Design of molecular sensors and switches based on luminescent ruthenium–terpyridine complexes bearing active methylene and triphenylphosphonium motifs as anion recognition sites: experimental and DFT/TD-DFT investigation

Abstract

Synthesis, characterization and thorough investigation of the photophysical and electrochemical properties of a new category of emissive homo- and heteroleptic Ru(II)-complexes derived from the [4′-(p-triphenylphosphonium methyl phenyl)-2,2′:6′,2′′-terpyridine]bromide (tpy-PhCH₂PPh₃Br) ligand have been executed in this work. Incorporation of the PhCH₂PPh₃⁺Br⁻ group at the terpyridine motif appropriately adjusts the triplet metal-to-ligand charge transfer (³MLCT) and metal-centered (³MC) excited states so that the complexes luminesce at room temperature (RT) having lifetimes within the range of 6.82–9.63 ns. The RT emission characteristics of the complexes get further enhanced via aggregation phenomena through the use of different solvent/non-solvent mixtures (DMSO/H₂O and DMSO/PhCH₃ mixtures). Temperature dependent emission spectral measurements indicate that the emission intensity, quantum yield and lifetime increase upon dropping down the temperature, thereby designated as the on-state, while the increase of temperature causes a reduction of the said properties, indicating the off-state and the process is fully reversible. Taking advantage of the active methylene group coupled with a phosphonium motif, anion sensing characteristics of the complexes are investigated systematically in DMSO through the use of various optical channels and spectroscopic tools. The complexes are very much sensitive to fluoride and to a lesser extent acetate and dihydrogen phosphate among the studied anions. In essence, the complexes function as sensors for temperature and fluoride ion. Computational investigations were also executed via density functional theory (DFT) and time-dependent (TD)-DFT to obtain a clear understanding of the electronic structures of the metalloreceptors, appropriate assignment of the spectral bands and their mode of interaction with selected anions.