The Synthesis Of Compounds With Enhanced Fluorescence In The Condensed State And Its Application

Fluorescent dyes with multifunctionality play a vital role both in the basic researches and potential applications of ionic detections, chem-sensors and/or biosensors, biolabeling and bioimaging, optoelectronic materials and devices, especially in the filed of organic electroluminescent materials and devices. Generally, most of the organic fluorophores exhibit high fluorescence quantum yield (ΦF) in the solution state. However, in the solid state or in aqueous media, these chromophores suffer from aggregation induced quenching (ACQ), which leads to a drastic decrease in the ΦF or even a complete loss. On the contrary, an abnormal and opposite phenomenon named aggregation induced emission (AIE) will avoid the issue of fluorescent quenching caused by aggregation. In the thesis, we were devoted to design and synthesis of the novel organic fluorescent materials with AIE attributes via utilizing newly synthetic methodology, and expanding the AIE active specis. On the basis of what we have achived, we believe that the development of AIE researches will be significantly promoted through the newly synthetic routine. The summarizations of this thesis are listed as follows:(1) A general approach to the design and synthesis of a new series of geminal-substituted tetraarylethene (g-TAE) chromophores with aggregation-induced emission (AIE) properties has been developed via Corey-Fuchs reaction and subsequent Suzuki/Stille coupling reactions. To demonstrate the synthetic simplicity and versatility of this approach, nine fluorophores with diverse electronic characteristics have been synthesized. The validation of the geminal structure and bulky steric effect on the bond lengths and dihedral angles is further confirmed by the X-ray crystallography of three chromophores. With respect to single crystals of2,2’-(2,2-diphenylethene-1,1-diyl)-dinaphthalene (DPDN2),1,1’-(2,2-diphenylethene-1,1-diyl)-dinaphthalene (DPDN1) and9,9’-(2,2-diphenylethene-1,1-diyl)dianthracene (DPDAn), intensive cofacial π-π stacking is absent in their molecular packing. The pseudopolymorphism of DPDAn crystals associated with different photoluminescence properties is investigated. In comparison to the solvent-free DPDAn crystal, the solvated DPDAn with embedded methanol or dichloromethane leads to some non-negligible conformational and packing alterations, which is accountable for distinct fluorescence properties. The AIE phenomena and optical properties of g-TAEs are probed with respect to the steric and electronic effects. Along with their electrochemical characteristics and piezofluorochromism, our work elucidates that this general approach can be utilized to develop a promising class of TAE materials with AIE characteristics for systematic investigations on the optoelectronic properties and crystal engineering.(2) I demonstrated a novel synthetic approach to ortho-methyl substituted tetraphenylethene materials:TPE, DMTPE and TMTPE. The introduction of ortho-methyl groups in DMTPE and TMTPE has a significant influence on the AIE effect, which is due to the enhancement of steric hindrance in the contiguous phenyl rings with ortho-groups. Through internal control via the introduction of ortho-groups, the RIR hypothesis as the basic mechanism for the AIE-active TPE derivatives is directly validated.(3) I have demonstrated a simple design of a donor-acceptor (D-A) system utilising TPE as the electron donor and naphthalimde (NI) as the electron acceptor. In this system TPE has dual ability to activate both ICT and AIE phenomena. Strong solvatochromism is observed with emission covering most part of the visible spectrum. This is the first example of a TPE-based fluorophore to exhibit both AIE and ICT behaviour without additional electron donating groups in the molecule.(4) I demonstrated a general and facile synthetic strategy to afford dibenzofulvene derivatives via Corey-Fuchs reaction and subsequent Suzuki coupling reactions. The weak intermolecular interactions in F-DAn are probably accountable for this piezofluorochromism with a bathochromic shift of λem (>80nm) reversibly altered between green and red emission under external stimuli, which is seldom reported. More importantly, we elucidated that aromatic hydrocarbon materials without any electron-withdrawing groups and heteroatoms can be served as promising candidates for piezofluorochromic materials.(5) The biodegradable SP-PCL nanoparticles, fabricated in the solvent mixture of dioxane and water, exhibited special behavior, which is similar to the well-known AIE phenomenon. The SP-PCL nanoparticles were capable of undergoing reversible fluorescence switching with alternating UV (<420nm) and visible (>420nm) light. The outstanding advantages of photoswitchable SP-PCL nanoparticles, e.g., great biocompatibility and biodegradability, endow them to act as excellent fluorescent probes in super-resolution imaging. The localization-based super-resolution fluorescence imaging results demonstrate that vicinal SP-PCL nanoparticles are discernible with sub-50nm resolution, which is much higher than that obtained with conventional fluorescent imaging. We demonstrated two-photon photoswitching and two-photon photoluminescence of SP/MCnanoparticles. The two-photon process photoswitched the SP-particles to MC-particles and also excited the MC-particles to emit red fluorescence. The photobleaching of MC nanoparticles was almost eliminated via using two-photon excitation.