Optical ON/OFF switching of intramolecular photoinduced charge separation in a donor-bridge-acceptor system containing dithienylethene

In the newly designed photoswitchable electron transfer compound 2, our pre viously published donor-bridge-acceptor system (anthracene-CH2-bithiophene- pyridinium) is modified by incorporation of the photoisomerizable dithienyl ethene (DTE) as a switching unit. In the open-ring form 2a, excitation of t he anthracene donor leads to an intramolecular charge separation proved by identification of the anthracene radical cation in transient absorption spe ctra (ON state). After photocyclization to the closed-ring isomer (2a --> 2 b), the intramolecular charge separation is completely suppressed (OFF stat e). The reversibility of the ON/OFF switching is verified. From UV-vis abso rption spectra and cyclovoltammetric studies it is deduced that in the open -ring isomer 2a (ON state) conjugation is markedly restricted within the di thienylethene bridge, whereas in the closed-ring isomer 2b (OFF state) the conjugation is extended over the whole dithienylethenepyridinium (DTEP) sub unit. Consequently, in 2 an enlarged conjugation within the bridge is not d ecisive for photoinduced charge separation. Instead the observed transfer p roperties can be understood by thermodynamic aspects.