INTRODUCTION
Background of pesticides detection
Pesticides play a crucial role in agricultural production, with their demand rising due to population growth and the need for enhanced agricultural productivity These chemical ingredients effectively manage pests and promote plant growth, significantly contributing to higher yields However, they also pose serious food safety concerns, responsible for over 200,000 deaths annually The health risks linked to pesticide consumption are increasingly evident, particularly in developing countries where farmers often lack adequate knowledge and understanding of safe pesticide use.
Nitro-aromatic compound (NAC) pesticides, such as parathion, nitrofen, fenitrothion, and mesotrione, are widely used in agriculture to protect crops but are significant contributors to environmental pollution due to their high toxicity and biological activity These pesticides can adversely affect humans, animals, and other organisms if misapplied, highlighting the importance of monitoring pesticide residues to mitigate risks Advances in detection methods, particularly traditional chromatographic techniques like gas chromatography (GC), have significantly improved our ability to identify these harmful substances.
High-performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS) are widely used methods for detecting pesticide residues, alongside techniques like electrochemical analysis and AChE-based biosensors While chromatographic methods offer high sensitivity and accuracy, they also entail significant equipment costs, complex operations, and limited portability Therefore, developing a new detection method that is fast, sensitive, reliable, and cost-effective is crucial for timely pesticide residue management.
No Names Structure WHO Acute Hazard
Ia = Extremely hazardous; II = Moderately hazardous; III = slightly hazardous; U Unlikely to present acute hazard in normal use; O = Obsolete as pesticide, not classified.
Fluorescence sensor for the detection of nitroaromatic pesticides
In recent years, fluorescence-based methods have gained popularity for their efficiency, simplicity, speed, and reliability These techniques utilize colorimetric and fluorimetric responses, with a significant focus on advanced fluorescence sensors Additionally, optical sensors that rely on photoluminescence (PL) quenching have garnered considerable interest due to their high sensitivity, affordability, and ease of use.
Fig 1-1: Schematic illustration of the working principle of the fluorescence quenching based sensor for NACs pesticide detection
In 2009, Mallard-Favier et al synthesized a novel peracetylated cyclodextrin trimer featuring three 1,2,3-triazole linkers, which demonstrated significant variations in fluorescence emission upon the addition of pendimethalin, achieving exceptionally low detection limits ranging from 0.8 to 4 aM.
In 2014, Kumar et al [11] synthesized a luminescent nanocrystal metal-organic framework (NMOF1) for chemosensing of the nitroaromatic-containing organophosphate pesticides such as parathion, methyl parathion, paraoxon and fenitrothion
In 2018, Hergert et al demonstrated the molecular chain effect in phenylene ethynylene oligomers for insecticide detection, revealing a notable increase in Stern-Volmer quenching that surpasses the molecular wire effect.
In 2018, Zhao et al developed a sensitive and selective pyranine-based sensor for detecting paraquat This innovative method operates effectively under optimized conditions, making it suitable for real-world sample analysis.
In 2019, Sun et al developed a molecularly imprinted polymer (MIP)-based fluorescent probe for the rapid and sensitive detection of mesotrione This innovative probe utilized biomass-derived carbon quantum dots (CQDs) encapsulated within MIPs through a sol-gel method, leveraging the CQDs@MIPs' ability to capture mesotrione and effectively quench fluorescence.
In 2019, Hu et al developed a colorimetric chemosensor that enables the rapid detection of dimethoate pesticides in agricultural products by utilizing the inhibition of gold nanoparticles' peroxidase-like activity Optical chemosensing methods employing functional materials and nanomaterials, including conjugated molecules, oligomers, polymers, macrocycles, and luminescent metal-organic frameworks, have demonstrated significant potential for the sensitive and selective detection of pesticides.
In 2020, Zhang et al [16] discovered that a carbazole-containing polymer, characterized by its significant inherent porosity, demonstrated notable fluorescence quenching when the concentration of trifluralin was increased, achieving a quenching level of up to 84%.
Thesis Objective
The functionalization of conjugated fluorophores and the understanding of molecular interactions between electron donors and acceptors are crucial for developing efficient fluorescence sensors This thesis aims to explore the use of conjugated molecular fluorescence as a chemosensor for detecting nitroaromatic pesticides The design focuses on pyrene derivatives combined with 4-(2-ethylhexyl)-4h-dithieno[3,2-b:2',3'-d]pyrrole or 9,9-dioctyl-9H-fluorene These innovative fluorescent chemosensors are anticipated to enhance our understanding of detection mechanisms, paving the way for the advancement of more effective fluorescent chemosensors in the future.
LITERATURE REVIEW
Fluorescence quenching theory
Fluorescence sensors operate on the principle that molecular interactions between analytes and fluorophores can either reduce fluorescence through quenching or enhance it by inhibiting the quenching effect These interactions may occur due to complex formation (static quenching), diffusive encounters (dynamic quenching), energy transfer, or photoinduced electron transfer (PET).
Fig 2-1: Quenching mechanisms of fluorescent sensor which is used in the process of detecting analytes And the effect of temperature on the effectiveness of dynamic (a) and static (b) quenching [17]
2.1.1 Static and Dynamic quenching mechanism
Static quenching happens when a non-emissive ground state is created through the interaction between a sensor and a quencher, resulting in the complex returning to the ground state without emitting photons, which decreases the initial fluorescence intensity In contrast, dynamic quenching involves excited-state electron transfer from the sensor to the quencher via collision, utilizing mechanisms of energy transfer or charge transfer.
The two quenching mechanisms exhibit distinct characteristics, which can be identified through time-resolved fluorescence decay measurements of the sensing materials In the case of static quenching, the fluorescence decay lifetime of the material remains constant, even as the concentration of the quencher increases.
The quenching of fluorescence occurs through the formation of a non-emissive fluorophore-quencher complex, where unbound molecules decay naturally In dynamic quenching, the process requires a collision between the quencher and the excited fluorescent sensor, making it diffusion-controlled Quenching happens when a photoexcited molecule briefly interacts with a colliding analyte, leading to a reduction in the average fluorescence lifetime as the quencher concentration increases To determine whether quenching is static or dynamic, the change in fluorescence lifetime in the presence and absence of quenchers is measured.
Dynamic quenching of fluorescence is described by the SternẻVolmer equation and the correlation of lifetime with quencher concentration can be expressed as:
酵 噺 な 髪 計 帖 岷芸峅""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""岫な岻
In fluorescence studies, 繋待 represents the intensity in the absence of a quencher, while 繋 denotes the intensity when a quencher is present Additionally, 酵 待 refers to the fluorescence lifetime of sensors before the addition of a quencher, and 酵 indicates the lifetime after the quencher is introduced at a specific concentration [Q] The dynamic Stern-Volmer quenching constant, 計 帖, is crucial for understanding the quenching process.
Static quenching involves analyzing how fluorescence intensity varies with quencher concentration, which can be understood through the association constant for complex formation This relationship is mathematically represented by a specific equation.
繋 噺 な 髪 計 聴 岷芸峅"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""岫ぬ岻 Where 計 聴 is the SternẻVolmer constant for the static quenching process
Now, for steady-state fluorescence quenching involving both collisional and static quenching, the quenching process is examined by Equation (4):
繋 噺 岫な 髪 計 帖 岷芸峅岻岫な 髪 計 聴 岷芸峅岻"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""岫ね岻
""""""噺 な 髪 岫計 帖 髪 計 聴 岻岷芸峅 髪 計 帖 計 聴 岷芸峅 態
At very low analyte concentrations, the relationship is linear due to minimal contribution from the quadratic term However, as concentrations increase, the plot begins to deviate from linearity, exhibiting an upward bend This behavior is effectively described by the Stern-Volmer equation, which can show an upward curvature at high quencher concentrations or fit an exponential model under certain conditions.
Static and dynamic quenching can be differentiated by their temperature and viscosity dependence Higher temperatures enhance diffusion, increasing dynamic quenching, while they typically promote the dissociation of weakly bound complexes in static quenching, leading to fewer non-emissive fluorophore-quencher complexes Analyzing the absorption spectra of the sensor provides another method for distinguishing between the two; dynamic quenching does not alter the absorption spectra since it only affects excited states, while ground-state complex formation in static quenching often disturbs the absorption spectrum Additionally, the bimodular quenching constant (kq), calculated as the ratio of the Stern-Volmer quenching constant (KSV) to the unquenched fluorescence lifetime (v0), serves as a discriminative measure, with dynamic quenching typically yielding kq values around 10^10 M^-1.
1 s -1 ; while for static quenching, the kq value is generally several orders larger than 10 10
Static quenching is more effective than dynamic quenching in pesticide detection because it offers a higher binding constant (KSV) for quenchers interacting with various fluorophore indicators, resulting in greater sensitivity While dynamic quenching fluorescent sensors can provide quicker and more reversible detection, they are limited by a significantly smaller KSV and lower overall sensitivity.
The energy transfer mechanism plays a crucial role in the development of various sensors, significantly enhancing fluorescence-quenching efficiency and sensitivity This mechanism encompasses three main types: Förster resonance energy transfer (FRET), Dexter energy transfer (DET), and Surface energy transfer (SET).
FRET, or Förster resonance energy transfer, is a phenomenon discovered by a German scientist in 1948, where the photonic energy from a donor fluorophore is transferred to an acceptor fluorophore, resulting in emission from the latter This energy transfer occurs without the release of a photon, facilitated by long-range dipole-dipole interactions between the fluorophore and quencher, typically over distances of 10 to 100 Å.
FRET, or Förster Resonance Energy Transfer, is an electrodynamic phenomenon explained by classical physics, occurring between an excited-state fluorophore and a ground-state quencher when their emission and absorption spectra overlap This process involves long-range dipole-dipole interactions and does not emit photons The efficiency of energy transfer is influenced by three key factors: the relative orientation of the donor and acceptor dipoles, the degree of spectral overlap between the donor's emission and the acceptor's absorption, and the distance separating the two molecules The probability of resonance energy transfer is directly related to the extent of this spectral overlap.
The effect of DET is based on electron transfer, not photon transfer and therefore requires a match between the redox potentials of donor and acceptor [17]
SET, or Surface Electron Transfer, is a relatively recent phenomenon primarily associated with metal nanoparticles, particularly those with metallic surfaces like gold nanoparticles, interacting with organic molecular dipoles The theoretical foundation for SET was laid in 1978 by R Chance and colleagues, with experimental validation occurring in the 2000s.
Photoinduced electron transfer (PET) involves the transfer of electrons between a sensor and a quencher, resulting in the formation of cation and anion radicals During this process, a complex forms between the electron donor and acceptor, which can return to the ground state without photon emission, although exciplex emission may occur in some instances PET is crucial in the fluorescence quenching process and offers valuable insights for the advancement of fluorescence sensors.
Photoinduced electron transfer (PET) can be categorized into reductive PET and oxidative PET In reductive PET, the sensor acts as an electron receptor, receiving electrons from an electron donor, driven by the energy gap between the lowest unoccupied molecular orbitals (LUMO) of the quencher and the highest occupied molecular orbitals (HOMO) of the sensor Conversely, oxidative PET involves the sensor donating electrons, with the driving force being the energy gap between the sensor's LUMO and the quencher's LUMO The presence of these energy gaps—whether between LUMO and HOMO or LUMO and LUMO—indicates that the quenching mechanism is indeed PET.
Fluorescent materials for chemosensor
Fluorescence-based sensors are highly sensitive and simplified tools widely used in diverse fields such as biomedical diagnosis, environmental monitoring, and food safety These sensors allow for signal changes to be easily detected using spectrofluorometers or even with the naked eye Recent advancements have focused on developing new fluorescent materials to enhance sensitivity, selectivity, and response times Various materials, including small fluorophores, conjugated polymers, supramolecular systems, aggregation-induced emission-active materials, and bio-inspired materials, have been utilized in the creation of innovative fluorescence sensing platforms.
Small fluorophores are essential in the development of fluorescence-based sensors due to their easy synthesis and diverse fluorescence quenching pathways Unlike conjugated polymeric systems, small molecule detection relies on different quenching mechanisms, primarily dynamic quenching, whereas polymer-based sensors often utilize static quenching Conducting polymers demonstrate higher quenching efficiency, as multiple excitons can be quenched by a single analyte molecule, while small molecules typically exhibit a one-to-one quenching ratio Recently, functional small molecule fluorophores, including oligofluorophores and self-assembling variants, have gained attention for their potential to enhance fluorescence sensing through improved binding and exciton migration capabilities.
Fig 2-2: The schematic illustration of molecular wire theory
In 2010, Lee et al introduced a dipyrene-appended calix[4]arene featuring two pyrene substituents aligned on the same side The study revealed that both monomer (375 nm) and excimer (470 nm) emissions were significantly quenched upon the introduction of TNT, a member of the nitroaromatic compounds family, in acetonitrile, achieving a detection limit as low as 1.1 nM.
In 2016, Z Zhong and colleagues developed an innovative irreversible fluorescent sensor, benzimidazo[2,1-a]benz[de]isoquinoline-7-one-13-(N-butylthioamide), specifically designed for detecting Hg 2+ This sensor demonstrated excellent selectivity and sensitivity through a unique Hg 2+-promoted selective desulfurization reaction, resulting in a remarkable 40-fold increase in fluorescent intensity exclusively in the presence of Hg 2+.
Fig 2-3: Fluorescence enhancement of sensor sensor benzimidazo[2,1-a] benz[de] isoquinoline-7-one-13-(N-butylthioamide) by reaction with Hg 2+
In 2018, Hergert et al reported on the synthesis and quenching behavior of water-soluble carboxylate-carrying phenylene ethynylene oligomers, ranging from monomer to tetramer, along with their polymers The study investigated their quenching behavior with various test analytes, including paraquat, mercury salts, and picric acid, in water, comparing the results to those of the conjugated polymer The findings revealed that for monovalent quenchers, only the molecular wire effect is relevant, while for divalent quenchers, multivalency effects play a significant role.
This thesis focuses on utilizing a functional small molecule to detect NACs pesticides, particularly triad molecules Research indicates that fluorescence quenching intensifies with the molecular weight of conjugated monomers, which can be explained by the molecular wire theory that highlights the increased diffusion length of excitons.
To improve exciton diffusion in simple small molecules, triad-system fluorophores have been developed and studied These triad systems consist of three units of small molecule fluorophores, or their fundamental building blocks, which endow them with distinctive properties due to their molecular structures.
Conjugated polymers (CPs) have emerged as effective tools for detecting nitrated fluorescence due to their extended exciton migration pathways and efficient electronic communication between quenchers along the polymer backbone Unlike small molecule fluorophores, CPs serve as excellent electron donors, with their donor capabilities enhanced by delocalized r* excited states that promote exciton migration and strengthen interactions with electron-deficient nitroaromatic analytes Swager et al demonstrated that in CP fluorescent sensors, the binding of a single receptor site can lead to significant quenching of all emitting units across the entire polymer molecule, outperforming single molecule systems.
In 2007, Aditya Narayanan and colleagues discovered that two conjugated polymers, P1 and P2, exhibit strong multiphoton absorption characteristics These polymers demonstrate exceptional sensitivity in detecting nitroaromatic compounds through multiphoton excited fluorescence.
Fig 2-4: Two conjugated polymers P1 and P2 with three-dimensional iptycene units
In 2013, F Chu, G Tsiminis, N.A Spooner, and T.M Monro introduced a groundbreaking fluorescent conjugated polymer sensor using poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) to detect liquid nitroaromatic compounds This innovative technique enabled the identification of 1,4-dinitrobenzene (DNB), an explosive nitroaromatic, in acetone at concentrations as low as 6.3 ppm within a minimal sampling volume of 27 nL The sensor quantifies DNB concentration through fluorescence decay lifetime analysis, achieving results in just a few minutes.
In 2016, Talbert, Levine, and colleagues discovered that PFBO-derived nanoparticles exhibited significant fluorescence enhancement when exposed to aromatic organochlorine pesticides This phenomenon enables the development of reversible pesticide detection systems, demonstrating notable generality and ease of reversibility.
Fig 2-5: 2,1,3-benzooxadiazole-alt-fluorene (PFBO) structure
Covalent polymers (CPs) offer high sensitivity for fluorescence detection, making them effective chemosensors across various environments, including aqueous, solid-state, and vapor phases However, their complex synthesis and purification processes pose significant challenges, limiting their practical application in real-world scenarios.
Supramolecular systems emphasize the importance of non-covalent interactions among molecules, which facilitate molecular recognition and self-assembly Key examples of these systems are macrocycles, dendrimers, supramolecular polymers, and metal-organic frameworks, with dendrimers and metal-organic frameworks being particularly noteworthy for their unique structural and functional properties.
(MOFs) are especially interesting and have a great potential to be applied in fluorescence sensors [19, 46]
In 2011, Shaw and co-workers reported on three generations of fluorescent carbazole dendrimers with spirobifluorene cores which interacted with NACs compound primarily via fluorescence quenching (Fig 1-7) [47]
Fig 2-6: Structures of dendrimers for three generations defined as G1ẻG3
2.2.4 Aggregation induced emission (AIE)- active materials and bio-inspired fluorescent materials
Aggregation induced emission (AIE)-active fluorescent materials have emerged as a significant area of research, presenting a phenomenon that contrasts with aggregation caused quenching In AIE materials, small molecules like tetraphenylethene (TPE), triphenylbenzene (TPB), and siloles exhibit efficient emission when aggregated These AIE-active polymers remain nearly non-luminescent in good solvents but emit strong fluorescence when aggregated in poor solvents or formed into thin solid films, making them effective as fluorescence sensors.
Fig 2-7: Small molecules for AIE materials such as tetraphenylethene (TPE), triphenylbenzene (TPB) and siloles
In recent years, bio-based materials like peptides, proteins, and DNA have gained traction for fluorescence-based detection, serving dual functions Firstly, these biomaterials can act as supportive layers, while secondly, their intrinsic fluorescent properties, particularly from tryptophan, tyrosine, and phenylalanine residues, are utilized in pesticide sensing The fluorescence of proteins, primarily attributed to tryptophan, can be effectively quenched by various agents, making them valuable tools for pesticide detection.
Molecular design of fluorophores
Understanding the relationship between structure and properties is essential for designing high-performance chemosensors, which helps in recognizing their nature and selecting ideal materials for applications Organic compounds such as pyrene, fluorene, dithienopyrrole, and carbazole exhibit unique electroconducting and fluorescence properties, making them suitable for various applications Improving the binding ability of these compounds to analytes remains an effective strategy for enhancing sensor performance.
Pyrene derivatives are popular fluorescent probes due to their high quantum yield, extensive electron-rich conjugated planes, and chemical stability in molecular labeling, making them effective for fluorescent sensing At dilute concentrations (e.g., < 0.001 M), they exhibit fluorescence peaks around 592-622 nm, which shift bathochromically to the visible range (642-822 nm) as dye concentration increases Both pyrene monomer and excimer emissions can be quenched by electron-deficient nitroaromatic compounds (NACs), with the planar structure of pyrene facilitating strong π-π stacking interactions with NACs A theoretical model suggests that the quenching mechanism involves the formation of excited fluorophore-quencher ion pairs, followed by electron transfer from pyrene to NACs.
The primary challenge with pyrene is its low fluorescence efficiency, which is attributed to the strong tendency of the fluorophore to aggregate at high concentrations or in pure crystal form To address this issue, researchers have introduced various peripheral attachments to the pyrene core, which induce structural twisting and inhibit excimer formation, leading to significant enhancements in photoluminescence properties, particularly in fluorescence quantum yield Despite these advancements, there is a limited number of studies focused on the development of novel pyrene-based compounds for detecting nitroaromatic compounds, as well as theoretical investigations into the underlying reaction mechanisms.
In 2009, Chen and colleagues conducted a study on the use of pyrene-functionalized ruthenium (Ru) nanoparticles for detecting NACs The metallic Ru core acted as a conductive medium, facilitating significant intraparticle charge delocalization, which resulted in a quenching efficiency over an order of magnitude greater than that of an equivalent concentration of 1-bromopyrene in DMF solution.
In 2011, Zhiqiang Wang and colleagues developed two innovative pyrene derivatives, 1,6-bis(3,5-diphenylphenyl)pyrene (BDPP) and 1,6-bis(2-naphthyl)pyrene (BNP), demonstrating their high quantum yields The findings suggest that BDPP exhibits exceptional efficiency and stability, making it a promising emitting material for non-doped deep-blue OLEDs.
In 2012, Nakorn Niamnont and colleagues developed novel tunable star-shaped triphenylamine fluorophores, incorporating pyrene (TEP, TAP) and corannulene (TEC, TAC), which exhibit varying fluorescence quenching sensitivity to nitro explosives The most sensitive fluorophore demonstrated the ability to detect TNT at the ng cm -2 scale, making this array valuable for the identification of nitroaromatic compounds (NACs).
Fig 2-8: (top) Structures of fluorophores; (bottom) HOMO-LUMO of TAP and TEP
In 2016, Sharad Chandrakant Deshmukh and his team developed an innovative amphiphilic pyrene derivative that enables selective sensing of metal ions and nitro explosives This compound facilitates efficient switching between excimer and monomer emissions, ensuring strong and stable excimer formation in aqueous environments.
Sensing applications of the excimer in the aqueous media were demonstrated in the present work through detections of metal ions and nitro explosives [68]
In 2021, Kovalev, I S and colleagues reported advancements in the tuning of pyrene-based chemosensors by modifying their structure or altering their environment They achieved impressive Stern-Volmer quenching constants, with values reaching as high as 2.28 × 10^4 M^-1 for structural modifications and 4.67 × 10^5 M^-1 for environmental changes.
Recent studies highlight the promising spectroscopic, photophysical, and thermal stability properties of fluorene and dithienopyrrole derivatives, making them ideal candidates for fluorescence sensors These organic fluorophores are favored due to their structural planarity, extended conjugation, and optimal HOMO-LUMO energy gap, coupled with strong intermolecular interactions.
This thesis focuses on the development of two pyrene-based compounds designed for the detection of nitroaromatic pesticides We synthesized two conjugated molecules that integrate pyrene with fluorene or dithienopyrrole groups, following established literature methods.
Overview of thesis
The research content of my thesis is divided into three parts namely Chapter 3, Chapter 4 and Chapter 5
Chapter 3 mainly focuses on the stages involved in the synthesis of novel conjugated monomers as well as the analytical methods used to analyze the chemical and optical properties of materials including: proton nuclear magnetic resonance spectroscopy ( 1 H NMR), infrared spectroscopy (FTIR), UV-visible absorption spectroscopy, and photoluminescence spectroscopy (PL) The synthesized triads 2PDTP and 2PDOF based on pyrene derivative combined with either 4-(2-ethylhexyl)-4h- dithieno[3,2-b:2',3'-d]pyrrole or 9,9-dioctyl-9H-fluorene, and the structure of monomers is depicted in Fig 2-9 below:
Fig 2-9: Structure of 2PDTP and 2PDOF
Chapter 4 will discuss the issue of synthesis and the results from chemical and optical properties of news monomers:
The synthesized and purified triads 2PDTP and 2PDOF will be characterized using FTIR and NMR spectroscopy to confirm the successful execution of the synthetic reactions and to analyze their chemical structures.
‚ The optical properties of 2PDTP and 2PDOF will be analyzed and investigated through UV-Vis absorption and PL spectroscopy
The study will assess the effectiveness of monomers as fluorescent sensors for detecting NACs pesticides using PL spectroscopy to analyze the materials mixed with the analytes The findings will enhance the understanding of the detection mechanism and the influence of functional groups on the materials' suitability as fluorescent sensors Chapter 5 will serve as the conclusion, summarizing key insights and providing clear answers and recommendations for future research.
EXPERIMENTAL
Materials and Reagents
N-bomosuccinimide (NBS, 99%), pyrene (98%), 3,3'-dibromo-2,2'-bithiophene (98%) and 2-ethylhexylamine (98%) were purchased from Acros Organic Mesotrione (98%), 4.4Ó-(9,9-dioctyl-9H-fluorene-2,7-dityl)bis(4,4,5,5-tetramethyl-1,3,2- dioxaborolane) (DOF, 99%), tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3,
The following chemicals were sourced from Sigma-Aldrich (St Louis): 4.4Ó-bis(diphenylphosphino)-3.3Ó-binaphthyl (BINAP, 98%), sodium tert-butoxide (t-BuONa, 97%), palladium(II) acetate (Pd(OAc)2, 98%), tricyclohexylphosphine tetrafluoroborate (P(Cy)3.HBF4, 97%), tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4, 99%), cesium carbonate (Cs2O3, 99%), and pivalic acid (PivOH, 99%).
MO, USA) Potassium carbonate (K2CO3, 99%) was purchased from Acros and used as received
Chloroform (CHCl3, 99.5%), dimethylformamide (DMF, 99%), and toluene (99.5%) were sourced from Fisher/Acros and dried with molecular sieves under nitrogen Additionally, dichloromethane (DCM, 99.8%), absolute ethanol (99%), and hexane (99%) were also obtained from Fisher/Acros and utilized without further purification.
Analysis and measurement methods
Column chromatography is an effective method for isolating individual chemical compounds from mixtures, relying on the differential adsorption of substances to an adsorbent This technique allows compounds to traverse the column at varying rates, resulting in their separation into distinct fractions Its versatility is highlighted by the use of different adsorbents—whether normal phase, reversed phase, or others—paired with a broad range of solvents Column chromatography can accommodate sample sizes from micrograms to kilograms, making it suitable for various applications A key benefit of this method is its cost-effectiveness and the disposability of the stationary phase, which minimizes the risk of cross-contamination and degradation from recycling The process can be facilitated by gravity or by employing compressed gas to drive the solvent through the column.
Thin-layer chromatography (TLC) is a valuable technique for predicting the behavior of compound mixtures during purification through column chromatography By optimizing the separation process with TLC, researchers can enhance the effectiveness of subsequent column chromatography.
Nuclear magnetic resonance spectroscopy (NMR)
Nuclear magnetic resonance (NMR) spectroscopy is a fundamental tool used by chemists to determine chemical structures through straightforward one-dimensional techniques These methods enable the detection of nuclear spins within a strong magnetic field Analyzing a basic first-order NMR spectrum involves assessing five key features: a) the number of signals, b) the chemical shifts that indicate the electronic environment of the nuclei, c) the multiplicity that reveals the number of neighboring non-equivalent magnetically active nuclei, d) the coupling constant (J), which measures the spacing in Hz between couplings, and e) the signal integral that quantifies the area under the signals.
NMR spectra were obtained using a Bruker Avance AM500 FT-NMR spectrometer at the Vietnam Academy of Science and Technology in Hanoi Chloroform-d (CDCl3) with tetramethylsilane (TMS) as an internal standard (0 ppm) served as the solvent for all molecules Chemical shift values are referenced to the solvent, with CDCl3 set at 7.26 ppm for protons The following abbreviations were employed for NMR signal multiplicity: s for singlet, d for doublet, t for triplet, m for multiplet, and br for broad.
All synthesized monomers were analyzed using 1H NMR spectroscopy to verify their structures For the analysis, the compounds were dissolved in deuterated chloroform, with a sample concentration ranging from 10 to 20 mg/ml for optimal 1H NMR measurements.
Infrared spectra were obtained using FTIR Bruker Tensor 27 at the National Key Laboratory for Polymer and Composite Materials This technique enables the detection of characteristic vibration frequencies of molecules based on their specific vibration modes Infrared spectroscopy is instrumental in identifying various functional groups and serves as valuable complementary information to validate proposed molecular structures.
Ultraviolet (UV) and visible radiation interact with matter, leading to electronic transitions where electrons are promoted from their ground state to higher energy levels The ultraviolet spectrum ranges from 100 to 400 nanometers, while the visible spectrum spans from 380 to 780 nanometers.
In photochemistry, two key terms describe changes in spectral band positions: bathochromic shift and hypsochromic shift A bathochromic shift, also known as red-shifted, occurs when a molecule's absorption spectrum moves to a longer wavelength, indicating lower energy Conversely, a hypsochromic shift, or blue-shifted, refers to the movement of a molecule's absorption spectrum to a shorter wavelength, signifying higher energy.
UV-Visible absorption spectroscopy was conducted using an ASEQ LR1 spectrophotometer at the National Key Laboratory for Polymer and Composite Materials, covering a wavelength range of 250-800 nm The absorption measurements were taken in various solutions based on the specified solvent, focusing on the relationship between absorbance and wavelength in nanometers.
Photoluminescence spectroscopy is an essential method for examining the optical and electronic properties of materials in their excited states, offering insights into recombination and relaxation processes This technique can be explored through two modes: excitation mode and emission mode In the excitation mode, one emission wavelength is fixed while the excitation wavelength is varied to create an excitation spectrum Our focus will be on photoluminescence emission, which involves analyzing the emission spectrum at a constant excitation wavelength Figure 3-1 provides a visual aid to enhance understanding of photoluminescence emission.
Fig 3-1: Jablonski diagram of electronic transitions, absorption spectrum and emission spectrum
When photons are absorbed by a molecule, the electrons become excited and transition to an excited electronic singlet state, resulting in a peak in the absorption spectrum This excitation energy then undergoes internal conformational conversion, dissipating energy as non-radiative relaxation The electrons eventually return to the ground state, emitting the remaining energy as fluorescence, which creates a peak in the emission spectrum The difference between the excitation energy and the emission energy is known as the Stokes shift.
Photoluminescence spectroscopy enables the investigation of charge transfer complexes within oligomers and the photo-induced charge transfer in blends of oligomers and fullerenes (PCBM) As charge transfer and energy transfer are nonradiative processes, photoluminescence measurements can effectively detect quenching or the lack of emission.
Photo luminescence spectra were recorded on an Ocean optics SF-2000 spectrometer over the wavelength range of 350/900 nm at National Key Laboratory for Polymer and Composite Materials.
Synthesis of 4-(2-ethylhexyl)-4h-dithieno[3,2-b:2',3'-d]pyrrole (DTP) [73]
A mixture of Pd2(dba)3 (94.25 mg, 0.10 mmol), BINAP (256.36 mg, 0.41 mmol), and t-BuONa (725.36 mg, 7.55 mmol) in 8 mL of toluene was prepared in a two-neck round-bottom flask under nitrogen To this solution, 3,3'-dibromo-2,2'-bithiophene (1111.78 mg, 3.43 mmol) and 2-ethylhexylamine (443.44 mg, 3.43 mmol) were added The reaction mixture underwent three freeze-pump-thaw cycles for degassing and was subsequently purged with nitrogen The reaction proceeded at 110 °C for 24 hours under nitrogen atmosphere After cooling to room temperature, the mixture was extracted three times with 20 mL of chloroform, and the combined organic layers were washed with 150 mL of 10% NaCl and dried.
The product was isolated using K2CO3 and filtered, followed by the removal of the solvent through rotary evaporation To purify the product, column chromatography was performed on a silica gel column with hexane as the eluent The final step involved drying the product in a vacuum oven at 50°C, resulting in a yellow viscous liquid with a yield of 56%.
Synthesis of 1-Bromopyrene [74]
In a 100 ml two-neck round bottom flask, 1.00 g (4.94 mmol) of pyrene was dissolved in 20 ml of anhydrous DMF and cooled to 0°C N-bromosuccinimide (0.97 g, 5.43 mmol) was then added slowly to the mixture, which was allowed to warm to room temperature and stirred overnight in the absence of light After the reaction was complete, it was quenched with ice water and extracted using 60 ml of chloroform The organic layer was washed with 150 ml of brine, dried over anhydrous K2CO3, and filtered to isolate the product The solvent was removed via rotary evaporation, and the product was precipitated with cold hexane, yielding a pure white powder The final product was dried in a vacuum oven at 50°C, resulting in a yield of 94%.
Synthesis of 4-(2-ethylhexyl)-2,6-di(pyren-1-yl)-4h-dithieno[3,2-b:2',3'- d]pyrrole (2PDTP)
To a solution of Pd(OAc)2 (6.74 mg, 0.03 mmol), P(Cy)3.HBF4 (22.09 mg, 0.06 mmol), Cs2CO3 (283.46 mg, 0.87 mmol) and PivOH (17.36 mg, 0.17 mmol) in toluene
In a two-neck round bottom flask, 1-bromopyrene (202.43 mg, 0.72 mmol) and DTP (84.53 mg, 0.29 mmol) were added under a nitrogen atmosphere The reaction mixture underwent three freeze-pump-thaw cycles to degas and was then purged with nitrogen The reaction proceeded at 110 °C for 48 hours under nitrogen After cooling to room temperature, the mixture was extracted with chloroform three times (3 × 20 ml), and the combined organic layers were washed with 10% NaCl (150 ml) and dried.
K2CO3 was isolated through filtration, followed by rotary evaporation of the solvent The resulting products were purified using column chromatography on a silica gel column with a hexane and DCM mixture (V/V = 40:1) as the eluent The final product, an orange powder (2PDTP), was dried in a vacuum oven at 50°C, achieving a yield of 52%.
1H-NMR (500 MHz, CDCl3+."h"*rro+