About half a century ago, Főrster and Kasper discovered that traditional organic chromophore such as pyrene was weakened with an increase in its solution concentration. It was soon recognized that this was a general phenomenon for many aromatic compounds. This concentration-quenching effect was found to be caused by the formation of sandwich-shaped (disc-like) excimers and exciplexes aided by the collisional interactions between the aromatic molecules in the excited and ground states.
In 2001, Tang¡¦group discovered such a system, in which luminogen aggregation played a constructive, instead of a destructive, role in the light-emitting process: a series of silole molecules were found to be non-luminescent in the solution state but emissive in the aggregated state. They coined the term ¡¥¡¥aggregation-induced emission¡¦¡¦ (AIE) or ¡§AIE enhancement¡¨ (AIEE) for this novel phenomenon which originated from the restricted intramolecular rotation (RIR) inherent from the chemical structures of the luminescent materials.
To verify the effect of molecular structure on the AIE properties of organic and polymeric materials, four approaches were attempted in this research.
(I) Aggregation-Induced Emission in Tetraphenylthiophene-Derived Organic Molecules and Vinyl Polymer
Organic molecules of tetraphenylthiophene (TP) and the derived model compound of TP-Qu and vinyl polymer of PS-Qu with the pendant group of TP-Qu were prepared and characterized to identify their photoluminescent responses toward the effect of AIE. During aggregate formation, the corresponding TP solutions greatly gained the emission intensity. In contrast, TP-Qu and PS-Qu in isolated or aggregated states emitted strongly with nearly the same emission intensity. RIR is the key factor deciding the AIE effect in different states. With four small phenyl rotors around the central thiophene stator, the RIR of the TP molecules in dilute solution is low but increases upon aggregate formations. In contrast, the bulky C-2 quinoline rotor of the TP-Qu molecule enhances the RIR in isolated state. With the inherent TP-Qu pendant groups, the emissive behavior of vinyl polymer PS-Qu is similar to the TP-Qu molecule.
(II) Aggregation-Induced Emission Enhancement of Diblock Copolymer Containing Tetraphenylthiophene-Quinoline Pendant Fluorphores by Selective Solvent Pairs
In this study, diblock copolymer of PSQu-PBS containing 25 mol% of fluorescent PSQu segments was synthesized and its aggregation-induced emission enhancement (AIEE) behavior was characterized and compared to PSQu homopolymer with 100 mol% of fluorescent units. With fewer (25 %) fluorescent units, solutions of diblock PSQu-PBS copolymer actually have higher (or comparable) emission intensities than the homopolymer PSQu solutions. Solutions of PSQu-PBS in THF/H2O of varied compositions emit essentially with the same intensity but in contrast, emissions of PSQu-PBS in THF/hexane increase with the increasing hexane content. Copolymer micelles formed in THF/hexane mixtures are supposed to have higher extent of aggregation, leading to more pronounced AIEE effect than micelles formed in THF/H2O.
(III) Tetraphenylthiophene-Functionalized Poly(N-isopropylacrylamide): Probing LCST with Aggregation-Induced Emission
A hydrophobic TP center with novel AIE property was chemically linked to two poly(N-isopropylacrylamide) (PNIPAM) chains to obtain thermoresponsive polymers to study the relationships between the lower critical solution transitions (LCSTs) and the AIE-operative fluorenscence emission. Three ethynyl-terminated PNIPAMs with different molecular weights were synthesized via controlled atom transfer radical polymerization (ATRP) using ethynyl-functionalized initiator. The PNIPAMs were then coupled with diazide-funtionalized TP (TPN3) via click reaction to obtain the desired TP-embedded polymers of Px (x = 1, 2, and 3). All three polymers show AIE-property from their solution fluorescence behavior in THF/hexane mixtures. In the aqueous solution, the TP-center served as a fluorogenic probe that reveals the LCSTs of polymers and its relation to the degree of TP labeling in terms of polymer concentration. The thermoresponsiveness of Px was demonstrated by the complete emission quench when heated at temperatures above LCST. Dissociation of the TP aggregates above LCST is responsible for the emission quench.
(IV) Influence of Molecular Weight on the Aggregation-Induced Emission of Vinyl Polymers Containing the Fluorescent 2,4,6-Triphenylpyridine Pendant Groups
Molecular weight effect on the AIEE property of vinyl polymers containing fluorescent 2,4,6-triphenylpyridine (TPP) pendant groups was evaluated in the fourth topic. The high and low Mw vinyl polymers of PDMPS¡VL and ¡VH were prepared through Click chemistry between azide¡VTPP derivative and acetylene¡Vfunctionalized polystyrenes. Solutions of the low Mw PDMPS¡VL exhibited the normal AIEE effect with continuous emission gains with increasing extent of aggregation upon nonsolvent inclusion. On the contrast, the high Mw PDMPS¡VH solutions emitted with constant intensity on all solutions with different extent of aggregation. Despite the varied solution behavior, the solid PDMPS-L and ¡VH films are all strong deep-blue emitter with high quantum yields of 84 and 82.5%, respectively. The emission behavior was explained by the conformational difference between the PDMPS¡VL and ¡VH chains, which were approached by computer simulation in this topic.
| Identifer | oai:union.ndltd.org:NSYSU/oai:NSYSU:etd-0201112-104746 |
| Date | 01 February 2012 |
| Creators | Lai, Chung-Tin |
| Contributors | Ming Chen, Jin-Long Hong, Wen-Chung Wu, Steve Lien-Chung Hsu, Yeo-Wan Chiang, Shiao-Wei Kuo |
| Publisher | NSYSU |
| Source Sets | NSYSU Electronic Thesis and Dissertation Archive |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.lib.nsysu.edu.tw/ETD-db/ETD-search/view_etd?URN=etd-0201112-104746 |
| Rights | user_define, Copyright information available at source archive |
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