Luminescent materials with aggregation-induced emission (AIE) characteristics have been of great interest in organic light-emitting diodes, organic light-emitting field-effect transistors, organic solid-sate lasers, solid-state lighting, two photon-absorption materials, mechanochromic materials, fluorescent probes for chemicals and bio-molecules as well as cell imaging. Therefore, development of new molecular design strategies and new AIE materials towards desired properties are of great importance.

In this thesis, research work is focused on two major areas of AIE materials, including modulation of singlet-triplet energy gap (ΔE_ST) of AIE materials and exploration of the phosphorescence and electroluminescent device performances thereof (Chapter II to IV), development of highl...[ Read more ]

Luminescent materials with aggregation-induced emission (AIE) characteristics have been of great interest in organic light-emitting diodes, organic light-emitting field-effect transistors, organic solid-sate lasers, solid-state lighting, two photon-absorption materials, mechanochromic materials, fluorescent probes for chemicals and bio-molecules as well as cell imaging. Therefore, development of new molecular design strategies and new AIE materials towards desired properties are of great importance.

In this thesis, research work is focused on two major areas of AIE materials, including modulation of singlet-triplet energy gap (ΔE_ST) of AIE materials and exploration of the phosphorescence and electroluminescent device performances thereof (Chapter II to IV), development of highly viscosity sensitive AIE molecules based on the restriction of intramlecular rotation (RIR) mechanism (Chapter V and VI). Efforts are devoted to the investigation on the relationship between the molecular structure and the properties of AIE materials, new molecular design strategies have been proposed and verified by comparative study.

In Chapter II, a series of triphenylethene (3TPE)-based AIE chromophores substituted with triphenylamine and phenylnitrile as donor and acceptor groups at different positions towards modulation of ΔE_ST, respectively, were designed and synthesized in good yields. Some molecules with larger ΔE_ST, the well-separated HOMO and LUMO orbitals and severe steric hindrance, emitted efficient long-lived phosphorescence with lifetime of up to several seconds in their glassy solutions at 77 K. Moreover, room temperature phosphorescence can be obtained from their crystals. The chromophores with highly twisted conformation exhibited improved mechanochromic behaviors with higher contrast. Electroluminescence devices using these molecules as emitting layers were fabricated, which exhibited external quantum efficiency equal or even exceed the theoretical values of singlet emitter-type devices. Particularly, the molecule with a small ΔE_ST value showed outstanding device performance with high luminance and efficiencies up to 36900 cd/m², 11.2 lm/W, 12.8 cd/A and 4.37%, respectively, considering that the solid-state quantum yield is only 42%.

In Chapter III, another two triphenylethene-cored AIE dyes substituted with triphenylamine and dimesitylboron as the strong donor and acceptor were designed and prepared. These two molecules exhibited very similar photophysical properties, including UV-vis absorption, photoluminescence, aggregation-enhanced emission and twisted intramolecular charge transfer characteristics, as well as electrochemical properties. Similarly, o-TPA-3TPE-p-DMB, which exhibits a larger ΔE_ST value, stronger steric hindrance and well separated HOMO/LUMO, showed much longer low temperature phosphorescence lifetime. Their electroluminescence were also investigated, and they exhibited efficient devices performances, with the luminance and efficiencies up to 6530/8460 cd/m², 5.6/4.8 lm/W, 6.3/6.8 cd/A and 3.00/2.81%, respectively.

In Chapter IV, a new tetrapenylethene (TPE)-based AIE luminogen DPATPEPy substituted with triphenylamino and pyridine salt as very strong donor and acceptor, respectively, has been designed and synthesized. With the synergy between the strong electron donating and withdrawing effect and conformational twist of the TPE linkage, well spatially separated HOMO-LUMO orbitals was achieved, offering DPATPEPy with extremely small ΔE_ST (0.07 eV). Such low ΔE_ST value promoted efficient delayed fluorescence in poly (methylmethacrylate) matrix doped with diluted dye molecules. However, the delayed fluorescence becomes much weaker in higher doping concentration and it cannot be detected in its thin film. Meanwhile the fluorescence of the thin film of the dye can be enhanced upon heating verifying its thermally activated nature.

In Chapter V, a novel molecular design strategy to enhance the viscosity sensitivity of AIE dye was proposed and verified. With an increase in the number of rotors, which can populate the non-radiative decay channels, enhance the quantum efficiency and enlarge the molecular size, the viscosity sensitivity of the newly designed AIE dyes was enhanced. Noteworthily, a high viscosity factor of 0.98 was achieved, which is the highest value reported so far.

In Chapter VI, with the synergy between AIE mechanism and ratiometry design principle, two new fluorescence ratiometric viscosity probes TPEAn-1 and TPEAn-2 with different conformational crowdedness were designed and synthesized. Their ratiometric measurement of viscosity was demonstrated with high sensitivity and improved reliability owing to its self-calibrating ability. And it was found that conformationally less crowded molecule exhibited much higher viscosity sensitivity. Moreover, the newly designed viscosity sensor was found to be two-photon absorption active, and the viscosity sensing process can be conducted via two-photon excited fluorescence (TPEF).

In the last chapter, a new kind of polymers with aggregation-enhanced emission characteristics has been synthesized via a new kind of alkyne polymerization. Interestingly, this polymer can be utilized to serve as a ‘light up’ sensor for hydrazine and a ‘turn off’ probe for explosives simultaneously.