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Fluorescence of certain metal 8-quinolinolates as a function of solvent and substituents
Spectroehimica Acta, 1960,Vol.16, pp. 49 to 57. PergamonPressLtd. Printedin NorthernIreland
Fluorescence of certain metal S-quinolinolates* as a function of solvent and substituents 0. POPOVYCH~ and L. B. ROGERS Department of Chemistry and Laboratory for Nuclear Science, Massachusetts lkstitute of Technology, Cambridge 39, Mass. (Received 1 June 1959) Absk&-Fluorescence spectra, intensities and efficiencies of zinc chelates of S-quinolinol (oxine), oxine-5-sulfonic acid, 2-methyl-oxine and 6:7-dichloro-oxine were obtained as a function of solvent and substitution. Shifts in fluorescence maxima did not always parallel corresponding solvent shifts for absorption maxima. The fluorescencespectra of chelates of gallium, indium, zinc, ~~esiurn and cadmium with 2-methyl-ox~e were also measured and compared. All except zinc experiencedblue shifts in the fluorescencebands, which were interpretedas arising from stark inhibition of resonance by the 2-substituent. Fluorescenceefficiencywas found to be a eensitivefunction of solvent, being highest in inert solvents. Substitution of the sulfonic-acid group on the ligand gave an unexpected rise in the fluorescenceefficiencyof the chelates, especially in dimethylformamide. Fluorescenceefficiency was found not to be adversely affected by steric hindrance. AnaIytical ~p~catio~ of the results are discussed.
Introduction ALTHOUGH many 8-quinolinolates
(oxinates) are known to fluoresce, investigations of the fluorescence of these chelates havs been largely confined to the development The only systematic spectral studies performed of specific analytical methods. on oxinates so’far [I] were concerned mainly with the variation of ~uorescence with metal ion. In part of the present investigation, the problem was reversed; one metal was kept constant, while variations were effected in the solvent and in the substituent on the ligand. The choice of substituent was restricted almost entirely to commercially available compounds; the choice of metal cation was more difficult. Aluminum, which would ordinarily have been selected because of the intense fluorescence and favorable solubility of its chelates, had to be rejected because it does not chelate with f-methyl-oxine. Other members of the aluminum family as well as most divalent metals were eliminated on the basis of their low fluorescence intensities or poor solubility. Zinc was selected by the process of elimination. All fluorimetric studies of oxinates were conducted in parallel with corresponding absorptiomet~~ experiments. Results Excitation
spectra
Fluorescence of oxinates is excited primarily by absorption corresponding to the low-frequency band, as in the case of protonated ligand in concentrated sulfuric * 8-Hydroxyquinoline;oxine. t Allied Chemical and Dye Fellow 1957-1958; present address: Esso Research Center, Linden, NJ. [l] W. E. OHNESORCEand L. B. ROGERS,Spectrochim. Acta 14, 27, 41 (1959). 4
49
O.POPOwCH
ROGEIW
andL.B.
acid [2]. Although the measured excitation spectra were poorly defined owing to the low intensities, the sample excitation spectrum in Fig. 1 shows that some of the fluorescence is also caused by absorption corresponding to the short-wavelength band. Even though excitation to a higher excited state does occur, this must 4w 20 310
I
320 ,
I
I
32,000
330
340
I
350
I
30,coo
360
370 360 390 400 410 42C
I
1
26,000
I
I
I
26,000
24,000
3,cm' Fig. 1. Excitation spectrum for 1.00X 10e4 M zinc-2-methyl-oxinate in chloroform using an excitation bandwidth of 13.2 rnp from a xenon lamp and measuring intmmity at 540 m,w
followed by a rapid internal conversion to the lowest excited level, from which all emission takes place. Table 1 shows that with the low concentrations employed (14)Ox 10-JM) the coincidence between absorption and excitation maxima is remarkably good.
be
Table 1. Comparison of excitation and absorption maxima of zinc oxinates (1.00 x 1O-4 M)
-
T
Absorption
Excitation
Difference
Solvent
Solution
A6
(44
9 (cm-l)
(&
t (cm-l)
(cm-l)
379 381 386
26,400 26,200 25,900
385 385 256
26,000 26,000 25,600
400 200 300
Oxinate
CHCl, THF* CHCl,
2-Methyl-oxinate * Tetrahydrofuran.
Fluorescence
of zirtc oxinates as a function
of solved
Regardless of the nature of substituent or solvent, fluorescence spectra of zinc oxinates appeared as single, broad, structureless bands with flat maxima. For that reason, only a sample spectrum of zinc oxinate in chloroform is shown (Fig. 2). The remaining fluorescence results are summarized in Table 2. Both observed intensities and those corrected for phototube response are tabulated. The former [2] 0. POPOVYCH
and L. B. ROQERS,
Spectrochim. A&
50
15,584
(1959).
Fluorescence of certain metal S-quinolinolates as a function of solvent and substituents
Fig. 2. Observed fluorescence spectrum of I.00 x 1OW M zinc oxinate in chloroform using 365 rnp radiation from a high-pressure mercury lamp. Table 2. Fluorescence maxima of zinc chelates as a function of solvents and substituents*
-
T
Fluorescence Chelate
2-methyl-oxinate 5:7-Dichloro-oxinate Oxinate-5-sulfonate (pH 6.45) (pH 5.04)
Afi (cm-l):
Solvent (mp)
(cz--l)
Intensity? obs. corr.
5408 525 520 560 535 525 640 555
18,500 19,000 19,200 17,900 18,700 19,000 18,500 18,000
8.20 4.70 3.50 2.90 12.0 5.30 11.6 5.72
18.9 9.34 6.64 8.66 26.3 10.5 26.8 15.9
31.9 14.8 16.4 19.2 47.5 96.3 50.3 28.0
7900 7400 7000 7000 7500 7500 7400 6600
525 525 530
19,000 19,000 18,900
3.55 5.30 10.2
7.06 10.5 21.2
8.73 16.7 73.6
8600 8600 6100
Iz
oxinate
Relative fluorescence
CHCl, C,H,OH* cc&* * Dmtt THFtt (C,H&O CHCl, CHCl, water
DMFtt
*
**
-
l Unless otherwise stated the solutions are 1.00 x lo+ M. t Quinine equivalents x 10. $ Energy difference between absorption and fluorescence maximum. 9 Observed maximum. l * Saturated solution. tt DMF is dimethylformamide; THF is tetrahydrofuran. $$ Intensity normalized with respect to absorption (see text).
51
NOlTIl.
ntensity$ j
0. POPOVYCR and
L. B. ROGERS
is useful for analytical purposes; the latter, as measure of the true fluorescence ability of a compound. The normalized intensity is the corrected intensity of emission divided by the average fraction of light absorbed (1 - T), in the wavelength interval used in the excitation. The last column states frequency differences between the tabulated fluorescence maxima and the corresponding (low-frequency) absorption maxima. Table 2 presents fluorescence maxima of zinc oxinate in the six solvents in which the shifts in the low-frequency absorption band have been studied earlier [3]. It is worth noting that the tabulated frequency differences between absorption and fluorescence maxima are not constant with solvent, indicating that solvent shifts of fluorescence bands are not always parallel to those of the absorption bands. The blue shift of an ethanol solution with respect to a chloroform solution of an oxinate has been observed before [l]. Of major interest is the observed dependence on solvent of the fluorescence efficiency. The highest fluorescence efficiencies are obtained for diethyl ether and tetrahydrofuran, while the commonly-used chloroform is in third place. Solutions of ethanol, dimethylformamide and carbon tetrachloride show considerably lower emittances. A decrease in fluorescence of oxinates on going from chloroform to ethanol has already been reported. Otherwise very little has been reported on the dependence of fluorescence efficiency on the nature of solvent. BOWEN and COATES[4] observed that solutions of rubrene showed decreasing fluorescence efficiencies in the solvent series : cyclohexane, hexane, benzene, toluene, dioxane, butanol, ethanol, acetone. Solvent quenching was interpreted as arising from solvent-solute interactions either before or after absorption. In the former case, complexes or “inner filters” trap exciting radiation. In the latter case, excited solute molecules lose their excess energy either by collision with solvent or by some other radiationless process dependent on solvent. From the present data it is apparent that highest fluorescence efficiencies are observed in relatively inert solvents, like ethers. These have little tendency to form complexes with either the ground or the excited state of the solute. The lower efficiencies observed for chloroform and especially for carbon tetrachloride are probably a function of the well-known quenching influence of halogens, which facilitates intersystem crossing to an excited triplet state [5]. Dimethylformamide seems to be a special case, as indicated by the abnormal red shift experienced by the absorption bands of zinc oxinate in that solvent. In view of our previous experience with N-methylformamide [3], it is not unlikely that dimethylformamide attacks the metal ion, loosening the entire chelate structure. Should this facilitate either partial dissociation in the excited state, or partial freeing of the n-electrons on the nitrogen, the reduction in fluorescence could easily be accounted for. The low emission efficiency observed in alcohol can be interpreted in terms of hydrogenbonding with the solvent, which results in quenching analogous to that found for aqueous solutions of protonated ligands reported in an earlier paper [Z]. [3] 0. POPOVYCH and L. B. ROGERS,J. Am. Chem. Sot.
81,4469 (1959). [4] E. J. BOWEN and E. COATES,J. Chem. Sot. 105 (1947). [5] M. KASHA, J. Chem. Phys. 20, 71 (1952).
52
Fluorescence of certain metal S-quinoliuolates as a function of solvent and substituents
Fluorescence of zinc oxinates as a function of substituent Fluorescence spectra, intensities and efficiencies of zinc oxinate, * zinc-oxinate5-sulfonate, zinc-Z-methyl-oxinate ‘and zinc-5:7-~c~oro-oxinate* have been compared, whenever possible, in the same solvent. Qualitatively, the shifts in the fluorescence maxima of substituted chelates are parallel to the corresponding changes in absorption maxima [6]. Notable exceptions to this behavior are the fluorescence spectra of zinc oxinate and zinc-oxinate-Csulfonate in dimethylformamide. While their absorption spectra are the same, the positions of their fluorescence maxima differ by 1000 cm- l, The different specific effects of ~methylform~ amide on the fluorescence of the two chelates are reflected even more in the observed fluorescence efficiencies. While the efficiency of the sulfonated chelate in dimethylformamide is very high (73~6), the efficiency of zinc oxinate in the same solvent is one of its lowest (19.2). Unfortunately no other common solvent for the two
4
5
6
a
7
9
IO
II
PH
Fig. 3. Ruorescence intensity of aqueous zinc-ox-inate-5-sulfona~ as a function of pH.
chelates could be found, but the possibility exists that zinc-oxinate-5-sulfonate is intrinsically more fluorescent than its unsubstituted analogue. The fact that zinc oxinate loses its fluorescence almost entirely in a 50-50 mixture of water and dimethylformamide, while zinc-oxinate-5-sulfonate fluoresces appreciably in pure water, may constitute ad~tional evidence to this effect. A broad re-examination of the effect of the sulfonic-acid group, which reputedly has no influence on fluorescence, is in order. The water-soluble zinc-oxinate-5sulfonate was interesting for another reason. When this substance was dissolved in distilled water, the resulting lo-* M solution had a pH of 6.45. We expected upon adding sodium hydroxide that the fluorescence intensity might rise slightly at first due to less extensive dissociation of chelate and then drop continuously in more alkaline solutions as the result of attack by hydroxide. Upon adding hydrochloric acid, a continuous reduction of fluorescence due to decomposition of the ohelate was expected. Fig. 3 shows that the experimentally observed behavior was entirely different. * Anhydrous zinc oxinate was used, which, unlike its dihydrate, was quite soluble in chloroform. In solid state the dihydrate of zinc-5,7-dichlom-oxinate was yellow and fluorescent, while the anhydrous product was orange and non-fluorescent. Both fluoresced in chloroform solutions. [6] 0. POPOVYCH.Ph.D. Thesis, M.I.T. (1959).
53
0. POPOVYCH and L. B. ROGERS Maximum fluorescence occurred at pH 5.05, where absorption spectra showed the chelate to be partially decomposed. In addition to a chelate peak at 361 rnp, there was another due to the ligand at 316 rnp. Calculations indicated that the concentration of residual chelate at pH 5.05 was about 6.6 x 1O-5 M compared to the 1-O x 1O-4 M originally dissolved. Thus, in the solution of maximum fluorescence the chelate-to-ligand ratio was approximately 1 : 1. OHNESORGE and ROGERS [l] had previously reported similar behavior for alcoholic solutions of aluminum oxinate. After extensive investigation, they concluded that the most probable explanation for the observed enhancement of fluorescence was the existence of a more strongly fluorescent intermediate. The present finding tends to corroborate this view. Zinc-2-methyl-oxinate shows more intense fluorescence than zinc oxinate. Although methyl groups are known to intensify fluorescence, the reason for this phenomenon is by no means clear. On the other hand, the dichloro-substituted chelate exhibits the expected reduction in fluorescence efficiency. This compound must be a victim of internal quenching of fluorescence via the triplet state. Dissociation of the chlorine-carbon bond is excluded as a quenching possibility due to the low energy of excitation at 365 mp. Injiuence
of 2methyl
substitution on spectral shifts
The object of this study was to investigate whether or not the spectral shifts caused by a given substituent on the ligand were dependent on the metal ion. For that purpose the fluorescence maxima, intensities and efficiencies of the 2methyl-oxinates of gallium, indium, zinc, magnesium and cadmium (Table 3) were compared with those of the oxinates of the same series (Table 4). Table 3. Fluorescence maxima of Z-methyl-oxinates (1.00 x 1O-4 M)
in chloroform
Relative fluorescence Metal
I (cm-l) Intensity
26.8 92.0 10.5 6.19 22.9
Norm. intensity:
50.3 134 14.7 16.8 112
solution with & fine suspension. z Intensity normalized with respect to absorption (see text).
t Saturated
It can be seen that fluorescence maxima of all 2-methyl-chelates, except zinc, have experienced blue shifts relative to their unsubstituted analogues. Earlier we observed similar, though somewhat greater, relative shifts in the corresponding 54
Fluorescence of certain metal S-quinolinolates as a function of solvent and substituents Table 4. Comparison of fluorescence maxima of 2-methyl-oxinates
-
!_
Oxinates
and oxinates in chloroform 2-Methyl-oxinates
_
Metal
I
Zna+ Ga3+ Ins+ Mg2+ Mg2+
F
bv)
(cm-l)
540* 520 530* 500 518
18,500 19,200 18,900 20,000 19,300
A* (cm-l)
7400 8300 7300
§
8100
-L
* Not corrected.
7 Values reported by OHNESORGEand ROGERS[I]. 2 In dimethylformamide. 3 Not measurable due to overlap of high- and low-frequency absorption bands.
of absorption spectra [6] and have attributed them to steric inhibition of A comparison of energy differences (absorption resonance by the 2substituent. minus fluorescence) for oxinates and 2-methyl-oxinates shows them to be greater, in general, for the latter. This means, of course, that fluorescence bands undergo smaller blue shifts than absorption bands. Apparently the excited state is not affected as much by steric considerations as the ground state. It is of interest to note that steric hindrance appears to have no adverse effect on the fluorescence efficiency. Thus, gallium-2-methyl-oxinate, which according to absorption data is sterically hindered, is much more fluorescent than zinc or indium chelates, both of which are less affected by steric effects. Zinc-2-methyloxinate, which experiences no blue shifts in absorption bands relative to oxinate, also shows no change in its fluorescence spectrum. pairs
Fluorescence
of magnesium chelates
Because the absorption behavior of magnesium oxinates was observed to differ from the usual properties of oxine chelates, the fluorescence spectra of magnesium chelates were subjected to a separate study. In order to eliminate the variation due to solvent, dimethylformamide was chosen as the common medium for magnesium chelates of oxine, oxine&sulfonic acid and 2-methyl-oxine. The results of this study are shown in Table 5. The striking property of magnesium chelates is their high fluorescence efficiency in dimethylformamide, relative to other solvents. The solvent shifts involved are also far from the ordinary. Thus, magnesium-oxinate-5-sulfonate, with its maximum in dimethylformamide at 450 rnp, is the first blue-fluorescing chelate encountered in this work. This large blue shift is specific for the fluorescence band, because absorption maxima for this chelate in chloroform and in dimethylformamide are identical. Analytical
implications
Possible improvements of existing fluorimetric with oxine can be seen. Limits of detectability 55
determinations of metal cations of oxinates can be extended to
0. POPOTYCH
and L. B. ROGERS
lower concentrations by the use of solvents in which fluorescence efficiency of chelates is higher than in the generally-employed chloroform. The recommended solvents are tetrahy~ofuran and dimethy~ormamide. Use of the latter for magnesium chelates and for zinc-ox~ate-5-sulfonate brings about a twofold increase in fluorescence, relative to the usual solvents, chloroform and water, respectively. Compared to chloroform, the above solvents have the disadvantage of Table 5. Fluorescence maxima of magnesium oxinates (l-00 x lo-4M) Jlelative fluoreecence Oxinate
Solvent
A
I
(m/d1
(cm-l)
I _-
L
Oxinate
CHCl, DMF CHCl, DMF Water DMF
2-Methyl-oxinate Oxinate-5-sulfonate
-
20,000 18,900 19,800 19,300 19,200 22,200
500 530 505 518
520 450
Intensity
Norm. intensity * -
37.3 71.9 B-19 83.9 28.6 18.0
73.9 134 10.8 179 42.2 129
i
* Intensity normalized with respect to absorption (see text).
miscibility with water. Thus, they cannot be used to extract chelates from aqueous Diethyl ether, which permits the highest fluorescence efficiency, is solutions. handicapped by its poor solub~ity properties for chelates. Other ethers may be worthy of investigation. Fluorimetric methods could also be improved by taking advantage of the 2-Methyl-oxinates, in addition to their known specific effects of substituents. selectivity in precipitation, also exhibit more intense fluorescence than unsubstituted oxinates. The sulfonic-acid group, too, deserves further attention Finally, it is not out of place to mention that the reputedly selective 5-nitroso-oxine is useless for fluorimetric purposes.
Experimental Methods for preparing the chelates were approximately the same as those reported in HOLLINGSHEAD [Tj. Chelates were usually dried for several hours at about 140°C to obtain the anhydrous product. Drying at room temperature over potassium hydroxide produced zinc chelates which were dihydrates and possessed markedly different properties. Attempts to prepare zinc chelates of B-nitroso-oxine and 5-amino-oxine were unsuccessful. The poor chelating properties of the former have been attributed to its acidic properties [8’J but more probably reflect the fact that it exists largely [7] R. G. W. HOLLIXWHEAD, [S] H. IRVING, E. J. BUTLER
O&ne aladits De&vat&esVols. I-IV. Butterwotihs, London (1954-1956). and M. F. RINQ, J. C&m. Sm. 1489 (194Q).
56
Fluorescence of certain metal 8-quinolinolates as a function of solvent and substituents
The 5-amino-oxine did produce a yellow prein the form of the oxime-quinone. cipitate which extracted into chloroform and fluoresced. However, it rapidly changed color owing to air oxidation, so it was reluctantly abandoned. Earlier attempts to form chelates had also failed [9]. Chemical analyses of the chelates for purity proved to be less satisfactory than An accurately weighed amount of chelate was a spectrophotometric procedure. dissolved in concentrated hydrochloric acid and diluted twentyfold to volume. The resulting solution was measured at two or more maxima and the absorbancy values compared with those obtained previously for the corresponding 8-hydroxyquinolinium ion. Discrepancies between observed and calculated absorbancies were usually less than 1 per cent relative. Apparatus and procedures for obtaining excitation and fluorescence spectra have been reported [ 1, 21. Acknowledgement--This Contract AT(30-l)-905.
[9]
A.
ALBERT
work was supported in part by the Atomic
and D. MAGRATH, Biochem. J. 41, 534 (1947).
57
Energy Commission under
Fluorescence of certain metal S-quinolinolates* as a function of solvent and substituents 0. POPOVYCH~ and L. B. ROGERS Department of Chemistry and Laboratory for Nuclear Science, Massachusetts lkstitute of Technology, Cambridge 39, Mass. (Received 1 June 1959) Absk&-Fluorescence spectra, intensities and efficiencies of zinc chelates of S-quinolinol (oxine), oxine-5-sulfonic acid, 2-methyl-oxine and 6:7-dichloro-oxine were obtained as a function of solvent and substitution. Shifts in fluorescence maxima did not always parallel corresponding solvent shifts for absorption maxima. The fluorescencespectra of chelates of gallium, indium, zinc, ~~esiurn and cadmium with 2-methyl-ox~e were also measured and compared. All except zinc experiencedblue shifts in the fluorescencebands, which were interpretedas arising from stark inhibition of resonance by the 2-substituent. Fluorescenceefficiencywas found to be a eensitivefunction of solvent, being highest in inert solvents. Substitution of the sulfonic-acid group on the ligand gave an unexpected rise in the fluorescenceefficiencyof the chelates, especially in dimethylformamide. Fluorescenceefficiency was found not to be adversely affected by steric hindrance. AnaIytical ~p~catio~ of the results are discussed.
Introduction ALTHOUGH many 8-quinolinolates
(oxinates) are known to fluoresce, investigations of the fluorescence of these chelates havs been largely confined to the development The only systematic spectral studies performed of specific analytical methods. on oxinates so’far [I] were concerned mainly with the variation of ~uorescence with metal ion. In part of the present investigation, the problem was reversed; one metal was kept constant, while variations were effected in the solvent and in the substituent on the ligand. The choice of substituent was restricted almost entirely to commercially available compounds; the choice of metal cation was more difficult. Aluminum, which would ordinarily have been selected because of the intense fluorescence and favorable solubility of its chelates, had to be rejected because it does not chelate with f-methyl-oxine. Other members of the aluminum family as well as most divalent metals were eliminated on the basis of their low fluorescence intensities or poor solubility. Zinc was selected by the process of elimination. All fluorimetric studies of oxinates were conducted in parallel with corresponding absorptiomet~~ experiments. Results Excitation
spectra
Fluorescence of oxinates is excited primarily by absorption corresponding to the low-frequency band, as in the case of protonated ligand in concentrated sulfuric * 8-Hydroxyquinoline;oxine. t Allied Chemical and Dye Fellow 1957-1958; present address: Esso Research Center, Linden, NJ. [l] W. E. OHNESORCEand L. B. ROGERS,Spectrochim. Acta 14, 27, 41 (1959). 4
49
O.POPOwCH
ROGEIW
andL.B.
acid [2]. Although the measured excitation spectra were poorly defined owing to the low intensities, the sample excitation spectrum in Fig. 1 shows that some of the fluorescence is also caused by absorption corresponding to the short-wavelength band. Even though excitation to a higher excited state does occur, this must 4w 20 310
I
320 ,
I
I
32,000
330
340
I
350
I
30,coo
360
370 360 390 400 410 42C
I
1
26,000
I
I
I
26,000
24,000
3,cm' Fig. 1. Excitation spectrum for 1.00X 10e4 M zinc-2-methyl-oxinate in chloroform using an excitation bandwidth of 13.2 rnp from a xenon lamp and measuring intmmity at 540 m,w
followed by a rapid internal conversion to the lowest excited level, from which all emission takes place. Table 1 shows that with the low concentrations employed (14)Ox 10-JM) the coincidence between absorption and excitation maxima is remarkably good.
be
Table 1. Comparison of excitation and absorption maxima of zinc oxinates (1.00 x 1O-4 M)
-
T
Absorption
Excitation
Difference
Solvent
Solution
A6
(44
9 (cm-l)
(&
t (cm-l)
(cm-l)
379 381 386
26,400 26,200 25,900
385 385 256
26,000 26,000 25,600
400 200 300
Oxinate
CHCl, THF* CHCl,
2-Methyl-oxinate * Tetrahydrofuran.
Fluorescence
of zirtc oxinates as a function
of solved
Regardless of the nature of substituent or solvent, fluorescence spectra of zinc oxinates appeared as single, broad, structureless bands with flat maxima. For that reason, only a sample spectrum of zinc oxinate in chloroform is shown (Fig. 2). The remaining fluorescence results are summarized in Table 2. Both observed intensities and those corrected for phototube response are tabulated. The former [2] 0. POPOVYCH
and L. B. ROQERS,
Spectrochim. A&
50
15,584
(1959).
Fluorescence of certain metal S-quinolinolates as a function of solvent and substituents
Fig. 2. Observed fluorescence spectrum of I.00 x 1OW M zinc oxinate in chloroform using 365 rnp radiation from a high-pressure mercury lamp. Table 2. Fluorescence maxima of zinc chelates as a function of solvents and substituents*
-
T
Fluorescence Chelate
2-methyl-oxinate 5:7-Dichloro-oxinate Oxinate-5-sulfonate (pH 6.45) (pH 5.04)
Afi (cm-l):
Solvent (mp)
(cz--l)
Intensity? obs. corr.
5408 525 520 560 535 525 640 555
18,500 19,000 19,200 17,900 18,700 19,000 18,500 18,000
8.20 4.70 3.50 2.90 12.0 5.30 11.6 5.72
18.9 9.34 6.64 8.66 26.3 10.5 26.8 15.9
31.9 14.8 16.4 19.2 47.5 96.3 50.3 28.0
7900 7400 7000 7000 7500 7500 7400 6600
525 525 530
19,000 19,000 18,900
3.55 5.30 10.2
7.06 10.5 21.2
8.73 16.7 73.6
8600 8600 6100
Iz
oxinate
Relative fluorescence
CHCl, C,H,OH* cc&* * Dmtt THFtt (C,H&O CHCl, CHCl, water
DMFtt
*
**
-
l Unless otherwise stated the solutions are 1.00 x lo+ M. t Quinine equivalents x 10. $ Energy difference between absorption and fluorescence maximum. 9 Observed maximum. l * Saturated solution. tt DMF is dimethylformamide; THF is tetrahydrofuran. $$ Intensity normalized with respect to absorption (see text).
51
NOlTIl.
ntensity$ j
0. POPOVYCR and
L. B. ROGERS
is useful for analytical purposes; the latter, as measure of the true fluorescence ability of a compound. The normalized intensity is the corrected intensity of emission divided by the average fraction of light absorbed (1 - T), in the wavelength interval used in the excitation. The last column states frequency differences between the tabulated fluorescence maxima and the corresponding (low-frequency) absorption maxima. Table 2 presents fluorescence maxima of zinc oxinate in the six solvents in which the shifts in the low-frequency absorption band have been studied earlier [3]. It is worth noting that the tabulated frequency differences between absorption and fluorescence maxima are not constant with solvent, indicating that solvent shifts of fluorescence bands are not always parallel to those of the absorption bands. The blue shift of an ethanol solution with respect to a chloroform solution of an oxinate has been observed before [l]. Of major interest is the observed dependence on solvent of the fluorescence efficiency. The highest fluorescence efficiencies are obtained for diethyl ether and tetrahydrofuran, while the commonly-used chloroform is in third place. Solutions of ethanol, dimethylformamide and carbon tetrachloride show considerably lower emittances. A decrease in fluorescence of oxinates on going from chloroform to ethanol has already been reported. Otherwise very little has been reported on the dependence of fluorescence efficiency on the nature of solvent. BOWEN and COATES[4] observed that solutions of rubrene showed decreasing fluorescence efficiencies in the solvent series : cyclohexane, hexane, benzene, toluene, dioxane, butanol, ethanol, acetone. Solvent quenching was interpreted as arising from solvent-solute interactions either before or after absorption. In the former case, complexes or “inner filters” trap exciting radiation. In the latter case, excited solute molecules lose their excess energy either by collision with solvent or by some other radiationless process dependent on solvent. From the present data it is apparent that highest fluorescence efficiencies are observed in relatively inert solvents, like ethers. These have little tendency to form complexes with either the ground or the excited state of the solute. The lower efficiencies observed for chloroform and especially for carbon tetrachloride are probably a function of the well-known quenching influence of halogens, which facilitates intersystem crossing to an excited triplet state [5]. Dimethylformamide seems to be a special case, as indicated by the abnormal red shift experienced by the absorption bands of zinc oxinate in that solvent. In view of our previous experience with N-methylformamide [3], it is not unlikely that dimethylformamide attacks the metal ion, loosening the entire chelate structure. Should this facilitate either partial dissociation in the excited state, or partial freeing of the n-electrons on the nitrogen, the reduction in fluorescence could easily be accounted for. The low emission efficiency observed in alcohol can be interpreted in terms of hydrogenbonding with the solvent, which results in quenching analogous to that found for aqueous solutions of protonated ligands reported in an earlier paper [Z]. [3] 0. POPOVYCH and L. B. ROGERS,J. Am. Chem. Sot.
81,4469 (1959). [4] E. J. BOWEN and E. COATES,J. Chem. Sot. 105 (1947). [5] M. KASHA, J. Chem. Phys. 20, 71 (1952).
52
Fluorescence of certain metal S-quinoliuolates as a function of solvent and substituents
Fluorescence of zinc oxinates as a function of substituent Fluorescence spectra, intensities and efficiencies of zinc oxinate, * zinc-oxinate5-sulfonate, zinc-Z-methyl-oxinate ‘and zinc-5:7-~c~oro-oxinate* have been compared, whenever possible, in the same solvent. Qualitatively, the shifts in the fluorescence maxima of substituted chelates are parallel to the corresponding changes in absorption maxima [6]. Notable exceptions to this behavior are the fluorescence spectra of zinc oxinate and zinc-oxinate-Csulfonate in dimethylformamide. While their absorption spectra are the same, the positions of their fluorescence maxima differ by 1000 cm- l, The different specific effects of ~methylform~ amide on the fluorescence of the two chelates are reflected even more in the observed fluorescence efficiencies. While the efficiency of the sulfonated chelate in dimethylformamide is very high (73~6), the efficiency of zinc oxinate in the same solvent is one of its lowest (19.2). Unfortunately no other common solvent for the two
4
5
6
a
7
9
IO
II
PH
Fig. 3. Ruorescence intensity of aqueous zinc-ox-inate-5-sulfona~ as a function of pH.
chelates could be found, but the possibility exists that zinc-oxinate-5-sulfonate is intrinsically more fluorescent than its unsubstituted analogue. The fact that zinc oxinate loses its fluorescence almost entirely in a 50-50 mixture of water and dimethylformamide, while zinc-oxinate-5-sulfonate fluoresces appreciably in pure water, may constitute ad~tional evidence to this effect. A broad re-examination of the effect of the sulfonic-acid group, which reputedly has no influence on fluorescence, is in order. The water-soluble zinc-oxinate-5sulfonate was interesting for another reason. When this substance was dissolved in distilled water, the resulting lo-* M solution had a pH of 6.45. We expected upon adding sodium hydroxide that the fluorescence intensity might rise slightly at first due to less extensive dissociation of chelate and then drop continuously in more alkaline solutions as the result of attack by hydroxide. Upon adding hydrochloric acid, a continuous reduction of fluorescence due to decomposition of the ohelate was expected. Fig. 3 shows that the experimentally observed behavior was entirely different. * Anhydrous zinc oxinate was used, which, unlike its dihydrate, was quite soluble in chloroform. In solid state the dihydrate of zinc-5,7-dichlom-oxinate was yellow and fluorescent, while the anhydrous product was orange and non-fluorescent. Both fluoresced in chloroform solutions. [6] 0. POPOVYCH.Ph.D. Thesis, M.I.T. (1959).
53
0. POPOVYCH and L. B. ROGERS Maximum fluorescence occurred at pH 5.05, where absorption spectra showed the chelate to be partially decomposed. In addition to a chelate peak at 361 rnp, there was another due to the ligand at 316 rnp. Calculations indicated that the concentration of residual chelate at pH 5.05 was about 6.6 x 1O-5 M compared to the 1-O x 1O-4 M originally dissolved. Thus, in the solution of maximum fluorescence the chelate-to-ligand ratio was approximately 1 : 1. OHNESORGE and ROGERS [l] had previously reported similar behavior for alcoholic solutions of aluminum oxinate. After extensive investigation, they concluded that the most probable explanation for the observed enhancement of fluorescence was the existence of a more strongly fluorescent intermediate. The present finding tends to corroborate this view. Zinc-2-methyl-oxinate shows more intense fluorescence than zinc oxinate. Although methyl groups are known to intensify fluorescence, the reason for this phenomenon is by no means clear. On the other hand, the dichloro-substituted chelate exhibits the expected reduction in fluorescence efficiency. This compound must be a victim of internal quenching of fluorescence via the triplet state. Dissociation of the chlorine-carbon bond is excluded as a quenching possibility due to the low energy of excitation at 365 mp. Injiuence
of 2methyl
substitution on spectral shifts
The object of this study was to investigate whether or not the spectral shifts caused by a given substituent on the ligand were dependent on the metal ion. For that purpose the fluorescence maxima, intensities and efficiencies of the 2methyl-oxinates of gallium, indium, zinc, magnesium and cadmium (Table 3) were compared with those of the oxinates of the same series (Table 4). Table 3. Fluorescence maxima of Z-methyl-oxinates (1.00 x 1O-4 M)
in chloroform
Relative fluorescence Metal
I (cm-l) Intensity
26.8 92.0 10.5 6.19 22.9
Norm. intensity:
50.3 134 14.7 16.8 112
solution with & fine suspension. z Intensity normalized with respect to absorption (see text).
t Saturated
It can be seen that fluorescence maxima of all 2-methyl-chelates, except zinc, have experienced blue shifts relative to their unsubstituted analogues. Earlier we observed similar, though somewhat greater, relative shifts in the corresponding 54
Fluorescence of certain metal S-quinolinolates as a function of solvent and substituents Table 4. Comparison of fluorescence maxima of 2-methyl-oxinates
-
!_
Oxinates
and oxinates in chloroform 2-Methyl-oxinates
_
Metal
I
Zna+ Ga3+ Ins+ Mg2+ Mg2+
F
bv)
(cm-l)
540* 520 530* 500 518
18,500 19,200 18,900 20,000 19,300
A* (cm-l)
7400 8300 7300
§
8100
-L
* Not corrected.
7 Values reported by OHNESORGEand ROGERS[I]. 2 In dimethylformamide. 3 Not measurable due to overlap of high- and low-frequency absorption bands.
of absorption spectra [6] and have attributed them to steric inhibition of A comparison of energy differences (absorption resonance by the 2substituent. minus fluorescence) for oxinates and 2-methyl-oxinates shows them to be greater, in general, for the latter. This means, of course, that fluorescence bands undergo smaller blue shifts than absorption bands. Apparently the excited state is not affected as much by steric considerations as the ground state. It is of interest to note that steric hindrance appears to have no adverse effect on the fluorescence efficiency. Thus, gallium-2-methyl-oxinate, which according to absorption data is sterically hindered, is much more fluorescent than zinc or indium chelates, both of which are less affected by steric effects. Zinc-2-methyloxinate, which experiences no blue shifts in absorption bands relative to oxinate, also shows no change in its fluorescence spectrum. pairs
Fluorescence
of magnesium chelates
Because the absorption behavior of magnesium oxinates was observed to differ from the usual properties of oxine chelates, the fluorescence spectra of magnesium chelates were subjected to a separate study. In order to eliminate the variation due to solvent, dimethylformamide was chosen as the common medium for magnesium chelates of oxine, oxine&sulfonic acid and 2-methyl-oxine. The results of this study are shown in Table 5. The striking property of magnesium chelates is their high fluorescence efficiency in dimethylformamide, relative to other solvents. The solvent shifts involved are also far from the ordinary. Thus, magnesium-oxinate-5-sulfonate, with its maximum in dimethylformamide at 450 rnp, is the first blue-fluorescing chelate encountered in this work. This large blue shift is specific for the fluorescence band, because absorption maxima for this chelate in chloroform and in dimethylformamide are identical. Analytical
implications
Possible improvements of existing fluorimetric with oxine can be seen. Limits of detectability 55
determinations of metal cations of oxinates can be extended to
0. POPOTYCH
and L. B. ROGERS
lower concentrations by the use of solvents in which fluorescence efficiency of chelates is higher than in the generally-employed chloroform. The recommended solvents are tetrahy~ofuran and dimethy~ormamide. Use of the latter for magnesium chelates and for zinc-ox~ate-5-sulfonate brings about a twofold increase in fluorescence, relative to the usual solvents, chloroform and water, respectively. Compared to chloroform, the above solvents have the disadvantage of Table 5. Fluorescence maxima of magnesium oxinates (l-00 x lo-4M) Jlelative fluoreecence Oxinate
Solvent
A
I
(m/d1
(cm-l)
I _-
L
Oxinate
CHCl, DMF CHCl, DMF Water DMF
2-Methyl-oxinate Oxinate-5-sulfonate
-
20,000 18,900 19,800 19,300 19,200 22,200
500 530 505 518
520 450
Intensity
Norm. intensity * -
37.3 71.9 B-19 83.9 28.6 18.0
73.9 134 10.8 179 42.2 129
i
* Intensity normalized with respect to absorption (see text).
miscibility with water. Thus, they cannot be used to extract chelates from aqueous Diethyl ether, which permits the highest fluorescence efficiency, is solutions. handicapped by its poor solub~ity properties for chelates. Other ethers may be worthy of investigation. Fluorimetric methods could also be improved by taking advantage of the 2-Methyl-oxinates, in addition to their known specific effects of substituents. selectivity in precipitation, also exhibit more intense fluorescence than unsubstituted oxinates. The sulfonic-acid group, too, deserves further attention Finally, it is not out of place to mention that the reputedly selective 5-nitroso-oxine is useless for fluorimetric purposes.
Experimental Methods for preparing the chelates were approximately the same as those reported in HOLLINGSHEAD [Tj. Chelates were usually dried for several hours at about 140°C to obtain the anhydrous product. Drying at room temperature over potassium hydroxide produced zinc chelates which were dihydrates and possessed markedly different properties. Attempts to prepare zinc chelates of B-nitroso-oxine and 5-amino-oxine were unsuccessful. The poor chelating properties of the former have been attributed to its acidic properties [8’J but more probably reflect the fact that it exists largely [7] R. G. W. HOLLIXWHEAD, [S] H. IRVING, E. J. BUTLER
O&ne aladits De&vat&esVols. I-IV. Butterwotihs, London (1954-1956). and M. F. RINQ, J. C&m. Sm. 1489 (194Q).
56
Fluorescence of certain metal 8-quinolinolates as a function of solvent and substituents
The 5-amino-oxine did produce a yellow prein the form of the oxime-quinone. cipitate which extracted into chloroform and fluoresced. However, it rapidly changed color owing to air oxidation, so it was reluctantly abandoned. Earlier attempts to form chelates had also failed [9]. Chemical analyses of the chelates for purity proved to be less satisfactory than An accurately weighed amount of chelate was a spectrophotometric procedure. dissolved in concentrated hydrochloric acid and diluted twentyfold to volume. The resulting solution was measured at two or more maxima and the absorbancy values compared with those obtained previously for the corresponding 8-hydroxyquinolinium ion. Discrepancies between observed and calculated absorbancies were usually less than 1 per cent relative. Apparatus and procedures for obtaining excitation and fluorescence spectra have been reported [ 1, 21. Acknowledgement--This Contract AT(30-l)-905.
[9]
A.
ALBERT
work was supported in part by the Atomic
and D. MAGRATH, Biochem. J. 41, 534 (1947).
57
Energy Commission under