Polarizability and dipole moment changes of the lowest 1ππ* state of azulene; an example of the use of the Stark effect on polar and nonpolar mixed molecular crystals

Abstract

The relatively high representative electric fields present at molecular sites in a polar molecular crystal, ${\text{∼ 10}}^7\hspace{0.17em}\mathrm{V}\hspace{0.17em}{\mathrm{cm}}^{\text{−1}}$ , make possible the determination of excited‐state dipolar and polarizability properties of a guest molecule dispersed in the polar host from a consideration of field induced moments. The determination of the magnitudes and directions of these guest properties is particularly dependent on an analysis interrelating Stark measurements made separately on the spectroscopically accessible states of the guest when dispersed in a polar and in nonpolar host lattice. The azulene‐benzophenone, azulene‐naphthalene mixed‐crystal systems were chosen to demonstrate this method of analysis for the lowest ¹ππ* state of azulene. The magnitude of the dipole moment change for azulene dispersed in the nonpolar (zero field) naphthalene lattice Δμ_zf = 1.21 D, has already been determined. Using this magnitude and the results of Stark measurements on azulene in benezophenone we find that (1) the molecular dipole reverses direction upon excitation, ${\mathrm{\Delta \; \mu }}_{\mathrm{zf}}\text{=−1.21\hspace{0.17em}}\mathrm{L\; D}$ and (2) the long axis polarizability is greater in the ground state than in the excited state, ${\mathrm{\Delta \; \alpha }}_{\mathrm{LL}}\text{=−3.5\hspace{0.17em}}{\mathrm{\AA }}^3$ . This value compares favorably with the theoretically calculated Δα_LL values of $\text{−4.4\hspace{0.17em}}{\mathrm{\AA }}^3$ and $\text{−3.75\hspace{0.17em}}{\mathrm{\AA }}^3$ .