# Synthetic Routes to Silsesquioxane-Based Systems as Photoactive Materials and Their Precursors
## 1 Introduction
describes hydrolytic condensation (\"corner capping\") of hepta(cyclohexyl)silesquioxane trisilanol with respective tri(methoxy)silylpropyl pyrylene diimide to obtain pyrylene diimide with T\({}_{8}\) unit, but only with a 23% yield [25]. This is described in Section 3.1, due to the ease of respective product comparison. The amidation or esterification processes between respective reactive groups are also worth mentioning. An interesting example of using (3-aminopropyl)hepta(isobutyl)silesquisoxane in the amidation of mono- and bis-anhydrides in microwave radiation was reported by [PERSON] et al. (Figure 2) [26]. However, irrespective of a fast reaction time, the reaction conditions, and especially the 4-fold excess of silsesquioxane that resulted in only (mostly) moderate to good yields of the respective products (**2a,b**, 15%-54%; **3a-c**, 37%-98%), which may not be satisfying. Figure 2. Synthetic path to obtain mono- and di-POSS-based imides and diimides. In their study, [PERSON] et al. present the photophysics of the compounds obtained. All products presented absorption bands (in solution, CHCl\({}_{3}\)) that were similar to and typical of their organic imide counterparts (\(\lambda_{\text{ab}}=294\) nm (**2a**), \(\lambda_{\text{ab}}=309\) nm (**2b**), \(\lambda_{\text{ab}}=335\), 350 nm (**3a**), \(\lambda_{\text{ab}}=342\), 360, 381 nm (**3b**), \(\lambda_{\text{ab}}=459\), 490, 526 nm (**3c**)). The emission spectra were much poorer for mono-POSS amide, as **2a** only had one wide band at ca. 330 nm, and for **2b** the strongest three peaks were at 362, 380 and 398 nm, respectively. However, the fluorescence quantum efficiency was very low for these two compounds, ca. \(\Phi=0.02\). On the other hand, for the dimides, the resulting emission spectra were more accurate and reflected the presence of diimide unit (compared with organic counterparts), and three \(\lambda_{\text{em}}\) peaks were present (362-390 nm for **3a**, 350-404 nm for **3b** and 533-620 nm for **3c**). Only in the case of **3c** was the \(\lambda_{\text{em}}\) red-shifted with \(\Phi\) that equaled 1.0. The low value of \(\Phi\) suggests quenching of fluorescence due to an aggregation of organic moieties. In this case, only **3c** had potential for use in optoelectronic device fabrication. A similar synthetic concept based on the amidation reaction of aminopropylPOSS and pyrylene anhydride, along with its consecutive amidation with methoxypolyethylene glycol amine, was presented by [PERSON] et al. (Figure 3) [27]. However, the isolation yield of the intermediate product after the first amidation reaction with aminopropylPOSS reagents resulted in only a 23% yield, while the sequent amidation reaction that was conducted to obtain the final product--asymmetric pyrylene-based diimide--with 87% yield. As a result, the overall yield of the final product was 20%. Nevertheless, this compound exhibited interesting photophysical properties. As reported, pyrylene diimides are strongly susceptible to concentration quenching due to intermolecular \(\pi\)-\(\pi\) stacking [28]. In POSS-based pyrylene diimides, the POSS unit prevents the aggregation of pyrylenes, resulting in decreased emission quenching, and these compounds exhibit a strong excimer-like red emission at ca. 620-660 nm (\(\lambda_{\text{ex}}=495\) nm) [26; 29]. The compounds described above were tested as chemosensors for variety of anions (F\({}^{-}\), Cl\({}^{-}\), Br\({}^{-}\), I\({}^{-}\), NO\({}^{3-}\), AcO\({}^{-}\), ClO\({}^{4-}\) and H\({}_{2}\)PO\({}^{4-}\)), but only in the case of fluorine anions was the red emission found to be quenched, along with its increasing concentration. Interestingly, this was a result of silsesquioxane cage hydrolysis catalyzed by F\({}^{-}\), as previously reported in [30; 31; 32]. The decomposition of POSS units enables a consecutive and induced aggregation of pyrylene units and results in quenching the fluorescence phenomenon. This, in turn, offers a new optical strategy for toxic F\({}^{-}\) ion sensing. [PERSON] et al.