Electronic States of 9,10-Diazaphenanthrene

The S1←S0 absorption spectrum of the diazaphenanthrene crystal has been measured at 4.2°K in polarized light. The strength of the transition and the coincidence of the (0, 0) band at 23 100 cm−1 in absorption and emission clearly indicates that the lowest singlet state is 1B1(nπ*). The T1←S0 spectrum has also been measured. It consists of weak sharp bands with an origin at 18 571 cm−1, and a broad continuum. The bands displayed Zeeman splitting in a magnetic field. The T1←S0 spectrum is polarized parallel to the S1←S0 transition. A comparison of the vibrations of the lowest singlet and triplet states with those of the ground state indicates that the state is 3ππ*. The spatial symmetry is assigned 3B2 by analogy with phenanthrene. The 3nπ*←S0 transition was not identified and some possible reasons for this are discussed. Some discussion of the efficiencies of radiationless transitions in o-diazines is also presented.