In the chapter on conjugation in Organic Chemistry by Clayden, Greeves, Warren and Wothers, there's a brief discussion of pigments with conjugated systems that includes the following paragraph (first edition, p. 171; emphasis mine):

Transitions from bonding to antibonding π orbitals are called π → π* transitions. A much smaller energy gap is available if we use electrons in a nonbonding orbital as the electrons start off much higher in energy and can be promoted to low-lying antibonding π orbitals. We call these transitions n → π*, where the ‘n’ stands for nonbonding. It is easy to find coloured compounds throughout the whole range of wavelengths by this means. The colour of blue jeans comes from the pigment indigo. The two nitrogen atoms provide the lone pairs that can be excited into the π* orbitals of the rest of the molecule. These are low in energy because of the two carbonyl groups. Yellow light is absorbed by this pigment and indigo-blue light transmitted.

The bolded statement is not elaborated on or, as far as I can tell, addressed directly in the rest of the text. I took "these" to mean "π* orbitals" rather than "lone pairs"; my best attempt at deducing an explanation is something like this:

The bonding orbitals in the carbonyl group are polarized towards the O atom, exposing a positive charge on C and shifting electron density in the π* orbitals toward C. These two effects combined mean that any electrons in π* experience a strong attraction to the carbonyl group, lowering the overall energy in π*.

Is this roughly correct?


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