Design, Synthesis, And Property Of Novel Mechanochromic Luminescence Materials

Mechanochromic materials are smart materials, of which the luminescence wavelength(color) and/or intensity of solid-state can be adjusted by mechanical perturbations. Recently, mechanochromic luminogens have been attracting increasing attention due to their fundamental importance and great potentials in practical applications, such as micro-stress sensing, optical information storage, memory chips, trademark anti-counterfeiting and photovoltaic devices and so on. In this dissertation, a series of high contrast mechanochromic materials with efficient emission in the solid state were designed and studied. Main content of this dissertation includes:(1). Novel mechanochromic materials based on twisted triphenylacrylonitrile were studied;(2).Crystallization-induced phosphorescence of benzil and its derivatives at room temperature and their force-induced phosphorescence switching-off were investigated as well;(3). Novel organic dyes which could simultaneously emit fluorescence and phosphorescence at room temperature were discovered, these novel organic dyes also have mechanochromic property. Specific studies are as follows: 1. Triphenylacrylonitrile(TPAN) and its derivatives were designed and synthesized. The photophysical properties, especially mechanochromism of these compounds were further studied. TPAN shows typical crystallization-induced emission features but no mechanochromicism. Fortunately, simple chemical modification can result in novel aggregation-induced emission(AIE) luminogens with efficient emission and mechanochromic properties. In a word, TPAN can be used as a chemical unit in preparing AIE luminogens and mechanochromic materials. 2. Electron donor–π-bridge–electron acceptor(D–π–A) structured luminogens based on TPAN and diarylamine were prepared by C-N coupling reaction. These D–π– luminogens, DPATPAN, Me DPATPAN and Ph NPATPAN, exhibit typical AIE characteristics with high solid state emission efficiency(even up to 100%) and switchable mechanochromism with high contrast(emission peak shifts up to 51 nm). Non-doped OLED devices of these compounds were also developed and they showed good performance, since they possess considerable emission efficiency in amorphous state(up to 73%). The maximal lumen efficiency(Lmax)/current efficiency(CEmax)/power efficiency(PEmax) and external quantum efficiency(EQE) for non-doped OLED device based on Ph NPATPAN respectively are 4323 cd m-2, 6.72 cd A-1, 2.70 lm W-1 and 2.2%. 3. Twisted chemical units, such as TPAN and tetraphenylethene(TPE), and plane molecules, such as dibenzothiophene and dibenzofuran, were joined together in conjugation to form twist-π-plane structured compounds. In crystal, the twisted chemical units can effectively prevent the molecules from forming π-π interactions, allowing efficient emission; while the crystal is destroyed, inter-molecular π-π interactions thus exciplex/excimer are formed and twisted units are planarized. All these effects synergistically lead to red-shift of fluorescence emission and lower emission efficiency. For example, dibenzothiophene-TPAN(DBTTPAN), its emission peak shifts from 433 nm to 519 nm, fluorescence quantum efficiency reduces from 35.65% to 5.44 % under force. The emission peak shift is as high as 86 nm and quantum efficiency reduction is close to 85%. These results suggest that the use of twisted chemical units conjugated with plane molecules can be effective in designing luminous color(wavelength) and luminous intensity dual-tunable high contrast mechanochromic materials. 4. Mechanochromic materials that are AIE active with high emission efficiency and thermo-stability, BPA2 TPAN and BNA2 TPAN have been designed and prepared. Their Tg values are higher than 170℃,while Td values higher than 450℃. Both luminogens exhibit high solid-state emission efficiency, 47.7% and 46.3% respectively for BPA2 TPAN and BNA2 TPAN. Traditional mechanochromic materials can easily restore their emission ability with the help of heat treatment or organic solvents fuming. However, the ground powders of BPA2 TPAN and BNA2 TPAN are not sensitive to heat treatment and need much more time to recover when fumed. They possess outstanding morphological stability besides obvious mechanochromism. Nondoped OLED devices of the luminogens show physiologically friendly orange light(603, 606 nm) with low color temperature(CT) values of 2093 and 1883 K, which are much lower than those of incandescent bulbs(2000~2500 K) or even candles(~1900 K), whereas their doped OLED devices emit yellow light(551, 559 nm) with significantly improved performance, whose maximal power, current and external quantum efficiencies are 8.3 lm/W, 12.2 cd/A and 4.2%, respectively. These results suggest that AIE luminogens are suitable to fabricate metal-free and nondoped low CT OLEDs with rational molecular design; meanwhile, their electroluminescence can be facilely modulated through doping technology. 5. A novel D–A conjugate(AN2TPAN) with sterically crowded and remarkably twisted conformations, consisting of arylamine and two triphenylacrylonitrile(TPAN) units, has been carefully designed and synthesized. Upon more drastic mechanical grinding, it emits redder light in the solid state as the proportion of amorphous state grows. Such multicolor mechanochromism for solid emitters under mild conditions has rarely been reported. And the nanosuspensions of AN2 TPAN obtained a large two-photon absorption(2PA) cross section of 5782 GM. Moreover, a multilayer non-doped OLED of AN2 TPAN was fabricated and performed excellently, whose Lmax/CEmax/PEmax/EQE are as high as 11430 cd m-2, 10.7 cd A-1, 4.9 lm W-1 and 3.3%, respectively. The multifunctionality and multicolor mechanochromism of AN2 TPAN make it promising material in optoelectronic applications. 6. Brand new efficient room temperature phosphorescence(RTP) emitters, benzil(BZL) and its derivatives have been discovered. These luminogens are nonluminescent in solvents and thin layer chromatography(TLC) plates, but become highly phosphorescent in crystal state at room temperature, exhibiting typical crystallization-induced phosphorescence(CIP) characteristics. The photoluminescent quantum yield of 4,4-dibromobenzil(DBBZL) crystal is up to 41.7%. The CIP phenomenon is ascribed to the restriction of intramolecular rotations in crystals owing to effective intermolecular interactions. Such intermolecular interactions greatly rigidify the molecular conformation and significantly decrease the nonradiative deactivation channels of the triplet excitons, thus giving boosted phosphorescent emission at room temperature. It’s worth noting that BZL and its derivatives even can emit phosphorescence at room temperature when doped inpolymethyl methacrylate(PMMA). Room temperature phosphorescence of emitters in amorphous state is very rare. Moreover, BZL and their derivatives could tune their phosphorescence emission under mechano-stimulation. Take DMe OBZL for example, its phosphorescence peak shifted from 526 to 568 nm and luminescence efficiency dropped drastically from 41.7% to 4.0%. The red-shift of phosphorescence emission can be attributed to planarizartion of their twisted molecular configuration while the reduction of emission efficiency can be ascribed to the loss of intermolecular interactions that can restrain non-radiative decay and the quenching of exposed sensitive triplet excitons. 7. Twisted D-π-A compounds, CZBP and its bromine substituted derivatives(BCZBP and DBCZBP) based on carbazole and benzophenone were synthesized. These novel luminogens simultaneously emit both fluorescence and phosphorescence in crystalline state at room temperature. For example, CZBP crystal emits white light under the radiation of a 365 nm ultraviolet(UV) lamp, which consists of blue fluorescence and orange phosphorescence. Moreover, the phosphorescence lifetime of CZBP surpass 517.87 ms. These dual-emissive twisted D-π-A lumingens also exhibit mechanochromism properties. Once the crystals were destroyed, the phosphorescence would disappear because triplet states are immediately quenched without the protection of crystal lattice, leading to a simultaneous dramatic reduction of luminescence efficiency. It’s the first report on mechanochromism realized by change of exciton species.