Synthesis And Properties Of π-gelators Based On Difluoroboron β-diketonate Complexes

Over the past decades, low molecular weight organogels (LMOGs), as animportant class of soft materials, have attracted substantial interest in supramolecularchemistry and material science because of their diverse applications in the fields oflubrication industry, cosmetic formulations, template synthesis, regenerative medicine,tissue engineering, self-healing materials and so on. In particular, LMOGs based onπ-conjugated organic compounds have currently received even more attention fortheir unique optoelectronic applications in multicolor display devices, light harvestingantennae, field efect transistors, photovoltaic devices, conducting materials,fluorescence sensors, etc. In particular, on account of high quantum yields, largemolar extinction coeffcients, strong emission in solid state, high electron affnitiesand sensitivity to the surrounding medium, difuoroboron β-diketonate complexes(BF2bdks) have fascinating potential in optoelectronic field. In this thesis, wesynthesized π-gelators based on BF2bdks and investigated their photophysical andself-assembly properties. Some creative results have been obtained, and outlinedbelow:(1) We synthesized triphenylamine functionalized β-diketone derivatives1-3.These compounds contains one, two and three β-diketone units respectively and theirdifuoroboron complexes1B-3B were synthesized by the reaction with BF3. It wasfound that the obtained compounds were highly emissive in toluene. For example,1-3emitted blue light and their fluorescence quantum yield(ΦF) were in the range of0.53-0.57.1B-3B emitted green light and their ΦFwere in the range of0.60-0.82. Thefluorescence of six compounds redshifted in solid state.1-3emitted green light andtheir ΦFwere in the range of0.10-0.43.1B-3B emitted orange light and their ΦFwerein the range of0.14-0.66. In addition, it was found that all compounds except2showgood gelation abilities. Compounds1,1B and3could form gels in polar solvents,while3B formed gels in nonpolar solvents. Although bis--diketone substitutedtriphenylamine2could not form organogel, its difuoroboron complex2B could gel DMSO due to the strong dipole-dipole interactions. The SEM image of xerogelindicated that three-dimensional networks consisting numerous nanofibersself-assembled by the gelator molecule were formed. The temperature-dependentUV-vis absorption, fuorescence and1H NMR spectral data suggested that π-πinteractions played a key role in the gel formation. It is interesting thataggregation-induced enhanced emission was achieved during the gelation of1,1Band2B with asymmetric structures, and the emission of symmetric compounds3and3B decreased to a certain degree upon gelation. The obtained gels based oncompounds1and3emitted strong green light and gels based on1B-3B emitted strongorange or red light. Combined with XRD pattern of the xerogels and the calculatedmolecular length, the molecular packing modes were proposed in the gels of1,1B,2Band3. It is thus clear that gelation process of-gelator is a good method for preparinghighly emissive organic nanomaterials.(2) We synthesized novel star-shaped difuoroboron β-diketonate complexes(TBC)3Ph and (TBF)3Ph, in which the terminal groups of triphenylaminefunctionalized difuoroboron β-diketonates were bridged by carbazole or fluorene tothe core of1,3,5-benzene. The star-shaped structure indeed can remarkably improvethe light-harvesting ability of difuoroboron β-diketonate complexes. It was found that(TBC)3Ph and (TBF)3Ph gave high molar extinction coefficients, for example, themaxof (TBF)3Ph reached3.11105M-1cm-1in THF, which was higher than otherreported difluoroboron β-diketonate complexes. Meanwhile,(TBC)3Ph and(TBF)3Ph emitted intense yellow light in toluene with ΦFof0.73and0.67,respectively, and strong red light emitting in solid states was also detected for(TBC)3Ph and (TBF)3Ph with ΦFof0.36and0.27, respectively. However, differentfrom our expectation,(TBC)3Ph and (TBF)3Ph did not exhibit gelation ability inmany kinds of organic solvents. It maybe resulted from the coexistence of theterminal group of nonplanar triphenylamine and the core of nonplanartricarbozolylbenzene or trifluorenylbenzene, which lead to excellent solubility inorganic solvents. Moreover, the electrochemical data illustrated that they hadconsiderably higher electron affinities than AlQ3, which indicated that they could beused as electron-transporting materials. Therefore, the two difuoroboron β-diketonatecomplexes (TBC)3Ph and (TBF)3Ph might have potential applications aselectron-transporters and emitting materials. (3) We synthesized a new π-gelator TCB based on difuoroboron β-diketonatecomplexes, in which the terminal group is tert-butyl carbazole. It has been found thatorganogel can give different emission depending on solvents. For example, organogelformed in o-Dichlorobenzene/Cyclohexane (v/v=1/2) emit orange light, whereasorganogel emit red light when the solvents was changed too-Dichlorobenzene/Cyclohexane (v/v=1/5), Chlorobenzene/Cyclohexane (v/v=1/2)or Toluene/Cyclohexane (v/v=1/2). The UV-vis absorption and fuorescence spectraldata during the gelation suggested that the packing mode of gelator in gel state isresponsible for the fluorescence colour. TCB formed H-aggregates ino-Dichlorobenzene/Cyclohexane (v/v=1/2) gel. Its fluorescence emission maximumwas around600nm and its xerogel blueshifted to577nm with ΦFof0.14. However,TCB formed J-aggregates in o-Dichlorobenzene/Cyclohexane (v/v=1/5) gel. Itsfluorescence emission maximum was around645nm and its xerogel redshifted to638nm with ΦFof0.18. Such control on the nature of the aggregate formed by changingthe type of solvent is highly desirable since it would be possible to control theoptoelectronic properties of the bulk materials formed from the same set of moleculesusing simple techniques.(4) We synthesized a new π-gelator HCB based on difuoroboron β-diketonatecomplexes, in which the terminal group is hexadecyl carbazole. HCB can form gels intoluene, ethylbenzene and some mixed solvents containing toluene. Thetime-dependent UV-vis absorption and fuorescence spectral data during the gelationsuggested that HCB fromed J-aggregates in gel state. SEM image of the xerogelillustrates scaffold-like3D networks were set up from numerous nanofibers indiameter of200-300nm. It is worth noting that emitting nanofibers were formed inethylbenzene gel HCB, and the fluorescence of the nanofibers-based film can bequenched efficiently and rapidly upon exposed to the vapors of n-butylamine,dibutylamine, triethylamine, triethylamine, cyclohexylamine, aniline and pyridine,instead of other common reagents. Notably, the response time of the nanofibers-basedfilm to aniline vapor reached ca.0.59s, and the detection limit was as low as ca.0.83ppm. Moreover, the good reversibility of the fuorescent nanofbers based on HCB inthe detection of amine vapors favors the practical applications. Therefore, thenanofibers based on compound HCB could be good sensor for detecting volatileorganic amines and pyridine. We deduced that the high sensitivity and very short decay lifetime of the sensory nanofbrils for gaseous amines originated from the highsurface-to-volume ratio and large interspace in the3D networks consisting of lots ofnanofbers (favoring the enhanced adsorption, accumulation and diffusion of gaseousmolecules), as well as the amplifed fuorescence quenching induced by the enhancedintermolecular exciton diffusion along the long axis of the1D nanostructures. Weprovide a strategy to prepare novel fluorescent sensory materials.