Photoisomerization of Azobenzene

Rotation around the double bond of azobenzene is restricted because it would distort the P orbital overlap between the nitrogen atoms. However, in the $n \rightarrow \pi^\*$ excited state ($S\_1$), the rotation is no longer hindered and it can isomerize. 


I don't understand why this is the case. You are taking an electron from the non-bonding orbital, and moving it into the pi-antibonding. However, the $\pi$-bonding is still present, so I see no reason why the rotation would be unrestrained. Is it something about the interaction between the electrons in the $\pi^\*$ with the $\pi$ that weakens the double bond? And if so, how?


![enter image description here](https://i.stack.imgur.com/17vCp.png)




Azobenzenes are one of the most widely used moietes in photochemically switchable systems. Nevertheless, the pathways for the *E*-*Z* isomerisation of azobenzene in the $S\_1$ and the $S\_2$ state have been in debate for decades and I'm not quite sure whether this is finally solved.



> 
> However, in the $n \rightarrow \pi^\*$ excited state ($S\_1$), the rotation is no longer hindered and it can isomerize.
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> 
> 


Yes and no. 


It is true that the $S\_1$ state arises from a $n \rightarrow \pi^\*$ transition in a weak, broad band with $\lambda\_{\mathrm{max}} = 432\,\mathrm{nm}$ and $\epsilon \approx 400\,\mathrm{L\,mol^{-1}\,cm^{-1}}$ in hexane. 


By the way, *E*-azobenzene exhibits another, much stronger absorption at $\lambda\_{\mathrm{max}} = 319\,\mathrm{nm}$ with $\epsilon \approx 22800\,\mathrm{L\,mol^{-1}\,cm^{-1}}$ in the same solvent. This $\pi\rightarrow\pi^\*$ transition to the $S\_2$ state would indeed allow for an isomerisation through an **one-bond-flip** rotational mechanism.


(For the UV data, see [1](http://dx.doi.org/10.1021/ja01547a010) and [2](http://dx.doi.org/10.1021/ja038327y)).


But - *you saw that coming after this lengthy introduction* - the situation is much more complicated!


You have neglected the fact that *E*-*Z* isomerisations can proceed by other mechanisms than just the simple one-bond-flip that is usually postulated for alkenes!


Forgive me for not getting into all details on this, but for polyenes, to give an example, the so-called **Bicycle Pedal** (**BP**) and **Hula twist** (**HT**) pathways have been proposed and confirmed as space-conserving alternatives to the **one-bon-flip** mechanism. The latter can be visualized as *"let's just flip the other side of the former double bond around like a rotor"*.


Back to the azobenzenes. Even from more recent works ([2](http://dx.doi.org/10.1021/ja038327y), [3](http://dx.doi.org/10.1021/jp031149a), [4](http://dx.doi.org/10.1038/ncomms6860), [5](http://dx.doi.org/10.1063/1.3000008)), it seems that the *E*-*Z* isomerisation is still not fully explained, although a plain rotation around the former double bond can be excluded. 


The two main fractions in the debate support mechanisms in which a contribution of **inversion** through a change of the $\ce{C-N-N}$ angle in the excited state is assumed. Whether this is the only effect and the reaction proceeds through a transition state with a linear $\ce{C-N-N}$ arrangement or whether an **inversion-assisted torsional pathway** provided the beter explanation is still unclear.


**References**


[1](http://dx.doi.org/10.1021/ja01547a010) George Zimmerman, Lue-Yung Chow, and Un-Jin Paik, *J. Am. Chem. Soc.*, **1958**, *80*, 3528-3531.


[2](http://dx.doi.org/10.1021/ja038327y) Alessandro Cembran, Fernando Bernardi, Marco Garavelli, Laura Gagliardi, and Giorgio Orlandi, *J. Am. Chem. Soc.*, **2004**, *126*, 3234-3243.


[3](http://dx.doi.org/10.1021/jp031149a) Eric Wei-Guang Diau, *J. Phys. Chem. A*, **2004**, *108*, 950-956.


[4](http://dx.doi.org/10.1038/ncomms6860) Eric M. M. Tan, Saeed Amirjalayer, Szymon Smolarek, Alexander Vdovin, Francesco Zerbetto, and Wybren Jan Buma, *Nature Communications*, **2015**, *6*, Article number: 5860.


[5](http://dx.doi.org/10.1063/1.3000008) Junfeng Shao, Yibo Lei, Zhenyi Wen, Yusheng Dou, and Zhisong Wang, 
*J. Chem. Phys.*, **2008**, *129*, 164111


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