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Robust optical oxygen sensors based on polymer-bound NIR-emitting platinum(ii)–benzoporphyrins

Several advanced optical oxygen sensor materials are presented. They are based on bright NIR-emitting platinum(II)–benzoporphyrins covalently incorporated into a variety of polymeric matrices. The dye– polymer conjugates are prepared either via Suzuki coupling of the brominated porphyrins to the styrene backbone or via co-polymerisation of the monomers with monostyryl porphyrin derivative. Importantly, in both strategies a highly stable C–C bond is obtained. The resulted materials benefit from excellent photophysical properties of the benzoporphyrin dyes (high brightness, emission in the NIR part of the spectrum) and high stability of the covalently grafted materials due to complete suppression of dye migration and leaching. This is demonstrated to be particularly important for operation of the sensors in harsh conditions e.g. during steam sterilization where the materials based on non-covalently grafted dyes showed significant drift of their calibration. Additionally, we present a new synthetic method for preparation of analytically pure benzoporphyrins via simple 1-step template condensation which a promising alternative to the commonly used Lindsey method.


Introduction
Optical oxygen sensors constitute widely-used analytical tools which are applied, for example, for the study of cellular function, 1,2 in medical diagnostics, 3,4 marine biology, 5 food packaging 6,7 and for process monitoring in industry and biotechnology. 8 In a general layout a phosphorescent indicator dye is physically entrapped into a matrix material which is a polymer (polystyrene, poly(methylmethacrylate), silicone, or ethylcellulose and their derivatives 9,10 ) or a sol-gel. 11,12 The matrix acts as a solvent and a support for the dye, as a permeation-selective barrier for the interfering chemical species and it also controls oxygen permeability. A suitable matrix can enhance both the specicity and selectivity of the sensor as well as its stability over time. A variety of oxygen indicators has been developed over the past decades 13 since the photophysical properties of the dyes have to be adjusted for a particular application. The development of dyes that are excitable in the red and NIR part of the electromagnetic spectrum has been of particular interest in recent years. 14 These indicator dyes are well suited for measurements in autouorescent or highly scattering media, such as in vivo samples in many bioanalytical applications. Oxygen indicators based on platinum(II) and palladium(II) complexes of porphyrin ketones 15 and porphyrin lactones, 16 and particularly of pi-extended benzo-and naphthoporphyrins [17][18][19] and related compounds 20 were found to be particularly promising for sensing applications. With the exception of water-dispersible oxygen-sensitive dendrimers (representing self-contained analytical tools primarily for imaging applications) 21,22 the above indicators are physically entrapped in a suitable polymeric matrix. Major restrictions of this approach are determined by the level of chemical and physical interaction between the indicator and its local environment. Low solubility of the indicator in the sensor matrix gives rise to leaching and aggregation; both processes seriously compromise the performance of an optical oxygen sensor over time. A lipophilic indicator will unlikely leach out of the sensor in aqueous environment but it may slowly migrate into the hydrophobic sensing support such as the very popular polyethylene terephthalate. Such migration processes are greatly accelerated at elevated temperatures, e.g. during steam sterilization of the sensors. Leaching can be particularly critical for materials with large surface to volume ratio, such as oxygensensitive nanoparticles. Covalent immobilisation of the indicators into polymeric matrices represents the optimal strategy to overcome the above limitations. Oxygen sensing materials based on covalently graed iridium(III) 23,24 ruthenium(II) [25][26][27][28] and platinum(II) or palladium(II) complexes [29][30][31][32][33] were reported. The meso-tetra(pentauorophenyl) porphyrin (TFPP) complexes, particularly, can be easily modied via nucleophilic substitution of the p-uorine atom of the pentauorophenyl groups. [29][30][31]33 Signicantly higher stability of the new materials compared to the sensors obtained via physical entrapment of the dyes was demonstrated. 31 To the best of our knowledge highly promising NIR indicators have not been covalently graed into oxygen-sensing materials. A versatile immobilisation strategy used for TFPP derivatives is not feasible since the meso-tetra(pentauorophenyl)-tetrabenzoporphyrin complexes does not form readily. 34 Herein, we present an efficient and versatile strategy towards advanced optical oxygen sensing materials based on platinum(II)-benzoporphyrin complexes covalently incorporated into a variety of relevant matrices. New brominated porphyrins can be either directly attached to the polymers or modied to give reactive monomers. Importantly, the synthetic strategy relies on Suzuki coupling which results in chemically stable C-C bond. It will be demonstrated how the covalent immobilisation improves the performance of the sensor materials compared to conventional sensors. In addition we will present a new efficient method for the synthesis of benzoporphyrins via template condensation which, compared to the previous reports, results in analytically pure products.

Synthesis of the brominated Pt(II)-porphyrin indicators
Brominated Pt(II)-benzoporphyrins serve as a synthetic platform for the covalent immobilisation of the indicator dye in various polymer matrices via Suzuki coupling. Depending on immobilisation strategy either tetra-or mono-substituted derivates can be used (Fig. 1). The tetra-substituted Pt(II) benzoporphyrin was initially synthesized using an adapted standard protocol. 35,36 This involved Lindsey-condensation 37 of 4,5,6,7tetrahydroisoindole with 4-bromobenzaldehyde yielding compound A1, the subsequent platination of the product (A2) and nally oxidation of the cyclohexene rings to yield the benzoporphyrin 3a. Surprisingly, the solubility of A1 was found to be rather low and the dye aggregated readily in organic solvents. On the other side, solubility of various metal-free tetraphenyltetracyclohexenoporphyrins, their Pt(II) complexes and the respective benzoporphyrins is known to be rather good. 34,36 The bromine atoms in the para-position of the meso-substituted phenyl rings appear to promote aggregation of the dyes. Due to low solubility of A1 in organic solvents, purication of the product was challenging; hence the crude product was used in the next step aer removal of the solvent.
In the second step, platination was performed in trimethylbenzene using Pt(C 6 H 5 CN) 2 Cl 2 as a precursor complex, which is a modication of the method published previously. 38 Trimethylbenzene allows for relatively high reaction temperatures (and therefore fast metalation) but in contrast to diphenylether can be easily removed under reduced pressure.
In the nal step, the brominated benzoporphyrin 3a, was obtained via oxidation of A2 with 2,3-dicyano-5,6-dichlorobenzoquinone (DDQ). Similarly to A1, the solubility of the Pt(II) benzoporphyrin was rather poor. The purication was possible only for diluted solutions.
One of the drawbacks of the above method is that it relies on 4,5,6,7-tetrahydroisoindole which is synthesised from not inexpensive ethyl isocyanoacetate. Template condensation represents an alternative method yielding Pt(II) benzoporphyrins in only 3 simple steps, starting from very cheap phthalimide and phenylacetic acid. However, the template condensation was reported to yield a number of benzyl-substituted side products which are extremely challenging to separate. 38,39 Preliminary experiments indicated that the condensation of phthalimide with 4-bromophenylacetic acid instead of phenylacetic acid resulted in a low yield of the zinc benzoporphyrin. Herein we present a new efficient method to yield analytically pure benzoporphyrins ( Fig. 1). By substituting phthalimide with dicyanobenzene, the reaction temperature could be reduced from 350 C to 280 C which greatly enhanced the yield of zinc tetra(4bromophenyl)tetrabenzoporphyrin. The yield of the target compound aer chromatographic purication was 6% which is rather good considering the low cost of the starting materials and simplicity of the procedure. Based on this novel protocol we were able to synthesize PtTPTBPBr 4 (compound 3a) in only three steps. The Pt(II) complex can be conveniently obtained aer demetalation of the respective zinc porphyrin and subsequent platination of the metal-free porphyrin. The 1 H NMR analysis and mass spectroscopic investigation indicate that no other porphyrin derivatives are formed and the product is identical to that obtained via Lindsey method. Considering high potential of zinc 40 and platinum(II) benzoporphyrins 41 for energy conversion applications the new method is expected to be particularly promising for simple and cost-efficient synthesis of these sensitizers. Many other commercially available dicyanobenzenes and phenylacetic acid derivatives can be used to provide necessary functionalities.
Mono-brominated Pt(II) porphyrin (3b) is also of great practical interest for covalent immobilisation since it only has a single site suitable for modication and hence does not act as cross-linker when it becomes incorporated into a polymer. The respective zinc complex was obtained via the modied template condensation using a mixture of phenylacetic-and 4-bromophenylacetic acids. As determined by MALDI-TOF, this approach results in a mixture of products with a degree of bromo-substitution ranging from 0 to 2 (Fig. S6 †). Chromatographic separation of these extremely similar compounds is virtually impossible. Due to bad ionization of the bromo-functionalized complexes, we were not able to reliably determine the contributions of the individual forms at this stage. However, as will be demonstrated in the following, a mixture of the mono-, dibromo-and unsubstituted porphyrins is suitable for preparation of soluble materials. Fig. 2 compares the absorption and emission spectra of the new brominated porphyrin dyes with those of the nonsubstituted dye. Evidently, the substitution has a very minor effect on the spectral properties of the dyes. A small bathochromic shi of the Q-band is observed for PtTPTBPBr (3b) and PtTPTBPBr 4 (3a) compared to PtTPTBP (1 and 5 nm, respectively). Similarly, the corresponding emission spectra shi bathochromically by 2 nm and 9 nm, respectively. Despite potential quenching of the "heavy" bromine atoms we did not observe any signicant change in the luminescence decay times and luminescence quantum yields upon bromination (Fig. 2). This correlates well to the results of Zhao and coworkers who demonstrated that the heavy atom effect of the halogen atoms in structurally related BODIPY dyes is much less efficient if they are positioned far away from the p-conjugated core of the chromophore. 42 The photostability of the dye is of particular interest for practical applications; especially in those cases where high light densities are used or measurements are performed for prolong time. PtTPTBPF 4 which has excellent photostability 34 was used for comparison. The photostability of the new compounds was found to be similar to that of PtTPTBPF 4 .

Preparation of the porphyrin-polymer composites
In order to covalently couple the indicators to the sensor matrix, two different strategies were used (Fig. 3). Both rely on Suzuki coupling and result in chemically stable C-C bonds. In the rst approach, polystyrene bearing different amounts of boronic acid residues (Table S1 ESI †) were prepared by copolymerisation of styrene and 4-vinylphenylboronic acid (4). The tetra-or mono-brominated Pt(II) benzoporphyrin was attached to the polymer in one step. The limitation of the approach is that the amount of the incorporated boronic acid groups is not easy to control and is lower than theoretically expected (e.g. 0.07 instead of 0.1 mol%). In order to avoid cross-linking when using the tetra-brominated porphyrin it should be used in excess to saturate all the boronic acid groups. The unbound dye is then removed during purication of the polymer.
In the second strategy, the mono-brominated dye was reacted with 4-vinylphenylboronic acid to produce reactive dye monomer (5). Using a standard protocol for Suzuki-coupling reactions, we were able to achieve acceptable yields of about 50% for the styryl-modied benzoporphyrin. The Heck reaction with the vinyl group of the boronic acid leads to the major side product. Due to the polarity of the boronic acid residue, the reactant as well as the side product could be readily removed by means of column chromatography. MALDI-TOF data (Fig. S7 †) indicate a mixture of PtTPTBP, PtTPTBPStyr and PtTPTBPStyr 2 (ratio 80 : 100 : 55) which is likely to reect the substitution pattern of the respective brominated complexes. Relatively high content of unmodied PtTPTBP and the di-substituted porphyrin in this mixture of products is not crucial for the subsequent co-polymerisation reactions since the unbound dye is removed in the purication process and the disubstituted porphyrin dye does cause signicant cross-linking due to rather low concentrations used.
Oxygen-sensitive polymers were obtained by co-polymerisation of different monomers with the styrene-modied dye, using radical polymerisation with AIBN as initiator. Reactions towards material 6b and 8 where carried out without solvent resulting in polymers with high PDI values, ranging from 1.20 to 5.44. These PDI values are not likely to have major inuence on the properties of the sensor. 1,1,1,3,3,3-Hexauoroisopropyl methacrylate was polymerised in the solution of tetrahydrofuran (THF) as the dye is insoluble in the monomer. In all procedures, unbound indicator was removed via precipitation of the polymer in methanol. The precipitation procedure was repeated until no more indicator dye could be found in the solution. In the case of material 8, functionalized poly(styreneco-maleic acid) particles were obtained by precipitation of a solution of the polymer in THF from water according to the reported procedure. 43 Overall, the co-polymerisation approach is preferable to the rst strategy (graing) due to its high versatility: the approach can be used for numerous monomers and it warrants a higher degree of control over the amount of indicator incorporated into the polymer. PtTPTBPBr physically entrapped in polystyrene is used as a reference for the co-polymerised and graed sensor materials. All three sensor materials possess similar phosphorescence lifetimes in the absence of oxygen. The sensitivities of all the materials are similar. It appears that the way of dye immobilisation only minor affects the calibration. The decay time plots (Fig. 4) are less linear compared to the luminescence intensity plots (Fig. S8 †) which is a typical case for the optical oxygen sensors. In fact, a linear t almost ideally describes the intensity plots (correlation coefficient r 2 ¼ 0.9997), but it is less adequate for the decay time plots (r 2 ¼ 0.997). The modied equation from the two-site model 44 adapted for the decay time plots (eqn (1), ESI †) delivers satisfactory results (r 2 ¼ 0.9996).

Properties of the sensing materials
The effect of dye loading on the luminescence lifetime for the graed sensor materials is shown in Fig. 5. A typical oxygen sensor based on non-covalently entrapped indicator would contain about 1-1.5 wt% of an indicator dye. As can be seen, only slight decrease of the luminescence decay time is visible for a higher concentration of 1.8 wt%. However, the phosphorescence lifetimes become markedly shorter upon further increase of dye loading (50.7, 47.3 and 38.0 ms at 0.33, 1.82 and 6.92 wt%, respectively). This self-quenching is likely to be due to aggregation of the dye at higher dye loadings.  (4) can be used for covalent grafting of either using tetra-(3a) or monobromo-(3b) substituted benzopophyrin. In the second strategy, the indicator is modified with styryl groups (5) and subsequent polymerisation with a variety of monomers is performed (styrene (6b), 1,1,1,3,3,3-hexafluoroisopropylmethacrylate (7) or maleic anhydride and styrene (8)). The latter can be used for the preparation of water dispersible nanoparticles (9). Importantly, the styryl-modied benzoporphyrin 5 can be easily incorporated in a variety of other polymeric matrixes. Hence, oxygen sensing materials covering a broad range of potential application elds can be produced (Fig. 6). For example, co-polymerisation with hexauoroisopropyl methacrylate results in signicantly more sensitive materials compared to polystyrene-based ones. Thus, this sensor is much better suitable for measurements at low oxygen. It should be mentioned that the same methodology can be adapted for graing of analogous Pd(II) benzoporpyhrins which possess about 7 times longer phosphorescence decay times. 34 The combination of Pd(II) benzoporphyrins and poly(hexa-uoroisopropyl methacrylate) would be suitable for trace oxygen sensing.
We also prepared a co-polymer of styrene and maleic acid with co-polymerised Pt(II) benzoporphyrin which was used to prepare nanoparticles via precipitation (9). The nanosensors (85 nm, PDI ¼ 0.15) are negatively charged at neutral pH (Zeta potential -40.9 mV) due to hydrolysis of maleic acid anhydride. Oxygen sensitivity is slightly lower than for the polystyrene materials (Fig. 6). Covalent graing of the indicators in the nanoparticle-based sensors is of particular importance due to their high surface to volume ratio, and therefore intense interaction with the environment (e.g. proteins and other species in biological probes). Additionally relatively small diffusion distances in the nanoparticles can aggravate leaching of the non-covalently bound dyes. Our approach overcomes this limitation.
To assess the effect of covalent immobilisation on the longterm performance of the sensor and the ability of covalent immobilisation to suppress effects associated with migration of the indicator dye, we prepared sensor lms on glass and polyethylene terephthalate (PET) support foil and exposed them to repeatable autoclave treatment for up to 15 hours at 135 C.    it is expected to be noticeable also at room temperature during prolong storage times. Obviously, the PET support is not suitable for the sensor lm based on the physically entrapped dye. For comparison, only very minor increase of the decay time is observed when the non-covalently embedded dye material is coated on glass support. Notably, glass support is less exible regarding processing and mechanical stability compared to PET. On the contrary, our new materials based on the covalently immobilised dye do not show any signicant migration into either supports used. In fact only minor change in the decay times is observed even aer 15 h of autoclavation which would produce and error of $18% in determination of pO 2 at air saturation providing that the sensor is not recalibrated aer sterilization. It should be mentioned that 30 min sterilization is typically sufficient and therefore the error in pO 2 quantication is virtually negligible. Thus, the new materials are particularly promising for application in harsh conditions (elevated temperatures, steam sterilization).
Suppressed migration of the covalently immobilised dye allowed us to directly apply the sensor composition to the PMMA ber in order to obtain cheap and fast responding beroptic sensors (Fig. 8). The physically entrapped indicator dyes would migrate into the PMMA core over time and compromise sensor performance. However, in the case of sensor materials incorporating covalently immobilised indicator dyes, direct coating of the PMMA ber allows for very thin sensor polymer layers without migration issues. The response times of such layers are very fast and are below 1 s.

Conclusions
We presented a simple and versatile strategy for the preparation of high performance oxygen-sensing materials. These materials rely on the NIR emitting bright and photostable Pt(II) benzoporphyrins covalently immobilised into different polymeric matrices. The new sensors overcome the limitations of their predecessors based on physically entrapped dyes. Migration and leaching are completely eliminated which not only ensures the reliable operation of the sensors in harsh conditions (such as elevated temperatures) but also enables higher exibility of sensor formats (e.g. fast responding ber-optic sensors based on PMMA bers). We also reported a novel straightforward and inexpensive synthetic route to the template-directed synthesis of (bromo-substituted) Pt(II)-benzoporphyrins. The new method not only allows preparation of reactive bromosubstituted porphyrins available for Suzuki coupling, but also other numerous porphyrins which can be used for a variety of emerging applications such as triplet-triplet annihilation upconversion.
Pt(II)-tetra-(4-bromophenyl)-cyclohexenoporphyrin (PtTPCH-PBr 4 , A2). The crude product (700 mg crude, 100 mg pure product, calculated using Lambert Beer law, 3 ¼ 250 000 M À1 cm À1 for the Soret band at 440 nm) was dissolved in TMB (60 ml) and heated to 150 C. Pt(C 6 H 5 CN)Cl 2 (90 mg, 0.192 mmol) was pre-dissolved in a small volume of TMB and was added to the reaction mixture. The formed HCl was removed by bubbling N 2 through the reaction mixture. The reaction progress was monitored via UV-Vis spectra. Aer 1 h, the reaction was cooled and the insoluble products (metallic platinum) removed by ltration. The ltrate was reuxed with 45 mg Pt(C 6 H 5 CN)Cl 2 . Aer the completion of the reaction, 50 ml hexane were added and the product was puried via chromatography on silica-gel (eluent: hexane/DCM, 2/1, v/v). Product fractions were determined via UV-Vis absorption spectra (see Fig. S1, ESI †). Yield: 52 mg, 51%. 1  Pt(II) tetra-(4-bromophenyl)-tetrabenzoporphyrin (PtTPTBPBr 4 , 3a). The platinated complex A2 (194.5 mg, 0.169 mmol) was heated to reux in 250 ml toluene. DDQ (385 mg, 1.696 mmol) was added. The red solution turned dark green aer 5 minutes. Reaction progress was monitored using UV-Vis spectroscopy. Aer completion of the reaction, the green organic phase was washed three times with 10 wt% Na 2 SO 3 solution. The solvent was removed under reduced pressure. Yield: 175 mg, 78%. 1  Zn-tetra-4-bromophenyl-tetrabenzoporhyrin (ZnTPTBPBr 4 , B1a). Zn-4-bromophenylaetate (6.330 g, 13.60 mmol), 4-bromophenylacetic acid (11.699 g, 54.40 mmol) and 1,2-dicyanobenzene (6.971 g, 54.40 mmol) were mixed and homogenized using a mortar. The solid mixture was split into equal portions of roughly 700 mg, placed into 2.5 ml Supelco® vials and compressed. The vials were sealed with a metal screw cap, placed into a pre-heated metal block at 140 C. The reagents were heated to a temperature of 280 C and le to react for 40 minutes while stirring and subsequently le to cool. The melt in each vial was dissolved in acetone. The dye was precipitated by adding a three-fold volume of EtOH-water (1/1, v/v) mixture. This operation was repeated three times. The product was further puried on an Al 2 O 3 column. Yield: 974 mg, 6%. UV-Vis (DCM): l max (rel. int.) ¼ 464 (1.00), 607 (0.06), 656 (0.22) nm.
Tetra-4-bromophenyl-tetrabenzoporhyrin (H 2 TPTBPBr 4 , B2a). ZnTPTBPBr 4 (500 mg, 0.419 mmol) was dissolved in 1 ml of acetone and methanesulfonic acid (3.82 g, 39.80 mmol) was added. The solution was stirred for 15 minutes at room temperature resulting in a color change from green to brown-red. The product was precipitated with water, and re-dissolved in acetone. Precipitation was repeated three times until the protonated form of the free ligand was no longer detectable in the absorption spectra (characteristic band at 504 nm, see Pt(II)-5-(4-bromophenyl)-10,15,20-tri(phenyl)-tetrabenzoporhyrin (PtTPTBPBr, 3b). Compounds B1b, B2b and 3b were synthesized in a similar way to the tetra-substituted benzoporphyrin. Their preparation is described in detail in the supplementary information.
Graing of PtTPTBPBr (3b) to polymer 4. The synthetic concept is exemplied by the following synthesis. The used quantities for polymers with different amount of dye loading are described in the supplementary information. The co-polymer 4 (799 mg, 7.67 mmol), PtTPTBPBr (3b) (10 mg, 0.009 mmol) and K 2 CO 3 (3 mg, 0.002 mmol) were dissolved in 20 ml toluene, 10 ml THF and 3 ml H 2 O in a Schlenk-ask. The solution was deoxygenated by rapid stirring under strong argon ow for 10 min. The catalyst Pd(PPh 3 ) 4 (1 mol%) was added under argon. The Schlenk ask was closed, heated to 70 C and le to react for 24 hours while stirring. To purify the product, the solution was added drop-wise to a ve-fold volume of methanol, resulting in the formation of a green precipitate. The suspension was ltered through a paper lter and re-dissolved in dichloromethane to give a solution containing 10 wt% of polymer. This step of dissolving and precipitation was repeated three times, until no more dye could be observed in the washing solutions. The polymer (6b) was dried in the oven at 70 C. Dye loading was calculated using Lambert Beers law, 3 ¼ 250 000 M À1 cm À1 at 430 nm in DCM ¼ 0.41 wt%. Yield: 589 mg, 72%. GPC data: Mn ¼ 118 950 g mol À1 , Mw ¼ 457 810 g mol À1 , Mz ¼ 1 320 760 g mol À1 , PDI ¼ 3.85.
Co-polymerisation of PtTPTBPStyr (5) with styrene. Styrene was ltered through aluminum oxide to remove the stabilizer. 550 ml of styrene (500 mg, 4.481 mmol) and 5.5 mg of PtTPTBPStyr (1.01 wt%) were placed in a Schlenk tube and deoxygenated for 15 minutes. 7.9 mg AIBN (1 mol%) was added, the Schlenk was closed and heated to 70 C for 2.5 hours. The polymer (6b) was dissolved in DCM and precipitated in a vefold volume of methanol three times and dried in the vacuum oven at 60 C. Dye loading was calculated using Lambert Beers law, 3 ¼ 250 000 M À1 cm À1 for the Soret band at 430 nm in DCM ¼ 1.0 wt%. Yield: 340 mg, 60%. GPC data: Mn ¼ 39 790 g mol À1 , Mw ¼ 92 470 g mol À1 , Mz ¼ 187 850 g mol À1 , PDI ¼ 2.32.
Preparation of PSMA nanoparticles. The PSMA-copolymer 8 was dissolved in THF giving a 0.5 wt% solution. The cocktail was added controlled with a pipette to a two-fold volume of H 2 O while stirring on a vortex with 1200 rpm. THF was allowed to evaporate for 2 hours under ambient air ow. Particle size: Z av 85 nm, PDI 0.15. Zeta potential ¼ À40.9 mV.
Preparation of sensor lms. Sensor lms of dened thickness were prepared by knife coating of "cocktails" onto PET foils using a Gardner 25 mm coating knife. The polystyrene-based "cocktails" typically contained 10 wt% of polymer dissolved in chloroform (HPLC-grade). The poly(1,1,1,3,3,3-hexa-uoroisopropyl methacrylate)-based "cocktail" contained 30 wt% of the polymer in THF. Aer coating, the sensor lms were dried for 24 hours at 70 C to ensure complete removal of solvent before characterization.
Fiber-optic sensors. The cladding of the 1 mm PMMA ber was removed with acetone. The core was coated by dipping the ber in a "cocktail" containing 2 wt% of the polymer (1.0 wt% PtTPTBPStyr co-polymerised in polystyrene (6b)) in ethylacetate. The ber was dried at 60 C for 24 hours to remove the solvent. The tip of the ber was cut off to ensure that the obtained signals resulted from side wall coating of the ber. The sensor response was measured by dipping the ber into a 2 wt% Na 2 SO 3 solution.
Autoclavation experiments 100 mg of the sensor material PS co-polymerised with compound 5 or 1 mg PtTPTBPBr 4 (3a) and 100 mg PS were dissolved in 1.0 g HPLC grade chloroform. These sensor "cocktails" were coated onto PET and microscopy slides using a 25 mm coating knife. The sensors were dried at 60 C for 12 h. The sensors were autoclaved (Sanoclav from Wolf, Germany) at 135 C for 15 h and phosphorescence lifetimes were determined under air and nitrogen in regular intervals.

Photostability
Dye solutions in water-free DMF were illuminated with and LED array (617 nm) at the following settings: 7.0 W, 7.88 V, 882 mA (photon ux: 5500 mmol s À1 m À2 ). Aer each measurement the cuvette was shortly unsealed and shaken to ensure oxygen saturation in the sample. The degradation was determined by calculating the average value of the three maximum absorption points in the Q-band.