2558 Y. HAAS, G. STEIN, AND M. TOMKIEWICZ

underlying rela'tionships between the various solution theory parameters used in this treatment. The success of this work is that the choice of segments based on equal segment molar volume considerations has at least been consistent. The possibility of being able to predict very accurately the activity coefficient of a particular solute in a multicomponent stationary phase is now very real. Experimental work on this problem is in progress.

Finally, the success of the quasilattice theory equa- tions does not necessarily vindicate the model but rather serves to pinpoint the type of factors that should be considered when a more sophisticated, possibly more realistic, solution model is proposed.

Acknowledgment. B. W. G. acknowledges gratefully the award of a postdoctoral fellowship from the Univer- sity of California at Los Angeles.

Fluorescence and Photochemistry of the Charge-Transfer Band in

Aqueous Europium( 111) Solutions

by Yehuda Haas, Gabriel Stein, and Micha Tomkiewicz Department of Physical Chemistry, The Hebrew Unisersity, Jerusalem, Israel (Received March IO , 1970)

Aqueous solutions of Eua+ show fluorescence upon excitation a t the charge-transfer band. This emission appears to consist of two distinct bands. Evidence is given to support the following assignments: one band is due to charge-transfer emission, the other to emission from an excited state of Eu2+. These assignments, as well as the formation of EuZ4 in the system, are further supported by photochemical experiments.

Introduction The broad absorption band found in aqueous solu-

tions of Eu3+ salts has been assigned by Jgrgensen' to a charge-transfer transition. This assignment has been substantiated by J@rgensen' and by Barnes and Day2 by observing the influence of different ligands on the wavelength of maximum absorption. We have recently reported3 the observation of new fluorescence bands in aqueous and acetonitrile solutions appearing on excita- tion in this absorption band. In this paper we report further details and give possible assignment to the band appearing in aqueous solutions. We also report some experiments on photochemical processes which occur on light absorption in the C T band.

Experimental Section Solutions of Eu(C104)X were prepared by dissolving

Euz03 99.97% (Fluka) in perchloric acid (Analar grade). Eu(C104)2 solutions were prepared from EuC03 (kindly supplied to us by Dr. 3Iayer of the Department of In- organic Chemistry) by dissolving i t in oxygen free per- chloric acid. Absorption spectra were obtained with a Cary Model 14 recording spectrophotometer and fluo- rescence spectra with a spectrofluorimeter described by Fe i t e l~on .~ A xenon arc served as a light source, thus limiting the excitation wavelength to 2 250 mp. Flash

technique wa8 used to follow the decay of Eu2+. The flash-photolysis setup was described by Ottolenghi and RabanL6

Photochemical Experiments. These were performed with a special apparatus, designed to measure products gas chromatographically. A quartz vessel was con- nected to the gas chromatograph; the solution in i t was flushed with argon and then illuminated with the ap- propriate lamp. After illumination, argon was again passed through the solution and into the gas chromato- graph.

We used a Varian Aerograph Model 90-P with active carbon column, enabling separation of hydrogen from air. Calibration was performed using aqueous solu- tions saturated with purified hydrogen (Matheson). Actinometry was done with uranyl oxalate-oxalic acid solution.

Results Absorption spectra in the region 250-330 mp were

found to follow Beer's law when the concentration was

(1) C. K. JZrgensen, Mol. Phys., 5, 271 (1963). (2) J. C. Barnes and P. Day, J . Chem. Soc., 3886 (1964). (3) Y. Haas and G. Stein, Chem. Phys. Lett., 3, 313 (1969). (4) J. Feitelson, J . Phya. Chem., 68, 391 (1964). (5) M. Ottolenghi and J. Rabani, ibid., 72, 593 (1968).

The Journal of Physical Chemistry, Vol. 74? No. 12, 1070

CHARGE-TRANSFER BAND IN AQUEOUS EUROPIUM(III) SOLUTIONS 2559

4

2

- 0 4

2

n

.,.... ..,. ,,A. ..... ,.------.\ -- "- *%. ' 1 " ~ . 1 1 . ...,.,.. %.

EXCITATION

EXCITATION

EXCITATION 270 rn)]

'*.I..

EXCITATION 260 my trLl 2 0 480

320 360 400 440

1 mJJ

Figure 1. perchlorate solutions upon excitation a t various wavelengths: -- , total luminescence normalized to same arbitrary scale; , , . , , ,, band I, excited at 260 mp, normalized as above; - - - - - - , band 11, obtained by subtracting band I from total luminescence.

CT emission spectra of aqueous europium

2 0 . 1 M . There is some increase of the molar absorp- tion with increasing pH; however, in the pH range 0-2 no effect on fluorescence was observed. At lower con- centrations (<0.1 M ) optical density was too low for accurate measurements. Such samples were prepared for use by dilution from more concentrated solutions.

Two distinct fluorescence spectra could be distin- guished. Excitation with wavelengths up to 260 mp gave a symmetric emission band with a maximum a t 350 mp. Excitation at longer wavelengths led to a broad, unsymmetrical emission band with a maximum varying between 360 and 380 mp, explained by increasing admixture of a component peaking -390 mp.

Figure 1 shows the changes in total luminescence of the solution upon excitation at different wavelengths. As stated above, excitation a t X <260 mp gave only the symmetrical band centered about 350 mp, hereafter called band I. The unsymmetric bands appearing upon excitation a t higher wavelengths were resolved, as shown in Figure 1, into two more symmetrical bands: band I and another band, which we name band 11, centered about 390 mp. It is seen that as the excita-

tion wavelength is increased, band I1 becomes stronger than band I. We could not obtain band I1 without band I. Nevertheless, the evidence presented shows conclusively that the total luminescence a t longer wave- lengths is not a distorted image of band I, but the super- position of two distinct bands.

Figure 2 shows the absorption band together with the two emission bands. It is evident from the figure that neither band is a mirror image of the absorption band. Excitation spectra, corrected for lamp and monochro- mator spectral response for the two bands, are given in Figure 3. The excitation spectrum of band I seems to coincide with the absorption spectrum.

It was found that the normalized intensity of band I was dependent upon the concentration. As shown in Figure 4, the intensity decreased with increasing con- centration in the 0.2-1 M range. At lower concentra- tions the intensity is constant within experimental error. The concentration quenching does not follow a simple Stern-Volmer law. Oxygen did not quench either fluorescence to a measurable degree. For con- centrations where no concentration quenching was ob- served, the quantum yield was 0.06 and 0.001 for bands I and 11, respectively, as measured against aqueous so- lutions of sodium salicylate as a standarda6

An oxygen-free solution of Eu(C104)2 was prepared. I ts spectrum did not change markedly when kept in the dark.

This solution did not show any fluorescence upon ex- citation at 260, 280, or 310 mp, i.e. , a t the well-known absorption bands of Eu2+ aqueous solutions. No emission mas observed even upon working with the most sensitive setups of the instrument. We can thus safely conclude that the quantum yield of fluorescence from Eu(I1) aqueous solutions is less than low6. This result is in agreement with previous

Discussion The charge-transfer band which is responsible for the

absorption is due to a transfer of an electron from an orbital centered mainly on the water molecule to an orbital centered mainly on the Eu(II1) i0n.l Thus the transition may be represented by the formula

Eu3+-H20 -% [Eu2+-HeO+]* (1)

assuming a one-electron transition and neglecting the in- fluence of other water molecules of the solvation shell. The excited species may lose its extra energy by one of the following processes: (1) radiative coupling to ground state; (2) nonradiative coupling to ground state either by intra- or by intermolecular processes; (3) transition to a new chemical species which we sug- gest, for reasons to become soon apparent, to be an ex- cited europous ion.

(6) (a) G. Weber and F. W. J. Teale, Trans. Faraday Soc., 53, 646 (1967) : (7) F. D. S. Butement, Trans. Faraday SOC., 44, 617 (1948).

(b) G. Stein and M. Tomkiewicz, in press.

The Journal of Physical Chemistry, Vol. 74, No. 18, 1970

2560

v)

t 3 h

- .-

e c g

Y. HAAS, G. STEIN, AND M. TOMKIEWICZ

I I I I I I

EMISSION

Band II Band I CHARGE TRANSFER ABSORPTION

Y (ctn-'. IO-')

Figure 2. normalized to unit height.

Absorption and emission CT spectra of aqueous europium perchlorate solutions. The spectra are

60

50

40

In 1 .- ' 30 e a

+ g I

20

1c

C I I I I I 210 290 310 330

Figure 3. Figure 1. light intensity :

Excitation spectra of band I and 11, derived from Data corrected for spectral dependence of exciting

A-A-A, band I ; 0-0-0, band 11.

Our results may be fully accounted for by considering these three processes. The assumption of an excited europous ion is crucial to this discussion, and a large part of the work was devoted to its verification.

The following possibilities exist for the fate of this, as yet unspecified, excited europous ion. (1) The for-

- i

I

I

200

&

IO0

Figure 4. Influence of concentration on the intensity of band I. Excitation at 260 mp. Reciprocal intensity is given in arbitrary units for emission a t 350 mp,

mation of the europous ion must be accompanied by oxidation of a water molecule to HzO +, equivalent to a hydroxyl radical

HzO+ ---t OH + H&q+ (2)

These two species are formed close to each other, so that reoxidation of the ion to the + 3 state by geminate re- combination with the hydroxyl radical is possible

(Eu'+)*. * *OH Eu3+0H- (3)

(2) solution, e.y., water, may take place by the process

Oxidation by other oxidizing agents present in the

(Eu'+)* + HaO+ + Eu3+ + H + HzO (4)

The Journal of Physical Chemistry, Vol. 7dV No. 12, 1070

CHARGE-TRANSFER BAND IN AQUEOUS EUROPIUM(III) SOLUTIONS 2561

>

w z w

P

I I

I !I I I I

/- /

D

r

Energy level diagram for the system Eu3+-H20. Figure 5 . r is internuclear distance along the critical coordinate responsible for the transitions observed. level of the charge-transfer complex [ E u ~ +-HzO]. excited state of the same charge-transfer complex. ground-state level of EuZ+ hydrate. same charge-transfer complex. of Eu2 + hydrate. I.M.C. denotes intermolecular crossing of the excited state of the charge-transfer complex into the excited state of Eu2+ hydrate. The transitions observed in the spectrum are denoted by arrows.

G is the ground-state F is an D is the

F is an excited state of the D is the ground-state level

E is an excited state of EuZ+ hydrate.

yielding atomic H. This process may result in the for- mation of molecular hydrogen. (3) There may be emission of a light quantum and transition to ground state. (4) Nonradiative coupling to ground state may also occur.

By considering the processes enumerated above, we can adequately account for our results with the aid of the following energy level diagram (Figure 5 ) .

This figure, given in the “critical vibration” approxi- mation shows the charge-transfer absorption band as a transition from ground state G to an excited state F with a shallow minimum as is common with this type of transition.* Absorption a t wavelengths shorter than about 270 my leads to dissociation within few vibra- tions, leading to an excited state of Eu2+, E, which may or may not fluoresce. Absorption a t wavelengths longer than 270 mp leads to relaxation to the lowest

vibrational level of the excited state. This process may lead finally to fluorescence from this level to ground state.

Thus the fluorescence from the yet undefined (Eu~+) * ion is our band I, and the emission from the CT excited state band 11. This energy level diagram is further supported by the excitation spectra. Band I seems to have an excitation spectrum coinciding with the ab- sorption spectrum. Band I1 has an excitation spec- trum beginning at 270 mp. The fact that neither band is a mirror image of the absorption spectrum is also readily accounted for by this model.

The deviation from Stern-Volmer kinetics is fur- ther evidence that band I does not result from the en- ergy level excited directly by the absorbed light. If the assignment of band I emission is correct, i t must be due to an energy level inaccessible by direct absorption of light from the ground state of Eu2+. That this is so is demonstrated by the absence of emission on exciting the accessible levels in Eu2+ solutions. This situation is reminiscent of the triplet phosphorescence in aromatic compounds, though no simple assignment of the state concerned suggests itself. Laser measurements3 indi- cate very short lifetimes, of the order of less than 10 nsec for one component and about 20-30 nsec of the second. These values, combined wiith the quantum yield, lead to a natural lifetime of about sec, another evidence for a forbidden transition.

The transition observed in einission may be an al- lowed one to a state with energy slightly higher than ground state. The pertinent energy diagram is shown in Figure 6.

The ground state g of Eu2+ can be radiatively cou- pled to an excited state f , which does not fluoresce but loses its energy to solvent, in the case of water most probably by interacting chemically with the formation of H atoms. The transition f + e is thus highly im- probable. If e, this other excited state of Eu2+, is somehow populated (e.g., by the mechanism suggested above for the C T complex) i t can return to ground state g via state d. The existence of bivalent europium in the solution exciting the charge transfer of Eu2+ cannot be proved by fluorescence measurements alone. As mentioned before, i t is highly probable that an excited species of Eu2+ would react rapidly with water. It is well known that aqueous solutions of Eu2+ are stable in the dark, but decompose rapidly upon illumination. There is no reason to suppose that excited states reached by means other than direct light absorption should be less reactive. They lead primarily to forma- tion of hydrogen atoms as in reaction 4 and then to hy- drogen molecules.

Thus, evolution of hydrogen upon illuminating the solution in the charge-transfer band should be a proof of the mechanism suggested.

(8) S . P. McGlynn, Chem. Rev., 58, 1113 (1958)

The Journal of Physical Chemistry, Vol. 74, No. 12, 1970

2562 Y. HAAS, G. STEIN, AND M. TQMKIEWICZ

f f

Figure 6. aqueous solution.

Possible energy level diagram for EuZf ion in

Another possibility of proving the existence of Eu2+ is direct observation of its characteristic absorption spectrum.

These considerations led to photochemical experi- ments, in which we tried to obtain hydrogen upon ir- radiating the solution in the charge-transfer absorp- tion band.

Hydrogen Formation Experiments in which solutions with different Eu3 +

(0.5-0.05 M ) concentration were illuminated in the ab- sence of O2 with the 2537 (E = 0.3) line gave negative results, and no evolution of molecular hydrogen was de- tected. It was concluded that without added H atom scavengers the inevitable impurities in the solution scavenged any H atoms produced a t the primary stage.

Changing the concentrations of ethanol had little effect on either absorption or fluorescence spectra. Illumination of a reference solution, identical with t'he test solutions but not containing Eu3+ ions, did not pro- duce hydrogen.

The results are given in Table I, and prove the for-

Table I

Quantum yield for

-----Conon, M-------- hydrogen Sample Eu'+ EtOH HClOd formation

1 0.048 0.07 2.38 6 2 0.048 0.007 2.38 3 3 0.048 0.007 0 . 2 6 . 5

mation of hydrogen. The quantum yield was cal- culated by taking into account only the absorption of the charge transfer band and cannot be clearly related to a definite mechanism. I ts high value makes i t prob- able that in some stage of the reaction sequence (Eu") is produced, which absorbs significantly a t 254 mp (E h/

2000). In addition to the formation of Eu2+ by light emission from the excited state, a process that is likely to occur is the reduction of Eu3+ by the radical formed in reactions 5 and 6

I n order to overcome this difficulty, ethanol was used as CH3CHOH + Eu3+ ---f a scavenger for H atomsg CH3CHO + Eu2+ + H+ (7)

CH3CH20H -k - H2 -k (5) A similar reaction has been observedl0 in the photo- Ethanol also scavenges OH radicals, according to chemistry of Fe3+ in aqueous solution.

CHaCHzOH + OH ---f H2O + CH8CIIOH (6) (9) C. Lifshitz and G. Stein, J . Chem. Soc., 3706 (19G2). (10) B. Behar and G. Stein, t o be published. yielding the same organic radical,

The Journal of Physical Chemistry, l'ol. 74, No . 19, 1970