3042
Mark Wrighton and Janet Markham
Quenching of the Luminescent State of Tris(2,2'-bipyridine)ruthenium(l I) by Electronic Energy Transfer Mark Wrighton" and Janet Markham Department of Chemistry, Massachusetts lnstitute of Technology, Cambridge, Massachusetts 02739 (Received July 23, 7973) Publication costs assisted by Undergraduate Research Opportunities Program of M. 1. T.
The quenching of Ru(bipy)32+ luminescence by anthracene, trans-2-styrylpyridine, trans-4-styrylpyridine, trans-stilbene, and cis-1,3-pentadiene has been studied in deoxygenated fluid solutions. Both the changes in quenching activity with triplet energy (ET)of the quencher and data for the Ru( bipy)s2+ sensitized olefin cis-trans isomerization support efficient triplet-triplet electronic energy transfer from the Ru( bipy)a2+ excited state to anthracene, the styrylpyridines, and stilbene. Electronic energy transfer to cis- 1,3-pentadiene is energetically unfavorable and little quenching activity could be detected using this quencher.
Discussion surrounding the deactivation of the excited state of Ru(bipy)s2+ either by electron transfer1 or by electronic energ? t r a n ~ f e rto ~ .quenchers ~ has not included consideration of experiments involving quenchers having a known behavior in a role as an acceptor of triplet excitation from a well-characterized donor. Adamsonl correlates the Stern-Volmer constants for quenching of the Ru(bipy)s2+ triplet state by C O ( N H ~ ) ~ ( X()X~ += F-, C1-, Br-, NH3) with the ease of reduction of the Co(II1) complex, but the results could be rationalized as well by correlating quenching rates with the energy of the lowest spin-allowed d-d absorption. The sensitized Co(II1) Co(I1) r e d u ~ t i o n , l ~ ~ athe ~ b sensitized aquation of PtC142-,2a and the sensitized reactions of oxalato complexes3c are not judicious choices to characterize the electronic energy transfer ability of a new triplet donor since the excited states responsible for these reactions are not well established. The qualitative observation2b of Ru(bipy)s2+ sensitized Cr(CN)63- emission provides unequivocal spectroscopic evidence to invoke electronic energy transfer as a t least one component of a quenching mechanism. It is known4 that certain organic quenchers can be used to characterize the donor properties of triplet sensitizers, and we now report some results of Ru(bipy)s2+ luminescence quenching by organic quenchers.
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Results Quenching of the luminescence of Ru(bipy)s2+ was investigated using anthracene (E,r = 42 kcal/m01),~transstilbene (ET = 49 kcal/m01),~trans-2-styrylpyridine (ET = 50 kcal/mol),6 trans-4-styrylpyridine (ET i= 50 kcal/ mo1),6 and cis-1,3-pentadiene (ET = 57 kcal/m01).~Typical Stern-Volmer plots for quenching are shown in Figures 1 and 2 and the data are summarized in Table I. From the two figures it is seen that the slope of the Stern-Volmer plot changes by a factor of about 3.5 in aerated compared to deoxygenated solutions consistent with competitive quenching of the Ru(bipy)32+ state by 0 2 . Little or no quenching activity could be ascribed to cis-1,3-pentadiene. In Figure 3 the quantum yield for the Ru(bipy)a2+ sensitized trans cis-stilbene isomerization as a function of trans-stilbene concentration is given and the limiting &.c
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The Journal of Physical Chemistry, Vol. 77, No. 26, 7973
+,
is 0.44 f 10%. The limiting was determined by repeated determinations of the observed quantum yield a t a given stilbene concentration followed by correction for incomplete quenching of the Ru(bipy)32+ triplets. The amount of quenching was measured by comparing the relative luminescence intensities of Ru(bipy)32+ with and without added trans-stilbene. As can be seen in Figure 3 the inverse of the limiting 4L+c falls in a position consistent with an extrapolated value from the concentration data. This latter result is taken as verification of the internal consistency of the results. An equivalent check of internal consistency for different stilbene concentrations is found in the comparison of the information in Figures 2 and 3. For example, consider the data a t 0.02 M and 0.05 M trans-stilbene; the @t-c values are 0.12 and 0.21, respectively, while the fractions of Ru(bipy)aZ+ triplets quenched are 0.225 and 0.422, respectively. Adjusting the q5,c values for the fact that 100% of the triplets were not intercepted gives 0.53 and 0.50 in good agreement with each other and the limiting value. Initial limiting trans cis isomerization quanR ~ ( b i p y ) 3 ~sensitized + tum yields for 4-styrylpyridine and 2-styrylpyridine were found to be 0.4 f 0.05. Finally, large amounts of the cis isomer are found a t the photostationary state achieved by Ru(bipy)s2+ sensitization: stilbene (95.0 f 1.0%), 2-styrylpyridine (91.5 k 2.0%), and 4-styrylpyridine (96.5 f 1.0%).
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Discussion The results outlined above are consistent with the conclusion that the Ru(bipy)s2+ behaves as a normal triplet donor (sensitizer) with respect to the acceptors studied. Both energetics and the isomerization data support this conclusion. The energetic dependence of the acceptor triplet level relative to the donor triplet level is as expected; the cis-1,3-pentadiene triplet at -57 kcal/mol is apparently inaccessible with the -49 kcal/mol available from the Ru(bipy)32+; having a nearly isoenergetic triplet level trans-stilbene, trans-4-styrylpyridine, and trans-2styrylpyridine (ET = 49 kcal/mol) quench but not as effectively as anthracene (ET = 42 kcal/mol) where the energy transfer is clearly exothermic. None of the quench-
3043
Quenching of the Luminescent State of Tris(2,2'-bipy)Ru(iI )
0.010
0.005
0.015
[Anthracene]
Figure 1. Stern-Voimer plot for quenching of Ru(bipy)s2+ luminescence by anthracene in aerated ( 0 )and degassed ( 0 )SOiutions at 25".
I
1
0.01
I d
I
I
0.02
0.03
0.04
0.05
0.06
[ trans-stilbene]
Figure 2. Stern-Volmer plot for quenching o f Ru(bipy)3*+ luminescence by trans-stilbene in aerated ( 0 ) and degassed ( 0 ) solutions at 25".
TABLE I: Quencihing of R ~ ( b i p y ) 3 ~Luminescence + ET kcal/mol
i
I
-
Qu enc her
Anthracene trans-2-Styrylpyridine trans-4-Styryipyridine trans-Stiibene cis-l,3-Pentadiene
42 -50 -50 49 57
k,ra
1500 30.9 29.2 14.7 -0
kab
2.2 X l o 9
4.5 x 105 4.3 x 106 2.1 X lo6
-Q
a s l o p e of Stern-Volmer plot, cf. Figure 1,2; k, is the quenching constant and T is the lifetime of the Ru(bipy)32* excited state in the absence of quenchers. Assuming 7 = 0.685 X sec, ref 2.
ers studied have an energetically available singlet excited state to account €orthe quenching activity. The fact that an electronically excited state of transstilbene is produced upon deactivation of the R u ( b i p y ) P excited state i B evidenced by the production of cis-stilbene from the trans isomer. This isomerization represents movement away from the thermodynamic ratio? of this isomeric pair. Similar reasoning can be used to invoke excited state formation for the trans-styrylpyridines. Further, the limiting +t.,c is near that for the benzophenone triplet sensitiz,ed reaction6 for all three olefins implicating very efficient electronic energy transfer from the Ru(bipy)32+ excited state resulting in information of the olefin triplet which decays in a characteristic way. Further, the large fraction of the cis isomers present a t the photostationary states is consistent with electronic energy transfer from a donor of substantially lower ET than the =57 kcall mol0 associated with cis olefin. Finally, we note that both the efficient quenchers, anthracene and trans-stilbene, are substantially more difficult to reduce (Eli2 us. sce = -2.2 and -1.94 V, respectively)8 than the poor quenchersl Co(NH3j63+ and Co(NH3)5F2+ (E1/2us. sce = -0.44,arid -0.33 V, respect i ~ e l y ) In . ~ this regard our results, at the very least, show that ease of reduction is not a necessity to deactivate the Ru(bipy)s2+ excited state with high quenching constants. Experimental Section Materials. The Ru(bipy)&lZ was a gift from G. S. Patterson. The quenchers used are commercially available: cis-l,3-pentadiene (Chemical Samples Co.) trans-stilbene and 4-styrylpyridine (Eastman Chemical Co.), 2-styrylpyridine (Chemical Procurement Labs, Inc.) and anthracene (Baker Chemical Co.). The same solvent system (ethanol: benzene 2:30 by volume) was used in all experiments. Quenching Experiments. Luminescence spectra were obtained using an Aminco-Bowman emission spectropho-
1
i_I___L_LJ IO
20
30
40
50
[m- s t i I b e n d - '
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Figure 3. Quantum yield determination for Ru(bipy)s2T sensitized trans cis-stilbene conversion as a function of transstilbene concentration .).( The limiting quantum yield ( A )was determined by compensating for lack of 100% quenching of the Ru (bipy)zz+ excited states in separate measurements. tometer. Typically 3.0-ml solutions of M Ru(bipy)32+ with variable quencher concentrations were placed in 13 x 100 mm test tubes with constrictions and degassed by several freeze-pump-thaw cycles. The samples were hermetically sealed and relative luminescence emission quantum yields measured. Isomerization of Olefins. Quantum yields for the Ru(bicis olefin conversion were meapy)3*+ sensitized trans sured using a merry-go-roundlo apparatus equipped with a 550-W Hanovia Hg lamp and Corning glass filters t o isolate the 436-nm Hg line. Light intensity was measured using ferrioxalate actinometry.ll Analysis for olefin isomerization was carried out using a Varian 1400 flame ionization gas chromatograph equipped with a 6 ft x in. 5% DEGS or a 6 ft X l/8 in. 3% SE-30 column operated at = 160". Prolonged irradiation a t 436 nm was required to achieve a photostationary state and darkening of the solutions was observed, but approach to the photostationary state was monotonic.
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Acknowledgment. We thank the Undergraduate Research Opportunities Program of M.I.T., the Uniroyal Foundation, and the National Science Foundation for support of this research. References a n d Notes (1) H. D. Gafney and A. W. Adamson, J. Amer. Chem. Soc., 94, 8238 (1972).
The Journal of Physica! Chemistry, Voi. 77, No. 26, 1973
3044
A.
(2) (a) J. N. Demas and A. W. Adamson, J. Amer. Chem. SOC., 93, 1800 (1971); (b) N. Sabbatini and V. Balzani, ibid., 94, 7587 '( 1972). (3) (a) P. Natarajan and J. F. Endicott, J. Amer. Chem. SOC.,94, 3635 (1972); 95, 2470 (1973); (b) P. Natarajan and J. F. Endicott, J. Phys. Chem., 77, 971 (1973); ( c ) J. N. Demas and A. W. Adamson, J. Amer. Chem. SOC.,95, 5159 (1973). (4) N. J. Turro, "Molecular Photochemistry," W. A. Benjamin, New York, N. Y., 1965, p 132. ( 5 ) J. Saittei, J. D'Agostino, E. D. Megarity, L. Metts, K. R. Neuberger, M. Wrighton, and 0. C. Zafiriou, Org. Phofochem., 3, 1 (1973).
Levy, D. Meyerstein, and M. Ottoienghi
(6) D. G. Whitten and M. T. McCali, J. Amer. Chem. SOC., 91, 5097 (1969). (7) G. Fischer, K . A. Muszkat, and E. Fischer, J . Chem. SOC. 6, 156 (1968). (8) L. Meites, "Polarographic Techniques," 2nd ed, Interscience, New York. N. Y . . 1965. 011671-711 (9) A. A. VlCek; Discus;. Faraday Soc., 26, 164 (1958). (10) F. G. Moses, R. S. H. Liu, and B. M. Monroe, Mol. Photochem., 1, 245 (1969), (11) C. G. Hatchard and C. A. Parker, Proc. Roy. SOC.,Ser. A, 235, 518 (1956),
Photodissociation of lodoaromatics in Solution A . Levy,* D. Meyerstein, Nuciear Research Centre-Negev,
Beer-Sheva 84190, israel
and M. Ottolenghi Department of Physical Chemistry, The Hebrew University, Jerusalem, lsrael (ReceivedJune 4, 7973) Pubiication costs assisted by the Nuciear Research Centre-Negev
Deiodination and isotopic-exchange processes are employed for determining the photodissociation yields of various iodoaromatic molecules in solution as a function of temperature and excitation wavelength. In the case of 1-iodonaphthalene the direct-excitation yields are compared with those obtained by photosensitization with benzophenone. The data indicate that dissociation takes place after thermal relaxation from either singlet or lowest triplet states. Photodissociation of these two excited states exhibits a different temperature dependence. Rate constants for the reaction of phenyl radicals with aromatic scavengers are determined and discussed along with the possibility of H-atom migration within the radical ring.
Introduction In previous publications1 we proposed a mechanism consisting of reactions 1-3 to account for the competition between the photoinduced exchange and deiodination in iodobenzene (PhI) solutions in the presence of radioactive iodine ( P I I ) .
Ph.
+
+
11311
I,
h
k
h~ 4 Ph*
k2
0,
21.
Ph.
+
---
PhI
+
1.
oxidation products
(1) (2)
(deiodination)
PhI131
+ 1. ( e x c h a n g e )
(3)
The final consequences of light absorption by the system are thus determined by the competition between dissolved oxygen and iodine on the phenyl radicals produced by photodissoc:ation of iodobenzene. Alternative mechanisms, such as exchange induced by photodissociation of 12, or uia excitation of the PhI.11311 charge-transfer complex, were ruled out. The observation that the photocleavage of iodobenzene (1) is wavelength dependent in the uv range, where part of the absorbed light leads directly to the triplet state of the molecule, raised questions relevant to the nature of the primary photodissociation step of iodoaromatics in solution. In the present work we have carried out photochemical experiments bearing principally on the following points. (a) The applicability of the proposed mechanism to other iodoaromatic molecules. (b) The details of the primary photodissociation step, such as the exact roles of the excited, thermalized, or nonthermalThe Journai of Physicai Chemistry, Vol. 77, No. 26, 1973
ized singlet and triplet states. (c) Properties of the aryl radical, related to its reactivity with added solutes and to the possibility of H atom migration along its ring. The results lead to a new insight into the photodissociation of iodoaromatics in solution for which only qualitative information was available.
Experimental Section (a) Materials. All details concerning iodobenzene, iodine, iodine-131 and methylcyclohexane have been previously described.l 0 - and m-iodotoluenes (BDH) were purified by vacuum distillation in a dry nitrogen atmosphere. In the case of the para isomer a preparative gaschromatographic procedure was employed. The purity of all isomers was checked by glc analysis and by uv spectroscopy. Benzene, chlorobenzene, toluene, benzonitrile, and benzophenone (all Merck, analytical grade) as well as 1-iodonaphthalene (Fluka purum) were used without further purification. ( b ) Procedure. Deiodination and exchange measurements a t various excitation wavelengths were carried out as previously described.1 Iodotoluene isomers were separated gas chromatographically using a diethylene glycol adipate column (Analabs G P 35A 78 in. diameter, 6.5 m long). The retention times obtained at loo", with a 35 C C / min rate of gas flow, for the ortho, para, and meta isomers, were 37.08, 38.83, and 39.33 min, respectively. Under the above conditions, ortho-para and ortho-meta mixtures were readily separable with a 5% sensitivity. However, due to the close values of the corresponding re-
