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Fluorescence Properties of Hydroxy- and Methoxyflavones and the Effect of Shift Reagents

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Fluorescence Properties of Hydroxy- and Methoxyflavones and the Effect of Shift Reagents Fluorescence Properties of Hydroxy- and Methoxyflavones and the Effect of Shift Reagents Otto S. Wolfbeis*a, Monowara Begum2, and Hans Geigerb a Institut für Organische Chemie, KF-Universität, A-8010 Graz, Austria b Institut für Chemie der Universität Hohenheim, D-7000 Stuttgart, West Germany Z. Naturforsch. 39b, 231 — 237 (1984); received September 23, 1983 Flavonoids, Fluorescence, Structure Elucidation, Shift Reagents The fluorescence spectra of 42 hydroxy- and methoxyflavones in methanol solution have been investigated. The following findings are considered to be useful in structure elucidation and identification of flavonoids: (a) The maxima of the absorption and fluorescence bands give, in most cases, a unique combination; (b) flavones exhibit exceptionally large Stokes’ shifts (6,800 to 10,000 cm-1); (c) most flavones fluoresce blue, but flavonols fluoresce yellow or green; the fluorescence of flavonols consists frequently of two bands; (d) fluorescence is zero or very weak for 5-hydroxyflavones; (e) 5-hydroxyflavones become increasingly fluorescent with in increasing number of oxygen functions being present in the molecule; a 3-hydroxy group has a particular beneficial effect; (f) hydroxyflavones fluoresce less intense than the corresponding methoxy­ flavones; (g) fluorescence intensity is distinctly higher in polar protic than in apolar solvents. The effects of three groups of shift reagents on the spectra has also been investigated. The first group involves water, 10% and 50% sulphuric acid. The second group consists of basic reagents (sodium acetate, sodium hydrogen carbonate and sodium carbonate), and the third group of complexing reagents such as aluminum trichloride, borax and magnesium sulphate. The following generalisations may be made: (a) Water causes a bathochromic shift in emission with 7-hydroxy- flavones, but a hypsochromic shift with flavonols; (b) 50% sulphuric acid is able to protonate (and thus to longware-shift the emission maximum) of all flavones except for the 3-, 5-, and 8 -hydroxy isomers; (c) sodium acetate and hydrogen carbonate cause characteristic shifts with 7-hydroxy- flavones and, occasionally, with 4'-hydroxyflavones lacking a 7-hydroxy group; (d) aluminum trichloride produces a longware shift and an enormous increae in fluorescence intensity with flavonols; (e) borax can be used to detect sensitively the presence of a 3',4'-dihydroxy group. Introduction which may be used to identify flavones. The aim of Ultraviolet spectrometry without and with added the present work is to provide a composition of the shift reagents is one of the most widely used methods solution spectra of 42 flavone aglycons in the pre­ for the identification of flavonoids [1]. In comparison sence and absence of various shift reagents. with U.V. methods, fluorimetry is a more sensitive means for the detection and quantitation of fluores­ cent compounds. In the most simple case the fluorescence band of a molecule is the mirror image of its longest-wave absorption band, shifted to longer wavelengths. So far fluorimetry would provide no more information than does absorptiometry. In addi­ The effects of the following shift reagents, which tion to previous work on the fluorescence of are readily available in every laboratory, were inves­ flavonoids [2] and of 6 ,8- and 2'-hydroxyflavones [3] tigated: on tic plates and in the solid state [4] some recent (1) Water. Since most of the used shift reagent solu­ studies on the fluorescence properties of selected tions are aqueous, we had to check the influence hydroxyflavones have revealed [5—9] that these of water. It turned out that water itself is a useful molecules may form exciplexes, undergo tautomeric reagent; changes or photodissociation. These processes are (2) 50% sulphuric acid and, less suitable, 10% sul­ highly dependent upon the solvent and its acidity and phuric acid; give rise to unusually longwave emission bands, (3) sodium acetate; (4) sodium hydrogen carbonate; (5) sodium carbonate; * Reprint requests to Dr. O. Wolfbeis. (6) sodium hydroxide; 0340-5087/84/0200-0231/S 01.00/0 (7) aluminum trichloride; 232 O. S. W olfbeis et al. ■ Fluorescence Properties of Hydroxy- and Methoxyflavones (8) borax (Na 2B40 7), and oxygen functions being present, the intensity begins (9) magnesium acetate/triethylamine. to increase. A 3-hydroxy group has a particularly Neu’s reagent (diphenyl boric acid /3-aminoethyl- beneficial effect (entries 30, 34 and 35 in Table II). ester), which is such a useful shift reagent for 5-Methoxyflavones are strongly fluorescent. Simi­ flavonoids absorbed on cellulose thin layers [3, 4], is larly, all other methoxyflavones are more strongly of little use for solution measurements. On the one fluorescent than the respective hydroxyflavones side it complexes only slowly to give weakly fluores­ (Tables II, III and IV). cent products, on the other side it exhibits a weak Addition of three volumes of water to one volume fluorescence itself. of the stock solution results in spectral shifts, changes in intensity and broadening of the bands. Batho- Results chromic shifts are observed with 2'- and 3'-hydroxy- With the exception of the 5-hydroxy derivatives all flavones and, less so, with their methyl esthers. In flavones are fluorescent in methanol solution. the case of flavonols, addition of water causes hypso- Fluorescence is distinctly lower in acetonitrile and chromic shifts of the longwave band, but opposite drops even more in cyclohexane. The relative inten­ shifts of the shortwave band. Compounds with a free sities are given qualitatively in the Tables. To allow a 7-hydroxy group frequently exhibit an additional rough estimation of the quantum yields we have band at around 520 nm when water is added. Minor compared our intensity scale with some measured shifts produced with other hydroxyflavones (Table quantum yields (Table I). The threshold of percep­ II) cannot be explained at present, but are neverthe­ tion of a well-adapted eye is in the intensity range 2 less useful in characterising and identifying specific to 3. flavones. Flavones with methoxy substituents but without Addition of 50% sulphuric acid results in longwave hydroxy substituents show only one single fluores­ shifts of 40 to 110 nm with all flavones lacking a cence band of rather high intensity. Those having 3-, 5- or 8-hydroxy group due to protonation. 5-Hy- one or more hydroxy groups (in particular in 3- or 7- droxyflavones with a 3'-methoxy group are, how­ position) may exhibit two fluorescence bands or un- ever, an exception in giving a weak green emission symmetric shoulders along with the main band. Ex­ too. cept for the flavonols all investigated hydroxy- and The effect of basic shift reagents has been investi­ methoxyflavones have their fluorescence maxima in gated with all compounds bearing hydroxy groups the 403 to 480 nm range. (Table III). We notice that extremely pure reagents 3-Hydroxyflavones (“flavonols”) show green or should be used to obtain reproducible results. In par­ yellow emissions with maxima at above 500 nm and ticular, they should be checked not to contain frequently a second, rather weak band in the blue aluminum, magnesium and other complex forming part of the spectrum [5, 6 , 9], ions. There are drastic differences in fluorescence inten­ Addition of one part of NaOAc solution to three sity evident. Generally, 5-hydroxyflavones fluoresce ml of the stock causes two effects in emission: The very weak, if at all. With an increasing number of one results from changing to an aqueous solvent sys­ tem and is similar to that produced by water. The Table I. Fluorescence quantum yields (0f) of some oxy- other results from the basicity of the reagent and is flavones in water at room temperature in comparison with very distinct for all hydroxyflavones having either a the relative intensity scale used in the Tables. 7- or 4'-hydroxy group (Table III). With 7-hydroxy- Flavone 0f Relative intensity Ref. flavones the maxima are shifted to above 500 nm, (scale 1 to 6) except for 7-hydroxy-3',4'-dimethoxyflavone and 3-OH 0.005 3a [9] some of the tetra- and pentahvdroxyflavones, with 7-OH 0.004 3a [7] which only shoulders at above 500 nm may be pro­ 4 -O H 0.02 4 [8 ] 4'-OM e 0.95 6 [8 ] duced. Unfortunately, the reagent is not specific for 7-hydroxyflavones, since 4'-hydroxvflavones without a Fluorescence intensities in the case of low quantum yields a 7-hydroxy group may also be affected. can not be estimated visually with accuracy. Therefore deviations of ± 1 in the relative intensity scale range 1 to With N aH C 0 3 reagent again the water and base 2 are possible. effects have to be discerned. Unlike NaOAc it pro- O. S. Wolfbeis et al. • Fluorescence Properties of Hydroxy- and Methoxyflavones 233 Table II. Fluorescence maxima (nm) of flavones in methanol and the effect of water and acidic shift reagents thereon. Intensities — given in brackets — range from 6 (very high) to 1 (very low) (see Table I). No. Flavone Fluorescencemaximum OH OMe Methanol +Water + 10% h 2s o 4 +50% H 2SO 1 3 _ 405, 531 (3) 412, 514 (3) 410, 528 (3) 412, 529 (3) 2 5 - a a - a - a - 3 - 5 459 (4) 473 (5) 464, 525 (4) 535 (3) 4 6 - 455 (4) 461 (4) 462 (4) 510 (3) 5 - 6 444 (5) 459 (6) 498 (5) 498 (4) 6 7 - 419, 543 (3) 540 (3) 432, 543 (4) 428, 542 (3) 7 - 7 404 (4) 414 (4) 440 (4) 440 (4) __C 8 8 - 471 ( 1) 475 (2) 480 ( 1) 9 - 8 475 (2) 495 (3) 485 (2) 468, 554 (2) 10 2' - 427 (2) 452 (3) 429, 520 (sh) 427, 514 (2) 11 - 2' 428 (3) 452 (4) 435, 509 (3) 516 (3) 12 3' - 428 ( 1) 452 (2) 450
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