Phosphorescence azabenzenes

The benzene and azabenzenes form iso-electronic series of molecules, as naphthalene and the azanaphthalenes also do. The ground state electronic and geometric structures are therefore quite similar within one series. The substitution of CH groups with nitrogens introduces lone-pair to 7r transitions, and lowers the benzene and naphthalene symmetries. Small and systematic trends are found for linear response properties of the azabenzenes [189]. Each molecule is, however, very specific with respect to phosphorescence due to the delicate nature of the SOC and electric dipole activity interactions. 

In the following we discuss the phosphorescence lifetimes of 3(n, x ) and 3(x,x ) states of the azabenzenes and the azanaphthalenes, which can differ by several orders of magnitude. The symmetry axes and geometries of the discussed compounds are reproduced in Fig. 14. Since benzene and naphthalene lack (n,x ) transitions their triplet state radiative lifetimes are, as for other hydrocarbons, considerably longer. 

Among the sequence of N-heterocycles, n.b. the 1,2,3,4 N-substituted azabenzenes, the response investigations have focussed on pyridine, pyrazine, pyridazine, pyrimidine, s-triazine, and s-tetrazine [151]. The calculations of phosphorescence of these compounds utilize x type complete active spaces, the general rule of thumb has been to use one correlating x orbital for each occupied x orbital. All azabenzenes, except pyridazine, take the same Hartree-Fock orbital configuration in C2V symmetry Ilai,7b2,2bi,la2, including 18 and 3 doubly occupied a and x orbitals, respectively. For pyridazine it reads 10ai,8b2,2bi,la2. 

The radiative lifetime of the order of 1 second and the small quantum yield of pyridine phosphorescence could be easily understood from the MCQR calculations [151]. For most of the other azabenzenes one finds that 3(rnr ) states also radiate in the order of 1 second. The averaged lifetimes in Table 20 for the 3(n, 7r ) states vary from 32 ms for pyrazine to 242 ms for tetrazine. The dependence of these lifetimes on the quality... 

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