Dipole Moments of the Lowest Singlet π* ← π States in p-Fluorophenol and p-Fluoroaniline

The Stark effect within the rotational fine structure is used to measure the change in dipole moment on excitation of p‐fluoroaniline and p‐fluorophenol. The spectra are observed in the 0–0 bands of the lowestlying singlet $\mathrm{\pi *\hspace{0.17em}\leftarrow \hspace{0.17em}\pi }$ transitions. The results are $\text{|\hspace{0.17em}Δμ\hspace{0.17em}|\hspace{0.17em}=\hspace{0.17em}0.44\hspace{0.17em}±\hspace{0.17em}0.10}\mathrm{D}$ for p‐fluorophenol and $\text{|\hspace{0.17em}Δμ\hspace{0.17em}|\hspace{0.17em}=\hspace{0.17em}0.82\hspace{0.17em}±\hspace{0.17em}0.09}\mathrm{D}$ for p‐fluoroaniline. If the sign of the change is taken to be positive, the results correlate well with the observed structural changes on excitation. This correlation indicates the extent of quinoidlike resonance structure to the electronic nature of the excited state.