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J. Phys. Chem. 1902, 86, 4284-4286
Doublet-Doublet Annihilation in Chromium(I I I ) Polypyridine Complexes Marla Teresa Indelll, Roberto Ballardlnl, Carlo Albert0 Blgnozzl, and Franco Scandola Istituto Chimico dell' Universttt5, Centro di Fotochimica CNR, Ferrara, Ita/y (Received: June 30, 1982; I n Final Form: August 26, 1982)
Under high-intensity pulsed laser excitation, the doublet states of Cr(4,7-Mezphen)33+ and Cr(phedS3+decay with mixed first- and second-orderkinetics. The dependence of the kinetics on laser intensity, presence of quenchers, and ionic strength demonstrates the occurrence of a doublet-doublet annihilation process in competition with the unimolecular doublet decay. The bimolecular rate constants for the annihilation process have been determined and found to be substantially lower than those for diffusion. Both energy-transfer and electron-transfer mechanisms for the annihilation process are discussed.
Although excited-state annihilation processes have been known for a long time in the field of organic photochemistry,' no evidence has been reported until now for the occurrence of such processes in the photochemistry of coordination compounds.2 Whether the reasons for this could be due to excited-state lifetimes or to other more fundamental differences between organic and inorganic chromophores3 was up to now open to speculation. We report here what appears to be a clear example of excited-state annihilation in chromium(II1) polypyridine complexes. The photophysics of Cr"' polypyridine complexes in aqueous solution has been studied in considerable detail, particularly by Serpone, Hoffman, and their associate^.^ Excitation of these complexes into any of the prominent UV or visible absorption bands leads to fast (subnanosecond) and efficient (@ = 1)5 population of the lowest doublet state, 2Egin Oh microsymmetry.6 This doublet state is remarkably long-lived (7 = 60-600 ps depending on the ligands)' and can be conveniently monitored by using either the moderately efficient phosphorescence (@ N 10-3)8v9 or the intense excited-state absorption (ESA) in the 400-600-nm range.1° The doublet undergoes hydrolysis in alkaline solutions but it is appreciably stable (@ I in acidic solutions." As far as bimolecular processes are concerned, both energy tran~fer~3~Jl and electron transfer to the doublet7J0J2J3have been studied. (1)C. A. Parker, "Photoluminescence of Solutions", Elsevier, Amsterdam, 1968;J. B. Birks, 'Photophysics of Organic Molecules", Wiley, New York, 1970. (2)Some evidence has been reported for intramicellar triplet-triplet annihilation of Ru(bpy),2+ on the surface of SDS micelles: U. Lachish, M. Ottolenghi, and J. Rabani, J. Am. Chem. SOC.,99,8062 (1977). (3)Another relatively common feature of organic photochemistry which up to now has no inorganic counterpart is exciplex formation (see, R. Ballardini, G. Varani, L. Moggi, and V. Balzani, J. Am. Chem. Soc., 96,7123(1974);99,688(1977);the reasons for this may lie in the planar structure of most organic cromophores as compared to the tridimensional situation of inorganic complexes. (4) M. A. Jamieson, N. Serpone, and M. Z. Hoffman, Coord. Chem. Reu., 39, 121 (1981). (5) F.Bolletta, M. Maestri, and V. Balzani, J. Phys. Chem., 80,2499 (1976);N. Serpone, M. A. Jamieaon, and M. 2.Hoffman, J. Chem. Soc., Chem. Commun., 1006 (1980). (6)The *E, state is in thermal equilibrium with the 2T,state so that the two states can be taken as one. (7)B. Brunschwig and N. Sutin, J.Am. Chem. Soc., 100,7568(1978). (8)A. D.Kirk and G. B. Porter, J. Phys. Chem., 84,887 (1980). (9)M. A. Jamieson, N. Serpone, M. S.Henry, and M. Z. Hoffman, Inorg. Chem., 18,214 (1979). (10)N. Serpone, M. A. Jamieson, M. S.Henry, M. 2. Hoffman, F. Bolletta, and M. Maestri, J. Am. Chem. Soc., 101,2907 (1979). (11)R. Sriram, M.S. Henry, and M. 2.Hoffman, Inorg. Chem., 18, 1727 (1979);N. Serpone, M. A. Jamieson, and M. 2.Hoffman, ibid., 20, 3983 (1981). (12)R. Ballardini, G. Varani, F. Scandola, and V. Balzani, J. Am. Chem. Soc., 98,7432 (1976). 0022-3654/82/2086-4284$01.25/0
TABLE I: Kinetic Parameters of the D o u b l e t Decaya k,, M-' s-' I . L , ~M k , , s-' Cr(4,7-Me2phen),)'
Cr(~hen),~+
1.5 1.5 3 x 10-2 3 x 10-3 1.5
2.5 X l o J c 2.3 X 1.6 x 103 1.6 x 103 4 . 2 x 103
2.7 X 10' 3.0 X l o s d 5.0 x 107 2.5 x i o 7 5.0 x io7
a Deaerated s o l u t i o n , p H 3 (H,SO,), unless otherwise noted. Ionic strength adjusted with Na,SO,. Value affected by impurities in t h e electrolyte. d Aerated solution.
Also, a number of peculiar phenomena, such as groundstate self-quenching14and anion-induced lifetime lengthening15 have been reported for these systems. When deaerated aqueous solutions of tris(4,7-dimethylphenantroline)chromium(III),Cr(4,7-Me2phen)33+, were flashed with a high-intensity 347-nm laser pulse (-1 X M absorbed photons/pulse)16 the decay of the phosphorescent intensity was clearly nonexponential (Figure 1). That this was not an experimental artifact was shown by measuring the decay of the doublet absorption under the same experimental conditions. Again the decay was clearly nonexponential (Figure 2). This behavior was found to be strongly dependent on the intensity of the exciting laser pulse. In fact, using low-intensity pulses (-1 X lo4 M absorbed photons/pulse) the doublet decay became strictly exponential (Figure 3), with a lifetime of 620 ps which agrees well with that reported by Brunschwig and Sutin7 for this complex. A qualitatively similar, though less pronounced, behavior was exhibited by tris(phenantroline)chromium(111),Cr(phen)2+. The results strongly suggest that the doublet decay can be described as a competition between an overall unimolecular (mainly radiationless) decay process (eq 1)and a bimolecular annihilation process (eq 2). In eq 1 and 2,
kl
(2Eg)C I ( N N ) ~ ~ + (4A2g)Cr(NN):+
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2(2E,) Cr(NN)33+
k2
products
(1) (2)
NN denotes the ligand, kl is the sum of all the unimolecular processes depopulating the doublet state, k2 is the bimolecular rate constant of the annihilation process, and the products of reaction 2 are not defined (see below). (13)R.Ballardini, G.Varani, M. T. Indelli, F. Scandola, and V. Balzani, J. Am. Chem. SOC., 100,7219 (1978). (14)R. Sriram, M. 2.Hoffman, M. A. Jamieson, and N. Serpone, J. Am. Chem. Soc., 102,1754 (1980). (15) M. S. Henry and M. Z. Hoffman, Adu. Chem. Ser., No. 168,91 (1978). (16)J&K System 2000 frequency-doubled ruby laser, Applied Photophysics detection system; half-width of laser pulse, 20 ns.
0 1982 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 86, No. 22, 1982 l
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Figure 1. Oscilloscope trace (a) and first-order plot (b) for the phosphorescence decay (A = 730 nm) obtained at high laser intensity (- 1 X lo4 M absorbed photons/pulse).
Flgure 3. Oscilloscope trace (a) and first-order plot (b) for the phosM phorescence decay obtained at low laser intensity (-1 X absorbed photons/pulse).
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Flgure 4. First-order plots of the doublet decays (open symbols, left scale) obtained at different laser intensities. 'Corrected first-order" plots (eq 3) of the same data (black symbols, right scale). Laser intensities (photon concentration/pulse): circles, lo-' M; triangles, 2 X M; squares, lo-' M.
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Flgure 2. Oscilloscope trace (a) and first-order plot (b) for the decay of the doublet absorption (A = 380 nm) obtained at hlgh laser intensity (-1 X l o 4 M absorbed photons/pulse).
According to standard kinetic analyses, the integrated rate equation for such a scheme is [DIo(kl + kz[Dlt) In = kit (3) [Dlt(kl + kZ[DlO) where [DIoand [Dit are the doublet concentrations measured at zero time (i.e., immediately after the laser pulse) and at time t , respectively.
The consistency of the proposed mechanism can be checked by plotting the experimental decay data according to eq 3, using k, values obtained from the good first-order plots (Figure 3) observed at low laser pulse intensity and trial values for k2. The consistency should be judged from the degree of linearity obtained and from the ability of a single kz value to fit decays obtained at different laser pulse intensities (i.e., at varying proportions of first- and second-order pathways). Plots of experimental data according to eq 3 are shown in Figure 4. They have been obtained from the experimental data of absorbance vs. time (or the corresponding phosphorescence intensity vs. time) by using a value of 6100 for the molar absorptivity of the Cr(4,7-Me2phen)33+ doublet at 380 nm, as obtained independently from a relative method based on the benzophenone triplet as standard."J8 In all the experiments, relatively dilute (17) R. Bensasson, C. Salet, and V. Balzani, J. A m . Chem. SOC.,98, 3722 (1976).
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The Journal of Physical Chemistry, Vol. 86, No. 22, 1982
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solutions were used (absorbance 0.7 at 347 nm) in order to minimize inhomogeneity of the transient concentration along the 1-cm optical path of the cell. The internal consistency of the data is seen to be quite good. The k , values obtained by this procedure are collected in Table I. Inspection of Table I shows that the mechanism fits well the experimental kinetics even when the proportion of first- to second-order decay is changed by the presence of quenchers (k2independent of kl in aerated and deaerated solutions). Moreover, the effect of ionic strength on k2 is as expected for a bimolecular reaction between two 3+ charged species.19 As to the mechanism of the annihilation process (eq 2), both energy-transfer and electron-transfer mechanisms should be considered. Energy transfer is the mechanism commonly thought to be responsible for the organic triplet-triplet annihilation process. In this case, selection rules allow processes leading to a quartet ground-state molecule and an excited one, either in a quartet or in a doublet state. Since the lowest doublet is the only observable state and intersystem crossing is very efficient, there is now experimental way to check the energy-transfer hypothesis. It should be noticed that, in such a case, the observed k z (18) When determining extinction coefficients from relative methods and high-intensity laser pulses, the two following conditions should be carefully observed: (i) the sample and the standard solutions must have the same absorbance at the laser wavelength, and (5) both the sample and standard solution concentrations must largely exceed the concentration of absorbed photons/pulse. Condition ii may not be obviously met, particularly if the sample substance has a high ground-state absorptivity. Optically very dense solutions (absorbance values higher than 3) had to be used in the present case to meet the above conditions. (19) The effect is so pronounced that a t low ionic strength the presence of the second-order component can easily pas8 unnoticed. Actually, this work was started by the observation of strikingly different doublet decays in dilute and concentrated acid solution, a feature which might be easily misinterpreted as a pH effect.
Letters
values (Table I) would be half of the true rate constants of eq 2, since one of the doublet states is recycled within the laser pulse. An electron-transfer mechanism leading to Crn and CrN (excited-state disproportionation) is an attractive possibility. Such mechanisms are not usually taken into consideration when dealing with triplet-triplet annihilation of organic molecules. However, they could be important for metal complexes exhibiting a rich redox chemistry. Unfortunately, the thermodynamic requirements of such a process cannot be established in this case, since the redox potential for oxidation of Cr"' to Cr'" has never been measured. Any oxidation potential lower than about 3 V vs. NHE, however, would make this process accessible. Experimental attempts to detect Cr" in the flashed solutions were unsuccessful. This result, on the other hand, is not conclusive since the very long excited-state lifetime, coupled with the presumably high rate of back comproportionation might well lead to undetectably low steadystate concentrations of primary redox products. As a final point, it should be remarked that the rate constants for the annihilation process are definitely lower than those for the diffusion-controlled limit. This is in contrast with what is observed usually for triplet-triplet annihilation.20 No matter what the mechanism is, the low values seem to require some degree of nonadiabaticity in the annihilation process. Energy-transfer processes involving Cr"' complexes are generally known to be nonadiabatic, to an extent depending on the nephelauxetic effect of the ligands.21 Acknowledgment. We thank Dr. C. Chiorboli for assistance in the computer work. (20) A. Yekta and N. J. Turro, Mol. Photochem., 3, 307 (1972). (21) V. Balzani, M. T. Indelli, M. Maestri, D. Sandrini, and F. Scandola, J . Phys. Chem., 84,852 (1980).
