Practice question

Stokes shift refers to fluorescence emission appearing at longer wavelength and lower energy than absorption. After excitation to upper vibrational level of S1, the fluorophore undergoes rapid vibrational relaxation, internal conversion and solvent reorientation, dumping small quanta of energy as heat before photon emission. This non-radiative dissipation in the excited manifold reduces energy gap for radiative return. Intersystem crossing instead populates triplet state producing phosphorescence. Radiative loss is the emitted photon itself. Hence non-radiative energy loss during S1 lifetime explains systematic red displacement of emission spectrum.