Question

what happens when chlorophyll and other pigments absorb light?

Answer 1

Well, very different things can happen, depending on the pigment. In the case of chlorophyll, it goes into a peculiar excited state which can survive for an unusually long time. Furthermore, the excitation energy can hope from chlorophyll molecule to chlorophyll molecule. In this way a large collection of chlorphyll molecules function as an "antenna" that collects many photons and funnels their energy down to a reaction center, where one particular chlorophyll will lose an electron.

The reducing power of this energetic electron is ultimately used to reduce atmospheric CO2 to carboyhydrates, e.g. glucose (C6H12O6). Not all of the photon energy ends up in the reduction of CO2, however. Some it wanders off as heat, or the random vibrations and rotations of the molecules involved.

This is an example in which the absorption of light ultimately produces a chemical reaction. A more mundane example of this is when dye molecules absorb light and the resulting energy results in a chemical reaction that fractures the dye molecule. The pieces rarely absorb light in the visual range, so the result is that the color produced by the dye fades. This is why dyed clothes fade in sunlight over time.

A related possiblity is an "internal" chemical reaction, and this is what happens with the visual pigments in your eyes. In this case, when the visual pigment retinal absorbs a photon, it undergoes an internal chemical reaction in which some of the atoms in the molecule switch sides. This changes the molecule from a long straight molecule to a bent molecule. When it gets bent, it no longer fits into its protein "socket" and it falls out. Special receptors pick up the newly freely-floating molecule, and that, ultimately, produces a signal in the nerve that tells your brain you've detected light. Meanwhile, an enzyme converts the retinal back to its straight form, and it reinserts into its socket, ready to absorb more light.

But in most simple inorganic pigments, like zinc oxide, the light is initially converted to some kind of internatl excitation of the molecule. For ultraviolet or visible light, usually some faster or more exotic motion of the electrons. (For infrared light, usually some vibrations of the atoms, and for microwaves, rotations.) Usually this energy eventually wanders off into random jostling motions, i.e. heat.

There are some pigments, called phosphorescent or fluorescent, in which a fair amount of the energy will be re-emitted as another photon of light, but at a lower frequency. For example, a fluorescent pigment will absorb ultraviolet light and re-emit visible light.

So those are the possibilities of which I can think:

(1) a chemical reaction.

(2) An "internal" chemical reaction (called an isomerization).

(3) Re-emission of light at a lower frequency.

(4) Heat.

Answer 2

Oh My God...

Answer 3

this is not chlorophyl it is AC6AzoC6,
but it is what happen when a molecule absorbs a single UV photon

Answer 4

thanks,everyone..