Bathophenanthroline Disulfonate Ligand-Induced Self-Assembly of Ir(III) Complexes in Water: An Intriguing Class of Photoluminescent Soft Materials

Abstract

Strong evidence of concentration-induced and dissolved electrolyte-induced chromophore aggregation has been universally observed in numerous water soluble bis-cyclometalated Ir(III) photosensitizers bearing the sulfonated diimine ligands bathophenanthroline disulfonate and bathocuproine disulfonate. This new class of aqueous-based soft materials was highly photoluminescent in their aggregated state where detailed spectroscopic investigations of this phenomenon revealed significant blue shifts of their respective photoluminescence emission spectra with concomitant increases in excited-state lifetimes and quantum yields initiating even at micromolar chromophore concentrations in water or upon the addition of a strong electrolyte. A combination of nanoscale particle characterization techniques, static and dynamic photoluminescence spectroscopic studies, along with atomistic molecular dynamics (MD) simulations of these soft materials suggests the formation of small, heterogeneous nanoaggregate structures, wherein the sulfonated diimine ancillary ligand serves as a pro-aggregating subunit in all instances. Importantly, the experimental and MD findings suggest the likelihood of discovering similar aqueous aggregation phenomena occurring in all transition-metal complexes bearing these water-solubilizing diimine ligands.

Water-Soluble Tris(cyclometalated) Iridium(III) Complexes for Aqueous Electron and Energy Transfer Photochemistry

“…All three strategies have been applied to heteroleptic cyclometalated iridium­(III) compounds (Figure ) by several researchers with different backgrounds and motivations. The Castellano group used bathophenanthroline disulfonate in combination with different cyclometalating ligands (Figure a), and they observed strongly concentration-dependent photoluminescence quantum yields and lifetimes due to aggregation of their iridium­(III) complexes in aqueous solution. , This phenomenon is reminiscent of aggregation-induced emission but is uncommon for octahedral metal complexes. The group of Lo used PEGylation to obtain water-soluble cyclometalated iridium­(III) complexes that can act as biological probes with low cytotoxicity and high cellular uptake efficiency (Figure b) .…”

“…Previously explored water-soluble cyclometalated iridium­(III) complexes (left) and the corresponding cyclometalating ligands (right); R = H, CF 3 , t Bu. − ,− …”

Photocatalytic Regeneration of a Nicotinamide Adenine Nucleotide Mimic with Water-Soluble Iridium(III) Complexes

“…Homoleptic tris­(cyclometalated) iridium­(III) complexes such as fac -[Ir­(ppy) 3 ] (ppy = 2-phenylpyridine) are promising photosensitizers for the regeneration of 1,4-NADH because they are strong and robust photoreductants with long-lived excited states. − However, these charge-neutral homoleptic tris­(cyclometalated) iridium­(III) complexes are very lipophilic and therefore usually insoluble in water . Several different groups previously obtained water-soluble cyclometalated iridium­(III) complexes, though commonly based on heteroleptic ligand frameworks, ,− which are often less photo-reducing and less photorobust than the homoleptic versions. ,, Our group recently developed a tri-sulfonated variant of fac -[Ir­(ppy) 3 ] with an excited-state oxidation potential ( E ox *) of −1.89 V versus SCE in an aqueous phosphate buffer at pH 7 (Table , [Ir­(sppy) 3 ] 3– ). ,, The excited-state electron donor properties turned out to be further tunable by ligand fluorination, resulting in [Ir­(Fsppy) 3 ] 3– ( E ox * = −1.85 V vs SCE, Table ) and [Ir­(dFsppy) 3 ] 3– ( E ox * = −1.76 V vs SCE, Table ). All three compounds (structures included in Figure ) are very well soluble in water, and they seem to have suitably strong excited-state electron donor properties for the photochemical regeneration of 1,4-NADH from NAD + .…”