Acid-base-driven multilevel switchimg of electronic coupling in 2,4-dimethylene five-member heterocycles

The ground state multiplicity of 2,4-dimethylene five-member heterocycles i s dependent on the protonation of the heteroatom. The relative singlet and triplet energy at pi-CASSCF-optimized geometries Varies and is dependent on the number of protons attached to the heteroatom. The 2,4-dimethylenecyclo pentadiene anion (X = CH-) and the 2,4-dimethylenepyrrole anion (X = N-) ar e ground state singlets while the 2,4-dimethylenecyclopentadiene (X = CH2) and 2,4-dimethylenepyrrole cation (X = NH2+) are ground state triplets. The neutral 2,4-dimethylenepyrrole (X = NH) is a ground state triplet with a w eaker preference than that calculated for the 2,4-dimethyrenepyrrole cation . Similarly, the protonated 2,4-dimethylenefurane (X = OH+) is a strongly p referred ground stale triplet while the neutral 2,4-dimethylenefurane (X = OH) has a weaker preference for the triplet ground state. Analogous relatio nships exist for the systems with the third-row elements. These properties of the 2,4-dimethylene five-member heterocycles allow for the construction of molecular organic ferromagnets with pH-controlled magnetism.