Synthesis, Characterization And Photophysical Properties Of New Highly Fluorescent AEE-active Cyclic Silole-based Derivatives

Synthetic materials with high solid-state fluorescence quantum yield are ofsignificant importance to efficient energy utilization in light-emitting devices. Silolesare five-membered silacyclics which can be considered as polyacetylene derivativespossessing additional σ*-π conjugation arising from the interaction between theσ*-orbital of two exocyclic-bonds on the silicon atom and the π*-orbital of thebutadiene moiety. The low-lying LUMO levels impart siloles with promisingoptoelectronic properties. Thus, siloles have wide and important applications in thefields of light-emitting devices. So far a number of conceptually different syntheticapproaches has been used to optimize siloles’ solid-state quantum yields by i.e.attaching conjugating substituents or highly fluorescent groups to the silicon atom,structural modification of peripheral phenyl moieties in siloles, catenation through the2,5-position to form polymers etc. However, most of the synthetic proceduresemployed are not always simple and mild. Few efforts are known that concentrat onthe incorporation of siloles into cyclosiloxane and siloles catenating through the1,1-position to form polysiloxane. Additionally, examples of contributions striving torationalize obtained solid state quantum yield of the newly synthesized silolederivatives based on considerations going beyond simplistic structural engineeringare scarce in the literature. Therefore, designing novel molecular silole-containingsystems, exploring alternative synthetic routes to newly designed siloles derivativesand gaining insights in the operative fluorescence mechanisms at a deeper level arekey issues on the path to the optimization of their solid-state quantum yields and aswell as of great importance to academic research.In this study, with silole as the main focus of the research, a series of new highlysolid-state fluorescent derivatives containing different fluorophores (silafluorene,1,3-diphenyl-9-silafluorene,2,3,4,5-tetraphenylsilole,-alumino diquinaldinate) aswell as separate fluorogenic and non-fluorogenic groups (Me, Ph) incorporated incyclosiloxanes were designed, synthesized, purified and fully characterized; the influence of separate groups, size of the cyclosiloxane rings and type of fluorophoreson the fluorescence properties of these silole derivatives were studied and discussed;the UV-Vis absorption, fluorescence emission, aggregated enhanced emission (AEE),fluorescence lifetime, fluorescence quantum yield of the silole derivatives in solution,thin film and crystalline state were studied systematically; the packing mode andpacking interactions of the silole derivatives in the unit cell were analyzed; themolecular orbital and excited state UV-Vis calculations of the silole derivatives wereperformed; mechanisms of solid-state fluorescence were studied based on the resultsof these calculations; ring-opening polymerization of two silole derivatives has beenattempted.The main content and results are summarized as follows:(1) The synthesis, characterization and photophysical properties ofsilafluorene-containing cyclosiloxane derivatives. Four ring-shapedsilafluorene-containing compounds were synthesized by co-hydrolysis reactions andfully characterized. Their optical properties in THF solution and solid state (thin filmand powder) were studied. These compounds have absorption around278nm and lowquantum yields in solution (Φfl=0.13-0.15) with fluorescence maxima at about355nm, but high quantum yields in solid state (powder, Φfl=0.35-0.54) with fluorescencemaxima at about369-403nm and shoulders around488-500nm. A discernable redshift in emission spectra for these compounds in solution as compared to the solidstate was observed. AEE studies revealed that these compounds are only weakly AEEactive (I/I0=1.5). The interaction analysis of silafluorene moieties in the unit cellshows that partial π-π interactions only present in the cyclotetrasiloxane containingfour silafluorene moieties. The other three compounds only have CH-π interactions.Excited state and MO calculations reveal that all involved optically allowed transitionorbitals (HOMO-1, LUMO, LUMO+1) are located on silafluorene moieties; thetransition gap decreases in solution/solid state comparison; the cyclotetrasiloxanecontaining four silafluorene moieties and cyclohexasiloxane containing twosilafluorene moieties have higher density of lowest-lying optically forbidden excitedstates in the relevant spectral range, thus, offering more relaxation pathways andallowing more efficient fluorescence decay.(2) The synthesis, characterization and photophysical properties of 2,3,4,5-tetraphenylsilole-containing cyclosiloxane derivatives. Three ring-shaped2,3,4,5-tetraphenylsilole-containing compounds were synthesized by condensationreactions and fully characterized. Their optical properties in THF solution and ascrystals were studied. These compounds show absorption peaks at366-375nm insolution. Fluorescent emission spectra of these compounds display peaks at490-500nm in solution and505-528nm in the solid state. Although these compounds werefound to have low quantum yields (Φfl≈0) in solution, they displayed remarkablyhigh solid state fluorescence quantum yields (Φfl=0.19-0.86). AEE studies revealedthat compounds are strongly AEE-active with I/I0=60. The results of the interactionanalysis in the crystal structure suggested that the intramolecular CH-π interactionsonly exhibit in cyclotrisiloxane containing Ph2Si-separate groups, and all compoundshave intermolecular CH-π interactions. The theoretical calculations demonstrated thatall MOs relevant for fluorescence processes (HOMO-1, LUMO, LUMO+1) of thesecompounds are located on the tetraphenylsilole units. The Ph2Si-containingcyclotrisiloxane has optically allowed transitions from S0to S2, while the other twoMe2Si-or Ph2Si-containing cyclotetrasiloxanes have optically allowed transitionsfrom S0to S4; the markedly different electronic structures allow for more relaxationpathways for efficient fluorescence decay and thus have lower fluorescence quantumyields in the latter.(3) The synthesis, characterization and photophysical properties ofsilole-containing cyclotetrasiloxane derivatives. Three ring-shapedsilole-containing compounds were synthesized by co-hydrolysis and condensationreactions and fully characterized. Their optical properties in THF solution and ascrystals were studied. The silole fluorophores in these compounds in solution haveabsorption peaks at279,291nm (silafluorene fluorophores),265nm(1,3-diphenyl-9-silafluorene fluorophores) and367nm (2,3,4,5-tetraphenylsilolefluorophores); fluorescence emission peaks at359nm (silafluorene fluorophores),375nm (1,3-diphenyl-9-silafluorene fluorophores) and491nm(2,3,4,5-tetraphenylsilole fluorophores). The fluorescence properties ofcyclotetrasiloxane containing two fluorophores (silafluorene and2,3,4,5-tetraphenylsilole) result mainly from tetraphenylsilole fluorophores. In THFsolution, cyclotetrasiloxane containing1,3-diphenyl-9-silafluorene fluorophores (Φfl =0.18) has larger fluorescence quantum yield than cyclotetrasiloxane containingsilafluorene fluorophores (Φfl=0.14), while the compound containing2,3,4,5-tetraphenylsilole has the lowest fluorescence quantum yield (Φfl≈0).However, all these compounds have high solid-state fluorescence quantum yields (Φfl=0.65-0.78). AEE studies show that cyclotetrasiloxane containing1,3-diphenyl-9-silafluorene fluorophores or silafluorene fluorophores do not havesignificant AEE (I/I0<2), while the compound containing2,3,4,5-tetraphenylsilolehas aggregation enhanced emission with I/I0≈18in THF/H2O mixtures. Packinganalysis of these compounds in the crystal structure reveals that these compounds allexhibit both π-π and CH-π interactions. The cyclotetrasiloxane containing twofluorophores (silafluorene and2,3,4,5-tetraphenylsilole) may have efficientfluorescence resonance energy transfer (FRET) due to the overlap of absorption peakand mission peak, which is consistent with the experimental fluorescence result.Excited state calculations predict that the optically allowed excited states correspondmainly to the (π-π*) HOMO-LUMO transitions in these compounds. The MOcalculations show that HOMO and LUMO are located exclusively on the silafluoreneunits for cyclotetrasiloxane containing silafluorene fluorophores,1,3-diphenyl-9-silafluorene units for cyclotetrasiloxane containing1,3-diphenyl-9-silafluorene fluorophores and tetraphenylsilole units forcyclotetrasiloxane containing two fluorophores (silafluorene and2,3,4,5-tetraphenylsilole), respectively.(4) The synthesis, characterization and photophysical properties of silolederivatives with~100%solid-state fluorescence quantum yields. Five silolederivatives containing2,3,4,5-tetraphenylsilole,1,3-diphenyl-9-silafluorene,silafluorene or aluminodiquinaldinate fluorophores were synthesized by condensationand acid-base reactions and fully characterized. Their optical properties in THFsolution and as crystals were studied. The trans isomers of2,3,4,5-tetraphenylsilolecyclotetrasiloxane containing PhMeSi-separate groups and the2,3,4,5-tetraphenylsilole cyclotrisiloxane containing Me2Si-separate groups havesimilar UV-Vis absorptions (366-373nm) and fluorescence emissions (497-500nm);their fluorescence quantum yields in THF solution are around zero, while solid-statequantum yields are~100%(0.97-0.99); AEE studies show that these two compounds have strong aggregation enhanced emission with I/I0=94; Packing analysis revealsthat these compounds all do not exhibit π-π interactions; the trans and cis isomers of2,3,4,5-tetraphenylsilole cyclotetrasiloxane containing PhMeSi-separate groups have1D chain packing mode with both intermolecular and intramolecular CH-πinteractions, while the2,3,4,5-tetraphenylsilole cyclotrisiloxane containing Me2Si-separate groups have3D network packing mode with only intermolecular CH-πinteractions. The Gaussian calculations suggest that the electronic situations in trans-and cis-cyclotetrasiloxanes containing tetraphenylsilole and PhMeSi-separate groupsdo not show any significant differences; the results further indicate that theexperimentally observed absorption spectra correspond to a transition into the secondexcited state rather than first, since the first excited state is optically forbidden inthese two compounds. All involved MOs in optically allowed transitions of thesecompounds are almost exclusively localized on the π-systems of the silole ring and(with a smaller contribution) appurtenant phenyl substituents of the tetraphenylsilolemoieties. The fluorescence properties of silole derivatives containing1,3-diphenyl-9-silafluorene and aluminodiquinaldinate fluorophores or silafluoreneand aluminodiquinaldinate fluorophores result mainly from aluminodiquinaldinatefluorophores (λem=461-476nm); these two compounds are not AEE-active; theirquantum yields in THF solution are0.31and0.34, while their solid-state quantumyields are0.92and0.79. Additionally, mechanochromic characteristic of crystallinesilole derivative containing silafluorene and aluminodiquinaldinate fluorophores wasobserved: a slightly red shift with λmax=481nm after grinding.(5) The ring-opening polymerization of cyclosiloxane and its application.The monomers of silafluorene and2,3,4,5-tetraphenylsilole cyclosiloanes were ringopened and catenated through1,1-position of the silicon by ring-openingpolymerization to give the poly(1,1-silole)s. These polymers can not be dissolved inany solvent at room temperature, but can be dissolved in ortho-dichlorbenzene over100°C. The octamethylcycloterasiloanes were ring opened and then reacted withnitrogenous siloxanes by polyconensation, the alkyl and phenyl groups wereincorporated in the side chains of these polymers by structurelly modification throughthe reaction wth epoxy compounds. The damping properties of silion rubber preparedby these polysloxanes and nano SiO2were studied. The silicon rubber prepared by polysiloxanes with different viscosity （r.t.,2.5×104mP s~8.0×104mP s）butsimilar content of nitrogenous pendant Si-O chain units（5%）show similar dampingproperties with tanδmax=0.42and Tg=-72℃~-50℃。The damping coefficient (tanδ)and glass transition temperature Tgof the silicon rubber prepared by polysiloxaneswith similar viscosity （r.t.,6.5×104mP s）but different content of nitrogenouspendant Si-O chain units become larger as the content of the side chain increases; thebest damping property of these silicon rubber with tanδmax>0.82was prepared by thepolysiloxae containing18%pendant Si-O chain units.