Indian Journal of ChemistryVol. 32A,June 1993, pp. 517-520

Quenching of excited uranyl ion during itsphotochemical reduction bytriphenylphosphine: Part III

M S Sidhu·, Paramjit Chahal & R J SinghDepartment of Chemistry, Guru Nanak Dev University,

Amritsar 143005

Received 13 August 1992; revised and accepted 4 December 1992

Relative rates of bimolecular quenching of exciteduranyl ion by some mono and disubstituted benzenederivatives have been measured during its photocheniicalreduction with triphenylphosphine. For the relatedcompounds in a series it has been found that substitutentgroups enriching the aromatic n-electron cloud due toresonance stabilization, show an enhanced photophysicalquenching action. The substituents decreasing the7t-e\ectron cloud and delocalization of positive charge overthe benzene ring decrease the quenching action.

Resonance phenomenon is of fundamental importancein chemistry. It not only makes a major contributiontowards the stabilization of molecules but is also used incorrelating the chemical and physical properties ofmolecules. In continuation of our earlier work 1.2,wepresent here the results of our studies on the effect of theresonance phenomenon exhibited by certain benzenederivatives (quenchers) on photophysical annihilation ofelectronically excited uranyl ion in polar medium. Anumber of studies on luminescence quenching of uranylion have been made using fluorescence spectrometer-".Recently, an indirect method based on the use ofUV-visible spectrophotometer has been proposed toevaluate Stern-Volmer quenching constants 1.2.Absenceof UV-visible spectral bands corresponding touranium(IV) formation on prolonged irradiation ofdioxouranium(VI) ion in the presence of quenchersunder investigation, shows the absence .of anyphotochemical reaction. However, presence of benzenederivatives suppresses the photochemical reaction ofuranyl ion with triphenylphosphine. Photochemistry ofuranyl ion in solution is important in view of thephotolytic separation of uranium from lanthanum andcerium", its usefulness in reprocessing the spent nuclearfuepo.13 and use of uranyl ion as an oxidisingagent3-6•1I•13.

ExperimentalAll the chemicals were of AR grade and these were

used as supplied. Vacuumpet and gloves were used for

Notes

experimental manipulations and experiments werecarried out under a ventilated hood provided with anexhaust fan. Other experimental details are described inthe earlier publications':". Waste chemicals weredissolved in alcohol (flammable solvent) and sprayedinto the fire box of an incinerator.

Results and discussionUpon optical excitation of linear uranyl ion O"max =

420 nm), an electron jumps from IT-bonding (U =10)orbital consisting of 5f16d and 2p atomic orbitals ofuranium and oxygen respectively to a ~rnon-bondingorbital on uraniumI2.13.

Consequently, positive charge localizes onaxial oxygen atoms for a donor-acceptor complexformation with electron rich triphenylphosphine(U01 + -PPh)* and benzene derivatives(U01 + X-C6H4- Y)* for photochemical andphotophysical deactivation respectively. Thus,dioxouranium(VI) ion may be reduced photochemicallyto uranium(IV) (A-.nax 652 nm) whereastriphenylphosphine is oxidised to the correspondingoxide':' (Scheme I).

2· 'a 2+(' )u02 · h" . uoz

2· kr uo2•• h~1 (2 )uoz 2

uar u~•• 2+

( 3)· zu~

u~'k,

v uer • opp~ (4)· ,pp~

uo2+'. H "q o(u~--Q ) ( 5)

2 X Y X y

2+ n' v uef- ~ (5 )(u!li----- )

X Y X' y

~

The optically excited dioxouranium(VI) ion, anelectron deficient species, is deactivated by electron richaromatic molecules'v+. Unlike cyclohexane' anddecalin-, which do not have aromatic sextet, benzenederivatives trigger off the competition between thephotochemical and photophysical quenching ofdioxouranium(Vl) ion by triphenylphosphine (Eq.4)and benzene derivatives themselves respectively. Onincreasing the concentration of quencher, a decrease inoptical density for uranium(IV) formation at A-.nax =

518 INDIAN JCHEM,SECA,JUNE 1993

O.2(}'-~""""""'-------'

.~IIIC••a 0.10;;u..Co.o

400 500 GOO 700

Wavclcngth Inm)

Fig. l=-Electronic spectral changes after 10min of irradiation bysunlight of solutions containing [U~ +1 = [PPh31 = 0.01 mol dm - 3

and[quencherJ(a)0.OO,(b)2x 1O-3,(c)4x IOJ,(d)6x 10-3• (e)8x 10-3, (I) tux 10-3, (g) 14x 10-3 mol dm ? respectively

6

AnisicliM .;;.•••¥

1.4

1.

1.2

• •1.1•

N.N-Oi .thyl~n il.n.

1.07~-------------------~4---/I, 8 12 IIi 20 24.10 co

4

3

2 2- CIllo rotolu ••.•• rr>--DiChlorob.nzpne

652nm (Fig 1) occurs, which is used to derive therelationship between the ratio of quantum yields foruraniwn(IV) formation in the absence and in thepresence of quencher (4)R and <flRrespectively) on thebasis of the Stern-Volmer equation,

<I>RI<flR= ODo/OD = 1+ KsJQ] ...(7)This relation yields a straight line with an intercept ofunity (Fig 2). r = I;rk; is a measure of lifetime of exciteduranyl ion in solution, whereas 00° and OD are opticaldensitiesat A.nax = 652 run for uranium(lV) formation in.the absence and in the presence of quenchers.Electronically excited uranyl ion has a lifetime r =2.4 x 10 - 6 see in aqueous solution 15• Value of thebimolecular quenching constant kq = Ksv/'t for variousbenzene derivatives can be readily calculated.

As observed in the cases of substituted arylaldehydes 7

and benzoic acids", the substitutions on the benzene ringplay a crucial role in determining the photophysicalquenching action of various benzene derivatives1.2. For aseries of compounds, the substituents enriching thearomatic n-electron cloud show an enhancedphotophysical quenching action as compared to thesubstituents decreasing the n-electron density over thearomatic. ring. Bimolecular quenching constant. values(kq) given in Table I indicate that like nitrobenzene',resonating structures (A) such as benzoates, phthalateand dichlorobenzene, where a positive charge

t L-. N. N-Oimpthyl-p-tolui<lin.

L.lsobutYlb.nzen.

o 5 15 20 25 30conc.ntration(mol <1m3)

5510 35 -360-1040 45 50

Fig. 2-Stern-Volmer plots at [VOrl = [PPh,l = 0.01 mol drn "?

NOTES 519

Table l+-Stern-Volmer (K.v) and bimolecular quenching constants (kq) of excited dioxouranium(VI) ion with some mono- anddisubstituted benzene derivatives {[UOi+J = [PPh3J = 0.010 mol drn-3; [H+J = 0.1 mol dm"; temp = 25±5° C}

Quencher

AnisidineN,N-DimethylanilineN-MethylanilineN,N-DiethylanilineN,N-Dimethyl-p-toluidineAnilineAnisoleIsobutyl benzene2-ChlorotolueneChlorobenzenem-DichlorobenzeneMethyl benzoateEthyl benzoateo-PhthalateNitrobenzeneCyC\ohexane

Ksv(dm3 mol-I)

210.5137.0142.0133.3121.0

80.068.5766.66

5.306.662.\03.801.66

kq(mol dm-see')

8.77 x \07

5.70 X \07

5.91 X 107

5.55 X \07

5.04 X 107

3.33 X \07

2.85 X 107

2.77 X \07

2.20 X \062.77 X 106

0.87 X \061.58 X \066.90 X 105

4.1 X \05

4.6 X 105

3.75 X 104

2

Ref.

2I

l.OO1.110.0902

delocalization over the aromatic ring reduces the electroncloud, show poor interaction with excited uranyl ion forits photophysical deactivation. On the other hand, likeanisole", where electron releasing groups (amines,rnethoxy, etc.) are attached to benzene ring (B),a negativecharge delocalization over the aromatic ring leads toresonance stabilization in polar medium. As in the caseof nitrosonium ion!", this results in donor-acceptorcomplex formation with electronicallyexcited uranyl ionfor enhanced photophysical deactivation. Multiplesubstitutions of electron releasing or electronwithdrawing groups have additive effect. Hence, thesechange kq values in their respective directions.

The electronically excited uranyl ion can be used as aprobe to look into the resonance phenomenon exhibited

by certain aromatic molecules which do notphotochemically reduce it to uranium(IV) in aqueousacetone solution. The nature of the inductive effect ofthe substituents on the benzene ring and resonancephenomenon of stabilization of benzene derivatives inpolar medium together determine the order of theirquenching action on electronically excited uranyl ion.This facileapproach may be useful to those who lack thefluorescence spectrometer.

AcknowledgementThe authors are grateful to the CSIR, New Delhi for

financial assistance [5(157)89 EMR II] and Prof. S SSandhu for constructive criticism.

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