Why Don't Colored Compounds Decolorize Immediately Under White Light?

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loom91
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Hi,

A few days ago, during a discussion at my chemistry class, I suddenly realized something very fundamentally puzzling about colour. We say that if in a molecule (say a conjugated organic system like beta-carotene) the HOMO-LUMO gap corresponds to a visible frequency of light, we observe that compound to be coloured because that frequency is absorbed and we see the complementary colour.

But consider this: when equilibrium has been established between the ground state and he excited state, the number of molecules getting excited in unit time is the same as the number getting deexcited. This should mean that the same amount of light being absorbed is also being emitted. If this were not true, then molecules would accumulate in the excited state.

This seems to imply that coloured compounds would decolorise after a short exposure to white light. While light does very gradually decolourise substances, it is due to photochemical decomposition and oxidation of dyes rather than reaching equilibrium. What is the explanation? Thanks.

Molu
 
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You mean that an electron excited to the previously-LUMO has other options than dropping back into the now-singly-occupied HOMO? And these options don't involve radiation? That seems too convenient...

Molu
 
But even within my viewing cone, the amount of light absorbed should be equal to the amount of light emitted. This seems like such an obvious question, yet I can't find the answer!

Molu
 
loom91 said:
You mean that an electron excited to the previously-LUMO has other options than dropping back into the now-singly-occupied HOMO? And these options don't involve radiation? That seems too convenient...

Molu

Your optically active molecules are in a complex environment with various sorts of interactions with neighbors. But you seem to have decided that the only way it can change state doesn't involve these. Maybe it's time for you to explain why you think it MUST involve radiation.
 
Dick said:
Your optically active molecules are in a complex environment with various sorts of interactions with neighbors. But you seem to have decided that the only way it can change state doesn't involve these. Maybe it's time for you to explain why you think it MUST involve radiation.

You are merely speculating. What is the actual explanation?

Molu
 
loom91 said:
You are merely speculating. What is the actual explanation?

Molu

The explanation is that the molecule can deexcite thermally by interacting with neighboring molecules - not only radiatively. I was waiting for you to realize this yourself.
 
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Dick said:
The explanation is that the molecule can deexcite thermally by interacting with neighboring molecules - not only radiatively. I was waiting for you to realize this yourself.

But why do photo-excitted molecules prefer to deexcite thermally rather than radiatively? And do you know this to be the explanation (i.e. have you read it in some reputable published source) or are you simply advancing a plausible hypothesis?

Molu
 
I know for a fact if I put a colored solution in sunlight that it doesn't instantly turn colorless from saturation or bleach from photodisintegration, it gets hot. I didn't feel the need to seek out a reputable published source.
 
Dick said:
I know for a fact if I put a colored solution in sunlight that it doesn't instantly turn colorless from saturation or bleach from photodisintegration, it gets hot. I didn't feel the need to seek out a reputable published source.

Alas! If theories of science could truly be verified that easily...

Molu
 
EDIT: post removed
 

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