Polarization of T→S Emission Spectra of Azines

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Abstract
Polarization of the n←π* triplet‐singlet emission bands have been determined for pyrazine and pyrimidine relative to the n→π*(1A→1W) and the first π→π*(1A→1Lb) absorption. The method consists in exciting into each absorption band separately and measuring the polarization of the resulting emission. The pyrazine emission is found to be polarized along the N–N axis and the pyrimidine emission along the axis that cuts through the bonds (x, in Fig. 1). The agreement with the theory is in two respects. In the theory the general prediction of in‐plane polarization for the n↔π* triplet‐singlet transitions comes out of the difference of the symmetry properties of the singlet and triplet spin functions. The specific prediction along the N–N axis for the pyrazine n↔π* triplet‐singlet transition comes from the symmetry properties of the spin‐orbit coupling operator. These predictions are independent of the form of the MO's and the operator chosen. The agreement with these predictions constitutes, then, an experimental verification of spin‐orbit coupling in phosphorescence spectra. The prediction of the polarization along the x axis (Fig. 1) for the pyrimidine emission comes from the molecular orbital theory. However, the basic reason for this prediction lies in the pyrimidine (π, π*) state wave functions retaining their benzene character. The agreement with this prediction shows the validity of such an approach.

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