$n-\pi^{\ast}$ SINGLET STATES OF THALAZINE.

Abstract

Phthalazine differs from other ortho-diazines, pyridazine and diazaphenanthrene, in having a lowest $\pi^{\ast}$orbital of $b_{2}$ rather than $a_{2}$ symmetry. This results in a symmetry forbidden $A_{2}\leftarrow A_{1}$ lowest excited promotion. Polarized absorption studies of single crystals of phthalazine and of phthalazine in naphthalene have enabled the locations and assignments of the lowest two excited $n\pi^{\ast}$ state of phthalazine to be made. In the crystal a weak 0.0 band is found at $25 365 cm^{-1} (0)$ followed by a stronger vibronic origin $(0^{\prime})$ at $25 875 cm^{-1}$. Most of the spectrum of the $A_{2}\leftarrow A_{1}$ transition is built on this false origin $(0^{\prime})$. A more intense band is found at $0+2168 cm^{-1}$ [or $0^{1}+1658 cm^{-1}$] which is not due to a totally symmetric mode built on $0^{\prime}$. Both $0^{\prime}$ and $0^{\prime}+1658$ act as origins for comparable spectra. Mixed crystal studies indicate in plane polarization for both systems. The strong $0+2168 cm^{-1}$ band is interpreted as part of a higher $n\pi^{\ast}$ state $B_{2}\leftarrow A_{1}$. The polarization and weakness of the transition are discussed.