“…To date, a large number of donor-acceptor assemblies with photo- and electro-active units have been constructed to study their structure-property relationships for a better understanding of the electron-transfer reactions between the donor and acceptor moieties [ 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 ]. Recently, a π-conjugated donor-acceptor system, in which ferrocene (Fc) was used as the donor, anthraquinone (Aq) as the acceptor, and ethynylene as the π-conjugated linker, has been studied in detail in solution and solid form by our group [ 22 , 23 , 24 , 25 , 26 , 27 ]. It is interesting that the conjugated Fc-Aq compounds are sensitive to the external stimulus of protonation, leading to an expanded π-conjugated system formed by a cyclocondensation reaction and simultaneously displaying significant changes in their optical, structural, and magnetic properties.…”