A Merry Dance Across the pi-Cloud: Tracking the Transformation of a 2,7-Substituted Dihydropyrene Through a Thermally Stimulated Single-Crystal-to-Single-Crystal Reaction

The novel 2,7-bis((TIPS)ethynyl)-10b,10c-diethyl-dihydropyrene photochrome (TIPS = triisopropylsilyl) was observed to undergo thermal rearrangement to 2,7-bis(TIPS-ethynyl)-pyrene in the solid state. The transformation proceeds stepwise with ethyl groups "dancing" out of the dihydropyrene core toward the periphery of the polycyclic framework. The formation of two regioisomers occurs initially and is followed by two progressive de-ethylations to give a pyrene. This transformation occurs in the solid crystalline state and represents a unique example of a single-crystal-to-single-crystal (SC-SC) transformation involving C-C bond cleavages with hydrocarbon fragments escaping the ordered crystalline structure. Detailed characterizations of the transformation by in situ temperature-dependent X-ray single-crystal diffraction supported by thermal analysis, HPLC-mass spectrometry experiments, NMR spectroscopy, and theoretical calculations, are reported.