# Synthetic Routes to Silsesquioxane-Based Systems as Photoactive Materials and Their Precursors
## 1 Introduction
### Heck Coupling for Obtaining Silylakenyl Units Anchored at the Si-O-Si Core
Both systems were proved to be thermally stable with \(T_{\mathrm{d}}\)\({}^{5\%}\) = 273 and 383 \({}^{\circ}\)C for the mono and octa products, respectively. Introduction of these azobenzene moieties reduced the crystallinity of the compound withthe increasing amount of these groups in the molecule. The trans-cis photoisomerisation was examined and supported by DFT (Density Functional Theory) calculations. These systems were fluorescent with a maximum emission wavelength \(\lambda_{\mathrm{em}}\) at about 400 nm, which enables their potential application as blue light emission materials. Octa-functional POSS-based OLEDs have attracted far more attention mainly due to the possibility to anchor chromophores moieties (i.e., eight groups) onto the Si-O-Si core. Heck coupling may be conducted in a few combinations and towards the formation of fully substituted products as well as without full substitution. Pioneering work was presented by [PERSON] et al. when they reported on the use octavinyl silsesquioxane (OVS) in a Heck coupling process with a mono-brominated version of a highly \(\pi\)-conjugated arene system in the presence of [Pd(PtBu\({}_{3}\))\({}_{2}\)], obtaining hexa-substituted OVS with a 75% yield (Figure 7) [43]. Incomplete substitution was found probably due to a steric hindrance of the chromophore used and steric availability of vinyl groups. The obtained compound was verified in terms of its thermal stability, showing very high \(T_{\mathrm{d}}\)\({}^{5\%}\) = 465 \({}^{\circ}\)C. Its film and solution (toluene) photoluminescence spectra revealed \(\lambda_{\mathrm{em}}\) = 430 and 434 nm respectively. The resulting electroluminescent device showed 18% improvement in external quantum efficiency over their small organic chromophore analogue. Analogously, not-fully-substituted pyrene-based T\({}_{8}\) cages were obtained by [PERSON] et al. in Heck coupling using a [Pd(OAc)\({}_{2}\)]/PPh\({}_{3}\)/NEt\({}_{3}\)-mediated reaction that, after two days, resulted in the formation of a product with four pyrene substituents (as the most abundant species) with  
Figure 6: Mono- and octa-azobenzene-functionalized T\({}_{8}\) derivatives obtained in a Heck coupling reaction. Figure 7: Heck coupling reaction leading to formation of T\({}_{8}\)-based systems with two functionalities. a 41% yield (Figure 7) [44]. The photophysical properties of the product were determined and broad emission bands \(\lambda_{\mathrm{em}}\) = 406, 426 and 494 nm of low intensity in DMSO and sharp \(\lambda_{\mathrm{em}}\) = 398, 421 and 445 nm of high intensity in THF were revealed. The compound revealed the solvent's polarity dependant fluorescence, the \(\pi\)-\(\pi^{\mathrm{t}}\) emission of sole pyrene moieties was observed in THF, while in DMSO the formation of a pyrene-pyrene excimer through space has been postulated (the authors suggest its confirmation due to a large Stokes shift of \(\Delta\lambda\) = 143 nm). This compound was also tested as a sensor for several types of anions and, interestingly, was proved to encapsulate fluoride that results in a \(\pi\)-\(\pi^{\mathrm{t}}\) fluorescence enhancement in DMSO. Recent research by [PERSON] et al. is related, with the synthesis of an analogous compound, but with four anthracene substituents (Figure 7) [45]. The synthetic procedure was based on Heck coupling reaction with the same catalytic system to obtain the desired product with 86% yield. Its photophysical parameters disclosed similar solvent's polarity dependant fluorescence. It was also used to perform additional tests for possibility to encapsulate a variety of anions (e.g., F\({}^{-}\), OH\({}^{-}\), CN\({}^{-}\) and PO\({}_{4}\)\({}^{3-}\)) by observation of fluorescence quenching due to from charge-transfer complex formation. This resulted in the possibility to use this compound as a sensor of the abovementioned anions. Heck coupling to yield pyrene derivatives was also performed by [PERSON] et al.