Celardin, F.

RERO DOC Digital Library

Zeitschrift für Physikalische Chemie Neue Folge, Bd. 106, S. 25© by Akademische Verlagsgesellschaft, Wiesbaden 1977—32 (1977)General Kinetics of Reversible Reactions in the ExcitedState. Application to Excited Singlet pK Determinationof Acridone, Xanthone and Thioxanthone*Byf. CelardinDepartment of Inorganic and Analytical Chemistry, University of Geneva,Geneva (Switzerland)(Received February 16, modified April 28, 1977)Kinetics / Excited state reversible reaction / Excited state acidity constants \Acridone / Xanthone j ThioxanthoneFrom steady state kinetics of a general excited state reversible reactionscheme, a linear relation between the total luminescence and the concentrationof one reactant is derived, showing that the equilibrium constant of certainexcited state reactions can be determined from the total fluorescence spectrawithout the separation of the components and irrespective of the ground stateequilibrium. The relation is checked by comparison with published results,and it is applied to the determination of excited state acidity constants ofacridone, xanthone and thioxanthone in ethanol-water (1:1).Aus der „steady state"-Kinetik eines allgemeinen Schemas reversiblerReaktionen angeregter Zustände wird eine lineare Beziehung zwischen der totalenLumineszenz und der Konzentration eines Reaktanten abgeleitet. Sie zeigt,daß die Gleichgewichtskonstante gewisser Reaktionen angeregter Zustände ausdem totalen Fluoreszenz-Spektrum ohne Separation der Komponenten und ohneRücksicht auf das Gleichgewicht im Grundzustand abgeleitet werden kann.Die Beziehung wird durch Vergleich mit veröffentlichten Ergebnissen geprüftund wird dann auf die Bestimmung von Säurekonstanten im angeregten Zu-stand von Acridon, Xanthon und Thioxanthon angewandt.The kinetic treatment of excited state acid-base reactions deve-loped mainly by the extensive studies of Weller [1] constitutea complementary approach and a good test of the experimental ap-* Presented at the meeting of "Société Chimique de Genève" on the 20thof December 1976.Brought to you by | Universitaetsbibliothek BaselAuthenticatedDownload Date | 7/11/17 11:22 AM26 F. Celard inplicability of Förster's elegant thermodynamic method [2]. Thefundamental and experimental limitations of "Förster's cycle" havebeen discussed by various authors [3—6].In the following, a kinetic calculation based on a general schemeof excited state reversible reactions, leading to a simple relationwhich allows determination of equilibrium constant from total fluores-cence spectra, without separation of the components and irrespectiveof the ground state equilibrium, is presented. The validity of thisrelation is checked by comparison of excited state acid-base equili-brium constants of some compounds with those obtained by otherworkers. Finally, a new excited state acid-base reaction involvingthioxanthone is reported and its equilibrium constant determined.A tentative correlation between the possible structures and themagnitudes of the excited state equilibrium constants, as well as thespectral characteristics of acridone, xanthone and thioxanthone, ispresented at the end of the paper.ExperimentalAll measurements were performed in ethanol-water (1:1 v/v)at 25 °.The various solutions were prepared from stock solutions of thereagents in ethanol (Merck G.R.) to which water was added in equalproportion. The pH was adjusted with sulfuric acid 98°/o (Merck G.R.)or sodium hydroxide (0.4 M) and measured by a Metrohm E 500digital pH-meter with Metrohm glass electrode. It should be pointedout that even in the very low pH range the meter readings were inaccord with the values calculated from the added acid amounts.Acridone (Fluka) was recrystallized from ethanol-water (m.p.354°); xanthone (Fluka puriss. m.p. 173—174°) and thioxanthone(Fluka purum) were used without further purification.Spectra and luminescence measurements were taken on Aminco-Bowman and Hitachi-Perkin Elmer MPF-2A spectrofluorimeters anda Beckman DB-G spectrophotometer.Results and discussionKineticsThe following reaction sequence is adopted as being a most generalrepresentation of a reversible reaction occurring in ground and ex-cited states:Brought to you by | Universitaetsbibliothek BaselAuthenticatedDownload Date | 7/11/17 11:22 AMGeneral Kinetics of Reversible Reactions in the Excited State 27A* + X (AX)*  I */,  k.{    */' ,  ki'A +  *- > AXwhere Ia, Iax are the intensities (einstein. s^1) absorbed by A andAX; I—Io (1—e"1), Io: intensity of incident beam; k¡, k/ : lumines-cence (fluorescence) rate constants;ki, k-i, k*, k-i: rate constants of forward and reverse reactions atground and excited states.In the above reaction scheme, it is assumed that the species(AX)* is the same, whether it is formed by reaction of A* with X orby direct excitation of AX. This is very probably so, when X isa hydronium ion [7]. This assumption may be incorrect when X isanother molecule of A, or some other reactant (metal ion or anotherorganic species) which is photoinactive at the same wavelength as A.Indeed, in differentiating between the encounter complex (^4*—X)and the excited complex (AX)*, the general mechanism as presentedhere, assumes that in the sequence A* -(- X ^ (A*—X)  ± (AX)* thediffusion controlled equilibrium relaxation is rate determining whencompared to intramolecular energy partition.From the reaction scheme, the total emission intensity can beexpressed as: h, ^': sum of rate constants of all photophysical proc-esses of A and AX, other than luminescence ;Ftotai = k, (A*) + kf (AX*) (1)and ifh*   (X) > 1andwheret0= (^ +  ^)- andt0'= (k/ + Zkt')-iBrought to you by | Universitaetsbibliothek BaselAuthenticatedDownload Date | 7/11/17 11:22 AM28 F. Celardina stationary state kinetic treatment yields : A*\— °  ' (  + Iax^ /óv{ >- ß* 0( ) +  0' (ZÌ   *\_   0'ß* ( ) (  +    ) , \{  >~ ß*   ( ) + to' ^whereis the excited state equilibrium stability constant.Upon substitution of (2) and (3) into (1) and with the additionalexperimental conditions such as :Ia = /o(l-e-*rf)« I0 eelF =  Ia ~     cl c : concentration of A or AX : fluorescence efficiencyIa  absorbed intensityone gets:with the limiting valuesand _(ß*) 1   0' +     '( ) ...(/?*)-! To' + TO (X) K 1lim  = (polim  =  'wherekfand  kf' + Ikt'Dividing (4) by 990 (or   ') and rearranging, the following linear rela-tions are derived:Brought to you by | Universitaetsbibliothek BaselAuthenticatedDownload Date | 7/11/17 11:22 AMGeneral Kinetics of Reversible Reactions in the Excited State 29Both relations (5) and (6) are applicable at the wavelengths where<po and  ' are not zero. If 950' = 0 at the wavelength of measurement,relation (5) is applicable; if, on the contrary,  = 0 then (6) may beused. In such cases (5) and (6) reduce to Stern-Volmer type rela-tions (7) and (8) respectively:«•. = 1+0*J*-(X), (7)The intercept divided by the slope gives the product of the excitedstate equilibrium constant with the ratio of the lifetimes of the speciesinvolved.Experimentally, it is sufficient to record the fluorescence inten-sities at the chosen wavelength, as a function of (A"). Indeed, it can Feasily be shown that in the present case = .The general expressions (5) and (6) were tested in the particularcase of excited state acid-base reactions. The data published by otherworkers (Table 1 ) gave linear plots and the values of ^pK* — logderived from the slopes were in good agreement with the previouslyreported ones.Table 1. Comparison of published and calculated acidity constantspK*—loj ToTillit. cale. -naphthol 2.50 [9] 2.48/¡-naphthol 2.82 [9] 2.332.5 [1] 2.312-naphthol-5-sulfonic acid 1.3(i|9| 1.55acridone 0.92 [8] 0.930.96xanthone 0.96 [10] 1.0 a0.52 bthioxanthone—0.540.52» in water, with HCIO4, ionic strength 1.0 M. (Ref. [10]).b in ethanol-water (1:1) with H2SO4.Brought to you by | Universitaetsbibliothek BaselAuthenticatedDownload Date | 7/11/17 11:22 AM30 F. CelardinExcited state acid-base reactions of acridone xanthoneand thioxanthoneThe excited state acid-base reactions of acridone in water—ethanol(1:1) were studied by Kokubun [8] who determined an increasedbasicity for the first excited singlet state. Our spectral observationsof two similar heterocyclic ketones, xanthone and thioxanthone, in300 400 500 [nm]I \I \ \I \s / iII1/ IX-/ s \300 500 [nm]Fig. 1. Unoorrected fluorescence excitation and emission spectra of acridone(1.7xl0-5M), xanthone (2.5X 10~6M) and thioxanthone (10~5M) in: (-)ethanol—water (1:1); ( — ) at pH 0.6, 0.7, 0.8 respectively; ( -) ethanol-H2SO4 (7N) (1:1)Brought to you by | Universitaetsbibliothek BaselAuthenticatedDownload Date | 7/11/17 11:22 AMGeneral Kinetics of Reversible Reactions in the Excited State 31the same solvent system, showed also increased basicities for thesemolecules in their Si states. A higher basicity was determined byIreland and Wyatt for xanthone Si in water alone [10] (Table 1).The excitation-fluorescence spectra of Fig. 1 show variations ofthe fluorescence spectra with increasing Ü30+ concentration whileexcitation spectra remain unchanged, indicating a proton exchangewhich is effective only in the excited state. It is also apparent fromthe bathochromic shift that the protonated forms have a lowerFig. 2. Plots-(   1] as a function of (HbjO)"1" for thioxanthone : a) at484 nm; b) at 520 nm(Si—So) energy gap. This shift is very distinct in the case of xanthoneand thioxanthone. The isoemissive points are at 448 nm for acridone,at 400 nm for xanthone and at 476 nm for thioxanthone.The values of ^pK* — log obtained from the slopes of linearplots for xanthone and thioxanthone are presented in Table 1. InFig. 2, the linear plots for thioxanthone at 484 nm and at 520 nm,show an intercept equal to unity at this latter wavelength, implyingthat the protonated form is the only emitter, while a different valueat 484 nm corresponds to both protonated and unprotonated speciesas emitters. The acidity constants derived are the same in bothexperiments.The values obtained for xanthone and thioxanthone are veryclose to each other, whereas for acridone it is almost twice as large.A tentative explanation for this difference may be proposed if it isBrought to you by | Universitaetsbibliothek BaselAuthenticatedDownload Date | 7/11/17 11:22 AM32 F. Celardinconsidered that the contributions of the charged canonical forms,as shown below, are greater in the excited singulet state.0 10  It appears that the excited forms of xanthone and thioxanthoneare similar, with the positive center on the aromatic nucleus. Foracridone the positive charge is localized on the nitrogen, and theexcited singlet state contains a higher contribution from a largerconjugate system. From the cases considered here, protonation on theoxygen of the carbonyl should give rise to a conjugate system com-patible with the bathochromic shift observed in the emission spectra.References1. A. Weller, Progress in Reaction Kinetics, vol. 1, ed. G. Porter, 1961, p. 187,and references therein (review).2. Th. Förster,  . Elektrochem., Ber. Bunsenges. physik. Chem. 54 (1950) 42.3. H. H. Jaffe and H. Lloyd Jones, J. org. Chemistry 30 (1965) 964.4. E. Vander Donckt, in: Elements de Photochimie avancée, ed. P. Courtot.Hermann, Paris 1972, p. 80 (review).5.  . R. Grabowski and A. Grabowska, Z. physik. Chem. Neue Folge 101(1976) 197.6. N. Abnal, M. Deumie and P. Viallet, J. Chim. physique Physico-Chim.biol. 66 (1969) 421.7. A. Welleb, Discuss. Faraday Soc. 27 (1959) 28.8. H. KoKUBUN,Z. Elektrochem., Ber. Bunsenges. physik. Chem. 62 (1958) 599.9. A. Welleb, Z. Elektrochem., Ber. Bunsenges. physik. Chem. 56 (1952) 662.10. J. F. Ibeland and P. A. H. Wyatt, J. C. S. Faraday I, 68 (1972) 1053.Brought to you by | Universitaetsbibliothek BaselAuthenticatedDownload Date | 7/11/17 11:22 AM

General Kinetics of Reversible Reactions in the Excited State. Application to Excited Singlet pK Determination of Acridone, Xanthone and Thioxanthone*

Abstract

Similar works

Full text

Available Versions

RERO DOC Digital Library