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

In summary, the conversation centers around the topic of why colored compounds do not decolorize after a short exposure to white light, as would be expected based on the concept of equilibrium between the ground and excited states. The explanation is that molecules can deexcite thermally by interacting with neighboring molecules, not just radiatively. While this is a plausible hypothesis, it has not been verified by a reputable published source. The conversation also touches on the idea of other options for molecules to deexcite and the potential role of interactions with neighboring molecules.
  • #1
loom91
404
0
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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  • #2
I guess this observation would imply there are other modes for a molecule to deexcite besides the extremes of reemittng the same wavelength and disintegrating.
 
  • #3
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
 
  • #4
Anyone to help?
 
  • #5
Anyone to help?
 
  • #6
Consider the solid angle that light is being emmitted into compared to the small solid angle of the light that is being reflected to your eye.
 
  • #7
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
 
  • #8
Anyone to help?
 
  • #9
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.
 
  • #10
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
 
  • #11
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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  • #12
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
 
  • #13
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.
 
  • #14
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
 
  • #15
EDIT: post removed
 

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

What is the physics of colour?

The physics of colour is the study of how light interacts with matter to produce the sensation of colour. It involves understanding the properties of light, such as wavelength and intensity, and how these properties are perceived by the human eye.

How is colour created?

Colour is created when an object absorbs some wavelengths of light and reflects others. The reflected light then enters our eyes and is interpreted by our brain as a specific colour. This process is known as selective absorption.

What is the difference between additive and subtractive colour mixing?

Additive colour mixing involves combining different coloured lights to create new colours, such as on a computer or TV screen. Subtractive colour mixing, on the other hand, involves mixing pigments or dyes that absorb certain wavelengths of light and reflect others, creating new colours.

What is the relationship between colour and energy?

Colour and energy are closely related in the physics of colour. Each colour corresponds to a specific wavelength of light, and shorter wavelengths (such as blue and violet) have higher energy levels, while longer wavelengths (such as red and orange) have lower energy levels.

How does the human eye perceive colour?

The human eye contains specialized cells called cones that are responsible for detecting and interpreting different wavelengths of light. These cones are sensitive to red, green, and blue light, and the combination of these signals allows us to perceive all the colours of the visible spectrum.

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