Ground- and first-excited-singlet-state electric dipole moments of some photochromic spirobenzopyrans in their spiropyran and merocyanine form

The dipole moments of four photochromic derivatives of indolinospirobenzopyran have been studied in their spiro and their open merocyanine form, in dioxane. Determinations have been done by electrooptical absorption measurements (EOAM). To prepare either a pure spiropyran or a pure merocyanine form solution, the experimental setup had to be modified by addition of a second light beam. In this way thermal equilibration between the spiropyran and the merocyanine form and switching from one form to the other induced by the measuring light beam could be avoided. The ground-state dipole moment of the spiro form amounts to (10-15) x 10(-30) C m and increases strongly to about 60 x 10(-30) C m with the excitation to the first excited singlet state. The ground-state dipole moment of the merocyanine form is much larger and amounts to (50-60) x 10(-30) C m. In contrast to the spiro form it decreases to about 40 x 10(-30) C m, when exciting to the first excited singlet state of the merocyanine form.