Fluorescence of organic compounds

A fluorescence spectrum is usually the mirror image of the absorption spectrum and a study of the fluorescence spectrum of a substance yields information concerning its molecular structure of essentially the same nature as that derived from its absorption spectrum. When vibrational structure appears in the fluorescence spectra of molecules in solution, a more or less complete vibrational analysis of the ground electronic state may be made analogous to that derived from Raman or infra-red spectra. 

Solutions of unsubstituted monoheterocyclic compounds, pyridine, pyrrol, furan and thiopene are non-fluorescent. According to Kasha and Reid17, the near ultra-violet absorption spectra of N-heterocyclic compounds like pyridine, pvrazine and phenazine include n tt transitions and the excited state undergoes, with high probability, a radiationless transition to a lower triplet metastable state and hence few molecules remain in the excited state long enough to fluoresce. The presence of molecules in the triplet state has been shown by their phosphorescence. 

Fusion of a benzene ring to a heterocyclic nucleus causes a bathochromic shift and increases the intensity of absorption. Thus, quinoline, isoquinoline, indole and quinolium and isoqumolium ions fluoresce in the ultra-violet. Acridine, acridone and carbazole exhibit visible fluorescence. Porphyrin18 and some of its derivatives are characterized by narrow well-defined vibrational bands and their fluorescence bands are in the red region of the spectrum. 

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Other than the obvious advantages of reduced fluorescence and high resolution, FT Raman is fast, safe and requires mmimal skill, making it a popular analytic tool for the characterization of organic compounds, polymers, inorganic materials and surfaces and has been employed in many biological applications [41]. 

Selected Examples of Organic Compounds of Biochemical, Pharmaceutical, and Environmental Significance That Show Natural Fluorescence or t Phosphorescence... 

Solid-surface luminescence analysis involves the measurement of fluorescence and phosphorescence of organic compounds adsorbed on solid materials. Several solid matrices such as filter paper, silica with a polyacrylate binder, sodium acetate, and cyclodextrins have been used in trace organic analysis. Recent monographs have considered the details of solid-surface luminescence analysis (1,2). Solid-surface room-temperature fluorescence (RTF) has been used for several years in organic trace analysis. However, solid-surface room-temperature phosphorescence (RTF) is a relatively new technique, and the experimental conditions for RTF are more critical than for RTF. 

We inferred that these properties might be exploited in a series of unique derivatizing reagents designed specifically for trace analysis of organic compounds using HPLC separation and fluorescence detection. The use of these pyridones for the analytical purposes reported here is based on their acidic properties. Treatment of a lH-2-pyridone with a base converts the pyridone to its salt. 

When porphyrins dissolved in strong mineral acids or in organic solvents are illuminated by ultraviolet hght, they emit a strong red fluorescence. This fluorescence is so characteristic that it is often used to detect small amounts of free porphyrins. The double bonds joining the pyrrole rings in the porphyrins are responsible for the characteristic absorption and fluorescence of these compounds these double bonds are absent in the porphyrinogens. 

A collection of UV spectra of plasticisers, fluorescent whitening agents (optical brighteners), UV absorbers, as well as of phenolic and aminic antioxidants was published by Hummel and Scholl [21]. UV absorbance data for isolated chromophores are listed elsewhere [22]. A general UV atlas of organic compounds is available [23]. 

Personov RI, AT shits LA, Bykovskaja LA (1972) The effect of fine structure appearance in laser-excited fluorescence spectra of organic compounds in solid solutions. Opt Commun 6 169-173... 

Emission-Excitation Matrix (EEM) fluorescence spectroscopy as a nondestructive and sensitive analytical technique was successfully applied in this study to characterize DOM in landfill leachte. The DOM is composed of complex mixture of organic compounds with different fluorescence properties. In particular, the EEM profiles of DOM show two well-defined peaks at Ex/Em=320-350 /400-420 nm, Ex/Em=320-350 /420-450 nm reasonably due to the presence of two different groups of fluorophores. An additional and less intense band at Ex/Em=280-290 /320-350 nm can be assigned to aromatic amino acids and phenol-like compounds. 

Waggot and Britcher [38] have discussed experimental considerations in the determination of organic carbon content of sewage effluent. Close attention is paid to the determination of particular classes of organic compounds in sewage including carbohydrates, amino acids, volatiles, steroids, phenols, surface active materials, fluorescent materials, organochlorine pesticides and ethylene diamine tetracetic acid. 

According to Kasha s rule, fluorescence from organic compounds usually originates from the lowest vibrational level of the lowest excited singlet state (Si). An exception to Kasha s rule is the hydrocarbon azulene (2) (Figure 4.5), which shows fluorescence from S2. 

Figure 7,1 typical absorbance and fluorescence properties of organic compounds. 

Other uses are to produce phosphorescence and fluorescence in organic compounds and for scintillation screens on instruments used to detect radiation. Radium salts were used in the past to paint the dials of luminous clock faces that glow in the dark. 

The apphcation of an optical transducer based on the fluorescence quenching effect of oxygen was described by Preininger et al. [3]. Another interesting technique is represented by the use of the luminous bacterium Photobacterium phos-phoreum [34]. This device is based on the correlation of the intensity of luminescence to the cellular assimilation of organic compounds from the wastewater. 

A remarkable number of organic compounds luminesce when subjected to consecutive oxidation-reduction (or reduction-oxidation) in aprotic solvents1-17 under conditions where anion radicals are oxidized or cation radicals are reduced. In many instances, the emission is identical with that of the normal solution fluorescence of the compound employed. In these instances the redox process has served to produce neutral molecules in an excited electronic state. These consecutive processes which result in emission are not special examples of oxidative chemiluminescence, but are more properly classified as electron transfer luminescence in solution since the sequence oxidation-reduction can be as effective as reduction-oxidation.8,10,12 A simple molecular orbital diagram, although it is a zeroth-order approximation of what might be involved under some conditions, provides a useful starting... 

The cerium(IV) oxidation reaction of many organic acids provides a sensitive and selective method for HPLC analysis of these compounds [116,117]. The oxidation of specific classes of organic compounds with cerium(lV), and the effects on the reaction of temperature, acidity, anion and catalyst, have been studied extensively [118-120]. The reaction produces cerium(HI) which is fluorescent and can be measured spectrofluori-metrically. The method has been applied successfully to the post-column reaction and detection of nmole amounts of organic acids by HPLC. 

Taylor, P.H., D angelo, J.A., Martin, M.C., Kasner, J.H., Dellinger, B. (1989) Laser photolysis/laser-induced fluorescence studies of reaction rates of OH with CH3C1, CH2C12, and CHC13 over an extended temperature range. Int. J. Chem. Kinet. 21, 829-846. Tewari, Y.B., Miller, M.M., Wasik, S.P., Martire, D.E. (1982) Aqueous solubility and octanol/water partition coefficient of organic compounds at 25.0°C. J. Chem. Eng. Data 27, 451 —454. 

Fluorescence spectroscopy is characterized by a greater selectivity when compared with other spectro-photometric techniques because there is an excitation and an emission spectra, with maxima usually quite characteristic of a particular compound. It is also selective because of the limited number of organic compounds that fluoresce. It has greater sensitivity than spectrophotometric methods in solution 10 9-10 12 M can usually be measured while on a thin-layer chromatogram some naturally fluorescent compounds may be detected instrumentally in sub-nanogram amounts. 

Solvatochromic fluorescent probe molecules have also been used to establish solvent polarity scales. The solvent-dependent fluorescence maximum of 4-amino-V-methylphthalimide was used by Zelinskii et al. to establish a universal scale for the effect of solvents on the electronic spectra of organic compounds [80, 213], More recently, a comprehensive Py scale of solvent polarity including 95 solvents has been proposed by Winnik et al. [222]. This is based on the relative band intensities of the vibronic bands I and III of the % - n emission spectrum of monomeric pyrene cf. Section 6.2.4. A significant enhancement is observed in the 0 0 vibronic band intensity h relative to the 0 2 vibronic band intensity /m with increasing solvent polarity. The ratio of emission intensities for bands I and III serves as an empirical measure of solvent polarity Py = /i/Zm [222]. However, there seems to be some difficulty in determining precise Py values, as shown by the varying Py values from different laboratories the reasons for these deviations have been investigated [223]. 

The fluorescence see Fluorescence) properties of organic compounds are well known and extensively studied. Many compounds, especially the polyaromatics, exhibit intense... 

A number of organic compounds have been identified as products of DON photodegradation. In the majority of published papers, they are characterized in bulk as amines measured as fluorescent ortho-phtaldialdehyde derivatives (Lindroth and Mopper, 1979). Figure 10.5 shows one of the few cases where organic products of DON phototransformation are presented as chemical moieties identified at the molecular level. 

Many redox reactions by colloidal nanoparticles have been reported. Three of the most-studied reactions are (1) the catalyzed electron transfer between ferricyanide and thiosulfate [8,19-21], (2) the catalytic reduction of fluorescent dyes by sodium borohydride [22, 23], and (3) the catalytic reduction of organic compounds (e.g., nitro-aryls [9] and alcohols [24]). These reactions have been studied extensively because they are easy to follow spectroscopically allowing for straightforward measurement of reaction kinetics. The third set of reactions has enormous industrial significance, where nitro compounds are reduced to their less toxic nitrate or amine counterparts [25, 26] and the electrooxidation of methanol is utilized for methanol fuel cells [27, 28]. 

The photocrosslinking of cinnamic polymers is accelerated by introduction of organic compounds. The absence of fluorescence of these sensitizers and their high intersystem crossing efficiencies (Sj —>T, ) show that triplet state is the precursor of the reaction. 

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