Reassignment of the n→π* Transitions of Phthalazine

We have measured the dipole‐moment changes for excitation to different vibronic levels of the near ultraviolet, 4200 Å, absorption bands of phthalazine (2, 3‐diazanaphthalene). The value of |Δμ| is found to be $\text{3.05 ± 0.10}\mathrm{D}$ (for $E_{\mathrm{c\prime }}{\text{= 0.54\hspace{0.17em}E}}_{\mathrm{eff}}$ ) for all vibronic transitions including bands associated with the so‐called second nπ^* state. Armed with this new information we have revised the previous analysis and show that the main bands belong to just one nπ^* state (¹A₂) and not to two nπ^* states as previously asserted. The analysis leads us to some new ideas on nonradiative processes in azines, especially 1,2‐diazines.