Effect of metal—ligand ratio on polarization of fluorescence from metal-8-quinolinol complexes

Specrrochimica Printed

Acna, Vol. 40A, No.

in Great

10, PP. 991-993.

1984 0

Bntain.

0584&8539,‘84 53.00 + 0.00 1984 Pergamon Press Ltd.

Effect of metal-ligand ratio on polarization of fluorescence from metal-&quinolinol complexes SALLY D. DOWLING

Department

of Chemistry,

University

and W. RUDOLF SEITZ

of New Hampshire,

Durham,

(Received 18 April 1984; injinalform

New Hampshire

03824, U.S.A.

6 June 1984)

Abstract-When excited at 370 nm in glycerol, 1:l complexes of 8-quinolinol with Ala+, Mg*+ and ZnZt fluoresce with polarizations of 0.327,0.301 and 0.314 respectively. As the ratio of ligand-to-metal increases, the polarizations decrease to limiting values, 0.200 and 0.168 for 2: 1 Mg’ + and Zn* + complexes and 0.111 for the 3: 1 A13+ complex. These changes indicate that excitation energy can transfer from one ligand to another on the same metal center. A simple model is developed to predict the extent of depolarization assuming (1) randomization of excitation energy among ligands associated with the same metal center, (2) the angle between emission transition moments is 60” and (3) that ligand electronic properties are independent of the complex stoichiometry. The predicted extent of depolarization is lower than experimental values. Shifts in excitation, emission and polarization spectra indicate that ligand electronic properties are not independent of ligand-metal ratio.

INTRODUCTION The fluorescence

of metal-S-quinolinol

complexes

tometer. Absorption spectra were obtained on a Varian Cary 219 spectrophotometer. Polarization and intensity data were obtained on an SLM 8000 photon counting spcctrofluorometer (SLM Instruments, Inc., Urbana, IL 61801, U.S.A.). The 45OW xenon arc source radiation passes through the grating monochromator and Glan Thompson polarizer before illuminating the sample. Emission is polarized by film polarizers and detected by photomultiplier tubes. There are two detectors for simultaneous measurement of parallel and perpendicular intensities. Excitation radiation is blocked by long pass filters (Schott Optical; Corion Corp.). Maximum throughput was obtained by using 8 mm slits (16 nm bandpass). Data acquisition for 1 s was electronically corrected for dark current background. Solvent and/or excess ligand background was subtracted prior to calculating polarization. Polarization values represent an average of five data acquisitions. In all cases, the relative standard deviation of intensity measurements was < 1 y0 resulting in polarizations precise to * 0.001.

has

been investigated in some depth[l-31. Because 8quinolinol is essentially nonfluorescent by itself but strongly fluorescent when associated with metal ions including Al(III), Mg(II), Zn(II), Be(II), Ca(I1) and Cd(II), it can be used for sensitive chemical analysis of these ions. However, because several ions form fluorescent complexes, selectivity is often a problem. It has been shown that binary mixtures of metal-8 quinolinol complexes can in some cases be resolved based on differences in the polarization of their fluorescence [4]. This prompted us to investigate the polarization of fluorescence from 8-quinolinol complexes as a function of the ligand-to-metal ratio. These data may not only be of value for analysis but may also provide structural information.

RESULTS

EXPERIMENTAL

1. Polarization

us @and-metal

ratio

Figure 1 shows polarization as a function of 8quinolinol-metal ratio for Al3 +, Mg2 + and Zn2 +. For all three metal ions, the polarizations are almost identical when the metal is in excess. Under these conditions only the 1: 1 complex can form, and the observed polarization is the intrinsic polarization for ligand fluorescence. As the ligand-metal ratio increases, polarization decreases, ultimately reaching a limiting value. The limiting values are similar for Mg2 + and Zn2+ but lower for A13+. Mg2+ and Zn2+ form 2: 1 complexes while Al3 + forms a 3: 1 complex [3].

1. Chemicals All chemicals were reagent grade or better and were used without further purification. Inorganic metal salts and KOH were Baker Analysed Reagents, as was 8-quinolinol. Glycerol was Aldrich Gold Label (99.5+ y0 spectrophotometric grade). Ethanol was absolute reagent grade. All water was deionized and glass distilled. Triethanolamine was distilled reagent grade. 2. Preparation of solutions Stock ligand solutions in absolute ethanol and metal salts in distilled water were diluted to volume with glycerol. In all cases, glycerol comprised at least 99.5 % of the total solvent system. 3. Instrumentation

2. Spectra

All special information was obtained at room temperature (18.5275°C) in glycerol solvent systems. Temperature was constant (within f 1°C) for each ligand system. Excitation and emission maxima for each ligand system were obtained by scanning on a Perkin-Elmer 204 fluorescence spectropho-

Figure 2 shows polarization spectra for the 1:l complexes (excess metal) and the limiting complexes (excess 8-quinilinol). The dependence of polarization on wavelength is similar for all complexes although 991

SALLYD. DOWLINGand W. RUWLF SEITZ

992

035

042

(a) r

i 03n I* q

025-

n

0 *

5 a

s2

0

s j 0240 .N b

025

a

P 015 -

2 n

018-

a

a li 012 -

Ol-

“““L

005-

0

320

Ligond /metal

340

360

Excitation

ratio

Fig. 1. Polarization of S-quinolinol as a function of ligand-metal ratio for excitation at 370 nm. A13+ a LI ~3; Mg2+ 1700 ; Zn’+***.

042

380

wovelength

400

420

(nm)

-(b)

0.36

there are changes in the ratio of polarization for the 1: 1 complex to the polarization of the limiting complex (Table 1). Absorption and emission spectra shift slightly with the stoichiometry of the complexes. Maxima are summarized in Table 2.

03

DISCUSSION The polarization of the 1: 1 complex fluorescence is the intrinsic polarization of 8-quinolinol fluorescence. The decrease in polarization with increasing ligand-tometal ratio indicates that excitation energy can transfer from one ligand to another on the same metal center. This is not at all unexpected since the distance between

0.06 ‘i

0 32( 1

I 340

I 360

Excitation

I 380 wovelength

I 400

I 420

tnm)

Table 1. Variation in experimental with excitation

Al

Mg

Zn

i (nm)

values of cos’w wavelength

Polarization 1:l 3:1

~ co? w

360 370 380 390 400

0.310 0.327 0.342 0.350 0.359

0.095 0.111 0.125 0.137 0.152

0.524 0.544 0.559 0.575 0.595

i(nm)

1:l

2:1

co? w

370 380 390

0.301 0.317 0.336

0.200 0.217 0.233

0.760 0.773 0.778

1bm)

1:l

2:1

370 380 390 400 410

0.314 0.332 0.347 0.364 0.369

0.168 0.191 0.210 0.225 0.241

320

0.672 0.700 0.717 0.725 0.749

340 Excitation

360

380 wavelength

400

420

(nm)

Fig. 2. Polarization spectra, 8-quinolinol complexes. Emission wavelength selected by a long pass filter with 50 %T at518nm.Bandpass:4nm.(a)A13+;l:lAAA,3:l~~U,(b) Zn’+;l:lAAA,2:1 q00,(c)Mg2+;1:lAAA,2:1000.

Metal-ligand

Table

2.

Wavelengths of maximum absorption emission for 8-quinolinol complexes

ratio and fluorescence

and

lmax (nm) Number of Metal

ligands

Absorption

third of the excitation energy would remain on the ligand originally excited while the other two ligands would each receive a third of the excitation energy. In general for a complex of composition ML,

Emission

CO82w = ; Al Al Zn Zn Mg Mg

1 3 1 2 1 2

361 384 376 393 370 366

993

475 505 515 575 500 498

ligands on the same metal center [5] is much less than the critical radius for Forster energy transfer. The critical radii were calculated to be 11.4, 13.8 and 9.9 A for Al3 +, Mg2+ and Zn2+, respectively, using literature lifetime data taken in dimethylformanide [3]. As would be expected, the Mg2+ and Zn2+ complexes have similar limiting polarization in the presence of excess ligand since both these metal ions form 2: 1 octahedral complexes. The 3: 1 Al3 + complex has a lower limiting polarization since in this case the excitation energy can redistribute over all three ligands. The extent to which fluorescence is depolarized by going from the 1: 1 to the highest complex can be expressed in terms of w, the angle of extrinsic depolarization. This is readily calculated from the equation [6]

[cos’ 0 + (n - l)cos21]

(2)

where 1 is the angle between the emission transition moments associated with ligands on the same metal center and w is the angle of extrinsic depolarization. (c) Since the complexes formed in this study are octahedral, the angle between ligands is 60”. If the emission transition moments are linear with respect to the S-quinolinol structure, then 1 is also 60”. This assumes a tram 2: 1 complex which is reasonable in a solvent, glycerol, which can form bidentate complexes. This simple model predicts cos* w = 0.5 for the 3: 1 complex and 0.625 for the 2:l complex. While the trends are correct, the calculated values are smaller than the experimental values. The simple model used for the calculation is not satisfactory. As the spectral shifts and wavelength dependence of cos2 w indicate assumption (a) is an approximation and does not rigorously hold. The other assumptions, particularly (c), may also be oversimplifications. Although we have not described the phenomenon quantitatively, we have successfully demonstrated that the polarization of ligand fluorescence is a function of complex composition, an observation which could be of value for obtaining both analytical and structural information.

(5) =(i43cos~w_ J (l) where P, is the polarization in the absence of any depolarizing process, i.e. the polarization observed for 1: 1 complex, and P is the observed limiting polarization when the highest complex is formed. Calculated values for cos’w are included in Table 1. There should be a relationship between w, the angle of extrinsic depolarization, and the structure of complex. Expected values for cos’w for the complex were calculated assuming the following: (a) The electronic properties of 8-quinolinol in a complex are not perturbed by the addition of the second and third 8-quinolinol ligands to a metal center. (b) The energy transfer process is fast relative to emission causing excitation to be randomly distributed among all the ligands. Thus, in a 3:l complex, one-

Acknowledgements-Partial support for this research was provided by NSF Grant CHE 80-25568. The Cary 219 spectrophotometer was purchased with support from NSF CHE 79-08399. REFERENCES Acta 15, 27 (1959). W. E. OHNESORGEand L. B. ROGERS,Spectrochim. Acta 15, 41 (1959). F. E. LTTLE, D. R. STOREY and M. E. JURICICH, Spectrochim. Acta 29A, 1357 (1973). A. G. STEPANOVA, P. L. TROPINAand 0. A. FAKEEVA, Russ. J. analyt. Chem. 33, 1782 (1978). L. L. MERRITT, R. T. CADY and B. W. MUNDY, Acfa Crystallogr. 7, 473 (1954). G. WEBER, in Fluorescence and Phosphorescence Analysis Ch. 8, (edited by D. M. HERCULES).Wiley-Interscience, New York (1966).

PI W. E. OHNESORGEand L. B. ROGERS,Spectrochim.

PI [31 [41 [51 [61