861
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A '
'A '
1.520
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Figure 1. Structures and assumed geometries of 4-methylumbelliferone ( I \ and its tautomer I l l ) . Terminal atoms are hydrogens. Other atoms are carbons and oxygens. H-C = 109 A.
shifts for both forms, a red shift of 6OOO cm-I in the fluorescence of 11relative to I emerges. This latter value compares favorably with the experimentally observed energy difference between the maximum of the two fluorescences of ca. 5200 cm-l. The results of the EHMO calculation appear to be consistent with the experimental findings supporting the suggested photobutomerism. For such a process we may, therefore, propose a feasible mechanism, illustrated in some detail in Figure 2. Here, irradiation into the first electronic transition of I produces the excited singlet I*. Deactivation of this state occurs mainly uia fluorescence hv, (I), intersystem crossing, and probably chemical relaxation. If, however, the pH and water content of the solution are appropriate so that protonation (and deprotonation) of I* can take place during its lifetime, the formation of the energetically more favorable II* is competing with these deactivation processes. The excited state TI* of the valence isomeric form subsequently decays to II, giving rise to a second fluorescence hut (IT). Ground-state 11 finally crosses over to the initial form I. (4) H. A. Skinner and H. 0. Pritchard, Chem. Rev., 55, 745 (1955). (5) "Tables of Interatomic Distances and Configuration in Molecules and lons,"Chem. SOC.,Spec. Pub/., No. 11 (1958).
Pioneering Research Laboratory U. S. Army Narick laboratories Natick, Massachusetts 01 760
J. R. Huber M. Nakashima* J. A. Sousa
Received August 9, 1972
I
Quenching of the R ~ ( d i p y ) 3+-~Phosphorescence by Cr(CN)s3-. Evidence for a Diffusion-Controlled Mechanism
II
Simplified potential energy diagram for the 4meth~~~mbellifero~e system.
Figure 2.
change. For the diagonal elements H,, of the Hamiltonian matrices, the following valence state ionization potentials I[eV] were used:4 I(I3.b -13.6; I(C) -21.40, -11.40; I ( 0 ) -35.30, -17.76. These values were also incorporated in the off-diagonal elements H,, = K S , (Hti H,,)/2 with K = 2.000. As usual, the orbital exponents were obtained using Slater's rules. Since no X-ray data of umbelliferone were available, the geometries were assumed5 as shown in Figure 1. Except f ~ the r methyl group, the structures are planar, and the bond angles are 120" unless otherwise indicated. The results of this calculation in terms of a simplified potential energy diagram are summarized in Figure 2. The ground-state energy of form I is lower than that of 11 by 4 ked. As expected, form I is the more stable configuration in the ground state. The first excited states of I and 11, on the other band, are predicted to lie 30,000 cm-l (experimental value --91,OOO cm-l)l and 24,000 cm-l above their respective ground states which places the excited state of TI 13.3 kcal below that of I (cf. Figure 2). Since form II is not stable in the ground state, the predicted red shift of its longest wavelength absorption band with respect to I is considered in the emission. If we assume similar Stokes
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Sir: The quenching of the phosphorescence emission of Ru(dipy)32+ by anionic CrfIII) complexes has recently been investigated2 and it was suggested that the observed quenching was a "static quenching," taking place between the counter ions in an ion pair formed by the ground-state complexes. To our knowledge, this would be the first example of a solely static quenching between complex ions in fluid solution. However, during a systematic investigation of the energy transfer between coordination compounds, we have found sure evidence against the static quenching hypothesis for the R~(dipy)3~+-Cr(CN)@system. Lifetime and phosphorescence intensity measurements were made on air-equilibrated aqueous solutions containing R ~ ( d i p y ) 3 ~(1.2 + x M) and various amounts of Cr(CN)s3-. Emission intensities were measured with a Turner Spectro 210 spectrofluorimeter, and li€etimes with an apparatus described elsewhere,3 which uses a Febetron 706 electron accelerator and a ZnSe target as a pulsed light source. We have thus observed that the triplet life(1) Work supported in part by the National Research Council of italy through Contract No. 71.01610.03 115.4416. (2) I. Fujita and H. Kobayashi. Ber. Bunsenges. Phys. Chem., 76, 115 (1972). (3) A. Hutton, G. Giro, S. Dellonte, and A. Bresccia. Int. J. Radiat. Phys. Chem., submitted for publication.
The Journal of Physical Chemisfry, Vol. 77, No. 6, 1973
COMMUNICATIONS PO THE EDITOR
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,,d 4..
3-
/ I
, /
I
1 [Cr(CN),3-] x
I
2
lo3
-
Figure 1. Quenching of the Ru(dipy)gzf phosphorescence emission (0)and lifetiime ( A ) by Cr(CN)63- in air-equilibrated sohtions: a, aqueous solutions of constant ionic strength (@ = 0.5); b, iaqueous solutions of variable ionic strength (see text); c, 20% glycerol-water solutions.
The bimolecular quenching rate constant ( k 4 = ks,/TO), obtained from ksvo and the experimentally determined lifetime of Ru(dipy)z2+6,7 is 2.1 X 1O1O M - I seccl, which is in good agreement with the encounter rate constant (3.2 x 1O1O M - l sec-I) calculated from the Debye equation for ionic species.8 As shown by points c in Figure 1, the quenching by C ~ ( C N ) Gis~ -less efficient in 20% glycerol aqueous solution than in pure water. This result is mainly due to the increase of the viscosity of the medium, and thus, it supports the hypothesis of a dynamic quenching. Fujita and Kobayash? found no difference between the quenchings in the two media reported above; since the association constant is not affected by the viscosity, these authors took this fact as another proof in favor of the static quenching. It is to be noted, however, that even the association constant may be influenced by a change of the medium, because of variations in the dielectric constant. For the R~(dipy)3~+-Cr(CN)6~system, calculations based on the Bjerrumg or the Fuoss10 equation show that the association constant, and thus the Stern-Volmer quenching constant if the static mechanism were to operate, should be about 30% higher in 20% glycerol-water than in water solutions. In conclusion, it appears evident that several parameters of the medium (such as, e.g., viscosity, dielectric constant, and ionic strength) must be taken into appropriate consideration in order to interpret correctly a quenching process between ionic species. Studies are now in progress in our laboratory in order to clarify the role played by these parameters.
time of R u ( r l i p ~ ) 3 ~is+ quenched in parallel with the phosphorescence emission (Figure 1). This clearly demonstrates that the quenching of the emitting excited state of Ru(dipy)32+ occur3 by a dynamic, diffusion-controlled process, and rules out the possibility of a static quenchAcknowledgments. We wish to thank Professor V. Baling, since this would affect the emission intensity but not zani for helpful discussions and Professors A. Breccia and the lifetime. G . Semerano (Laboratorio di Fotochimica e Radiazioni We have also found that the Stern-Volmer quenching d’Alta Energia del CNR, Bologna) for the suggestion to constant, k ,§,, shows a decrease with increasing ionic use the Febetron equipment for lifetime measuring. strength. This dependence is clearly shown in Figure 1, where the points i n line a refer to solutions at constant ionic strength ( p = 0.5, adjusted by adding KCl), and (4) In a static mechanism, kSv is, under certain condltions, equal to the association constant of the ion pair. points in curve b refer to solutions whose ionic strength (5) S . Glasstone, “Physical Chemistry,” 2nd ed, Macmillan, London, was not adjusted and, thus, increased with increasing 1953,p1116. [Cr(CN)63-] ( p =: 1.6 x 10-3, 6.4 x 10-3, 12.4 x re(6) T O = 0.40 p e c in air-equilibrated solution; we also obtained a lifetime of 0.65 psec for deoxygenated solution, in good agreement spectively). The same effect had been observed by Fujita with a previous report.7 and KobayashP and it was taken as evidence for the stat(7) N. J. Demas and A. W. Adamson. J. Amer. Chem. SOC., 93, 1800 (1971). ic quenching since, of course, p influences the association (8) P. Debye, Trans. Electrochem. SOC., 82, 265 (1942 in this calcuconstant of t,he ion pair.* However, the ionic strength is lation we assumed an encounter radius value of 5 i l ’ t o r both complexes. also expected to influence the encounter rate constant be(9) N. Bjerrum, Kgl. Dan. Vidensk. Seisk.. Mat-Fys. Medd., 7, 3 tween two ionic species in the same way, so that the de(1926). pendence of k,, on ,u cannot discriminate between the (10) R. M. Fuoss, J. Amer. Chem. Soc., 80,5059 (1958). static and dynamic mechanism. The k,, value calculated from the quenching data of our most dilute solution is 4.9 F. Bolletia lstituto Chimico ”G. Ciamician” x IO3 M - I , and the corresponding value at p = 0 (calcuM. Maestri dell‘ Universita lated with the usual equation5 log k = log ko 1 . 1 3 2 2 ~ 2 ~40126 Bologna, lfaiy L. Moggi* u1J2) is ksvQ = 8 6 X lo3 M - I . This value is in good agreement with that obtained by extrapolation in ref 2. Received December 4, 1972
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The Journal of Physical Chemistry, Voi. 77, No. 6, ‘1973
