Comprehensive Overview of Fluorimetry and Fluorescence Mechanisms in Analytical Chemistry

Comprehensive Overview of Fluorimetry and Fluorescence Mechanisms in Analytical Chemistry

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  Presented by: Prof.Samruddhi S. Khonde Assistant Professor P. R. Patil Institute of Pharmacy, Talegaon (S.P.) Fluorimetry Unit I -IMA
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  Fluorimetry Fluorimetry is ananalytical technique based on the principle that when molecules absorb light, they are promoted to an excited state and, upon returning to the ground state, emit light known as fluorescence. In fluorescence, the emitted light has a longer wavelength (lower energy) than the absorbed light, a phenomenon called the Stokes shift. Unlike phosphorescence, fluorescence is instantaneous and ceases when the excitation source is removed. The technique measures fluorescence intensity after excitation at a specific wavelength, using a light source such as a xenon or mercury lamp and detecting the emission with a photodetector. Known for its high sensitivity and selectivity, fluorimetry can detect trace analytes in complex mixtures, with performance influenced by factors like quantum yield, sample purity, and instrument quality. It is widely applied in biochemistry, environmental monitoring, and pharmaceutical analysis for precise and reliable measurements.
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  What is Fluorescence? •Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. • In simpler terms, a material absorbs light at one wavelength and then re-emits light at a longer wavelength. This process involves a molecule absorbing energy, moving to an excited state, and then returning to its ground state by releasing energy as light. • When a molecule absorbs UV/visible light, it is promoted to an excited state and then returns to the ground state by emitting light. The emitted light has a longer wavelength (lower energy) than the absorbed light due to energy loss in the excited state. This difference between absorption and emission wavelengths is called the Stokes shift, named after physicist George Gabriel Stokes. The Stokes shift results from processes like vibrational relaxation and solvent reorganization in the excited state, ensuring fluorescence emission always occurs at a longer wavelength than absorption. Fig: Jablonski diagram explaining the occurrence of fluorescence
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  Types of Luminescence Luminescence- The emission of light by a substance that has absorbed energy, not resulting from heat. Often called “cold light”, it occurs because of electronic transitions within molecules or atoms. Fluorescence - A type of luminescence in which a substance absorbs light (usually UV or visible) and immediately re-emits light of a longer wavelength. The emission ceases almost instantly (within nanoseconds to microseconds) once the excitation source is removed. Ex. - Fluorescent dyes, highlighter inks, and certain minerals glow under UV light. Phosphorescence - A type of luminescence where the emission of light continues for a noticeable time (seconds to hours) after the excitation source is removed. This occurs due to forbidden triplet-to-singlet electronic transitions, which are slower. Ex.- Glow-in-the-dark toys, watch dials, and safety signs that glow after the lights are off. Key differences between Fluorescence and Phosphorescence: Fluorescence stops immediately when the excitation light is switched off; phosphorescence continues for some time. Fluorescence arises from singlet-singlet electronic transitions; phosphorescence involves triplet-singlet transitions which are spin-forbidden, causing longer emission times.
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  Type Description EmissionDuration Example Fluorescence Immediate light emission during excitation; stops as soon as excitation ceases. Nanoseconds to microseconds Fluorescent dyes, minerals Phosphorescence Delayed light emission continuing after excitation source is removed due to forbidden spin transitions. Seconds to hours Glow-in-the-dark toys, watch dials Chemiluminescence Light produced by chemical reactions (without light excitation). Varies Glow sticks Bioluminescence A type of chemiluminescence occurring in living organisms. Varies Fireflies, certain marine animals Electroluminescence Light emission caused by electric current or field. Instantaneous LED lights, OLED displays Thermoluminescence Light emission upon heating previously energized material. Variable Quartz mineral dating Types of Luminescence Luminescence: Emission of light by a substance without involving heat (cold light).
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  • Fluorescence takesplace through a process involving the absorption of light by a molecule, followed by emission of light at a longer wavelength. The steps are as follows: • Excitation: When a molecule absorbs incident photons, electrons are promoted from the ground state to an excited electronic state. This absorption occurs at a specific wavelength characteristic of the molecule. • Vibrational Relaxation: Almost immediately (within nanoseconds), the excited molecule undergoes non-radiative relaxation, losing some energy as heat and settling into the lowest vibrational level of the excited state. Emission: The molecule then returns to the ground state by emitting a photon of lower energy (longer wavelength) than the absorbed photon. This emitted light is the fluorescence. The entire process is rapid and occurs within nanoseconds, and the emitted fluorescence can be detected and measured. The difference in wavelength between absorption and emission (Stokes shift) is due to energy lost during vibrational relaxation Fluorescence Process
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  Mechanism of Fluorescence A.Excitation : When a molecule absorbs incident photons, electrons are promoted from the ground state/ground electronic singlet state (denoted as S0) to an excited electronic state/higher electronic singlet excited state (denoted as Sn) (Sn, where n≥1, commonly S1 or S2). This absorption occurs at a specific wavelength characteristic of the molecule. The electron is excited to a vibrational sublevel of the excited electronic state; these vibrational sublevels arise because each electronic state contains multiple vibrational energy levels. Fluorescence takes place through a process involving the absorption of light by a molecule, followed by emission of light at a longer wavelength. The steps are as follows: Excitation, Vibrational Relaxation and Internal Conversion, Emission (Fluorescence). B. Vibrational Relaxation and Internal Conversion: After excitation, the molecule undergoes vibrational relaxation, a non-radiative process in which the molecule loses excess vibrational energy by transferring it as heat to its surroundings (solvent or lattice). This relaxation is very fast and brings the molecule down to the lowest vibrational level of the first excited singlet state S1. If the electron is initially excited to a higher electronic excited state Sn (where n>1), internal conversion occurs: a non- radiative transition down to the lower excited state S1 without change in spin. Both vibrational relaxation and internal conversion ensure that fluorescence typically starts from this lowest vibrational level of S1.
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  Mechanism of Fluorescence C.Emission (Fluorescence): From the lowest vibrational level of the excited singlet state S1, the electron relaxes back to various vibrational levels of the ground state S0, emitting a photon of light in the process. This photon has less energy (longer wavelength) than the originally absorbed photon, due to the energy lost during vibrational relaxation. This difference in wavelength is known as the Stokes shift. The fluorescence emission is very fast, occurring within nanoseconds. Once the electron returns to the ground state, the molecule is ready to be excited again. Fluorescence emission stops immediately when the excitation source is removed. Fig: Jablonski diagram illustrating the principle of fluorescence with the transition from ground state to higher electronic states
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  • Why electronictransition is necessary? Electronic transitions are necessary because they allow molecules to absorb or emit energy, leading to various physical and chemical changes such as fluorescence, phosphorescence, and photochemical reactions. These transitions enable molecules to move between different energy states, which is fundamental for processes like spectroscopy, energy transfer, and chemical reactivity. ELECTRONIC TRANSITION Electronic transitions are key to fluorescence, involving electrons moving between energy levels within a molecule. The two main types of transitions involved are: •π → π*: An electron from a bonding pi (π) orbital is excited to an antibonding pi-star (π*) orbital, common in conjugated systems . •n → π*: A non-bonding (lone pair) electron, often on heteroatoms like oxygen or nitrogen, is excited to a π* orbital. • When a molecule absorbs light, electrons are promoted to higher energy states. As they return to their original state, they emit photons, producing fluorescence . • The emitted light's wavelength is longer (lower energy) than the absorbed light due to energy losses during relaxation . • These electronic transitions enable molecules to absorb specific wavelengths and emit light, forming the basis of fluorescence spectroscopy.
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  Electron Spin andEnergy Levels • Electrons are fundamental particles that possess an intrinsic property called spin, which can be visualized as a tiny magnetic moment. The spin of an electron can take on one of two possible orientations: up (+1/2) or down (−1/2). • This property is crucial because it influences how electrons occupy atomic or molecular orbitals and how they interact with each other. • In an atom or molecule, electrons occupy specific regions of space called orbitals, which are associated with particular energy levels. These orbitals are arranged in energy hierarchies, and electrons fill them according to principles such as the Pauli exclusion principle and Hund's rule. • Pauli Exclusion Principle: No two electrons in the same atom can have identical quantum numbers; thus, each orbital can hold a maximum of two electrons with opposite spins. • Hund's Rule: Electrons will fill degenerate orbitals singly with parallel spins before pairing occurs, minimizing electron-electron repulsion and stabilizing the configuration.
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  Singlet Electronic States SingletState: • In a singlet state, all electrons within the molecule are paired. This means each electron has an opposite spin to its partner, resulting in no net magnetic moment. The total spin quantum number S in this state is zero because the spins cancel each other out. ▪ The spin multiplicity, which indicates the number of possible orientations of the total spin, is calculated as 2S + 1, giving a value of 1 for singlet states. ▪ These states are typically associated with molecules or atoms where electrons occupy orbitals in pairs, leading to a stable, non-magnetic configuration. ▪ Singlet states are often involved in fluorescence processes, where the molecule returns to the ground state by emitting light without changing its spin configuration. ▪ Example: Benzene in its ground and excited singlet state. • All electrons are paired with opposite spins. • Most ground states (S₀) and first excited states (S₁) are singlet.Total spin quantum number S = 0. • Spin multiplicity = 2S + 1 = 1.
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  Doublet Electronic States DoubletState: • A doublet state occurs when there is one unpaired electron in the molecule or atom. Since this unpaired electron has a spin quantum number S = 1/2, the total spin is half-integer. • The spin multiplicity for a doublet is 2, reflecting the two possible orientations of the unpaired electron's spin (up or down). • Doublet states are common in radicals and certain ions, which exhibit paramagnetic behavior due to the presence of unpaired electrons. • Transitions involving doublet states are important in many spectroscopic techniques, such as Electron Spin Resonance (ESR). One unpaired electron. S = 1/2. Spin multiplicity = 2.
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  Triplet States Triplet State: •In a triplet state, two unpaired electrons occupy different orbitals but have parallel spins (both spin-up or both spin-down). The total spin quantum number S is 1, indicating a higher spin multiplicity. • The spin multiplicity for a triplet state is 3, corresponding to three possible orientations of the total spin (e.g., S_z = -1, 0, +1). • Triplet states are typically lower in energy than singlet states due to exchange interactions and are involved in phosphorescence, where the molecule relaxes from an excited triplet state to the ground state, often emitting light over longer timescale. Two unpaired electrons with parallel spins. S = 1. Spin multiplicity: 2S+1=3 → Triplet
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  Internal And ExternalConversions Internal Conversion (IC) and Intersystem Crossing (ISC) are two important non-radiative processes that influence the fluorescence behavior of molecules. ❑ Internal Conversion (IC): • It involves a transition between electronic states of the same spin multiplicity, such as from the first excited singlet state (S₁) to the ground singlet state (S₀). • This process occurs without the emission of a photon, meaning the excess energy is dissipated as vibrational energy (heat) within the molecule. • IC is facilitated by the overlap of vibrational levels and is typically very rapid, competing directly with fluorescence, which is the radiative emission from S₁ to S₀. ❑ Intersystem Crossing (ISC): ▪ It is a transition between states of different spin multiplicities, for example from the singlet excited state (S₁) to the triplet state (T₁). ▪ Like IC, ISC is a non-radiative process and does not involve photon emission. ▪ It is often slower than IC but still significant, especially in molecules containing heavy atoms that enhance spin-orbit coupling, making ISC more probable.
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  The Jablonski Diagram •Blue arrows (up) — absorption. The molecule jumps from S₀ to S₁ or S₂ almost instantly. • Gray dashed (S₂→S₁) — internal conversion. Silent, heat-only drop to the lowest excited singlet. • Gray dashed (diagonal, S₁→T1) — intersystem crossing. The "spin flip" step, slower and less likely. • Teal arrow (down) — fluorescence. S₁→S₀, fast, releases light. • Coral dashed arrow (down) — phosphorescence. T1→S₀, slow, releases light with a delay — this is your "afterglow."
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  Internal And ExternalConversions ❑ External Conversion (EC)/ external quenching • It is a non-radiative process where the excited molecule transfers its energy directly to the surrounding environment, such as neighbouring molecules or the solvent. • The energy is dissipated as heat, increasing the thermal energy of the surroundings, rather than being emitted as light. • External conversion occurs through collisions or interactions with the environment, facilitating energy transfer away from the molecule. • Factors such as solvent polarity, viscosity, and the molecular environment influence the rate of external conversion. This process competes with fluorescence, reducing the overall emission intensity by providing an alternative pathway for energy dissipation. Fig: Jablonski diagram illustrating possible Internal, intersystem and External conversion
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  Factors Affecting Fluorescence FactorsAffecting Fluorescence •Temperature: An increase in temperature generally decreases fluorescence intensity because higher thermal energy enhances non-radiative decay processes, such as internal conversion and external conversion, which dissipate the excited-state energy as heat rather than light . •Solvent: The polarity of the solvent can affect the electronic environment of the fluorophore, influencing both the fluorescence yield and the emission wavelength. Viscosity impacts the molecular motion; higher viscosity reduces collisional quenching and internal conversion, often resulting in increased fluorescence efficiency .
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  Factors Affecting Fluorescence •pH: Manyfluorophores are sensitive to pH because their ionization state changes with pH, affecting their electronic structure and, consequently, their ability to fluoresce. Fluorescence often occurs only within specific pH ranges where the fluorophore remains in its optimal ionization form . •Concentration: At high concentrations, fluorophores tend to undergo self-quenching due to interactions such as excimer formation or energy transfer between molecules. This leads to a decrease in fluorescence intensity, a phenomenon known as concentration quenching . •Quenchers: Molecules like oxygen (O₂), halides, and heavy metals can reduce fluorescence by facilitating non-radiative energy transfer from the excited fluorophore to the quencher. These quenchers effectively deactivate the excited state, lowering the fluorescence signal.
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  Quenching • Quenching isa process that results in the reduction or complete suppression of fluorescence emission from a fluorophore. It occurs when an excited fluorophore interacts with another molecule or environmental factor that provides a non-radiative pathway for the excited state to relax back to the ground state without emitting a photon. • Quenching is a non radiative energy transfer from excited species to the other molecules • Why does quenching happen? Quenching happens due to various mechanisms, primarily because of interactions between the excited fluorophore and quenchers such as oxygen, metal ions, or other molecules capable of accepting energy or electrons. These interactions facilitate non- radiative decay processes, effectively competing with the fluorescence emission pathway. As a result, the fluorescence intensity decreases, which can be useful in studying molecular interactions or environmental conditions.
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  Quenching • How doesquenching work?/ (types of quenching) ➢Dynamic (Collisional) Quenching ➢Static Quenching ➢Concentration Quenching ➢Chemical Quenching ➢ Dynamic (Collisional) Quenching Occurs when the excited fluorophore collides with a quencher molecule during its excited state lifetime. The collision transfers energy non-radiatively, returning the fluorophore to the ground state without photon emission. The efficiency of dynamic quenching depends on factors like diffusion rates, temperature, and quencher concentration. Halides ions such as iodide, chloride, cause collisional quenching. Ex. Quenching of Quinine sulphate drug by chloride ions.
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  Quenching ➢ Static Quenching: Involvesthe formation of a non-fluorescent complex between the fluorophore and quencher in the ground state. When excited, these complexes do not fluoresce, leading to decreased overall fluorescence. Static quenching is influenced by the binding affinity between the fluorophore and quencher. ➢Concentration Quenching: The concentration quenching is also known as self quenching, concentration quenching occurs when concentrations of fluorescent molecules increases in the sample solution. Fluorescent intensity decreases with increase in the high concentration in the sample.
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  Quenching ➢Chemical Quenching: Chemical quenchingmay take place due to change in Ph, presence of oxygen, presence of halides, electron withdrawing groups and heavy metals. ▪ Change in pH: pH range control is necessary for the molecules to exhibit Fluorescence. Ex., Aniline specifically, exhibit fluorescence in the range of pH 5-13, when excited at wavelength of 230 nm. It does not show fluorescence below pH 5 and above 13. ▪ Presence of oxygen: Presence of oxygen cause oxidation of fluorescent molecules which further results in decrease in fluorescence. ▪ Presence of halides and electron withdrawing groups: Presence of halides like, chloride and iodide often tends to decrease the fluorescence. Presence of electron withdrawing group like, -NO2, -COOH, -CHO, also causes quenching. ▪ Presence of heavy metals: Heavy metals present on the sample always the causes the reduction of the fluorescence because of collision and complex formation. eg., quenching of quinine sulphate in the presence of iodide ions.
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  Instrumentation of Fluorimeter Afluorimeter is an instrument used to measure the fluorescence emitted by a substance when it is excited by a specific wavelength of light. ❖ Light Source: Xenon arc lamp (mostly used), High Pressure Mercury Lamps, Mercury Xenon Arc Lamp, Tungsten – Halogen Lamps, Light emitting diodes (LEDs) ❖ Monochromators(Primary Monochromator): Monochromators are used to disperse white light into the various colors or wavelengths. This dispersion can be accomplished using prisms or diffraction gratings. ❖ Sample Holder ❖ Monochromators (Secondary Monochromator): selects emission wavelength, positioned at 90° ❖ Detectors: Photomultiplier tube (PMT) Detectors is attached at a viewing angle (usually around 90 degrees), which prevents the transmitted or reflected incident light to reach the detector.
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  Fig: The figureshows a fluorescence schematic.
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  Instrumentation of Fluorimeter ❖LightSource: • Provides the excitation light necessary to excite the fluorophores. • Common sources include xenon arc lamps, mercury lamps, and LEDs. • Xenon lamps emit a broad spectrum suitable for various wavelengths, mercury lamps provide specific lines, and LEDs offer monochromatic excitation with high efficiency. • The choice depends on the application and the wavelength range required ❖ Monochromators/Filters: • These components select specific excitation and emission wavelengths. • Monochromators use prisms or diffraction gratings to disperse light and isolate desired wavelengths, while filters (bandpass, long-pass, short-pass) allow only certain wavelengths to pass. • Precise wavelength selection enhances sensitivity and specificity in fluorescence measurements.
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  ❖ Sample Holder: •Typically, a quartz cuvette, which is transparent to UV and visible light, allowing the excitation light to pass through the sample and the emitted fluorescence to be detected. • Quartz is preferred because of its chemical inertness and broad transmission range, making it suitable for various samples. ❖ Detector: • Usually a Photomultiplier tube (PMT), positioned at approximately 90° to the excitation beam to minimize scattered light. The PMT converts the emitted fluorescence photons into an electrical signal, which is then amplified and measured. • Its high sensitivity makes it ideal for detecting low-intensity fluorescence signals . ❖Polarizers: • Used in fluorescence polarization studies to analyze the rotational mobility of molecules. By polarizing the excitation light and analyzing the emitted light, information about molecular size, shape, and interactions can be obtained. Polarizers are essential for studies involving molecular dynamics .
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  Applications of Fluorimetry •Pharmaceuticals: Drug assays, impurity detection. • Biochemistry: Protein/DNA quantification, enzyme activity studies. • Environmental: Pollutant detection in water/air samples. • Clinical: Diagnosis via body fluid analysis. • Forensics: Trace evidence detection.