Polarization of Trarr;S Emission Spectra of Azines

摘要

Polarization of thenlarr;pgr;*triplethyphen;singlet emission bands have been determined for pyrazine and pyrimidine relative to thenrarr;pgr;*(1Ararr;1W) and the first pgr;rarr;pgr;*(1Ararr;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 Nsngbnd;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 inhyphen;plane polarization for thenharr;pgr;*triplethyphen;singlet transitions comes out of the difference of the symmetry properties of the singlet and triplet spin functions. The specific prediction along the Nsngbnd;N axis for the pyrazinenharr;pgr;*triplethyphen;singlet transition comes from the symmetry properties of the spinhyphen;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 spinhyphen;orbit coupling in phosphorescence spectra. The prediction of the polarization along thexaxis (Fig. 1) for the pyrimidine emission comes from the molecular orbital theory. However, the basic reason for this prediction lies in the pyrimidine (pgr;, pgr;*) state wave functions retaining their benzene character. The agreement with this prediction shows the validity of such an approach.