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

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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-thedark 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. 歐亞書局

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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.

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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…

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

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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

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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.

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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 "glowin-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.

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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.

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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%)

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The photoluminescence (PL) properties of the pyridine-based polymers, poly(ppyridylvinylene), poly(ppyridine), and poly(ppyridylvinylene-pphenylenevinylene) (PPyVPV) was studied. PL efficiencies are usually high…

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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


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TABLE 15-1 Effect of Substitution on the fluorescence of Benzene

Electron donating groups usually increase φF. 歐亞書局


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Rigid Planar Structure (Fluorene) has more efficiency of fluorescence than biphenyl. [See next page]

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Rigid Planar Structure 9H-fluorene

φF = 1.0

φF = 0.2

Fluorene: It has a violet fluorescence, hence its name. Rigid plane – promotes fluointensity. Non-rigid – decreases fluointensity

φF = 0.8

Skoog, Hollar, Nieman, Principles of Instrumental Analysis, Saunders College Publishing, Philadelphia, 1998.

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not fluorescent Ingle and Crouch, Spectrochemical Analysis

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.

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Metals Metals other than certain lanthanides and actinides (with f-f transitions) are usually not themselves fluorescent.

A number of organometallic complexes are fluorescent. 歐亞書局


FIGURE 15-5 Spectra for phenanthrene: E, excitation; F, Fluorescence [lower energy]; P, phosphorescence [even lower]. 歐亞書局


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FIGURE 15-7(a) Synchronous fluorescence spectra. In (a), the excitation (black) and emission spectra (blue) of tetracene are shown. 歐亞書局


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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 electron from a ‘bonding’ π orbital to an ‘antibonding’ π orbital designated as π*. 1996, 68, 2266 歐亞書局

pi stacking (also called π−π stacking) refers to attractive, noncovalent interactions between aromatic rings.

The benzene dimer is the prototypical system for the study of pi stacking, and is experimentally bound by 8–12 kJ/mol (2–3 kcal/mol) in the gas phase with a separation of 4.96 Å between the centers of mass. Analysis of the aromatic amino acids phenylalanine, tyrosine, histidine, and tryptophan indicates that dimers of these side chains have many possible stabilizing interactions at distances larger than the average van der Waals radii.[2] Pi stacking (of benzene groups) is prevalent in protein crystal structures, and also contributes to the interactions between smallmolecules and proteins. As a result, pi-pi and cation-pi interactions are important factors in rational drug design.[16] 歐亞書局

Luminescence is rare in nonaromatic hydrocarbons. Nonaromatic Unsaturated Hydrocarbons (like rubbers)

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Seyhan Ege, Organic Chemistry, D.C. Heath and Company, Lexington, MA, 1989.

Aromatic Hydrocarbons

Fluorescent Low lying π – π* singlet state Phosphorescence is weak because there are no n-electrons (for high-conjugation molecules). See Table 12-10. 歐亞書局


Heterocyclic Aromatics

Aromatics containing carbonyl or heteroatoms (with n-electrons) are more likely to phosphorescence. n – π* transition promotes intersystem crossing..  Fluorescence is often weaker. 歐亞書局


Aromatic Substituents

• Electron-donating groups usually increase φF. • Electron-withdrawing groups usually decrease φF (φF= fluorescence efficiency). See Table 15-1. 歐亞書局


Halogen Substituents

Internal Heavy Atom Effect Heavy atom (I, Br, Cl, or F) Promotes intersystem crossing. φF decreases as MW increases. φP increases as MW increases. τP decreases as MW increases. 歐亞書局


P3HT, Poly(3-hexylthiophene-2,5-diyl)

π-conjugated system in a long chain

Phosphorescence emission? P3HT does emit weak phosphorescence at 826nm if excited by a 600 nm source.

太陽能光電

Regioregular poly(3-hexylthiophene-2,5-diyl). Commonly known as P3HT.

UV-Vis Abs. and Fluo. Intensity

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FIGURE 15-8 Components of a fluorometer or spectrofluorometer. Source radiation is split into two beams. The sample beam passes through the excitation wavelength selector to the sample. The emitted fluorescence is isolated by the emission wavelength selector before striking the transducer. The reference beam is attenuated before striking the transducer. The electronics and computer system compute the ratio of the fluorescence intensity to the reference beam intensity, which cancels the effect of source intensity fluctuations. 歐亞書局


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Anthracene = three benzene rings conjugated in line.

FIGURE 15-9 Fluorescence spectra for 1 ppm anthracene in alcohol: (a) excitation spectrum; (b) emission spectrum.

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FGIURE 15-10 A typical fluorometer. 歐亞書局


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FIGURE 15-11 A spectrofluorometer. 歐亞書局


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FIGURE 15-12(a) Three-dimensional spectrofluorometer. (a) Schematic of an optical system for obtaining total luminescence spectra with a CCD detector. 歐亞書局


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The End (Chap. 15, Luminescence Spectroscopy)

Next: FTIR Spectroscopy [Chaps. 17, 18]

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Absorption and re-emission of UV/Vis light - Luminescence: Note: Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength.[1] It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. However, when the absorbed electromagnetic radiation is intense, it is possible for one electron to absorb two photons; this two-photon absorption can lead to emission of radiation having a shorter wavelength than the absorbed radiation. The most striking examples of fluorescence occur when the absorbed radiation is in the ultraviolet region of the spectrum, and thus invisible to the human eye, and the emitted light is in the visible region. Fluorescence has many practical applications, including mineralogy, gemology, chemical sensors (fluorescence spectroscopy), dyes, biological detectors, and, most commonly, fluorescent lamps (日光燈).

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Rock-forming elements rare earth metals: atomic number = 57-71 + Two other metals : Z=21 (Sc=scandium), and 39 (Y=yitrium)

39 歐亞書局

Note: “Fluorescence” is derived from the word “Fluorite” – a natural mineral crystal that glows upon absorbing UV. Fluorite: is a halide mineral composed of calcium fluoride, CaF2. [photo below]

Photo above shows “fluorescence” is applied to microscopy. 螢光顯微鏡歐亞書局

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Practical applciations of phosphrescent materials: Common pigments used in phosphorescent materials include zinc sulfide (ZnS) and strontium aluminate. Use of zinc sulfide for safety related products dates back to the 1930s. However, the development of strontium aluminate, with a luminance approximately 10 times greater than zinc sulfide, has relegated most zinc sulfide based products to the novelty category. Strontium aluminate (鋁酸鍶 SrAl2O4) based pigments are now used in exit signs, pathway marking, and other safety related signage.[2] ZnS

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SrAl2O4

To make a glow-in-the-dark toy, what you want is a phosphor that is energized by normal light (photoluminescence) and that has a very long persistence [by phosphorescence]. Two phosphors that have these properties are Zinc Sulfide and Strontium Aluminate. Strontium Aluminate is newer -- it's what you see in the "super" glow-in-the-dark toys. It has a much longer persistence than Zinc Sulfide does.

The phosphor is mixed into a plastic and molded to make most glowin-the-dark stuff.

Some others use Chemiluminescent Signal Devices. 歐亞書局

Luminescence (or photoluminescence) includes fluorescence and phosphorescence. Photoluminescence (PL) 指物質吸收光子（或電磁波）後重新輻射出光子（或電磁波）的過程。從量子力學理論上，這一過程可以描述為物質吸收光子躍遷到較高能級的激發態後返回低能態，同時放出光子的過程。

光致發光(PL) 可按延遲時間分為螢光（Fluorescence）和磷光 (Phosphorescence)。產生螢光之方式大致分為兩類，分別為以高於或等於能隙能量之光子照射樣品來產生額外載子，或以電子注入之方式增加載子濃度以增加螢光光子產生之機率. 此兩類方式分別稱為光激發螢光(photoluminescence，以下簡稱PL), 及電激發螢光(EL)，LED的發光原理便為電激發螢光. NOTE: Ironically, white phosphorus (from which phosphorescence

takes its name) does not actually exhibit this property, but rather chemiluminescence. 歐亞書局

FIGURE 15-2 Partial energy-level diagram for a photoluminescent system (fluorescence and phosphorescence). 歐亞書局


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Principles of Instrumental Analysis

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