Blue Emitting Star-Shaped and Octasilsesquioxane-Based Polyanions Bearing Boron Clusters. Photophysical and Thermal Properties

High boron content systems were prepared by the peripheral functionalisation of 1,3,5-triphenylbenzene (TPB) and octavinylsilsesquioxane (OVS) with two different anionic boron clusters: closo-dodecaborate (B12) and cobaltabisdicarbollide (COSAN). TPB was successfully decorated with three cluster units by an oxonium ring-opening reaction, while OVS was bonded to eight clusters by catalysed metathesis cross-coupling. The resulting compounds were spectroscopically characterised, and their solution-state photophysical properties analysed. For TPB, the presence of COSAN dramatically quenches the fluorescence emission (λem = 369 nm; ΦF = 0.8%), while B12-substituted TPB shows an appreciable emission efficiency (λem = 394 nm; ΦF = 12.8%). For octasilsesquioxanes, the presence of either COSAN or B12 seems to be responsible for ∼80 nm bathochromic shift with respect to the core emission, but both cases show low emission fluorescence (ΦF = 1.4–1.8%). In addition, a remarkable improvement of the thermal stability of OVS was observed after its functionalisation with these boron clusters.

In addition to the properties described for carboranes, the presence of a cobalt metal centre in the cobaltabisdicarbollide anion (COSAN, [3,3 -Co(C 2 B 9 H 11 ) 2 ] -) confers to these cluster unique

Scheme 1. Synthesis of compounds 4 and 5.
On the other hand, functionalisation of the octavinylsilsesquioxane (OVS) cube with eight closododecaborate units was performed via cross-metathesis reaction, as was previously described for COSAN and FESAN derivatives by our group [43]. Based on that strategy, a new closo-dodecaborate precursor bearing a terminal styrene group was prepared by ring-opening reaction of 2 with 4vinylphenol (Scheme 2a). The evolution of the reaction was also monitored by 11 B NMR spectroscopy, and compound 6 was obtained in 72% yield. A successful cross-metathesis reaction of OVS and 6 was achieved using the first generation Grubbs catalyst [101] in CH2Cl2 at reflux for 60 h, and around 20% of the excess of 6 was added to ensure the complete functionalisation of the central cube. The crossmetathesis reaction was monitored by 1 H NMR upon the disappearance of the vinyl proton resonances from the Si-CH=CH2 of OVS ( Figure 1). The regio-and stereoselective E-isomers of T8-B12 Scheme 1. Synthesis of compounds 4 and 5.
On the other hand, functionalisation of the octavinylsilsesquioxane (OVS) cube with eight closo-dodecaborate units was performed via cross-metathesis reaction, as was previously described for COSAN and FESAN derivatives by our group [43]. Based on that strategy, a new closo-dodecaborate precursor bearing a terminal styrene group was prepared by ring-opening reaction of 2 with 4-vinylphenol (Scheme 2a). The evolution of the reaction was also monitored by 11 B NMR spectroscopy, and compound 6 was obtained in 72% yield. A successful cross-metathesis reaction of OVS and 6 was achieved using the first generation Grubbs catalyst [101] in CH 2 Cl 2 at reflux for 60 h, and around 20% of the excess of 6 was added to ensure the complete functionalisation of the central cube. The cross-metathesis reaction was monitored by 1 H NMR upon the disappearance of the vinyl proton resonances from the Si-CH=CH 2 of OVS ( Figure 1). The regio-and stereoselective E-isomers of T 8 -B 12

Photophysical Properties
The solution-state photophysical behaviour of compounds, 4-6, T8-B12, as well as T8-COSAN ( Figure S1, previously synthesised by us [43]), were studied by UV-Vis and fluorescence (PL) spectroscopy. Results of the optical properties are shown in Figure 2 and listed in Table 1.
Inspecting the absorption spectra of 4 and 5 in CH3CN (Figure 2a), high energy bands found around 270 nm are attributed mainly to the π-π* transitions occurring within the aromatic core, which are redshifted compared to the maximum at 254 nm reported for TPB [103,104]. This 270 nm band is the main absorption observed for 4, but additional bands are identified for COSAN derivative 5: A band at 313 nm and another near 370 nm, which are endorsed to the metallacarborane unit substituted with the O-(CH2)2-O-(CH2)2-moiety [105]. Additionally, a weak band around 450-460 nm due to the d-d transition in Co III is also observed [106]. Molar extinction coefficients (ε) were determined at their higher absorptions, giving a value of 6.8 × 10 4 M −1 ·cm −1 for 4, which increases up to 8.3 × 10 4 M −1 ·cm −1 for compound 5, probably due to some contribution of COSAN cluster at that wavelength range [106]. These ε values are also in the range of TPB substituted by hydrosilylation with three COSAN units (ε = 8.6·10 4 M −1 ·cm −1 ) [76]. Regarding the emission properties, both compounds exhibit a single broadband when excited at their maxima absorption, but a different fluorescence (PL) behaviour is pointed out. In this regard, a remarkable higher PL emission intensity of compound 4, with a quantum yield value of F = 12.8% is observed, while COSAN derivative 5 do The molecular structures of all the compounds were established using standard spectroscopy techniques: IR-ATR, NMR ( 1 H, 13 C{ 1 H}, and 11 B{ 1 H}) ( Figure S2-S17), and elemental analysis.
The IR-ATR spectra for those compounds bearing the closo-dodecaborate cluster show the typical υ(B-H) strong bands below 2500 cm −1 (4, 6, and T 8 -B 12 ), while compound 5 exhibit this band around 2534 cm −1 . For T 8 -B 12 , the characteristic broadband around 1090 cm −1 due to the vibration frequency of the Si-O bond is also evident ( Figure S17). The 1 H NMR spectra of all compounds show the aromatic resonances corresponding to the phenyl moieties in the region from δ 6.97 to 7.80 ppm, and the -CH 2 Ogroups are identified in their usual range δ 3.43-4.24 ppm. Additionally, COSAN derivative 5 displays two resonances at 4.29 and 4.32 ppm undoubtedly attributed to the C c -H protons, while for 6 appears the characteristic vinylic (CH=CH 2 ) distribution as two doublets and one doublet of doublets in the range 5.08-6.69 ppm (Figure 1b). In contrast to 6, in the 1 H NMR of T 8 -B 12 the vinylic signals have disappeared after the cross-metathesis reaction, giving rise to the new vinylene (CH=CH) resonances as two doublets with an 18 Hz trans coupling constant at 6.26 and 7.39 ppm (Figure 1c). The 11 B{ 1 H} NMR spectrum of 5 shows the typical 1:1:1:1:2:3:3:2:2:1:1 pattern from δ +25 to −28 ppm [95,96,102], and the lowest field resonance is assigned to the [B(8)-O]. For those compounds bearing the closo-dodecaborate cluster (4, 6, and T 8 -B 12 ), the 11 B{ 1 H} NMR spectra show a simpler distribution pattern with a ratio of 1:5:5:1, from +8 to −21 ppm, assigning again the lowest field resonance to the [B(1)-O] [98]. The 13 C{ 1 H} NMR spectra also established the structure of compounds, showing the resonances of aromatic and vinylic carbons for all of them in the region δ 110-160 ppm. Moreover, the terminal vinylic CH=CH 2 signal at 110.6 ppm displayed for 6 has disappeared after the coupling reaction with OVS, which further confirms the successful synthesis of T 8 -B 12 . The 13 C{ 1 H} NMR spectra of the aliphatic ether moieties are shown from δ 67 to 74 ppm, and for compound 5 two additional resonances displayed at 55.2 and 47.2 ppm are attributed to the C c from COSAN.

Photophysical Properties
The solution-state photophysical behaviour of compounds, 4-6, T 8 -B 12 , as well as T 8 -COSAN ( Figure S1, previously synthesised by us [43]), were studied by UV-Vis and fluorescence (PL) spectroscopy. Results of the optical properties are shown in Figure 2 and listed in Table 1. Similarly, the optical properties of T8-B12 and T8-COSAN were analysed through their UV-Vis absorption and PL spectra in CH3CN ( Table 1). The UV-Vis spectra of both POSS cages show their absorption maxima as a broadband in the range 270-272 nm, which is endorsed mainly to the styrene moieties [8,43]. Although this band is red-shifted with respect to free styrene (251 nm), this effect is in good agreement with the UV-Vis spectra of previously reported styrenyl-containing POSS with different spacers [8,88]. As described for compound 5, the presence of COSAN units in T8-COSAN is responsible for the additional absorption bands near 310 and 370 nm, as well as the weak absorption around 450-460 nm. Regarding the extinction coefficients, the COSAN cluster in T8-COSAN increases the ε value with respect to T8-B12 at their high energy absorption band (from 186,000 to 233,000), due to the nearness of the metallacarborane band at 310 nm. On the other hand, T8-B12 and T8-COSAN show nearly identical PL spectra, with emission maxima at λem = 405-406 nm after excitation at their maxima absorbance ( Figure 2b, Table 1). This emission is in the range to that obtained for carborane-functionalised T8 through a styrene group (λem = 370-414 nm), but it is remarkably redshifted compared to the maximum emission (em = 326 nm) of (p-methoxystyrenyl)8OS [88]. Therefore, the presence of either closo-dodecaborate or COSAN clusters as substituents in the styrenylOS cube seems to be responsible for this ∼80 nm bathochromic shift. This could be due to a combination of phenomena, as the high electron delocalisation within the OS cube or possible interactions of the functional groups linked to the cage in the excited state [8,107]. In fact, the emission maxima of T8-B12 and T8-COSAN is even more redshifted than those observed for silsesquioxanes substituted with more conjugated systems, e.g., (stilbene-vinyl)8OS emitting in a range λem = 375-388 nm in different solvents, (p-methyl-stilbene-vinyl)8OS with λem = 398 nm [88,108], and even (carborane-vinylstilbene)8OS hybrids with λem = 391-392 nm [7]. Regarding the PL quantum yield, CH3CN solutions of T8-B12 and T8-COSAN were analysed. Differently to the equivalent (pmethoxy-styrenyl)8OS core that shows a F = 12% [88], the polyanionic T8 hybrids here prepared show  Inspecting the absorption spectra of 4 and 5 in CH 3 CN (Figure 2a), high energy bands found around 270 nm are attributed mainly to the π-π* transitions occurring within the aromatic core, which are redshifted compared to the maximum at 254 nm reported for TPB [103,104]. This 270 nm band is the main absorption observed for 4, but additional bands are identified for COSAN derivative 5: A band at 313 nm and another near 370 nm, which are endorsed to the metallacarborane unit substituted with the O-(CH 2 ) 2 -O-(CH 2 ) 2 -moiety [105]. Additionally, a weak band around 450-460 nm due to the d-d transition in Co III is also observed [106]. Molar extinction coefficients (ε) were determined at their higher absorptions, giving a value of 6.8 × 10 4 M −1 ·cm −1 for 4, which increases up to 8.3 × 10 4 M −1 ·cm −1 for compound 5, probably due to some contribution of COSAN cluster at that wavelength range [106].
These ε values are also in the range of TPB substituted by hydrosilylation with three COSAN units (ε = 8.6·10 4 M −1 ·cm −1 ) [76]. Regarding the emission properties, both compounds exhibit a single broadband when excited at their maxima absorption, but a different fluorescence (PL) behaviour is pointed out. In this regard, a remarkable higher PL emission intensity of compound 4, with a quantum yield value of Φ F = 12.8% is observed, while COSAN derivative 5 do not show appreciable emission, Φ F = 0.8%. The low intensity of emission of compound 5 has been ascribed to a quenching process when the COSAN cluster is chemically bonded to the fluorescent core, which has been previously observed for other COSAN-containing fluorophores [1,76]. Nevertheless, when TPB was substituted with closo-carborane units through a -CH 2 -spacer, their Φ F could be modulated from 26% to 55% depending on the substituents at the second C c [9]. In that case, the presence of closo-carborane clusters increased the emission efficiency compared to that reported for pristine TPB (Φ F = 10% in CH 2 Cl 2 ) [75]. These results confirm that the inherent nature of COSAN induces a notable PL quenching of TPB, while this is not occurring for compound 4 substituted with closo-dodecaborate cluster. It should be noted that comparable behaviour was observed when 2 and 3 were appended to organotin fluorophores: While the presence of COSAN reduced the emission efficiency to Φ F ≤ 7%, the substitution of the tin complexes with one closo-dodecaborate unit increased this value up to Φ F = 49% [96]. In addition to the different PL emission efficiency of 4 and 5, the former exhibits a maximum emission (λ em ) centred at 364 nm, that is in the region of carboranes-bearing TPB [9], but blue-shifted by 25 nm with respect to the weak emission band of 5.
Similarly, the optical properties of T 8 -B 12 and T 8 -COSAN were analysed through their UV-Vis absorption and PL spectra in CH 3 CN ( Table 1). The UV-Vis spectra of both POSS cages show their absorption maxima as a broadband in the range 270-272 nm, which is endorsed mainly to the styrene moieties [8,43]. Although this band is red-shifted with respect to free styrene (251 nm), this effect is in good agreement with the UV-Vis spectra of previously reported styrenyl-containing POSS with different spacers [8,88]. As described for compound 5, the presence of COSAN units in T 8 -COSAN is responsible for the additional absorption bands near 310 and 370 nm, as well as the weak absorption around 450-460 nm. Regarding the extinction coefficients, the COSAN cluster in T 8 -COSAN increases the ε value with respect to T 8 -B 12 at their high energy absorption band (from 186,000 to 233,000), due to the nearness of the metallacarborane band at 310 nm. On the other hand, T 8 -B 12 and T 8 -COSAN show nearly identical PL spectra, with emission maxima at λ em = 405-406 nm after excitation at their maxima absorbance ( Figure 2b, Table 1). This emission is in the range to that obtained for carborane-functionalised T 8 through a styrene group (λ em = 370-414 nm), but it is remarkably redshifted compared to the maximum emission (λ em = 326 nm) of (p-methoxy-styrenyl) 8 OS [88]. Therefore, the presence of either closo-dodecaborate or COSAN clusters as substituents in the styrenylOS cube seems to be responsible for this ∼80 nm bathochromic shift. This could be due to a combination of phenomena, as the high electron delocalisation within the OS cube or possible interactions of the functional groups linked to the cage in the excited state [8,107]. In fact, the emission maxima of T 8 -B 12 and T 8 -COSAN is even more redshifted than those observed for silsesquioxanes substituted with more conjugated systems, e.g., (stilbene-vinyl) 8 OS emitting in a range λ em = 375-388 nm in different solvents, (p-methyl-stilbene-vinyl) 8 OS with λ em = 398 nm [88,108], and even (carborane-vinylstilbene) 8 OS hybrids with λ em = 391-392 nm [7]. Regarding the PL quantum yield, CH 3 CN solutions of T 8 -B 12 and T 8 -COSAN were analysed. Differently to the equivalent (p-methoxy-styrenyl) 8 OS core that shows a Φ F = 12% [88], the polyanionic T 8 hybrids here prepared show low emission efficiencies, with values Φ F = 1.4-1.8%. It is also worth noting that, in contrast to carborane-substituted T 8 , either through styrene or vinyl-stilbene moieties [8,43], in this work there is no apparent relationship between the type of boron cluster and the emission efficiency behaviour. Nevertheless, it is evident that incorporation of anionic boron clusters to the POSS cube gives rise to low emissive materials in the solution.

Thermal Stability of Anionic-Boron Clusters Containing T 8
Silsesquioxanes have been used as additives in polymers and composites to improve their thermal, mechanical, and oxidative resistance [109,110]. Additionally, it has been stated that boron clusters have high thermal stability (vide supra). To study the influence of thermal behaviour when these two families of compounds are combined, we reported the thermal stability of the T 8 -COSAN hybrid [43], and the thermogravimetric analysis (TGA) of T 8 -B 12 under argon was here studied (Figure 3). The TGA results show a very different behaviour when nonfunctionalised OVS is compared to T 8 -B 12 and T 8 -COSAN. Pristine OVS shows a sharp fall near 290 • C, and after heating up to 900 • C, 18% of the initial weight was recovered [111]. On the contrary, the boron cluster-containing OVS hybrids undergo a gradual weight reduction between 300-500 • C, and a residual mass of around 58% and 88% were respectively recovered, after heating up to 900 • C for T 8 -B 12 and T 8 -COSAN, respectively. In T 8 -B 12 the percentage of organic part is around 82%, whereas only 11% would correspond to the percentage of hydrogen. Therefore, after burning these materials, we may propose that the total of hydrogen and a percentage of the organic part (around 31%) are lost; being this percentage higher for T 8 -B 12 with regards to T 8 -COSAN (around 4.4%). This result confirms once again that binding anionic boron clusters to the T 8 cube cause a significant increase in the thermal stability of the final materials; moreover, those materials containing metallacarborane fragments are the most thermally stable.

Thermal Stability of Anionic-Boron Clusters Containing T8
Silsesquioxanes have been used as additives in polymers and composites to improve their thermal, mechanical, and oxidative resistance [109,110]. Additionally, it has been stated that boron clusters have high thermal stability (vide supra). To study the influence of thermal behaviour when these two families of compounds are combined, we reported the thermal stability of the T8-COSAN hybrid [43], and the thermogravimetric analysis (TGA) of T8-B12 under argon was here studied (Figure 3). The TGA results show a very different behaviour when nonfunctionalised OVS is compared to T8-B12 and T8-COSAN. Pristine OVS shows a sharp fall near 290 °C, and after heating up to 900 °C, 18% of the initial weight was recovered [111]. On the contrary, the boron clustercontaining OVS hybrids undergo a gradual weight reduction between 300-500 °C, and a residual mass of around 58% and 88% were respectively recovered, after heating up to 900 °C for T8-B12 and T8-COSAN, respectively. In T8-B12 the percentage of organic part is around 82%, whereas only 11% would correspond to the percentage of hydrogen. Therefore, after burning these materials, we may propose that the total of hydrogen and a percentage of the organic part (around 31%) are lost; being this percentage higher for T8-B12 with regards to T8-COSAN (around 4.4%). This result confirms once again that binding anionic boron clusters to the T8 cube cause a significant increase in the thermal stability of the final materials; moreover, those materials containing metallacarborane fragments are the most thermally stable.

Materials
All reactions were performed under an atmosphere of dinitrogen employing standard Schlenk techniques. Acetonitrile and dichloromethane were purchased from Merck and distilled from sodium benzophenone before use. Commercial grade acetone, hexane, ethyl acetate, tetrahydrofuran, and methanol were used without further purification. Compounds

Instrumentation
Elemental analyses were performed using a Thermo (Carlo Erba) Flash 2000 Elemental Analyser microanalyzer (EA Consumables, Inc., Pennsauken, NJ, USA). ATR-IR spectra were recorded on a

Instrumentation
Elemental analyses were performed using a Thermo (Carlo Erba) Flash 2000 Elemental Analyser microanalyzer (EA Consumables, Inc., Pennsauken, NJ, USA). ATR-IR spectra were recorded on a JASCO FT/IR-4700 spectrometer on a high-resolution (Madrid, Spain). The 1 H-NMR (300.13 MHz), 11 B{ 1 H} (96.29 MHz), and 13 C{ 1 H} NMR (75.47 MHz) spectra were recorded on a Bruker ARX 300 spectrometer (Bellerica, MA, USA). All NMR spectra were recorded in CD 3 COCD 3 solutions at 25 • C. Chemical shift values for 11 B{ 1 H} NMR spectra were referenced to external BF 3 ·OEt 2 , and those for 1 H and 13 C{ 1 H} NMR were referenced to SiMe 4 (TMS). Chemical shifts are reported in units of parts per million downfield from the reference, and all coupling constants are reported in Hertz. UV-Vis spectra were recorded on a VARIANT Cary 5 UV-Vis-NIR spectrophotometer (Santa Clara, CA, USA), using a spectroscopic grade ACN (Sigma-Aldrich, Merck Life Science, Madrid, Spain), in normal quartz cuvette having 1 cm path length, for different solutions for each compound in the range 2 × 10 −6 to 5 × 10 −6 M in order to calculate the molar extinction coefficients (ε). The fluorescence emission spectra and excitation spectra for all samples were recorded in a VARIANT Cary Eclipse fluorescence spectrometer. No fluorescent contaminants were detected on excitation in the wavelength region of experimental interest. The fluorescence quantum yields were determined by the "single point method" and repeated three times with similar optical density for reproducibility [112], against quinine sulfate in a 0.5 M aqueous sulfuric acid with φ F = 0.54 as a standard [113].

Synthesis of 5
A solution of 1 (68 mg, 0.193 mmol) and NaH (57.7% dispersion, 28 mg, 0.66 mmol) in 5 mL of dry THF was stirred under nitrogen for 1 h at room temperature. Then, 3 (250 mg, 0.61 mmol) was added and the mixture was refluxed for 60 h. The reaction was cooled down and quenched with CH 3 OH (1 mL), water (3 mL), and a few drops of acetic acid (1 M). The organic solvents were removed under vacuum, and the crude product was dissolved in 10 mL diethyl ether and extracted with water (3 × 10 mL). The organic layer was dried over MgSO 4 and the volatiles were reduced under vacuum.   (75.47 MHz) spectra were recorded on a Bruker ARX 30 spectrometer (Bellerica, MA, USA). All NMR spectra were recorded in CD3COCD3 solutions at 25 °C Chemical shift values for 11 B{ 1 H} NMR spectra were referenced to external BF3·OEt2, and those for 1 H and 13 C{ 1 H} NMR were referenced to SiMe4 (TMS). Chemical shifts are reported in units of parts pe million downfield from the reference, and all coupling constants are reported in Hertz. UV-V spectra were recorded on a VARIANT Cary 5 UV-Vis-NIR spectrophotometer (Santa Clara, CA USA), using a spectroscopic grade ACN (Sigma-Aldrich, Merck Life Science, Madrid, Spain), i normal quartz cuvette having 1 cm path length, for different solutions for each compound in th range 2 × 10 −6 to 5 × 10 −6 M in order to calculate the molar extinction coefficients (ε). The fluorescenc emission spectra and excitation spectra for all samples were recorded in a VARIANT Cary Eclips fluorescence spectrometer. No fluorescent contaminants were detected on excitation in th wavelength region of experimental interest. The fluorescence quantum yields were determined b the "single point method" and repeated three times with similar optical density for reproducibilit [112], against quinine sulfate in a 0.5 M aqueous sulfuric acid with ɸF = 0.54 as a standard [113].

Synthesis of T 8 -B 12
A 10 mL round-bottomed flask was charged under nitrogen with OVS (15 mg, 0.024 mmol), compound 6 (190 mg, 0.227 mmol), and the first generation Grubbs catalyst (12 mg, 0.014 mmol) in 6 mL of CH 2 Cl 2 . The solution was stirred and refluxed for three days. The solvent was removed under vacuum. The residue was treated with a mixture of THF/MeOH (1:10) to obtain a grey solid.

Conclusions
A set of 1,3,5-triphenylbenzene and octasilsesquioxane-based hybrids decorated with three (4, 5) and eight closo-decahydro-dodecaborate and cobaltabisdicarbollide (T 8 -B 12, T 8 -COSAN), respectively, have been successfully synthesised, isolated, and fully characterised. Although they possess different types of fluorophores, all of them show a similar maxima absorption wavelength, which is red-shifted with regard to the nonsubstituted scaffolds. The molar extinction coefficient is correlated with the type of boron cluster, and proportional to the number of clusters attached to the core molecules. It is worth noting that a significant red-shift of the emission maxima (λ em 369-406 nm) up to 80 nm for the T 8 hybrids, as well as an important drop of the fluorescence efficiencies were produced after linking these anionic boron clusters to both scaffolds. These results confirm once again that the B 12 and COSAN clusters produce a significant quenching of the fluorescence in the solution. Notably, binding anionic boron clusters to the OVS provide materials with an extraordinary thermal stability.