= Solution
\b[Vibrational and solvent relaxation remove energy before emission, producing a Stokes shift.] <Absorption> initially excites a vertical transition from the populated ground-state vibrational levels to excited-state vibrational levels. <Vibrational relaxation> and solvent rearrangement then bring the molecule near the relaxed lowest excited singlet level before <fluorescence>. Emission ends in vibrationally excited levels of the electronic ground state, so the emitted photon generally has less energy and a longer wavelength. This explanation does not require a change of electronic multiplicity.
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