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

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Homework Help Overview

The discussion revolves around the behavior of colored compounds, particularly in relation to their interaction with light. The original poster raises a question about why these compounds do not immediately decolorize under white light, despite the expectation that absorbed light should be re-emitted, leading to equilibrium between excited and ground states.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants explore the mechanisms of deexcitation in molecules, questioning whether non-radiative processes play a significant role. There is a focus on the implications of thermal interactions versus radiative decay.

Discussion Status

The discussion is ongoing, with participants expressing curiosity and skepticism about the proposed mechanisms of deexcitation. Some guidance has been offered regarding thermal interactions, but there remains a lack of consensus on the reasons behind these processes.

Contextual Notes

Participants are navigating complex interactions within optically active molecules and questioning the assumptions about radiation being the sole method of deexcitation. There is an acknowledgment of the need for reputable sources to support claims made during the discussion.

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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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.
 
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
 
Anyone to help?
 
Anyone to help?
 
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.
 
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
 
Anyone to help?
 
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.
 
Last edited:
  • #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
 

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