Interpretation of the Stark effect in molecular crystals

A theory is developed for the Stark effect of guest molecules in host molecular crystals. Local field effects are treated within a point-dipole linear-polarizability approximation, and expressions are derived for the effective dipole moment of molecules in a pure host crystal subjected to an external electric field. The changes induced in the effective dipoles by the introduction of a guest molecule are determined, and from them is obtained the energy of the doped crystal relative to the pure host crystal. A general expression for the linear Stark splitting for a guest molecule is derived. It is shown that the splitting is given by 2Δμ · F, which has been previously used to analyse experimental measurements, only for a non-polar host crystal when all polarizability differences are neglected. With this approximation, published results for azulene in a naphthalene host are reanalysed using the present more rigorous treatment of the local field F. The dipole moment difference between the ground state and the ¹A₁ excited state of azulene is found to be 2.15 D, and is directed nearly parallel to the crystal c′-axis. However, the neglected polarizability changes could contribute 20% of the measured splitting.