Luminescent Hydrogel Particles Prepared by Self-Assembly of β‑Cyclodextrin Polymer and Octahedral Molybdenum Cluster Complexes Kaplan Kirakci,* ,† Va ́ clav S ̌ ícha, † Josef Holub, † Pavel Kuba ́ t, ‡ and Kamil Lang* ,† † Institute of Inorganic Chemistry of the AS CR, v.v.i, Husinec-R ̌ ež 1001, 250 68 R ̌ ež , Czech Republic ‡ J. Heyrovsky ́ Institute of Physical Chemistry of the AS CR, v.v.i, Dolejs ̌ kova 3, 182 23 Praha 8, Czech Republic * S Supporting Information ABSTRACT: A series of luminescent octahedral molybdenum cluster complexes were obtained by treating Na 2 [Mo 6 I 8 (OMe) 6 ] with icosahedral closo-dicarbaborane C-carboxylic acids in refluxing tetrahydrofuran. The study of the photophysical properties of Na 2 [Mo 6 I 8 (1-OOC-1,2-closo-C 2 B 10 H 11 ) 6 ] (1), Na 2 [Mo 6 I 8 (1-OOC-1,7- closo -C 2 B 10 H 11 ) 6 ] ( 2 ), and Na 2 [Mo 6 I 8 (1-OOC-1,12-closo-C 2 B 10 H 11 ) 6 ] (3) in acetonitrile revealed a red luminescence with high quantum yields up to 0.93 for 2, an efficient quenching of the luminescence by oxygen, and high quantum yields of singlet oxygen formation of approx- imately 0.7. Self-assembly between compound 2 and β- cyclodextrin polymer led to monodisperse hydrogel particles with a diameter of approximately 200 nm and unchanged luminescence spectra and kinetics features over 14 days. In contrast, bare cluster complex 2 in water formed aggregates and hydrolyzed over the time as indicated by a progressive red shift of the luminescence maxima. The invariance of key photophysical parameters of the hydrogel particles coupled with a high oxygen sensitivity of the luminescence are attractive features for long- term biological experiments involving optical oxygen probing. In addition, this hydrogel is a singlet oxygen sensitizer in water with promising properties for photodynamic therapy. ■ INTRODUCTION Among the variety of phosphorescent dyes, the octahedral molybdenum cluster complexes [Mo 6 L 14 ] 2- have recently emerged as relevant building blocks for the construction of photofunctional materials thanks to promising photophysical properties and simple synthetic protocols. Upon excitation from the UV to green spectral regions, they form long-lived triplet states that relax via a red luminescence with quantum efficiencies close to 100%. 1 Their luminescence is quenched by molecular oxygen forming the reactive singlet oxygen, O 2 ( 1 Δ g ), with quantum yields up to 0.92. 2,3 Such properties have already been exploited in various functional materials such as optical oxygen sensors, 4 light convertors for photovoltaics, 5 or photocatalysts. 6 The [Mo 6 L 14 ] 2- cluster complexes are constructed from an octahedron of molybdenum atoms (Mo II ) surrounded by eight face-capping halogen and six inorganic or organic apical ligands. The photophysical proper- ties are strongly affected by the nature of ligands. In this respect, the coordination of carboxylates to the {Mo 6 I 8 } 4+ core can lead to desired intrinsic properties in terms of luminescence efficiency, oxygen sensing ability, and singlet oxygen production. 3,7 In addition, the use of carboxylates as apical ligands allows for an additional functionalization of the clusters providing the propensity to form liquid crystals via mesogenic ligands, 8 an increased absorption and the antenna effect, 9 or the ability to copolymerize with methacrylate monomers. 10 The low stability of molybdenum clusters in water at physiological pH remains a barrier for biological applications such as oxygen probing or singlet oxygen sensitization. Indeed, the archetypal molybdenum halides [Mo 6 X 14 ] 2- (X = Cl, Br, and I) are known to be stable only in concentrated aqueous solutions of their related acid HX and an increase of the pH causes the formation of aqua-hydroxo complexes [Mo 6 X x (OH) y (H 2 O) z ] n . 11 From a practical point of view, this hydrolysis is detrimental as it leads to microsized aggregates with poorly defined photophysical properties. In addition, such aggregates have shown an acute toxicity for living organisms, which restricts the use of cluster complexes in biological applications. 11 These difficulties have been overcome by the embedding of cluster complexes in silica nanoparticles or in polystyrene beads, which have shown promising results for luminescent probing or singlet oxygen sensitization. 1,12,13 The methods used to date are based on the covalent binding of the complexes to a corresponding matrix and causes the changes in photophysical properties, thus limiting the control over them. Received: September 3, 2014 Article pubs.acs.org/IC © XXXX American Chemical Society A dx.doi.org/10.1021/ic502144z | Inorg. Chem. XXXX, XXX, XXX-XXX