Synthesis and photoswitching studies of OPE-embedded difurylperfluorocyclopentenes

We report the synthesis and photochemical behavior of five photochromic molecular switches 7a-e with attached molecular wires based on differently substituted oligo(phenylene ethynylene) (OPE) building blocks. The switchable molecular wires 7a-e were built in a convergent approach from substituted iodotolans 6a-e and 1,2-bis(2-methyl-5-ethynylfuran-3-yl)perfluorocyclopentene 5 by 2-fold Sonogashira coupling. Compound 5 was prepared from the corresponding bis-aldehyde 2 by Wittig-type olefination with [PPh3CHBr2]Br·CH3CN, followed by elimination to the bromoalkyne under mild phase-transfer conditions at 0 °C. Halogen-metal exchange with i-PrMgCl·LiCl and hydrolysis furnished 5 in good overall yield. Substituents R(1) and R(2) in the OPE portion were either electron-withdrawing or electron-donating, and their influence on the photostability and photoswitching characteristics of 7a-e was studied. All resulting molecules show reversible photochromism between the colorless off and the deeply colored on states when irradiated with light of 313 and 576 nm wavelengths, respectively. The quantum yields of these photoreactions increased when electron-withdrawing groups were used. This was further corroborated by reversible protonation/deprotonation of 7e (R(1) = NMe2, R(2) = H) for which the ring-closing quantum yield increased 10-fold upon switching off the donor by protonation.