Direct measurement of the dipole moment of a metastable merocyanine by electromechanical interferometry

The permanent dipole moments of an indolinespirobenzopyran and its merocyanine form in a poly(isobutyl methacrylate) host were determined by electromechanical interferometry. This technique allows the determination of the piezoelectric and electrostrictive properties of thin films for which the polarization of the sample can be determined. The sample was exposed to an external electric field and the polarization was measured as a function of time before and after UV irradiation. Results show that the orientation of spiropyran and merocyanine molecules in the direction of the field is fast compared with the time scale of the photoisomer decay kinetics, allowing the determination of dipole moments from polarization values in the limit of thermal equilibrium. The experiments reveal a pronounced increase in the dipole moment upon ring opening with μ₀ = 6.4 D for the spirobenzopyran and μ₀ = 14.2 D for the merocyanine form.