scroll down to item 4, Empirical Rules for Caluclating Uv/Vis Absorptions