Proc. Indian Acad. Sci. (Chem. Sci.), Vol. 104, No. 2, April 1992, pp. 325-330.
9 Printed in India.
Fluorescence quenching of coumarin 314 by Ce(IIl) ions
L F M I S M A I L
Department of Chemistry, Faculty of Science, A1-Azhar University (Girls), Cairo, Egypt Abstract. The fluorescence of coumarin 314 in water and ethanol is quenched statically via ground state complexation with cerium (III) ions. The temperature dependence of the quenching rate constant, (/(sv), determined from linear Stern-Volmer plots, indicate that the complexation is an enthalpy-dominating process. The Ksv determined in ethanol is larger than that in water indicating the important competitive role played by the H-bonding interaction. The activation energy is also evaluated. The experimental findings verify the ability of coumarin 314 to form ground and excited state complexes.
Keywords. Coumarin; quenching; fluorescence.
1. Introduction
C o u m a r i n s are a class of dyes which have been used as active media for tuning lasers, as well as solar energy concentrators (Schaefer 1973; Jones et al 1980, 1985; Reynolds a n d Drexhage 1980; K u b i n and Fletcher 1983; Vogel et al 1988; Abdel-Mottaleb et al 1989). An important factor that limits its lasing and luminescent solar concentrator characteristics is the fluorescence quenching. This p h e n o m e n a has been studied in a variety of dyes (Lopez et at I988, 1989; Drexhage 1977; Weber and L a m b e 1976).
Few fundamental studies on the molecular complex formation by inorganic ions as well as organic c o m p o u n d s have been carried out (Livingston and Ke 1950; Livingston et al 1952; K a n o et al 1983, 1987).
In this work the fluorescence quenching of coumarin 314 by cerium (III) ions in aqueous and ethanolic solutions has been studied. The quenching process is studied at various temperatures in order to evaluate some thermodynamic parameters.
0 I1 c o u m a r i n 314
CtHs
2. Experimental details
Coumarin 314 was of laser grade (Eastman K o d a k Company) and was used as received.
Optically pure solvents were used.
325
326 L F M Ismail
The absorption spectra were measured on a Perkin-Elmer Lambda 3B spectro- photometer. A Shimadzu RF 510 spectrofluorometer equipped with a temperature regulated cell holder was used to record the fluorescence and excitation spectra.
3. R e s u l t s a n d d i s c u s s i o n
Photophysical properties of coumarin 314 have been extensively studied (Abdel- Mottaleb et al 1989). The interaction of coumarin 314 and cerium (III) ions is studied in water and ethanol. The absorption spectrum of coumarin 314 is changed upon the addition of Ce(III) ions. Up to a Ce(III) ion-concentration of 1"4 x 10-4 M, an isosbestic point is observed (figure 1). The Benesi-Hildebrand (Kano et al 1987) plot could not be applied under the present experimental conditions.
The fluorescence of coumarin 314 is effectively quenched by Ce(III) ions (figure 2).
Although the fluorescence intensity of coumarin 314 decreases with increasing Ce(III) ion concentration, no shift in the fluorescence maximum is observed.
Competition between static and dynamic fluorescence quenching analysed in terms of the Stern-Volmer equation (Stern and Volmer 1919) gives
lo/1 = (1 + Ksv [Q](1 + K[Q]),
(1)
Relative fluorescence intensity
8 0
6 0
4 0
2 0
0
4 5 0 4 8 0 510 5 4 0
X nm
r ' l
Figure 1.
in EtOH.
0 . 0 0 hi I l x 1 0 - 6 M ---g- 2 x 1 0 - 6 M
4 x 1 0 - 6 M x @x10-6 M
Absorption spectral changes of coumarin 314 upon the addition of Ce(III) ion
Fluorescence quenchino of coumarin 314 by Ce(III) ions 327 Absorbance
0 . 0 8
0 . 0 6
0"0"
0 . 0 2
0 , , , i
8 9 0 4 1 0 4 8 0 4 5 0 4 7 0
X n m
Figure 2.
EtOH.
0 . 0 0 M " 4 - l x 1 0 - 6 M ~< 2 x 1 0 - 6 M 4 x 1 0 - 6 M --x-- ttx10-5 M
Fluorescence spectral changes of coumarin 314 upon the addition of Ce(III) in
where Io and I are the fluorescence intensities in the absence and the presence of quencher, Ksv is the Stern-Volmer constant and K is the equilibrium constant of nonfluorescent 1:1 ground state complex responsible for static quenching, [Q] is the free quencher concentration. For low quencher concentration, (1) reduces to (Mignel et al 1986)
o r
lo/I = 1 + (Ksv + K)[Q]
Io/I = 1 + Ksv[Q]. (2)
A linear Stern-Volmer plot is obtained when coumarin 314 is excited at 437 and 435 nm in ethanol and water respectively, which corresponds to the isosbestic point observed on the absorption spectra.
The efficiency of the fluorescence quenching of coumarin 314 by Ce(III) ions decreases with increasing temperature (figure 3). The observed temperature dependency should be ascribed to the formation of a thermally dissociable molecular complex.
The determined Ksv value from the linear plot of I o / ! - 1 vs [Ce(III)] at various temperatures (figure 3) in ethanol and water are summarized in table 1. The activation energy of the complex formed in ethanol and water is calculated by the least squares
328 L F M lsmail
(a)
I0/I-1 0 . 8
0.6 "
0.4
0.2
0 I I
0 0 . 0 2 0 . 0 4 0 . 0 6 O JOB 0.1
[Q]
2 8 8 K " 2 g S K ~ 2 9 8 K 0 S 0 8 K
(b)
1 O/1-1
0 . 8
0 . 4
0.2
0
4- o
O
I I I
4 8 12
[Q| xl0 6
288 K --I--- 293K - , x - 298k
~03K -..,x- ~0~K
Figure ~ Stem-Volmerplots ~ r t h e quenchingofcoumafin 314 by Ce(III)ionsatdifferent temperaturesin H 2 0 (a) and EtOH (~.
Fluorescence quenching of coumarin 314 by Ce(III) ions 329
T a b l e 1. Temperature dependence of Stern-Volmer rate constants
Ksv and the activation energy AEa for the complexation ofcoumarin 314 with Ce(III) ions.
In Ksv(M- 1) AE~
T(K) in EtOH : in H 2 0 in EtOH : in H 2 0
288 3"54 0-97
293 3'47 0"94
298 3'41 0"90
303 3"31 0'88
- - 2 1 " 4 - - 5 " 6
analysis of log Ksv vs 1/T plot according to the linear relation Ksv = A e x p ( - AEa/RT).
The present study indicates the strong ability of coumarin 314 for the excited state complex formation in ethanol rather than in water. On the basis of the fluorescence behaviour, it has been inferred previously that most of these coumarins are strongly quenched in water due to H-bonding complex formation. These findings verify the ability of coumarin 314 to form molecular complexes.
Acknowledgement
I am deeply indebted to Prof M S A Abdel-Mottaleb, Faculty of Science, Ain Shams University, for his critical discussion and also for the facilities offered in his laboratory.
References
Abdel-Mottaleb M S A, Antonious M S, Abo-Aly M M, Ismail L F M, EI-Sayed B A and Sherief A M K 1989 d. Photochem. PhotobioL AJO 259
Drexhage K M (ed.) 1977 in Dye lasers (Berlin: Springer)
Jones G II, Jackson W R, Choi C and Bergmark W R 1985 J. Phys. Chem. 89 294 Jones G II, Jackson W R and Halpern M A 1980 Chem. Phys. Lett. 72 391 Kano K, Nakajima T and Hashimate S 1987 J. Phys. Chem. 91 6614 Kano K, Sato T, Yamada S and Ogawa T 1983 J. Phys. Chem. 87 566 Kubin R F and Fletcher A N 1983 Chem. Phys. Lett. 99 49
Livingston R and Ke C L 1950 J. Am. Chem. Soc. 72 909
Livingston R, Thompson L and Ramarao M V 1952 J. Am. Chem. Soc. 74 1073 Lopez A F, Ruizojeda P and Lopez A I 1989 J. Lumin. 44 105
Lopez A F, Ruiz Ojeda P and Lopez A I 1988 J. Photochem. Photobiol. A45 313 Miguel M D G M, Formosinho S J and Leitao M L P 1986 J. Photochem. 33 290 Reynolds G A and Drexhage K H 1980 Opt. Commun. 13 222
Schaefer F P 1973 Laser dyes (Berlin: Springer Verlag) Stern O and Volmer M 1919 Phys. Z 20 183
Vogel M, Rettig W, Sens R and Drexhage K H 1986 Chem. Phys. Lett. 147 452 Weber W H and Lambe 1976 J. Appl. Opt. 15 2299
