Switchable polarity solvent (SPS) systems: probing solvatoswitching with a spiropyran (SP)–merocyanine (MC) photoswitch

Abstract

The switchable polarity solvent (SPS) of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and an alcohol (e.g.1-propanol) reversibly switches to a higher polarity ionic liquid, [DBUH⁺][RCO₃⁻], when treated with CO₂. A long-lived species with unique properties was detected in an investigation into the use of SPS to control the lifetime of the merocyanine (MC) form in a spiropyran (SP)–MC molecular photoswitch. Irradiation of SP in 1-propanol (PrOH) in the presence of DBU generates a new species (λ_max = 420 nm). This species converts to MC upon bubbling with CO₂, which produces [DBUH⁺][PrOCOO⁻]. It is proposed that a mixture of 1,2 and 1,4 alkoxide addition products form as a result of nucleophilic attack on the conjugated diene system of MC, where alkoxide formation arises from equilibration of highly basic DBU and the alcohol. These adducts revert to MC upon application of CO₂ or addition of acid. Determination of the overall equilibrium constant for alkoxide adduct formation involving DBU was afforded through Benesi–Hildebrand analysis.