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Principles of Instrumental Analysis Principles of Instrumental Analysis Chapter 15 Molecular Luminescence Spectrometry -Emission of visible lights 1. Fluorescence spectroscopy 2. Phosphorescence spectroscopy 3. Chemiluminescence spectroscopy 1 Luminescence is the emission of light by a substance [thus, complimentary to absorption]. It occurs when an electron returns to the electronic ground state from an excited state and loses it's excess energy as a photon. Luminescence spectroscopy is a collective name given to three related spectroscopic techniques. They are; -Molecular fluorescence spectroscopy -Molecular phosphorescence spectroscopy -Chemiluminescence spectroscopy 歐亞書局 Note 2 - Phosphorescence is a specific type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower time scales of the re-emission are associated with "forbidden" energy state transitions in quantum mechanics. As these transitions occur very slowly in certain materials, absorbed radiation may be re-emitted at a lower intensity for up to several hours after the original excitation. Commonly seen examples of phosphorescent materials are the glow-in-the- dark toys, paint, and clock dials that glow for some time after being charged with a bright light such as in any normal reading or room light. Typically the glowing then slowly fades out within minutes (or up to a few hours) in a dark room. “Phosphorescence” is original derived from phophorous (P). “phosphors” has a different meaning: Phosphors are transition metal compounds or rare earth compounds of various types. 3 歐亞書局 Note: Chemiluminescence occurs when a chemical reaction produces an electronically excited species which emits a photon in order to reach the ground state. These sort of reactions can be encountered in biological systems; the effect is then known as “bioluminescence”. The number of chemical reactions which produce chemiluminescence is small. However, some of the compounds which do react to produce this phenomenon are environmentally significant. Light production in fireflies is due to a type of chemical reaction called bioluminescence A good example of chemiluminescence is the determination of nitric oxide: * NO + O3 NO2 + O2 * NO2 NO2 + hv (λ = 600 - 2800 nm) The glow of phosphorus itself originates from oxidation of the white (but not red) phosphorus— a process now termed chemiluminescence. The glow of phosphorus itself originates from oxidation of the white (but not red) phosphorus— a process now termed chemiluminescence. 歐亞書局 Possible physical process following absorption of a photon by a molecule. 1. Fluorescence occurs when the molecule returns to the electronic ground state, from the excited singlet state, by emission of a photon. 2. Phosphorescence will be explained later… 歐亞書局 Singlet state: All electrons in the molecule are spin-paired. Triplet state: One set of electron spins is unpaired. FIGURE 15-1 Electronic spin states of molecules. In (a) the ground electronic state is shown. In the lowest energy, or ground, state, the spins are always paired, and the state is said to be a singlet state. In (b) and (c), excited electronic states are shown. If the spins remain paired in the excited state, the molecule is in an excited singlet state (b). If the spins become unpaired, the molecule is in an excited triplet state (c). 歐亞書局 Ch15 Molecular Luminescence Spectrometry P.401 fluorescence vs. phosphorescence Fluorescence arises from the decay of the excited singlet state S1 to the ground state S0. Phosphorescence arises from the decay of the excited triplet state T1 to the ground state S0. Note: Photoluminescence (PL) is different from Electroluminescence. Electroluminescence (EL) is an optical phenomenon and electrical phenomenon in which a material emits light in response to the passage of an electric current or to a strong electric field. - LED is one example. 歐亞書局 Jablonski energy diagram - Fluorescence vs. phosphorescence 歐亞書局 Why Time delay in phosphorescent emission? Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower time scales of the re-emission are associated with "forbidden" energy state transitions in quantum mechanics. As these transitions occur very slowly in certain materials, absorbed radiation may be re-emitted at a lower intensity for up to several hours after the original excitation. 歐亞書局 More detailed interpretation of phosphorescence Students should read these texts carefully themselves! In the special case of phosphorescence, the absorbed photon energy undergoes an unusual intersystem crossing into an energy state of higher spin multiplicity (see term symbol), usually a triplet state. As a result, the energy can become trapped in the triplet state with only classically "forbidden" transitions available to return to the lower energy state. These transitions, although "forbidden", will still occur in quantum mechanics but are kinetically unfavored and thus progress at significantly slower time scales. Most phosphorescent compounds are still relatively fast emitters, with triplet lifetimes on the order of milliseconds. However, some compounds have triplet lifetimes up to minutes or even hours, allowing these substances to effectively store light energy in the form of very slowly degrading excited electron states. If the phosphorescent quantum yield is high, these substances will release significant amounts of light over long time scales, creating so-called "glow- in-the-dark" materials. 歐亞書局 Emission of a photon from the singlet excited state to the singlet ground state—or between any two energy levels with the same spin—is called fluorescence. The probability of fluorescence is very high and the average lifetime of an electron in the excited state is only 10–5–10–8 s. In some cases, an electron in a singlet excited state is transformed to a triplet excited state (Figure 10.47c) in which its spin is no longer paired with the ground state. Emission between a triplet excited state and a singlet ground state—or between any two energy levels that differ in their respective spin states–is called phosphorescence. Because the average lifetime for phosphorescence ranges from 10– 4–104 s, phosphorescence may continue for some time after removing the excitation source. 歐亞書局 Quinine occurs naturally in the bark of the cinchona tree, though it has also been synthesized in the laboratory. FIGURE 15-3 Fluorescence excitation and emission spectra for a solution of quinine. 歐亞書局 Ch15 Molecular Luminescence Spectrometry P.403 Quinine is a strongly fluorescent compound in dilute acid solution with two excitation wavelengths (250 and 350 nm) and a fluorescence emission at 450 nm. The following UV-Vis spectra are for various conc. (0.02% - 0.2%) 歐亞書局 The photoluminescence (PL) properties of the pyridine-based polymers, poly(p- pyridylvinylene), poly(p- pyridine), and poly(p- pyridylvinylene-p- phenylenevinylene) (PPyVPV) was studied. PL efficiencies are usually high… 歐亞書局 Ch15 Molecular Luminescence Spectrometry P.405 Fluorescence is often weaker. Aromatics containing carbonyl or heteroatoms (like what shown here) are more likely to phosphorescence, but not fluorescence. We will explain these molecules later. 歐亞書局 Ch15 Molecular Luminescence Spectrometry P.405 TABLE 15-1 Effect of Substitution on the fluorescence of Benzene Electron donating groups usually increase φF. 歐亞書局 Ch15 Molecular Luminescence Spectrometry P.406 Rigid Planar Structure (Fluorene) has more efficiency of fluorescence than biphenyl. [See next page] 歐亞書局 Ch15 Molecular Luminescence Spectrometry P.406 Rigid Planar Structure 9H-fluorene φF = 1.0 φF = 0.2 Fluorene: It has a violet fluorescence, hence its name. Rigid plane – promotes fluo- intensity. Non-rigid – decreases fluo- φ = 0.8 not fluorescent intensity F Skoog, Hollar, Nieman, Principles of Instrumental Analysis, Ingle and Crouch, Spectrochemical Analysis 歐亞書局Saunders College Publishing, Philadelphia, 1998. Increased Conjugation [Rings sharing common bonds] φF increases as conjugation increases [more bonded rings]. φP decreases as conjugation increases. -Hypsochromic effect (i.e., blue shift) and -bathochromic shift (i.e., red shift – shift to longer wavelength). 歐亞書局 Ingle and Crouch, Spectrochemical Analysis Cisplatin, PtCl2(NH3)2 A platinum atom with four ligands. Inorganic types Metal complexes - not themselves fluorescent. 歐亞書局 Ch15 Molecular Luminescence Spectrometry P.406 Metals Metals other than certain lanthanides and actinides (with f-f transitions) are usually not themselves fluorescent. A number of organometallic complexes are fluorescent. Skoog, Hollar, Nieman, Principles of Instrumental Analysis, Saunders College Publishing, Philadelphia, 1998. 歐亞書局 FIGURE 15-5 Spectra for phenanthrene: E, excitation; F, Fluorescence [lower energy]; P, phosphorescence [even lower]. 歐亞書局 Ch15 Molecular Luminescence Spectrometry P.410 FIGURE 15-7(a) Synchronous fluorescence spectra. In (a), the excitation (black) and emission spectra (blue) of tetracene are shown. 歐亞書局 Ch15 Molecular Luminescence Spectrometry P.411 Fluorescence or Phosphorescence? π – π* transitions are most favorable for fluorescence. ε is high (100 – 1000 times greater than n – π*) . kF is also high (absorption and spontaneous emission are related). Fluorescence lifetime is short (10-7 – 10-9 s for π – π* vs. 10-5 – 10-7 s for n – π*). Note: Definition of π − π* transition (discussed earlier): An electronic transition described approximately as a promotion of an
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