$I_2$ in solution is typically absorbing around $525 \; nm$ this is why it appear to be magenta, I recently learn that with some solvent :

  • Benzene
  • Toluene
  • $o$-Xylene
  • $p$-Xylene

The coordination complex between the solvent and $I_2$ is also absorbing around $300 \; nm$ I guess there is some explanation with the MO diagram ?


Iodine is highly polarisable and will form a (Mulliken) charge-transfer complex with many aromatics. Very many pairs of donor - acceptor pairs do this and have been studied for approx $100$ years, see for example tetracyanoethylene–pyrene (J. Chem. Phys. 105,1996 p2287) for an ultrafast time resolved study. In absorbing a photon the ground state complex DA transfers an electron directly to a charge transfer state $DA \overset{h\nu} \to \mathrm D^+A^-$. The interaction is thus that of all the orbitals in one molecule with all those in another.


R. S. Mulliken and W. B. Person, 'Molecular Complexes' Wiley, New York, 1969. and G. Briegleb, 'Elektronen Donator Acceptor Komplexe' Springer, Berlin, 1961.


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