Knowledge Bank

Polarizations of $T\to S$ emission bands have been determined for pyramidine and pyrimidine relative to $n\to \pi \ast (1W\leftarrow 1A)$ and first $\pi \to \pi \ast (1Lb\leftarrow 1A)$ transitions. The method consists in exciting at $77\circ K$ into both $n\to \pi \ast$ and first $\pi \to \pi \ast$ absorption bands separately using narrow band monochromatic light and measuring the polarization of the resulting emission bands. I. The phosphorescence of both axines is found to be unambiguously polarized perpendicular to the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}W\leftarrow 1A(n\to \pi \ast )$ absorption and is thus polarized in the plane of the molecule. This result is completely consistent with spin-orbit coupling theory for a transition involving promotion of a $\sigma$-electron to a $\pi$-orbital and thus can be taken as the experimental verification of spin-orbit coupling in molecules. II. The pyrazine phosphorescence is polarized $\perp$ to the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Lb\leftarrow 1A(\pi \leftarrow \pi \ast )$ transition. The pyrimidine emission polarization indicates both $|$ and $\perp$ components to the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Lb\leftarrow 1A$ transition. These results are consistent with the Clementi-Kasha-Sidman central field spin-orbit coupling scheme. In a central field approximation only A coupling states are predicted for pyrazine, whereas both A & B states are predicated for pyrimidine.