Switching of the inter-component photoinduced electron- and energy-transfer properties of a Ru(II)-aza-crown-Re(I) complex;: effects of changing temperature, and of incorporation of Ba2+ ion into the macrocyclic spacer between the chromophores

The binuclear complex [{Ru(bipy)(2)}(mu-L-1){Re(CO)(3)(H2O)}][PF6](3) (Ru-crw-Re), and its mononuclear counterpart [Ru(bipy)(2)(L-1)][PF6](2), were synthesised. The bridging ligand L-1 contains two bipyridyl binding sites which are attached via methylene spacers to each of the N atoms of a diaza-18-crown-6 macrocycle spacer (crw), such that addition of a metal ion to the macrocycle affects the pathway linking the two chromophores. The absorption spectra, luminescence spectra and excited-state lifetimes, and electrochemical properties of acetonitrile solutions of these complexes were investigated. Luminescence studies on Ru-crw-Re showed the presence of two competing types of photoinduced intramolecular process involving the Re-based excited state as a donor, viz.: (i) electron transfer from an amine group(s) of the aza-crown ether; and (ii) Re-->Ru energy transfer. These processes were monitored by time-resolved luminescence spectroscopy both at room temperature and at 77 K, and in the presence and absence of a cation (Ba2+) hosted within the aza-crown macrocyclic unit. In fluid solution at room temperature, there is no photoinduced Re-->Ru energy transfer; instead photoinduced electron transfer occurs in the absence of Ba2+ (k(el)=1.2x10(10) s(-1)), which is inhibited by the presence of Ba2+. In frozen solvent (77 K), only Re-->Ru photoinduced energy transfer occurs, with k(en)=2x10(8) s(-1) in the absence of Ba2+ and k(en)(Ba)=7x10(6) s(-1) in the presence of Ba2+. Thus, Re-->Ru energy transfer is strongly affected by Ba2+ being hosted within the aza-crown cavity, i.e. a switching effect. The role of the diverse factors affecting the two processes is discussed in detail.