While I am all for analogies to convey the basis of complicated 
processes to the ignorant, the analogy must be accurate. But in this 
case, if a _scientist_ asks about bleaching from the triplet state why 
does the explanation have to be dumbed down to such an (inaccurate) 
level?  Surely the basis of chemical reactions in terms of electrons and 
electron pairing should not be so unfamiliar after undergraduate physics 
and chemistry?

A good explanation is given in Encyclopeda Britannica and should be 
within the grasp of most I think.

http://www.britannica.com/EBchecked/topic/457736/photochemical-reaction/277509/Consequences-of-photoexcitation#ref=ref499215

Even if you can't remember the simple reason for more rapid oxidation 
from the excited state (due to singlet oxygen production, I think, but 
probably other possibilities also exist in complex molecular systems), 
you point your undergraduates there rather than use horribly inaccurate 
analogies. If nothing else, a reason to learn this explanation is that 
it is the basis of life on earth as it explains how light can be 
harnessed to chemical reactions!

Cheers Mark
P.S. Tobias, if you wonder why I object to your analogy it is because a 
power surge does not involve switching off the computer!!!!


> John,
>     From the non physicist's point of view the answer could go 
> something like this. If you have a power surge it can fry your 
> computer. But if your computer is not plugged into the mains then it 
> would take a very big power surge indeed to do the damage. A molecule 
> in the ground state can of course be damaged by free radical attack 
> but no more or less than other molecules. But once a molecule has 
> absorbed a photon then it is not in the ground state any more. To 
> continue my hoaky analogy, a chromophore in light is like your 
> computer plugged in to the mains.
>
>     Hope this helps. The physicists (and musicians) can go for the 
> triplets.
>
>     Tobias
>
>
>> Hi Everyone,  this question follows on from a helpful discussion that 
>> we had about photobleaching back in November.  I have recently tried 
>> to explain to a group of colleagues about the mechanism of 
>> photobleaching.  The answer is based on the transition of molecules 
>> from the excited singlet state (S1) to the triplet state (T1) which 
>> is long-lived and therefore more susceptible to bleaching by free 
>> radicals (my entire discussion of this is below).
>>
>> My question that arises from my attempted answer is: why are excited 
>> molecules more susceptible to oxidative attack than ground state 
>> molecules.  I hope I'm not completely mucking up the mechanism here. 
>> Would the physicists out there please help.
>>
>> Thanks, John.
>>
>> The original answer: When excited, fluorophores generally transition 
>> from singlet ground state (S0) to singlet excited state (S1). 
>> Relaxation from S1 to S0 results in emission of heat and light 
>> (fluorescence). Lifetime in S1 is in the nano to pico second range 
>> and allows very little time for the excited molecule to interact with 
>> free radicals. Periodically, however, an excited molecule will do a 
>> transition from S1 to the triplet excited state (T1 - the physics of 
>> this is a bit difficult to understand). T1 is a very long-lived state 
>> - molecules can remain in T1 for up to the microsecond range - i.e. a 
>> thousand to a million times longer than for normal S1 state. It is 
>> during this long T1 state that molecules are attacked by free 
>> radicals and destroyed.
>>
>> -- 
>> Runions signature
>>
>> (Sent from my cra%#y non-Blackberry electronic device that still has 
>> wires)
>>
>>
>>
>> *********************************
>> John Runions, Ph.D.
>> School of Life Sciences
>> Oxford Brookes University
>> Oxford, UK
>> OX3 0BP
>>
>> email:  <mailto:[log in to unmask]>[log in to unmask]
>> phone: +44 (0) 1865 483 964
>>
>> <http://www.brookes.ac.uk/lifesci/runions/HTMLpages/index.html%21>Runions' 
>> lab web site
>>
>>
>>
>> Visit <http://www.illuminatedcell.com/ER.html>The Illuminated Plant 
>> Cell dot com
>> Oxford Brookes Master's in 
>> <http://www.brookes.ac.uk/studying/courses/postgraduate/2007/bmt>Bioimaging 
>> with Molecular Technology