TUNING THE SINGLET-TRIPLET ENERGY-GAP IN A NON-KEKULE SERIES BY DESIGNED STRUCTURAL VARIATION - THE SINGLET STAGES OF N-SUBSTITUTED-3,4-DIMETHYLENEPYRROLE BIRADICALS

Semiempirical quantum chemical calculations (AM1/CI and PM3/CI) confir m the qualitative perturbational prediction that electron-withdrawing groups on the ring nitrogen of a 3,4-dimethylenepyrrole should diminis h the energy separation of the singlet and triplet states to near zero . Syntheses of a series of precursors of such biradicals have been dev eloped. Study of the chemistry and spectroscopy of the biradicals has revealed persistent singlet states for the cases where the substituent is methyl, isobutyryl, and pivaloyl. In the cases of N-arenesulfonyl- 3,4-dimethylenepyrroles, both a singlet and a triplet form can be obse rved as persistent species. In this paper, the properties of the singl ets in this series are described. Although energy transfer from the ex cited triplet state of the sensitizer xanthone to the diazene precurso r of N-p-toluenesulfonyl-3,4-dimethylenepyrrole is observed, by nanose cond time-resolved spectroscopy, the chemical behavior of the biradica l intermediate is the same as that observed in the direct photolysis o r thermolysis of the diazene. The reactive form of the biradical under these conditions appears to be the singlet.