[DavidF]

Hey folks,

I've been asked why is it that when you cool or freeze Stilbenes (or say, other fluorophores), their flourescence raises ?

i couldn't find a justification for my hypothesis, which is the probablity of quenching in a cold body gets smaller.

can you help please ?


Thanks

[Yggdrasil]

Well, most quenching reactions are due to bimolecular reactions with quenching molecules like molecular oxygen.  So, how would cooling/freezing affect the rate of bimolecular collisions?

However, I think that you may want to consider other effects as well.  Fluorescence is just one of many pathways by which a molecule can relax from the excited state.  Thus, the quantum yield of a fluorophore depends on the competition between fluorescence and non-radiative relaxation pathways.  One major non-radiative relaxation pathway is internal conversion.  While I'm not clear on the details (so you may want to look up more on this subject), molecular vibrations and rotations are involved in relaxation via internal conversion.  Therefore, hindering/reducing rotation and/or vibration of a molecule can help reduce the rate of relaxation through internal conversion and thus increase the fraction of molecules relaxing via emission of a photon.

[Johnny010]

Say you have a molecule (call it S₁), when excited it will become S₁*

Now S1* can return to its ground state in various ways:

Florescence (k_f)                               Rate_f=k_f[S₁*]
Interal-system-crossing (k_isc)             Rate_isc=k_isc[S₁*]
Interal conversion (k_ic)                      Rate_ic=k_ic[S₁*]
Quenching (k_q)                                 Rate_q=k_q[Q][S₁*]

Fluorescence is where the molecule in the excited state loses energy by a photon and thus returns to its ground state.
Internal conversion is where the gained energy of the S1* is crossed into vibrational energy (more vib levels become available)
Intersystem crossing causes the formation of triplet states T₁
Quenching involves the removal of these triplet states usually by collision with a second molecule (quencher - Q). It is a slow process.

These processes are all unimolecular except for quenching (a bimolecular process) which relays on collisions of molecules. As temperature decreases, less molecules are quenched therefore more will fluoresce than will be quenched.

I hope this explained it .

[Golden_4_Life]

So, under freezing temperatures the number of molecules that relax from their excited state is shifted in favor of the "flourescence" pathway rather than towards molecuar/vibrational collisions viz "Brownian motion events" and quenching, as a means of dispersing Energy.