# 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  
Heck coupling (HC), also known as the Mizoroki-Heck reaction, is generally a palladium catalyzed coupling process between unsaturated C=C bonds (here, Si-HC=CH\({}_{2}\)) and an aryl halide (usually -I, -Br) resulting in the formation of alkenyl-substituted arenes. It occurs in the presence of additional phosphine ligand and equimolar amount of a base, although there are many variations of this process, a wide scope and tolerance to many organic functional groups is observed [40]. Mono-functional silsesquioxanes are generally less frequently applied in obtaining compounds with interesting photophysical properties. Nevertheless, there are a few interesting examples to present that reflect the application of Heck coupling as a method for their synthesis. It is worth mentioning that the compounds are characterized by better thermo- and photostability. [PERSON] et al. presented the synthesis of boron dipyrromethane (BODIPY) derivatives of POSS and their photophysical characterization (Figure 5) [41]. The elaborated synthetic procedure was not so effective, and enabled the respective mono- and disilsesquioxane substituted BODIPY derivatives (BODIPY-POSS) to be obtained with low (9%) to high yields (88%). Their photophysical properties were determined in EtOAc solution and poly(methyl methacrylate) (PMMA) film. Maxima absorption wavelengths were similar in both systems, with \(\lambda_{\text{ab}}\) in a range of 521 to 536 nm, and maxima fluorescence emission wavelengths \(\lambda_{\text{em}}\) ranging from 532 to 575 nm. Thermal stability was improved for both systems, i.e. mono- and disilsesquioxane substituted BODIPY derivatives. The most photostable hybrid dyes were the systems with a POSS unit linked to the BODIPY by one or two ethylene bonds (\(\pi\)-conjugation). The results obtained indicate a possibility for these systems to be applied where high thermo- and photostability are required. Monovinylhepta(cyclohexyl)silsesquioxane was reported as an efficient reagent towards Heck coupling (mediated by Pd(P\({}^{\text{f}}\)Bu\({}_{3}\))\({}_{2}\)) with p-iodophenyl pyrylene dimimide by [PERSON] et al. [25]. This compound synthesized with a moderate 37% yield when compared with an analogous one, obtained via stoichiometric hydrolytic condensation (mediated by Mc\({}_{\text{e}}\)H\({}_{4}\)SO\({}_{3}\)H) of hepta(cyclohexyl)silsesquioxane trisilanol with respective tri(methoxy)silylpropyl pyrylene dimide (23% yield). These two systems, where POSS unit and pyrylene dimide moiety were separated either via an aliphatic (propyl) or styryl spacer, were analyzed in terms of their absorption/emission spectra and compared to their pyrylene dimide counterparts. The type of the linker (i.e., aliphatic or arene) did not bear any influence on the photophysics of these. The analyses as expected showed slight red-shift emissions of up to 10 nm in solution (CHCl\({}_{3}\)), but with very good photoluminescence quantum yields (up to 100%). The analogous analysis for solids resulted in higher red-shifted emission spectra due to the expected aggregation and \(\pi\)-\(\pi\) intermolecular interactions. However, the photoluminescence quantum yields for the POSS-based systems were twice as high as for the respected organic analogue that suggests suppressed aggregation. This favors its possible use in the fabrication of efficient emissive devices. This part is presented in this section for the ease of product comparison, despite the fact that it also concerns condensation reactions. Heck coupling was also applied by [PERSON] et al. in the synthesis of thermally stable mono- and octazobenzene-functionalized T\({}_{8}\)-based dyes (Figure 6) [42]. The procedure enabled formation of the designed compound with yields of 67% and 80%; however, the reaction conditions were more severe  
Figure 5: Respective structures for BODIPY derivatives of POSS. in comparison with the previous example.