P2O5‑Promoted Cyclization of Di[aryl(hetaryl)methyl] Malonic Acids as a Pathway to Fused Spiro[4.4]nonane-1,6-Diones

Conventional spiro-linked conjugated materials are attractive for organic optoelectronic applications due to the unique combination of their optical and electronic properties. However, spiro-linked conjugated materials with conjugation extension directed along the main axis of the molecule are still only rare examples among the vast number of spiro-linked conjugated materials. Herein, the synthesis, leading to π-extended spiro-linked conjugated materialsspiro[4.4]­nonane-1,6-diones and spiro­[5.5]­undecane-1,7-dioneshas been developed and optimized. The proposed design concept starts from readily available malonic esters and contains several steps: double alkylation of malonic ester with bromomethylaryl­(hetaryl)­s; conversion of a malonic ester into the corresponding malonic acid; electrophilic spirocyclization of the latter into the annulated spiro[4.4]­nonane-1,6-dione or spiro­[5.5]­undecane-1,7-dione in the presence of phosphorus pentoxide. On the basis of these insights, the developed method yielded spiro-linked conjugated cores fused with benzene, thiophene, and naphthalene, decorated with active halogen atoms. The structures of the synthesized spirocycles were determined by single-crystal X-ray diffraction analysis. Benzene fused spiro[4.4]­nonane-1,6-dione decorated with bromine atoms was transformed into V-shape phenylene-thiophene co-oligomer type spirodimers via Stille coupling. The spiro-bis­(4-n-dodecylphenyl)-2,2′-bithiophene derivative possessed high photoluminescence properties in both solution and solid state with a photoluminescence quantum yield (PL QY) of 38%.