Rhenium fac-tricarbonyl bisimine complexes: luminescence modulation by hydrophobically driven intramolecular interactions

Abstract

A triplet metal-to-ligand charge transfer emitting cationic Re^I complex (1c) functionalised with a C₁₂ alkyl chain possesses unique solvent-dependent photophysical properties. In acetonitrile solution the luminescence properties of 1c are typical of related fac-{Re(CO)₃(diimine)L}⁺ species with emission at 555 nm (τ = 135 ns, Φ_em = 1.7%) whereas in water, emission was blue-shifted to 523 nm with an increase in luminescence lifetime (688 ns) and quantum yield (9.2%). These unusual properties are attributed to a dynamic intramolecular mechanism involving fold-back of the alkyl chain onto or around the coordinated 2,2′-bipyridine ligand, thus shielding the excited state from the surrounding water solvent. Comparison of 1c with Re^I complexes either lacking a chain or incorporating varying chain lengths (C₈ and C₁₆) showed these properties to be unique to 1c. The intramolecular fold-back conformation was shown to be highly temperature dependent between 278 and 318 K, with elevated temperatures resulting in far less effective shielding. These unique photophysical properties can therefore be exploited in aqueous environments through interaction with lipophilic entities such as liposomes or biomolecules such as human serum albumin, which both result in a reverted red-shifted emission for 1c at 552–555 nm.