Structure and Dipole Moment of the First Electronically Excited State of Pyridine N-Oxide

A study is presented of the lowest‐energy singlet—singlet transition of pyridine N‐oxide in mixed crystals with p‐dichlorobenzene at 4.2°K. Polarization and Stark effect measurements on the electronic orgin of the transition (at 29 599.6 cm⁻¹) show that the transition is ${\pi }^{*}{\mathrm{\leftarrow \; \pi ,}}^1{B}_2{\leftarrow }^1{A}_1.$ The spectrum of d₅‐pyridine N‐oxide is blue shifted by 123 cm⁻¹ and its absorption and emission spectra are quite similar to those of h₅. The vibrational analysis of the absorption and fluorescence spectrum, and the Franck—Condon patterns, suggest that the equilibrium geometry is little affected by electronic excitation. Stark effect measurements on the electronic origin band yield a value of $\text{Δ μ=0.71 ± 0.03\hspace{0.17em}}\mathrm{D}$ (assuming ${\text{E ≡ 0.60\hspace{0.17em}E}}_{\mathrm{eff}})$ for the change in dipole moment on electronic excitation; where E and E_eff are the applied and effective Stark electric fields corresponding to an external field directed down the a′ crystallographic axis of the p‐dichlorobenzene host crystal. This small change is much less than predicted by π‐electron theory.