Today, we were discussing optical activity in class and our teacher said that some drugs are kept in dark coloured bottles to avoid direct exposure to sunlight. Same is the case with some other drugs in which it's written that the direct exposure to sunlight should be avoided.

The reason he gave for the above was that the one form of isomer can get converted to other form in direct sunlight. But as far as i know, photolytic decomposition can happen. Is the conversion of d to l isomer and vice versa possible in sunlight?

  • 1
    $\begingroup$ Cis/trans isomerisation can occur. $\endgroup$
    – Mithoron
    Nov 9, 2017 at 15:54
  • $\begingroup$ I wonder if there is some confusion here about language. Optical activity refers to the rotation of plan-polarized light, where one enantiomer rotates the light in one direction and the other rotates the light to the same degree in the opposite direction. This is a separate phenomenon from photosensitivity, where a compound can decompose or isomerize (or otherwise be affected) by exposure to light. $\endgroup$ Nov 10, 2017 at 12:31
  • $\begingroup$ @MichaelLauman - no, I was actually asking about the conversion of one enantiomer to the other form in presence of sunlight. Not exactly about photosensitivity or optical activity but just wanted to know if the enantiomeric conversion is possible or not(in sunlight's presence). $\endgroup$ Nov 10, 2017 at 14:04

2 Answers 2


Conversion of D to L isomer or vice versa is about the last thing to worry about in this situation, even if it may be facilitated by sunlight to a significant extent (which I doubt). Also, storage of chemicals in dark coloured bottles is by no means limited to compounds that have optical isomers, or even to drugs in general. Other examples are numerous; permanganate solutions and nitric acid come to mind.

Photons of visible light have pretty decent energy of a few eV. That's way higher than the energy of thermal motion at room temperature. When a molecule is hit by a photon, it is hit hard. If a molecule is fragile or strained in any way, it will probably break. Photons come in droves, and after a while your precious organic sample turns into a brown goo. (It would probably do so even in the dark, but that would take a bit longer.)

That's what's wrong about sunlight in the first place.

  • $\begingroup$ So dextro form can be converted to laveo form or vice versa by exposure to sunlight? $\endgroup$ Nov 9, 2017 at 16:34
  • $\begingroup$ Just to strengthen the answer. This is not specific to drug and conversion between stereoisomers is, IF it happens to be, the last to worry about. $\endgroup$
    – Alchimista
    Nov 9, 2017 at 17:01
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    $\begingroup$ @anamikaSingh I already said that in my answer, but if that's too long for you, here is a brief summary: no. $\endgroup$ Nov 9, 2017 at 18:19
  • $\begingroup$ @IvanNeretin - i read the answer but at that time i got confused btw the dextro(d) and laveo(l) and the D and L system used for naming the optical isomers. As in ,i got confused in the representation. Sorry for that. Anyways, thanks for clearing up the matter! =) $\endgroup$ Nov 9, 2017 at 18:51

Isomerism is only one (and probably a minor) pathway.

Many substances have chromophores that absorb light. Absorption leads to a high energy intermediate that is more reactive and able to access degradation pathways that were unavailable from the ground state. This is different from thermal excitation because the energizing photon carries a lot more energy and directly promotes an electronic transition between two states.

  • $\begingroup$ @Zhe - Just asking, in photosystems PS-I and PS-II , the central 'chlorophyll a' molecule can be called a chromophore? $\endgroup$ Nov 9, 2017 at 19:00
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    $\begingroup$ Yeah, it absorbs light. It's still pretty destructive. If I remember correctly, PS-II has to be rebuilt every 20 minutes or something like that. @anamikaSingh $\endgroup$
    – Zhe
    Nov 9, 2017 at 19:39
  • $\begingroup$ @Zhe- I Didn't know that they are regenerated at that rate! Thanks! $\endgroup$ Nov 10, 2017 at 0:49
  • $\begingroup$ @anamikaSingh I think I heard Dan Nocera say that during a talk once, but that was a long time ago. Please double check before quoting. $\endgroup$
    – Zhe
    Nov 10, 2017 at 1:12

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