In the absorption spectrum of porphyrins, the intense soret band is attributed to the S0 to S2 transition whereas the q bands are attributed to the S0 to S1 transition. It is clearly understandable why there is one Soret band but not so intuitive to understand why there are 4 Q bands. Very often the Gouterman's four orbital model is invoked to explain the number of bands, but can anyone explain this four orbital model in very simple terms, without dragging into the complex jargons of computational/theoretical chemistry?
From what do these additional HOMO LUMO states arise that we end up having 4 Q bands and not just one?
Anyone please!
In freebase porphyrins the Q band is split due to vibrational excitations. Therefore, two bands are produced due to transition from ground state to two vibrational states of the excited state [Q(0,0) and Q(1,0)]. Now the presence of the NH protons breaks the symmetry and as a result the above mentioned bands are further split into two bands each. The X and Y components are no longer degenerate and therefore we see 4 Q bands Qx(0,0), Qy(0,0), Qx(1,0) and Qy(1,0).
Will this explanation hold good when we consider dithiaporphyrin which has 2 diagonally opposite S atoms in the core and still shows 4 Q bands?
Dear Dr. Vijayendra Shetti ,
Yes this will still remain a valid explanation in case of dithiaporphyrin. The idea is, in case of square symmetry the x and y components of the excited state remain degenerate, however the degeneracy in porphyrins is removed by the NH axis, while in your case the diagonally opposite sulfur atoms serves the same purpose.
Thank you Mr. Debangsu ! I also thank Dr. Santosh for initiating this discussion.
thanks Mr Debangsu, Dr. Vijayendra and Hosseini for your inputs.
Let me ask another couple of questions here:
Why and when can one see this resolution of the transition dipole into x and y components and
why does this not happen for the Soret band and applicable only to the Q bands?
Will appreciate your inputs on these too!
Dear Dr. Santosh,
I think the answer to your 1st question lies in the symmetry of the system concerned (which I have mentioned in my earlier reply).
For your second question the answer would be no. Infact this happens in case of soret band also, however we must keep in mind that in metalloporphyrins the Soret band is largely affected by the metals.
For details please see the following paper which deals with all your queries in detailed manner
Inorg. Chem. 1980, 19, 386-391
Can you explain about the variation in the intensities of Qx and Qy bands when free base porphyrin is substituted by electron releasing groups? I have attached picture I am concerning about. Thank you
Where do the N, M and L bands arise from in the near UV region of the spectrum?
Thanks for nice explanation, I have one small query , why anti-aromatic porphyrinoids like isophlorins does not show Q band
This discussion about the spectral details of porphyrins is very helpful. I would like to add a questions here,
Sometimes soret band shows a splitting in porphyrins UV-vis, is it valid in case of hyperporphyrins?
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