# Synthetic Routes to Silsesquioxane-Based Systems as Photoactive Materials and Their Precursors
## 1 Introduction  
Functionalized polyhedral oligomeric silsesquioxanes (POSSs) [1] with a general (RSiO\({}_{3/2}\))\({}_{n}\) formula are inorganic-organic systems composed of a well-defined Si-O-Si core with organic moieties that have become the representative building blocks for hybrid materials (Figure 1). Figure 1: The hybrid (i.e., organic-inorganic) nanostructures of respective cubic T\({}_{8}\), DDSQ and T\({}_{10}\) silsesquioxanes. Due to their finely tunable physical and chemical properties, POSSs meet the requirements of both science and industry [2; 3]. The silsesquioxane family group contains a small number of well-defined 3D structures. The most recognizable and best known of this is cubic T\({}_{8}\) with either one or eight functional group derivatives [3]. Recently, the so-called double-decker-type silsesquioxane (DDSQ) structure, which differs from the symmetric, cubic one, and features opened (M\({}_{4}\)T\({}_{8}\) = DDSQ-4 OSi [4]) or closed (D\({}_{2}\)T\({}_{8}\) = DDSQ-2 Si [5]) frameworks with either two or four reactive moieties, has also gained respective interest [6]. The physicochemical properties of functionalized silsesquioxanes are a particularly important aspect of the research on their synthesis. This is due to their chemical and spatial structure resulting in a hybridic (i.e., organic-inorganic) nature. The presence of a regular element of the Si-O-Si framework corresponds with the silica architecture places silsesquioxanes as fillers (nanofililers) with a perfectly defined structure and nanometric dimensions for polymers modifiers [7]. The well-defined sizes of their molecules, and the presence of different amount and type of functional groups at the POSS core, allows their use in the precise deposition/embedding in the polymer matrix and obtain composite materials with unique properties [2; 3; 8; 9]. The obtained organic-inorganic hybrid materials are characterized by a whole range of interesting physicochemical features (e.g., thermal, mechanical, optical and chemical). This is related to their solubility change, increase in decomposition and glass transition temperatures, improvement of dielectric properties, reduction of heat transfer coefficients, and improvement of oxidation and fire resistance, as well as their impact on the hardness of obtained materials [2; 8; 10; 11]. Due to these aspects, functional silsesquioxanes possess great application potential [2] and could be applied as nanofillers and components of polymers [8; 11], building blocks and synthons for a variety of advanced materials with tailored properties [12; 13], such as silica models [14], (super)hydrophobic coatings [2], dendrimers and metal carriers. They were applied as immobilizing phase in catalysts [13; 15], medicine (e.g., as drugs carriers, components of artificial tissues or in stomatology [16; 17]) and opto- and electroluminescent (EL) materials [18]. Organic light emitting diodes (OLEDs) are a highly targeted area of technology because of their expected utility in flat panel displays. There have been discussions over the past decade concerning the family of small molecules and polymers that is best suited for OLEDs [18; 19; 20; 21; 22; 23]. Small molecules can be highly purified and vacuum-deposited in multi-layer stacks, which is important both for display lifetime and efficiency. However, vacuum deposition techniques are expensive and limit the practical application, which causes problems achieving full colour displays at high volume. On the other hand, polymers are generally not as pure as small molecules, but can access larger display sizes and full colour at less substantial expense via solution-based deposition techniques. Nanocomposite materials based on a silsesquioxane architecture that combines the advantages of both small-molecule and organic polymer fragments have been applied to OLEDs. They were introduced by [PERSON] et al. in 2003 [24], with the compounds containing a spherical \"silica\" core with a hole-transporting functionalized periphery (chromophores).