Vibronic Perturbations and Phosphorescence of Phthalazine

The phosphorescence and EPR spectra of phthalazine (2,3‐diazanaphthalene) were studied in rigid glass solutions at 77°K. It is shown that the perturbation of the zero‐point level of the emitting ${}^3\mathrm{\pi \pi *}$ state by the nearly ${}^3\mathrm{n\pi *}$ states leads to a number of spectral features which are not expected from a $\mathrm{\pi *\hspace{0.17em}\to \hspace{0.17em}\pi }$ phosphorescence. Among these are a blue shift of the spectrum in hydroxyl solvents, a positive polarization of the 0, 0 band with respect to $\mathrm{\pi \hspace{0.17em}\to \hspace{0.17em}\pi *}$ ($L_b$ and $B_b$ ) excitations, and a severe modification of the vibrational structure of the emission by halogenated solvents. It is concluded that when the Born–Oppenheimer approximation breaks down, the usual optical criteria for distinguishing $\mathrm{n\pi *}$ and $\mathrm{\pi \pi *}$ triplet states become invalid or at best ambiguous. A strong optical criterion based on the polarized phosphorescence spectrum is presented. The low quantum yield of the phosphorescence and the strong solvent dependence of the emission yield and lifetime were ascribed to the dependence of the radiationless transition on the vibronic coupling between $\mathrm{n\pi *}$ and $\mathrm{\pi \pi *}$ states.