Hetero-atom stabilization of zwitterionic non-Kekulé molecules: A DFT study on energy gaps between zwitterionic singlet and biradical triplet states

Energy gaps between a biradical triplet (BR-T) state and a zwitterionic singlet (ZI-S) state in tetramethyleneethane (TME) π-systems stabilized by hetero-atoms are discussed. Unsymmetrical introduction of both electron-donating and -withdrawing groups into the TME system caused substantial stabilization of the ZI-S. Thus, the energy separation between ZI-S and BR-T states in 2-dicyanomethylene-3-(1,3-dithiolenylidene)cyclopenta-1,3-diene was as small as 0.62 kcal mol−1, in contrast to that in 2,3-dimethylenecyclopenta-1,3-diene at 22.5 kcal mol−1, using the B3LYP/6-31G* level of calculation. In a series of 2,3-dimethylene-5-heterocyclopentadiene, the ZI-S state was stabilized by donor-atoms in the order of X (hetero-atoms)=O>S>NH>NMe. The ZI-S states of 3,4-dimethylenepyrroles (NH and N-Me) and 3,4-dimethylene-1,2-diazole lie below the BR-T by 3.1, 4.4 and 10.3 kcal mol−1, respectively. Cyclic 10-π-delocalization in hetero-pentalene further stabilizes the ZI-S, and 2,5-dimethylpyrazolo[3,4-c]pyrazole is a ground state singlet molecule lying below the triplet by 49.5 kcal mol−1. Time dependent-density functional theory (TD-DFT) calculations have successfully predicted the absorption spectra of these TMEs in the ZI-S state.