Frank-Condon Principle - Potential Energy Surfaces

[elemis]

Theory and my Understanding:

So I understand how the frank condon principle let's us effect electronic transitions instantaneously, since the motion of nuclei (on the timescale of such electronic transitions) is quite slow.

Consequently, when a photon of light is absorbed you can have an electron being promoted from the S^x₀ to the S^y₁ where S represents the singlet state and x and y are vibrational levels (x>y)

My Question

What effects how much the upper curve is translated over to the right-hand-side, with respect, to the lower curve ?

My Interpretation

When an electron is promoted from S⁰₀ to S²₁, for example, the electron is being put into an anti-bonding orbital consequently weakening bonds and leading to a greater vibrations.

The more anti-bonding character the S₁ state has the greater the amount by which the upper curve is translated over to the right-hand-side and hence the larger the vibrations.

 

[DrDu]

Yes, your explanation sounds good.

 

[elemis]

Yes, your explanation sounds good.

Thanks! I just have one last question, if you imagine a porphyrin ring versus simple benzene. It is my belief that the upper curve (see above diagram) is displaced more to the right in benzene.

This is because a single electronic excitation in a massive porphyrin ring (a multi electron system) is unlikely to change the bonding character of the molecule by that much.

 

[DrDu]

Yes, but it depends on the localization of the orbitals that are involved.