J. Phys. Chem. 1984,88, 2685-2686 The intensities of the transient signals were not reduced by successive laser excitation. These results lead to the conclusion that SI TI isc is effective and the nonphosphorescent character is not due to a photochemical process but to a very fast TI So nonradiative transition. In the foregoing discussion we have shown * that the TI state of phthalazine has both 3mr*and 3 n ~ character. This is consistent with the prediction of the theory proposed by Lim, Kanamaru, and Wassam2Ivz2that a triplet state with a strong
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(21) N. Kanamaru and E. C. Lim, J . Chem. Phys., 65, 4055 (1976). (22) W. A. Wassam, Jr., and E. C. Lim, J . Chem. Phys., 68,433 (1978).
2685
3nT*-3 aa* coupling exhibits very fast nonradiative decay. The present work presents the first direct demonstration of the strongly mixed nature a nonphosphorescent triplet state. Further investigations including the study of the temperature and host dependences of the signals are currently in progress. Acknowledgment. We thank Mr. K. Fujii and JEOL Co. for providing a preamplifier with a fast time resolution and a good S/N ratio. This research was supported by a Grant-In-Aid from The Mitsubishi Foundation. Registry No. Phthalazine, 253-52-1; biphenyl, 92-52-4.
Excited-State Absorption of Tris(phenanthroline)rhodium(I I I). A Handle on the Excited-State Behavior of a Powerful Photochemical Oxidant Maria Teresa Indelli, Angela Carioli, and Pranco Scandola* Istituto Chimico dell’Universitri and Centro di Studio sulla Fotochimica e Reattivitci degli Stati Eccitati dei Composit di Coordinazione del CNR, 441 00 Ferrara, Italy (Received: March 2, 1984)
Laser flash photolysis of R h ( ~ h e n ) ~in~aqueous + solution at room temperature and in 1:l water:ethylene glycol clear glasses at 77 K reveals an intense transient absorption. On the basis of comparison with emission lifetimes under the same experimental conditions, the transient absorption is assigned as an excited-state absorption (ESA) associated with the ligand-centered r-r* triplet state. The ESA offers a precious handle on the otherwise elusive (practically nonemitting, at rcom temperature) excited state of this powerful photochemical oxidant. Some aspects of the photophysics of Rh(~hen)~’+ are also elucidated by ESA, emission, and electron-transfer quenching studies. In particular, it is shown that (i), at room temperature, the T-T* triplet is in fast thermal equilibrium with a metal-centered d-d triplet state and (ii) the quantum yield of intersystem crossing at room temperature is unity.
Polypyridine-transition-metalcomplexes constitute a remarkable and widely studied class of redox photosensitizers, particularly from the standpoint of the photochemical conversion of solar energy.’-* Excited-state redox potentials, estimated on the basis of ground-state potentials and spectroscopic energies, indicate that very powerful excited-state oxidants or reductants can be found within this family, depending essentially on the type of central metal i ~ n . l - ~In principle, tris(phenanthroline)rhodium(III), R h ( ~ h e n ) , ~ and + , its bipyridine congener have outstanding oxidizing properties in their lowest excited state, with Eovalues of approximately +2 V.9 In spite of this, the practical use of these photosensitizers in excited-state electron transfer studies has been extremely limited.g-’2 This is no doubt due to the fact that
(1) Balzani, V.; Bolletta, F.; Gandolfi, M. T.; Maestri, M. Top. Curr. Chem. 1978, 75, 1. (2) Balzani, V.; Scandola, F. In “Photochemical Conversion and Storage of Solar Energy”; Connolly, J. S., Ed.; Academic Press: New York, 1981; p 97. (3) Balzani, V.; Scandola, F. In “Energy Resources through Photochemistry and Catalysis”; Gratzel, M., Ed.; Academic Press: New York, 1983; Chapter 1. (4) Sutin, N. J . Photochem. 1979, 10, 19. (5) Sutin, N.; Creutz, C. Pure Appl. Chem. 1980, 52, 2717. (6) Whitten, D. G. Acc. Chem. Res. 1980, 13, 83. (7) Kalyanasundaram, K. Coord. Chem. Reu. 1982, 46, 159. (8) Jamieson, M. A.; Serpone, N.; Hoffman, M. Z. Coord. Chem. Reu. 1981, 39, 121. (9) Ballardini, R.; Varani, G.; Balzani, V. J . Am. Chem. SOC. 1980, 202, 1719. (10) Lehn, J. M.; Sauvage, J. P. Nouu. J . Chim. 1977, I , 449. ( 1 I ) Indelli, M. T.; Ballardini, R.; Scandola, F. J. Phys. Chem., in press.
0022-3654/84/2088-2685$01 -~ SO10 ., I
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rhodium(II1)-polypyridine complexes are essentially nonluminescent in room-temperature fluid s o l ~ t i o n s , ~a Jfeature ~ which has made the monitoring of excited-state bimolecular processes with these systems a very difficult task. We have now discovered that these complexes exhibit prominent excited-state absorption (ESA) patterns in the visible. This finding brings into play from the experimental standpoint a variety of excited-state redox reactions of these sensitizers. On the other hand, a new, powerful tool is available for investigating the photophysics of these systems. We report here on the ESA properties of Rh(phen)?+ and on some preliminary photophysical results obtained on this molecule. When aqueous solutions of R h ( ~ h e n ) were ~ ~ + flashed at 298 K with 347-nm pulses from a ruby laser,14 an intense transient absorption was observed in the visible region, which decayed in the submicrosecond time scale. The decay of the transient absorption was strictly exponential over the whole spectral range with a constant lifetime of 250 f 20 ns. The transient spectrum (“initial” absorbance values, taken at the end of the pulse risetime, ca. 20 ns, converted into molar absorptivities by means of a relative (1 2 ) By contrast, the ground-state electron-transfer reactivity of rhodium(111)-polypyridine complexes has been widely studied: Kirch, M.; Lehn, J. M.; Sauvage, J. P. Helv. Chim. Acta 1979, 62, 1345. Creutz, C.; Keller, A. D.; Schwarz, H. A.; Sutin, N.; Zipp, A. P. In “Mechanistic Aspects of Inorganic Reactions”; Rorabacher, D. B.; Endicott, J. F. Eds., American Chemical Society: Washington, DC, 1982; ACS Symp. Ser. No. 198, p 385, and references cited therein. Creutz, C.; Keller, A. D.; Sutin, N.;Zipp, A. P. J . Am. Chem. SOC.1982, 104, 3618. (1 3) Bolletta, F.; Rossi, A.; Barigelletti, F.; Dellonte, S.; Balzani, V. Gazz. Chim. Ital. 1981, 1 1 1 , 155. (14) J&K System 2000 frequency-doubled ruby laser, Applied Photophysics detection system; half-width of laser pulse, 20 ns.
0 1984 American Chemical Society
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2686 The Journal of Physical Chemistry, Vol. 88, No. 13, 1984
'. I
I
I
40 0
500
600
'@
a'
a(nm)
Figure 1. Excited-state absorption (ESA) spectrum of Rh(phen)?+ in aqueous solution.
actinometric method, see below) is shown in Figure 1. It consists of a broad-band system peaking at 490 nm. When the same experiments were performed on solutions of R h ( ~ h e n ) , ~in+ 1:l waterethylene glycol clear glasses at 77 K,15 an identical transient spectrum was obtained. In these conditions, the transient lifetime was enormously increased ( T = 48 f 2 ms) with respect to room temperature. The emitting properties of Rh(phen)?+ have been characterized to a different extent at 77 K and room temperature. At 77 K, in a number of glassy solvents including 1:l water:ethylene glycol, R h ( ~ h e n ) ~has ~ +been found to exhibit an intense (a = 1),l6 = 22 200 cm-') which has been well-structured emission ( p assigned to a ligand-centered (LC) r a * phosphorescence.l6-l* At this temperature, the phosphorescence lifetime is reported to + be 48 m s . I 7 Until recently, no emission from R h ( ~ h e n ) ~at~room temperature had been o b ~ e r v e d .Bolletta ~ et al.,I3 however, have reported the observation of an exceedingly weak (@ N 10-s)19 emission in CH3CN at room temperature. This emission is broad and structureless, red shifted (A, = 17 300 cm-I), and much shorter lived ( T = 190 ns) with respect to the low-temperature emission. Bolletta et al. assigned this emission to a metal-centered (MC) d-d phosphorescence which they calculated to lie in this spectral region.13 We find that in room-temperature aqueous solution, both the 17 300- and 22 200-cm-l emissions are observable (the d-d one as a definite band and the a-a* one as a shoulder on the tail of the exciting light). The two-band profile is more (15) Low-temperature experiments were performed by using an Oxford Instruments DW 704 cryostatic equipment with quartz windows and standard 1-cm spectrofluorimetric cuvettes. (16) Hillis, J. E.; De Armond, M. K. J. Lumin. 1971, 4, 273. (17) De Armond, M. K.; Hillis, J. E. J. Chem. Phys. 1971, 54, 2247. (18) Crosby, G. A.; Elfring, W. H., Jr. J. Phys. Chem. 1976, 80, 2206. (19) Bolletta, F., private communication.
Letters evident in laser experiments, where the emissions can be distinguished temporally from excitation light. The important observation here is that the two emissions decay with the same lifetime of 250 f 20 ns. We interpret this finding as indicating that, at 298 K, the LC r-a* triplet state undergoes fast (subnanosecond) thermal equilibration with the lowest M C d-d triplet state. For times beyond nanosecond range, the two states behave in many respects (e.g., radiationless decay, bimolecular quenching) as a single excited state. The comparison between the lifetime of the transient absorption and that of the luminescence at both 298 and 77 K allows us to assign unequivocally the transient as an ESA. The fact that the shape and initial intensity of the ESA is unchanged when the temperature is changed from 77 to 298 K shows that (i) the observed ESA spectrum is that of the aa* LC triplet; (ii) in the fast equilibrium at room temperature the emitting d-d state is (iii) inonly present in a small fractional concentration (-3%); tersystem crossing to the aa* LC triplet is 100% efficient even at room temperature. In order to find further evidence for this last conclusion, the following laser scavenging experiment was performed. First, a relative actinometric method based on benzophenone triplet ESA20,z1was used to measure the absolute concentration of absorbed photons/laser pulse. Then, complete quenching of excited R h ( ~ h e n ) , ~was + achieved in deaerated solution by using Fe,? as a reductive quencher (eq 1, kq = 4.0 X lo9, = 1 M) and the
+
* R h ( ~ h e n ) ~ ~Fe,?+ +
k,
+
R h ( ~ h e n ) , ~ + Fe,:+
(1)
R h ( ~ h e n ) ~formed ~ + was quantitatively scavenged (to saturation) by methylviologen (MVz+) (eq 2) yielding the easily detectableZ2
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+
R h ( ~ h e n ) ~ ~MV+ + (2) R h ( ~ h e n ) ~+~MVz+ + reduced methylviologen cation (MV+). The result was that MV+ was formed in 1:l ratio with respect to the photons absorbed by R h ( ~ h e n ) , ~ +This . demonstrates that the yield of scavengeable products of eq 1 (cage escape yield) is unityz3and, more important, confirms that the efficiency of intersystem crossing to the triplet manifold in R h ( ~ h e n ) ~ is ~unity + even at room temperature. Work is in progress toward a deeper understanding of the photophysics of R h ( ~ h e n ) by ~ ~means + of detailed temperature dependence studies of ESA and emission. Acknowledgment. Partial support by the Progetto Finalizzato C N R Chimica Fine e Secondaria is gratefully acknowledged. Registry No. R h ( ~ h e n ) ~ , +47837-61-6. , (20) Bensasson, R.; Salet, C.; Balzani, V. J . Am. Chem. SOC.1976, 98, 3722. (21) Indelli, M. T.; Ballardini, R.; Bignozzi, C. A,; Scandola, F. J. Phys. Chem. 1982, 86, 4284. (22) Kalyanasundaram, K.; Kiwi, J.; Gratzel, M. Helu. Chim. Acta 1978, 61, 2270. (23) This result is not unexpected, based on previous knowledge of (i) the high cage escape yields in reductive quenching of R h ( ~ h e n ) ~by~ methoxy+ benzenes" and (ii) the slightly nonadiabatic, high intrinsic barrier behavior of the Fe,92+/3+ couple.24 (24) Balzani, V.; Scandola, F.; Orlandi, G.; Sabbatini, N.; Indelli, M. T. J. Am. Chem. SOC.1981, 103, 3370.
