Aggregation induced emission macromolecules: structures, properties and their supramolecular applications

Rananaware, A 2017, Aggregation induced emission macromolecules: structures, properties and their supramolecular applications, Doctor of Philosophy (PhD), Science, RMIT University.


Document type: Thesis
Collection: Theses

Title Aggregation induced emission macromolecules: structures, properties and their supramolecular applications
Author(s) Rananaware, A
Year 2017
Abstract Luminescence is the process of light emission by luminescent materials. In recent years, the
development of luminescent materials gained a lot of attention due to their promising applications in
various fields such as electroluminescent devices including organic light emitting diodes (OLEDs),
organic field effect transistors (OFETs), sensors, and so on. However, these luminescent materials
were suffered from the fluorescence quenching in their solid state. The conventional luminophores
shows stronger light emission in the solution state and emission becomes weaker in the condensed
phase i.e. in thin solid films or aggregate state. Simply, the light emissions of these traditional
molecules were suffered from aggregation of molecules. This process of fluorescence quenching was
referred as Aggregation Caused Quenching (ACQ). The ACQ effect of many luminogens was creating
obstacles for these luminophores while applying in the real-world applications in solid state or
aqueous phase, as they show aggregation in these phases. Multiple processes have been discovered
including physical, chemical and engineering attempts to alleviate the problem of fluorescence
quenching upon aggregation however only limited success was achieved. The development of such
luminogen which will be able to show the enhanced emission upon aggregation and in the solid state
was suggested the solution for emission quenching effect (ACQ) of common luminogens.

Later, in 2001, the phenomenon of Aggregation Induced Emission (AIE) was reported by Tang group
which is exactly opposite to the earlier ACQ effect. In this AIE process, the luminophores showed
enhanced emission in the solid and aggregate state and their emission is reduced in the dilute solution
studies. The 1,1,2,3,4,5-hexaphenyl-1H-silole (HPS) molecule is the first AIE gen discovered which
showed turn “off” and “on” emission in the solution and solid state. It was found that the restriction
of intramolecular motions including rotations and vibrations causes the molecule to be emissive in the
aggregate state. The invention of AIE and understanding of its mechanism plays positive role in
presenting a new platform to investigate the functionally useful luminescent materials for real world
applications. The variety of luminogens (AIEgens) with high emission efficiency (quantum yield) in the
condensed phase have been developed and becomes the promising candidates for many
technological applications such as fluorescent sensors, OLEDs, solid lasers, fluorescence gels, biomarkers,
etc.

Among these AIEgens, Tetraphenylethylene (TPE) and its derivatives are extensively studied and have
gained tremendous attention due to its characteristics features as compared to the rest of AIEgens.
TPE is easy to synthesize, shows bright emission in the solid state and non-emissive in the solution. It
has very simple, stable molecular structure and is readily accessible for structural modification.
Moreover, it gives high quantum efficiency, also useful for solving the problem of ACQ effect of many
common luminogen and behaves as mechanochromic luminescent material in the solid state.
Considering all factors in mind, many AIE active TPE luminogens have been introduced for multiple
applications such as chemo sensors, bio-probes, solid-state emitters, luminescent gel, etc. In this body
of work, the synthesis of several aggregations induced emission macro-molecules especially
tetraphenylethylene derivatives, their properties and various supramolecular applications such as pH
sensors, multifunctional mechanochromic luminescent probe, artificial light harvesting antenna, an
AIE activity of the sterically hindered system, AIE active NDIs and BHJ solar cells, have been presented.
TPE derivatives have been employed as fluorescent sensors for detection of metal ions such as Cu2+,
Hg2+, Ag+, Cr3+, Al3+ and so on.

However, a TPE based reversible fluorescent pH sensor for a solution and live cells is rarely reported.
The synthesis of TPE-pyridyl derivative and its utilization for pH detection in the solution and live cells were
investigated. This derivative undergoes protonation in acidic medium and deprotonation in the basic medium
which makes it a reversible probe as pH sensor. The process of reversibility of protonation and deprotonation
process in a different medium is confirmed by a colour change, UV-Vis, fluorescence study and self-assembly
of protonated and unprotonated species. After treating this dye in PC-3 cells, cells showed green fluorescence
at acidic pH and blue at neutral and basic pH. Furthermore, the substituted derivative of Py-TPE salt has been
shown the mechanochromism property in the solid state. When it was used for cellular imaging, DNA
marking and cell-cycle analysis, it showed better results/performance as compared to commercially
available and commonly used dyes.

Photosynthesis is the vital process in plants which involves the production of chemical energy from
CO2 and H2O in the presence of sunlight where LH2 complex plays an important role as it involves in
the absorption, storage and transfer of light energy to the reaction centre. This natural harvesting
complex has commonly explored the model in making artificial light harvesting systems where
porphyrins are arranged in the ring of turbines and involves in the light transportation process. Taking
advantage, the next derivative has been designed to mimic the nature using TPE and porphyrin,
synthesized and utilised as an artificial light harvesting antenna. The SEM and TEM results confirm the
formation of ring shaped morphology reminiscent with LH2 complex and time resolved absorption
spectroscopy results confirm the absorption and energy transfer process by the derivative.

So far, the developed AIE active derivatives are simple in structure, however, the AIE activity in the
stearically hindered system is rarely reported. Moreover, the AIEgens with mechanochromic
properties is extensively studied and widely shown the application in smart materials such as security
papers, storage device, etc. However, very few TPE based derivatives have shown the
mechanochromism behaviour. The HTCA, a stearically hindered AIEgen have been developed and its
AIE activity, mechanochromic properties are investigated. This derivative is highly luminescent in the
aggregate state, and its mechanochromic properties make it a promising candidate for smart materials
applications.

The well-known naphthalene diimide (NDI) is extensively studied and widely gained attention due to
its electro-optical properties and applications in the numerous field. However, it severely suffered
from luminescence quenching in the solid state which results in the limitation of application of NDI
derivatives as luminescent materials in different areas. To convert the ACQ effect of NDIs into AIE,
tetraphenylethylene have been incorporated on to the NDI core. The mono, di and tetra-substitution
of TPE on NDI core converted ACQ active moiety into AIE active moiety and shown that it can used as
luminescent material in the solid state. Furthermore, the application of TPE derivative in solar cell
devices has been studied. TPE based four-directional, non-fullerene acceptor; 4D has been developed
for solution processable BHJ device. 4D was prepared by simple reaction between Br4-TPE and DPPboronate
ester using Suzuki conditions. High solubility in organic solvents, thermal stability,
absorption in the visible region, red-shift in emission, and comparable energy level with P3HT makes
it a promising candidate for BHJ device.

The device prepared using 4D as an acceptor with P3HT (1:1.2) gives an excellent high voltage current
Voc 1.18 V and recorded a PCE of 3.86%. Overall, this thesis provided the knowledge and further applications
of TPE derivatives in different fields. To my knowledge, the derivatives synthesised and shown here are the first
examples in the literature of their possible applications.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Science
Subjects Optical Properties of Materials
Organic Chemical Synthesis
Nanochemistry and Supramolecular Chemistry
Keyword(s) Aggregation induced emission
AIEgens
Tetraphenylethylene derivatives
TPE based solar cells
Artificial light harvesting antenna
pH sensor
Multifunctional bioprobe, DNA marker
Naphthalene diimide
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