Visible-light photochromism of triarylamine- or ferrocene-bound diethynylethenes that switches electronic communication between redox sites and luminescence

Redox-active ferrocene- and triarylamine-terminated diethynylethene derivatives have been synthesized and their photochromic properties and switching behavior based on through-bond electronic communication between the two redox sites, as well as their emissions, have been examined. Both bis(ferrocenylethynyl)ethene I and bis(triarylaminoethynyl)ethene 2 show visible-light photochromism induced by donor-acceptor charge-transfer (CT) transitions from the ferrocene or triarylamine to the diethynylethene moieties. The reversibility and quantum yields of the photochromism of 2 (Phi(E -> Z) = 6.1 x 10(-2). Phi(Z -> E) = 1.4 x 10(-2)) are far higher than those of 1 (Phi(E -> Z) = 8.6 x 10(-6). Phi(Z -> E) = 2.5 x 10(-6)). The higher efficiency in 2 may be attributed to the absence of the heavy atom effect and of a low-lying (LF)-L-3 state, which are characteristic of ferrocenyl compounds. This proposition is further supported by the fact that bis(ferrocenylbuta-1,3-diynyl)ethene 3, which, unlike 1, is free from steric interference between the two ferrocenyl groups in the Z form, does not show a significant improvement in its photoisomerization quantum yields (Phi E -> Z = 6.2 x 10(-5), Phi(Z -> E) = 3.4 x 10(-5)). The visible-light photochromism of I and 2 is accompanied by a switch in the strength of the electronic communication between the two redox sites in their mixed-valence states (Delta E-0t values are 70 and 48 mV for (E)-1 and (Z)-1. and 74 and 63 mV for (E)-2 and (Z)-2). In the case of 2, further evaluations were carried out through intervalence charge-transfer (IVCT) band analyses and DFT calculations. We have also demonstrated that steric repulsion between the methyl ester moieties in the Z form is implicated in the reduction in the through-bond electronic communication. Compound 2 exhibits photoluminescence, which is more efficient in the E form than in the Z form, whereas 1 and 3 show no photoluminescence.