It has to do with promotion of electrons from a bonding orbital to an anti-bonding orbital. These orbitals are out of phase (usually for pi*) and not between nuclei.
http://chemwiki.ucdavis.edu/Theoretical_Chemistry/Chemical_Bonding/Pictorial_Molecular_Orbital_Theory/Bonding_and_antibonding_orbitals
It has to do with promotion of electrons from a bonding orbital to an anti-bonding orbital. These orbitals are out of phase (usually for pi*) and not between nuclei.
http://chemwiki.ucdavis.edu/Theoretical_Chemistry/Chemical_Bonding/Pictorial_Molecular_Orbital_Theory/Bonding_and_antibonding_orbitals
You can can think along the following intuitive way: a (classical) dipole moment indicates the magnitude of of charge and distance. In the excited state the distance between the nuclei is increased, because the molecule has absorbed energy. As a result a result, the dipole moment in the excited state is larger. You can search for Frank Condon factor and fluorescence; you can get a nice pictorial idea of why this is so.
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