Hugh D. Burrows' and Sebastiao J. Formosinho University of Coimbra Coimbra, Portugal
I I
Uranyl Luminescence Quenching An experiment in photochemistry and kinetics
While a numher of good student experiments in preparative photochemistry have been reported in the literature, there appear to be f& fewer reports df studies in the mechanistic and kinetic aspects which can conveniently be carried out during 1-2 oeriods. We have been interested in the ohotochemistrv of the uranyl ion, and it is shown in the present article that the quenching of uranyl luminescence by alcohols provides a convenient experiment in this area. A laree numher of svstems involvine U O- P and oreanic substrates have long been known t o he photosensitive &3), with reaction normallv leadine t o oxidation of the suhstrate. Alcohols, for exampli, are ph&oxidized in the presence of uranyl salts to yield the corresponding carbnnyl compounds. Uranyl ion possesses both a characteristic greenish-yellow luminescence (1-3) and a strong lowest excited state ahsorption around h9Onm ( 4 , s )and studirs of the decay of the uranyl ion excited state in the presence of alcohuls by either luminescence lifetime or flash photolysis measurements indicate that there is a himoler~~lar reaction hetween excited uranvl ion and the alcohd. Further. esr studies (GIshow that the drirnary alcohol species after photolysis is n d r i a ~ l ya free radical resulting from loss of a n a-hydrogen atom, e.g.
-
-
+
+
+
(U022+)* (CH&2HOH UOzt (CHB)&OH Ht (1) T h e radical can then he further oxidized t o give the corresponding ketone These reactions are then normally followed by the disproportionation of the uranium(V) species (7). In addition t o such direct studies of the reaction between excited uranyl inn and suhstrate, the rate constant for the initial oxidation step (1)can also conveniently be obtained by Stern-Volmer analysis of static studies on the quenching of uranvl luminescence (8). . . We have made this the basis of a mechanistic photochemistry experiment. T h e initial reaction between excited uranvl ion and alcohol could occur either bv electron-transfer
+
(U022+)* RH
-
U02+ + RH.+
RH'+-.R-+Ht
(3) (4)
or by direct hydrogen-atom abstraction
+
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(UOs2+)* RH U02H2+ + R(5) T h e objective of this experiment is to distinguish between these two mechanisms. Experimental Experiments are carried out using solutions of uranyl nitrate heaahydrate (10V M )in distilled water. It is preferable not to use deionized water, as this normally contains organic impurities which can also quench the U0z2+luminescence. Luminescence spectra are recorded using 1-cm cuvettes in any conventional fluorimeter with ~
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Presented in part at the International Symposium on Interdiseiplinary Aspects of Photochemistry, Barcelona, Spain, July 1976. Present address: Chemistry Department, University of Ife, Ile-Ife, Nigeria. ' A further possibility for the quenching is that this involves a purely physical mechanism, such as exciplex formation,and that the chemical step oecurs after this. However, theoretical calculations (12) suggest that this is unlikely.
Figure 1. Luminescence spectrum of 3 ml aqueous uranyl nlhatesolution in the presence of various quantities of isapropanol. the emission excited at 436 nm, and observed in the 480-560 nm region. Even a less expensive filter fluorometer should also work well. Uranyl luminescence is unaffected by the presence of oxygen, so that degassing of solutions is unnecessary. For the quenching studies, solutions are most conveniently prepared by adding pl quantities of the alcohols methanol. d4-methanol.ethanol. isoorooanol. and t-butanol
in the cell is negligible. Luminescence spectra are recorded for a number of substrate concentrations and, from the quenching data, Stern-Volmer plots are drawn for each substrate. Stern-Volmer kinetics ( 9 ) consider the competition between emission by an excited state (eon. . , (61) . .. and theauenehineof that state bvas~eciesQ . . .(7)..and show that there is a linear relation*hip between rhc rnrio oiil~mr?srenre intenslt) in theprcicnre 111an81 ahitnrr (I,, ofqurnchcrnnd the queneher concentration (8) Aa+A+hv (6) A*+Q-A+X
(7)
1011= 1 + K,[Q] (8) eraohicallv. The Stern-Volmerconstant can then be determined .. . , The Stem-\'olmrr ronsranr :;the product ~f the qtwnehmg rste cunrtant. k(,, and the lifetime uf the cmiiiim in the nhsrnce of quenchrr, r K,, = kQi (9) so that knwing thelifttime of the ~ m i s s ~ ~ ~ n , ~ h ~ ~ ~ t mw r e nmn rh~np, hc determined in each rase. The hfetime
