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

Abstract

The binuclear complex [{Ru(bipy)₂}(μ-L¹){Re(CO)₃(H₂O)}][PF₆]₃ (Ru-crw-Re), and its mononuclear counterpart [Ru(bipy)₂(L¹)][PF₆]₂, were synthesised. The bridging ligand L¹ 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 = 1.2 × 10¹⁰ 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 = 2 × 10⁸ s⁻¹ in the absence of Ba²⁺ and k_en^Ba = 7 × 10⁶ 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.