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The dimensionless displacement strength for the equal-frequency single-vibration model in displaced-oscillator Franck-Condon progression. It equals the mean number of excited vibrational quanta in its zero-temperature overlap distribution.

An organic semiconductor has conjugated molecular units with strong intramolecular bonding and comparatively weak intermolecular electronic overlap. Optical excitation commonly promotes a $\pi$ electron from an occupied molecular orbital toward an unoccupied one. Weak dielectric screening and narrow bands favour bound Frenkel excitons, sometimes mixed with intermolecular charge-transfer states, rather than immediate production of free carriers. The optical excitation energy is below the unbound electron–hole threshold by the exciton binding energy, with relaxation conventions specified when comparing measured gaps.

The electric-dipole interaction selects bright states through their transition dipoles, producing polarization-dependent absorption in oriented samples. From a singlet ground state, strong electric-dipole absorption normally preserves spin and reaches singlet excitons; direct triplet excitation is weak without spin mixing. The Franck-Condon principle treats excitation as vertical on the nuclear-motion timescale. For one displaced harmonic vibration at low temperature, the displaced-oscillator Franck-Condon progression has relative strengthsHere the Huang-Rhys factor $S$ measures the displacement and $E_{00}$ is the zero-vibration transition energy. Vibrational relaxation before emission produces a Stokes shift. Molecular packing and excitonic coupling shift or split bright features and change their intensities; disorder and vibrations broaden them. Consequently a universal parabolic-band absorption edge is inappropriate for a localized molecular excitation.^[ref]