Synthesis of liquid caged hydrocarbon fuel by intramolecular [2+2] photocycloaddition for advanced propulsion

Intramolecular [2+2] photocycloaddition is a reliable way to synthesize high-strained cyclobutyl-containing structures for high-value chemicals. Here we propose a photosensitized dimer of methyl cyclopentadiene (DMCPD) intramolecular [2+2] cycloaddition to synthesize an intriguing liquid caged fuel, namely 3, 8-dimethylpentacyclic [5.3.0.0 2,5 .0 3,9 .0 4.8 ] decane (MPCD), with two four-membered rings, four five-membered rings and two methyl branched chains. The reaction conditions are optimized to obtain a high DMCPD conversion of 80.95% as well as a high MPCD selectivity of 74.19%. Based on the quenching experiment and low-temperature phosphorescence test, intramolecular [2+2] photocycloaddition complies with the Dexter triplet-energy-transfer mechanism, which is essentially double electron exchange between excited photosensitizer and DMCPD. Through kinetic studies, the reaction order ( α =1) and the reaction kinetic equation related to light intensity and DMCPD concentration are obtained. MPCD has excellent physicochemical properties, with a high density of 0.964 g·cm -3 , high net heat of combustion of 42.63 MJ·kg -1 , relatively low kinetic viscosity of 25.03 mm 2 ·s -1 at (20°C), and high specific impulse of 328.19 s.