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 1 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 (Φ_E→Z = 6.1 &times 10^-2, Φ _Z→E = 1.4 &times 10^-2) are far higher than those of 1 (Φ_E→Z = 8.6 × 10^-6, Φ_Z→E = 2.5 &times 10^-6). The higher efficiency in 2 may be attributed to the absence of the heavy atom effect and of a low-lying ³LF 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 (Φ_E→Z = 6.2 &times 10^-5 Φ _Z→E = 3.4×10^-5). The visible-light photochromism of 1 and 2 is accompanied by a switch in the strength of the electronic communication between the two redox sites in their mixed-valence states (ΔE⁰' 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.