38
J. Phys. Chem. 1981, 85, 38-46
Laser-Induced, Sequential Two-Photon Redox Processes of Transition-Metal Complexes. Effects of Coordination Environment, Laser Pulse Intensity, and Excited-State Lifetimes‘ R. Sriram, John F. Endicott,” and Klrkwood M. Cunnlngham Department of Chemistry, Wayne State Unlversity, Detroit, Michigan 48202 (Received: August 27, 1979; In Final Form: September 30, 1980)
Several cobalt(II1) and chromium(II1) complexes have been found to exhibit enhanced yields of oxidized ligand/reduced metal products for focused nitrogen laser as compared to CW 337-nm irradiations. Over an appreciable range of laser beam intensities, the enhanced rates of formation of redox products have been found to be second order in incident intensity, Io. At very high intensities a “saturation”of the effect has been observed, with the rates of product formation approaching a first-order dependence on Io. For cobalt(II1) complexes, the laser-enhanced redox effect has been found to increase when C O ( N H ~ )is~perdeuterated ~+ and to disappear when a coordinated ammonia is replaced by H20. This is in accord with the proposal that the effect is due to absorption of a second 337-nm photon by an intermediate ligand field excited state, and the expectation that excited-statelifetimes should increase with perdeuteration and decrease when coordinated NH3is replaced by water. The effect of variation of excited-state lifetime on laser-enhancedredox yields has been systematically investigated by using tr~ns-cr(NH~)~(NCS)*-. For this complex, the laser photolyses have been carried out by using various acetone/water mixtures to “tune” the lifetime, TLF, of the intermediate 2E excited state. It has been found that the dependence of the laser-enhanced redox yields on TW and Io are in excellent agreement with the predictions of a simple stationary-state model.
Introduction Photoinduced homolytic processes of transition-metal complexes have been extensively investigateda2 A matter of concern in the various models proposed to account for these processes has been the role of nonspectroscopic excited states, usually states differing in spin multiplicity from the ground ~ t a t e . ~Attempts .~ to probe the susceptibility of nonspectroscopic states to metal ligand homolysis by using chemical sensitizers4s5have proved ambiguous owing to a variety of chemical complications.6-8 Several years ago we began the investigation of photohomolytic processes stimulated by sequential two-photon absorption in the substrate of interestsg In principle this technique could provide an alternative means for the exploration of the photochemical behavior of reactive nonspectroscopic states. Thus, in fluid solutions one expects (1)Partial reports of this work were made a t the 173rd National Meeting of the American Chemical Society, New Orleans, LA, March 1977 (INOR 107)and at the Second joint Conference of the CIC and ACS, Montreal, Canada, May-June 1977 (INOR 045). (2)For reviews see: (a) Adamson, A. W. Discuss. Faraday SOC. 1960, 29, 163. (b) Adamson, A. W.; Waltz, W. L.; Zinato, E.; Watts, D. W.; Fleischauer, P. D.; Lindholm, R. D. Chem. Rev. 1968,68,541.(c) Balzani, V.; Carassiti, V. “Photochemistry of Coordination Compounds”; Academic Press: New York, 1970. (d) Endicott, J. F. In “Concepts of Inorganic Photochemistry”; Adamson, A. W., Fleischauer, P. D., Eds.; Wiley: New York, 1975,Chapter 3. (3)(a) Endicott, J. F.; Ferraudi, G. J.; Barber, J. R. J.Phys. Chem. 1975, 79,630. (b) Endicott, J. F. Inorg. Chern. 1975,14,2129. (c) Endicott, J. F.; Ferraudi, G. J. Ibid. 1975,14,3133.(d) Mok, C. Y.; Endicott, J. F. J. Am. Chem. SOC.1977,99,1276. (4)(a) Natarajan, P.; Endicott, J. F. J.Am. Chem. SOC.1972,94,5909. (b) Natarajan, P.; Endicott, J. F. J.Phys. Chem. 1973,77,2049. (5)Hall, B. S.;Dahnke, K. F.; Fratoni, S. S., Jr.; Perone, S. P. J.Phys. Chem. 1977,81,866. (6)(a) Gafney, H.D.; Adamson, A. W. J. Am. Chem. Soc. 1972,94, 8238. (b) Gafney, H. D.; Adamson, A. W. J. Phys. Chem. 1972,76,1105. (7)(a) Balzani, V.;Moggi, L.; Manfrin, M. F.; Bolletta, F.; Laurence, G. S. Coord. Chem. Reu. 1975,15, 321. (b) Balzani, V.;Bolletta, F.; Gandolfi, M. T.; Maestri, M. Top. Curr. Chem. 1978, 75,1. (8)Navon, G.;Sutin, N. Inorg. Chem. 1974,13, 2159. (9)(a) Cunningham, K. M.; Endicott, J. F. J. Chem. SOC.,Chem. Commun. 1974,1024.(b) Sriram, R.; Endicott, J. F.; Pyke, S. C. J. Am. Chem. SOC.1977,99,4824. 0022-3654/81/2085-0038$01 .OO/O
vibrational relaxation and other thermalization processes to be rapid,213J0J1especially in systems for which the quantum yield of products is nearly wavelength independent. Furthermore, intersystem crossing processes between excited states of different spin multiplicities seem to be very rapid for transition-metal complexes.12J3 Therefore, in complexes with ligand field states lowest in energy, ligand field excitation would generally be expected to produce a thermalized ligand field excited state with spin multiplicity different from the ground state. This new state, *(LF)o,might persist for a short time and will often be substitutionally labile.13-ls The spin-allowed electronic transitions of *(LF), would provide photochemical access to electronic manifolds not directly populated from the ground state. While sequential two-photon photochemistry is crucial in chlorophyll photosystems,1618 and while there have been many studies of the two-photon photochemistry and spectroscopy of organic dye moleculeslSz1 (IO) Adamson, A. W. In “Concepts of Inorganic Photochemistry”; Adamson, A. W., Fleischauer, P. D., Eds.; Wiley: New York, 1975; Chapter 10,p 413. (11)Rapid thermalization, even in excited states undergoing homolysis, has been inferred from picosecond studies of org&oco6alamins: Endicott, J. F.; Netzel, T. J.Am. Chem. SOC.1979,101,4000. (12)(a) Kirk, A. D.; Hoggard, P. E.; Porter, G. B.; Rockley, M. G.; Windsor, M. W. Chem. Phys. Lett. 1976, 37, 199. (b) Pyke, S. C.; Windsor, M. W. J. Am. Chem. SOC.1978,100,6518. (13)Bergkamp, M.A.;Watts, R. J.; Ford, P. C.; Brannon, J.; Magde, D. Chem. Phys. Lett. 1978,59,125. (14)(a) Adamson, A. W.; Walters, R. T.; Fukuda, R.; Gutierrez, A. R. J. Am. Chem. SOC. 1978,100,5241.(b) Gutierrez, A. R.; Adamson, A. W. J. Phys. Chem. 1978,82,902. (15)Pyke, S. C.; Ogasawara, M.; Kevan, L.; Endicott, J. F. J. Phys. Chem. 1978,82,302. (16)Calvin, M.Acc. Chem. Res. 1978,11, 369. (17)Bolton, J. R. Science 1978,202,705. (18)(a) Fong, F. K. “Theory of Molecular Relaxation”; Wiley: New York, 1975; (b) Fong, F. K. J. Am. Chem. SOC.1976, 98, 7840. (c) Winograd, N.; Shepard, A.; Karweik, D. H.; Koester, V. J.; Fong, F. K. Ibid. 1976,98,2369. (19)McClain, W. M. Acc. Chem. Res. 1974,7, 129. (20)Wright, J. C. Top. Appl. Phys. 1976,15, 249. (21)(a) Naquivi, K. R.; Sharma, D. K.; Hoytink, G. J. Chem. Phys. Lett. 1973,22,5. (b) Ibid. 1973,22,221. 0 1981 American Chemical Society
Laser-Induced, Two-Photon Redox Processes
The Journal of Physical Chemistry, Vol. 85, No. 7, 198 1 39
TABLE I: R e d o x Q u a n t u m Yields f o r Some Co(II1) and Cr(II1) Complexes usin 3 3 7 - n m Pulsed L a s e P a n d C o n t i n u o u s Irradiation
f
t
t
@(CO2+)aJ laser
complex Co( NH,),
Co( ND,),
@(COZ+)b @(CO*+)b a t L, a t 254 n m
