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Polymeric ligands for molybdenum and rhenium clusters

Final Report Summary - POLYMORE (Polymeric ligands for molybdenum and rhenium clusters)

Octahedral molybdenum and rhenium metal cluster complexes (MCCs) of general formula {M6Q8}Y6 (where M is either Re or Mo, Q are halcogens or chalcogens and L are terminal organic or inorganic ligands) show bright photoluminescence in the red and near-infrared regions with high quantum yields and photoluminescence life times of 1-100 μs. Additionally, the cluster complexes are extremely robust, generate singlet oxygen efficiently and readily survive heating and photolysis. They also have the capability for undergoing reversible oxidation with retention of the original structure. The properties of the cluster complexes can be tuned readily simply by substitution of the outer ligands in which is easily achievable using conventional solution-based chemical transformations. Such a combination of physical and chemical properties makes molybdenum and rhenium cluster complexes highly attractive for numerous applications in biological sciences (e.g. bioimaging, biolabeling) and as materials for light-emitting/harvesting devices (organic solar cells, OLEDs etc.). However, naked clusters are not ideally suited for any of these applications, as they must be supported by a media that tailors them to a certain application. Incorporating inorganic Mo/Re cluster complexes into organic polymer matrices (PMs) offers great opportunities in terms of integrating the excellent photophysical properties of metal clusters with the bespoke properties of various organic polymer matrices and thus to create designer materials tailored to specific applications. The principal scientific goals of PolyMoRe were therefore to 1) develop bespoke MCCs; 2) incorporate them into organic PMs in order to generate novel photo-luminescent materials and 3) evaluate the materials in biological and materials science applications.

To target certain biological or materials-based applications PolyMoRe focused on several classes of materials:

i. Photoluminescent microspheres
To develop photoluminescent microspheres for biological applications we focused primarily on two types on materials based on either i) polystyrene, due its low-toxicity and the ability of polystyrene-based microspheres to enter cell by a non-endocytosis-mediated process or ii) biodegradable polymers, such as PLA and PLGA. We have also synthesised and characterised a number of novel metal cluster complexes, such as (Bu4N)2[Mo6I8(NO3)6], (Bu4N)2[Mo6I8(PhSO3)6] and (Bu4N)2[Mo6I8(OTs)6]. These compounds display bright long-lived red phosphorescence in both deaerated solution and the solid phase with high quantum yields and therefore they were used as a luminescent dopants for polymeric microspheres
Main results: Complex [Mo6I8(NO3)6]2– bears labile terminal nitrato ligands and consequently is a handy precursor for the preparation of new luminescent materials through substitution of the NO3– groups by an appropriately functionalised polymer matrix. To demonstrate this approach, we have shown that cluster complex [Mo6I8(NO3)6]2– can be readily immobilised into the polymer matrix of polystyrene microspheres bearing bespoke thiol-based functionalization. Thiol groups are entities that bind molybdenum atoms irreversibly. Characterization of the conjugate material revealed that it shows emission in the red and near-infrared regions arising from metal cluster complex, though with slightly lower quantum yield (4% rather 26% as for the cluster precursor). Such an emission profile is especially promising for bioimaging applications due to relatively low absorption, at these wavelengths, by biological tissues. In contrast, to the cluster complex [Mo6I8(NO3)6]2–, photoluminescence from the conjugate material {Mo6I8}@PS-SH does not show any significant quenching clearly indicating the strong shielding effect that the polymer matrix confers against oxygen and solvent quenching of the photoluminescence from the {Mo6I8}4+ cluster core. We also demonstrated the low toxicity of {Mo6I8}@PS-SH and its ability to absorb antibodies. This work therefore provides a platform for exploiting these readily available and highly luminescent octahedral molybdenum cluster complexes in biological applications, such as, bioimaging, cell delivery etc.

ii. Photoluminescent, transparent plastics
Photoluminescent plastics based on transparent polymers are essential for many materials-based applications such as in photonics (materials for waveguides and optical fibres) or in photovoltaics (luminescent solar concentrators – devices that concentrate solar radiation to increase the electrical output of a solar cell). The approach that we used was to develop such materials centred on the development of photoluminescent polymerisable metal cluster complexes and co-polymerisation of these bespoke complexes with organic monomers - methyl methacrylate (MMA) and styrene to generate materials based on PMMA and polystyrene – polymers that naturally have high transparency, good mechanical and thermal properties and therefore make a good PMs for the applications outlined above.
Main results: By treating the neutral complexes trans-[{Re6Q8}(TBP)4(OH)2] with vinyl-benzoic acid we have synthesised novel, readily polymerizable, photoluminescent octahedral cluster complexes trans-[{Re6Q8}(TBP)4(VB)2], where VB = vinyl benzoate and Q = S or Se. These compounds have reasonably good solubility and therefore they were easily copolymerised with MMA and styrene organic monomers using solution-based techniques to produce processable photoluminescent hybrid materials. The resultant materials are entirely new and show photoluminescent quantum yields of up to 8% in solid state.
In an alternative approach we have developed entirely different methodology, in which polymerisable organic counter ions were combined with the inorganic di-anionic cluster to make a polymerisable cluster precursor. Specifically, we generated a complex that combines both a polymerisable organic cation [2-(methacryloyloxy)ethyl] dimethyldodecylammonium (dMDAEMA+) and a photoluminescent cluster anion [{Mo6I8}(OTs)6]. The quantum yield of the resultant material, (dMDAEMA)2[{Mo6I8}(OTs)6], is up to 65% in a deaerated solution. This precursor was used to make photoluminescent PMMA-based materials. We demonstrated that the resultant polymers can be processed readily by using conventional solution-based techniques, for example by electrospinning – a technique used to make micro- and nano-fibres.

iii. Electroluminescent hybrid materials
To examine, the potential of metal cluster based materials in electroluminescent applications, namely organic light emitting diodes (OLEDs), we developed materials based on the well-known electroluminescent polymer poly(N-vinylcarbazole) (PVK) and [{Re6Q8}(TBP)4(VB)2]. PVK is commonly used in association with a phosphorescent dopant to improve the efficiency of, or to tune the colour of OLEDs. Accordingly, we designed a number of OLEDs to assess the performance of the hybrid materials in these devices.
Main results: [{Re6Q8}(TBP)4(VB)2] -10%(w/w)@PVK was incorporated into OLED devices. The measurements of the electroluminescent emission spectra of the devices indicated emission maxima from both host (PVK) and the cluster complexes. Our data show also that the relative intensities of the blue (PVK host) and red (rhenium cluster) emission peaks changes with increasing applied voltage. At lower voltages, emission from PVK dominates over the emission of the cluster complex while at the higher voltages this situation is reversed with the cluster emission now being the predominant emission. Such behaviour results in an ability to tune the device in terms of colour emission with a shift from blue to red simply by increasing the applied voltage. We also demonstrated that rhenium metal cluster complexes, Re6Q8(TBP)4(OH)2 (Q=S,Se), can be used within the active layer of OLEDs without the presence of PVK. Whilst still requiring optimization (on-going in the company Polar OLED Ltd), by developing these rhenium cluster-based OLED devices we have successfully achieved (and published) the first direct evidence of the electroluminescent properties of rhenium clusters and indeed, to the best of our knowledge, of any member of the family of 24-electron hexanuclear cluster complexes of molybdenum, tungsten or rhenium. Once OLEDs with optimal properties have been identified a ready route to commercialisation, and thus economic impact, already exists. The materials and their methods of production form the basis of a patent filed by Aston University and the company, XXXX, who are conducting on-going optimisation evaluation studies, are set up to develop commercial-scale OLED production should the devices be deemed suitable for commercial exploitation.

Project web-page: http://www.aston.ac.uk/eas/staff/a-z/dr-olga-efremova/polymore/

Contact details:
Dr Olga Efremova
Department of Chemistry
University of Hull
Cottingham Road
Hull, HU6 7RX
UK
E: o.efremova@hull.ac.uk T: +44 (0)1482 465417 M: +44 (0)7583 012446

Dr Andrew Sutherland
CEAC
Aston University
Aston Triangle
Birmingham, B4 7ET
UK
E: a.j.sutherland@aston.ac.uk T: +44 (0)121 204 3425 M: +44 (0)7788 240897