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 previously published donor-bridge-acceptor system (anthracene-CH2-bithiophene-pyridinium) is modified by incorporation of the photoisomerizable dithienylethene (DTE) as a switching unit. In the open-ring form 2a, excitation of the anthracene donor leads to an intramolecular charge separation proved by identification of the anthracene radical cation in transient absorption spectra (ON state). After photocyclization to the closed-ring isomer (2a --> 2b), the intramolecular charge separation is completely suppressed (OFF state). The reversibility of the ON/OFF switching is verified. From UV-vis absorption spectra and cyclovoltammetric studies it is deduced that in the open-ring isomer 2a (ON state) conjugation is markedly restricted within the dithienylethene bridge, whereas in the closed-ring isomer 2b (OFF state) the conjugation is extended over the whole dithienylethenepyridinium (DTEP) subunit. Consequently, in 2 an enlarged conjugation within the bridge is not decisive for photoinduced charge separation. Instead the observed transfer properties can be understood by thermodynamic aspects.