# Chemical Forums

## Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: eltornado90 on February 08, 2018, 05:05:51 PM

Title: Fluorescence
Post by: eltornado90 on February 08, 2018, 05:05:51 PM
Hi,

So I have this problem :

The fluorescence intensity emitted at λemi = 455 nm is almost six times larger when the excitation made at λexc = 250 nm with respect to λexc = 360 nm. According to the fluorescence equation (IF = 2.303⋅ΦF⋅Io⋅ε⋅c⋅b), propose an explanation for this fact knowing that c and b are constant.

The first thing I can say is that small wavelenghts are related to high energy. Can someone help me with this ?

Thank you.
Title: Re: Fluorescence
Post by: Arkcon on February 08, 2018, 11:58:12 PM
The fluorescence intensity emitted at λemi = 455 nm is almost six times larger when the excitation made at λexc = 250 nm with respect to λexc = 360 nm.

That's a given, so good.

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According to the fluorescence equation (IF = 2.303⋅ΦF⋅Io⋅ε⋅c⋅b), propose an explanation for this fact knowing that c and b are constant.

And are the variables that are not constant related to wavelengths?

Quote
The first thing I can say is that small wavelenghts are related to high energy. Can someone help me with this ?

That's good.  Can you compare, with a formula, the energy difference between the two wavelengths?

Quote
Thank you.

FWIW, I agree with reddit: https://www.reddit.com/r/chemhelp/comments/7vzfl5/fluorescence_and_intensity/ that the lower wavelength interacts with the molecule better.  Otherwise, we're not really using fluorescence, but instead Raman or Raleigh scattering.
Title: Re: Fluorescence
Post by: Corribus on February 09, 2018, 03:25:49 AM
Yes, it is most likely relative absorptivity at the incident wavelength. Without knowing molecular structure or at least having an absorption spectrum it's impossible to say why. Essentially, all things being equal, if a molecule absorbs more light, it radiates a proportional amount via fluorescence. (Although, there are exceptions - the quantum yield can be wavelength dependent in some situations.) There is a resonance condition for absorption, and depending on the molecular structure (or solvent), the absorption bands may shift to higher or lower energy (or intensity). This is why molecular compounds can appear to be different colors by eye. So, the molecule in question absorbs better at the one wavelength than the other.

Raleigh scattering really has nothing to do with this problem.