While looking at how substituents affect the absorption spectra of molecules, I came across this:
Substituents alter the emitted wavelengths of these molecules by affecting the electron densities. Electron Withdrawing Groups (or EWGs) withdraw electron density, thereby making the arrangement more stable. The energy level of the orbital affected subsequently lowers, causing a typically-noticeable red shift in the maximum wavelength. Alternately, Electron Donating Groups (EDGs, for short), add to the electron densities of the molecule. This causes the affected orbital to be less stable, and requires more energy to obtain. The orbital affected depends on the placement of the substituent. A useful mnemonic device in this matter is HOLE: HOMO Odd, LUMO Even. This explains that the affected orbital alternates between odd and even placements.
Why does the orbital affected depend on the placement of the substituent? I thought substituents that caused bathochromic shifts just extended conjugation and hence reduced the HOMO-LUMO gap? Is this odd-even rule only applicable to azulene, or can it be applied to other chromophores?