Spin—Orbit Coupling and the Radiationless Processes in Nitrogen Heterocyclics

Abstract

The matrix elements of the spin—orbit operator between the zero-order (spin-free) (n, π*) states of nitrogen heterocyclics are examined. It is found that generally, to the first order, there is no spin—orbit coupling between singlet and triplet states of the same configuration. The coupling between states of different configurations due to differences in the occupancy of the π* orbitals, but having the same nonbonding orbital, is found to have matrix elements similar to those resulting from the coupling between (π, π*) states, which McClure has shown to be unimportant. For the diazines (or polyazines), the coupling between different configurations due to a different occupancy of the nonbonding orbitals is found to have matrix elements which cancel one another. The general conclusion is thus reached that spin—orbit (s.o.) coupling between singlet and triplet (n, π*) states is unimportant to the first order. Using the observed polarization and lifetime data, it is estimated that the ratio |〈S(n,π*)|Hs.o.|T(n,π*)〉〈S(π,π*)|Hs.o.|T(n,π*)〉|2has an upper value of 10—3. These results as well as vibrational overlap considerations suggest that the enhancement of the efficiency of the intersystem crossing in nitrogen heterocyclics with the lowest singlet state of the (n, π*) type might be due to an S(n, π*)→T(π, π*) rather than an S(n, π*)→T(n, π*) radiationless process. An alternative explanation to the one previously given for the observation of fluorescence of 9, 10-diazaphenanthrene and s-tetrazine is thus suggested. In these molecules, the strong interaction between the (n, π*) singlet levels might cause the lowest S(n, π*) excited level to have a lower energy value than the (π, π*) triplet level. This would therefore lead to a retardation of the intersystem crossing process and allow the fluorescence to be observed. The characteristics of the observed weak phosphorescence of 9,10-diazaphenanthrene give spectroscopic support for this proposal. For nitrogen heterocyclic molecules with the lowest singlet and lowest triplet states of the (π, π*) types, an examination of the quinoline total emission in comparison with that of the parent hydrocarbon, nath-thalene, suggests an increase in the efficiency of the intersystem crossing process in quinoline. This is suggested to be due to the efficient S1(π, π*)→T2(n, π*)→T1(π, π*) radiationless process. A probable enhancement of the internal conversion processes in these molecules is suggested to be due to an increase in the density of states and the efficiency of the S(π, π*)↔S(n, π*) radiationless process due to a large vibrational overlap integral.