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I know that energy is absorbed due to chromophores which enable electron transition which uses up energy. But I am a bit unsure about what happens after the absorption and where this absorbed energy went.

I am not very sure if the energy is dissipated since it would contradict with the law that energy can't be destroyed. If the energy is lost as heat, then it is a bit contradictory again according to the equation $E= hc/\lambda$. The wavelength would be big for infra-red and thus, the energy lost must be less than the absorbed energy from UV.

So how exactly are these flavonoids shielding UV radiation? And also, if these flavonoids are in the excited state, it makes me feel that they might be quite reactive and may form free radicals.

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    $\begingroup$ The energy adsorbed may be re-emitted as another photon (the phenomenon known as fluorescence) or dissipated into thermal energy or used up in a chemical reaction (a subject of photo-chemistry) $\endgroup$ – permeakra Jul 14 '15 at 7:09
  • $\begingroup$ "The wavelength would be big for infra-red and thus, the energy lost must be less than the absorbed energy from UV." Who said you could only emit a single photon? Why can't we absorb a photon of say, 4eV (about 300nm) and emit 20 photons of 0.2eV (about 6.2 micron)? $\endgroup$ – chipbuster Jul 15 '15 at 3:43
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So how exactly are these flavonoids shielding UV radiation?

Due to the structure of the flavonoids, there are many transitions to absorb the UV radiation. Also, there are many transitions to release the energy. Most of the time, it is released in 2 steps (2 photons) which equal the energy of the photon that exited the molecule.

Fluorescence emits a photon that is close to the same energy as the first photon. For quercetin, the excitation is at 440 nm and the emission is at 540 nm.

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My guess is that they absorb UV because they have conjugated pi bonds in their structure (i.e. $\ce{= - = - =}$ structures). A system of three of these in a row might form a box of the right length to absorb at around 250 nm (since that is what hexatriene absorbs at), which is ultraviolet.

check out Wikipedia.

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  • $\begingroup$ This could be improved with the inclusion of references. $\endgroup$ – user15489 Aug 14 '15 at 4:13

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