Topic overview

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Conjugation allows p-orbitals to overlap and π electrons to delocalize. As conjugation extends, the energy gap for a π→π* transition decreases, generally shifting the absorption maximum (λmax) to a longer wavelength (a bathochromic or red shift) and increasing absorption intensity (a hyperchromic effect). Ethene and 1,3-butadiene illustrate this shift; extensive conjugation in β-carotene moves absorption into the visible region. The topic also distinguishes π→π* and n→π* transitions and introduces Woodward–Fieser rules as an empirical way to estimate λmax. Planarity, aromatic substitution, auxochromes, ring geometry, cumulenes, and solvent polarity can modify spectral behavior, so conjugation length is an important guide rather than the only factor in interpretation.