Frequency dependent properties of zero-phonon lines are considered on the basis of persistent hole burning in the inhomogeneous optical bands of dye-doped polymer glasses. The energy spread of optical transitions was expressed as a difference of Lennard-Jones (L-J) potentials of the ground state and the excited state. Pressure shift coefficients of spectral holes depend on the difference of the first derivatives of L-J potentials. From the second derivatives the quadratic electron phonon coupling constants were calculated and compared to temperature induced shift and broadening of spectral holes. A softening of local phonons in glasses or defect crystals containing empty space can be associated with the vanishing of force constants at intermolecular distances exceeding the equilibrium value by a mere 11%. (c) 2005 Elsevier B.V. All rights reserved.