# Can different groups within a single molecule photoisomerize at different wavelengths?

Azobenzene undergoes trans → cis photoisomerization when irradiated with light with a wavelength of 300–400 nm.

By adding electron withdrawing groups, the wavelength can be red-shifted.

If I were to make the molecule shown below, would it be possible to selectively photoisomerize around each of the $\ce{N=N}$ at different wavelengths?

• Yes you can increase wavelength by substituents and by increasing conjugation length also, for example a simple polyene such as butadiene vs. carotene or a substituted polyene such as retinal. But no, you cannot just excite one bond in the whole conjugated part of the molecule. Using tailored femtosecond pulses there has been much effort to do 'bond selective' chemistry but what the results mean is still hotly disputed. Oct 26, 2016 at 17:00
• I don't remember the reference any more, but there was a way to have selectivity that was between cis- and trans- double bonds. I think as long as the energy difference is big enough relative to the spread in your input light, you should be able to have some selectivity. The only thing to watch out for is that as you increase substitution, you're going to start disfavoring the cis- form, which I gather is what you want to make.
– Zhe
Oct 26, 2016 at 17:09
• Oct 26, 2016 at 17:20
• Thanks Porphyrin and Zhe. I am trying to design a photoswitchable ion binder for a supramolecular chemistry class. I am trying to design a way to 'lock' the molecule in a particular conformation using a thiol-disulfide dynamic interchange. I would like to be able to switch between which of the rings is locked to the other by using different wavelengths. Pretty far fetched idea, needs lots of refining, but it would be very nice to be able to rotate around a bond selectively. Thanks again Klaus, I asked the azobenzene question the other time :) Still much to learn about photochemistry! :p
– Max
Oct 26, 2016 at 17:27
• cis and trans selectivity by wavelength, see for example hemithioindigo. Disclaimer: My former group.
– Jan
Oct 26, 2016 at 19:50