Knowledge Bank

While most N-heterocycles phosphoresce with moderate yields in a variety of matrices, a few, particularly those with ortho-nitrogens, show a striking solvent effect, A dramatic appearance of phosphorescence from non-emitting, hydrocarbon solutions at $77\circ K$ of cinnoline, 4-methylcinnoline, phthalazine, and 9,10-diazaphenanthrene is seen with the addition of small amounts of strongly hydrogen-bonding alcohols. An investigation of the role of hydrogen-bonding in the luminescence behavior of these molecules, along with appropriate spectral data is presented which suggests chat in hydrocarbon solutions a near degeneracy exists between the lowest $\pi \pi \ast$ and a higher $n\pi \ast$ triplet, and that the Low yield of phosphorescence results from a rapid triplet radiationless decay. The pseudo-Jahn-Teller effect is offered as an explanation of this fast decay.